changeset 31345:1bba15125d8d

8015086: add interned strings to the shared archive. Summary: Support saving interned strings in shared CDS archive. Reviewed-by: coleenp, iklam, pliden
author jiangli
date Fri, 12 Jun 2015 17:29:14 -0400
parents 2316eb7a0358
children a70d45c06136
files hotspot/src/share/vm/classfile/compactHashtable.cpp hotspot/src/share/vm/classfile/compactHashtable.hpp hotspot/src/share/vm/classfile/javaClasses.hpp hotspot/src/share/vm/classfile/stringTable.cpp hotspot/src/share/vm/classfile/stringTable.hpp hotspot/src/share/vm/classfile/symbolTable.cpp hotspot/src/share/vm/gc/g1/g1StringDedupThread.cpp hotspot/src/share/vm/gc/g1/g1StringDedupThread.hpp hotspot/src/share/vm/memory/filemap.cpp hotspot/src/share/vm/memory/filemap.hpp hotspot/src/share/vm/memory/metaspace.cpp hotspot/src/share/vm/memory/metaspaceShared.cpp hotspot/src/share/vm/memory/metaspaceShared.hpp hotspot/src/share/vm/memory/universe.cpp hotspot/src/share/vm/prims/whitebox.cpp hotspot/src/share/vm/runtime/synchronizer.cpp hotspot/src/share/vm/services/virtualMemoryTracker.cpp
diffstat 17 files changed, 724 insertions(+), 116 deletions(-) [+]
line wrap: on
line diff
--- a/hotspot/src/share/vm/classfile/compactHashtable.cpp	Fri Jun 12 12:55:32 2015 +0200
+++ b/hotspot/src/share/vm/classfile/compactHashtable.cpp	Fri Jun 12 17:29:14 2015 -0400
@@ -32,11 +32,11 @@
 //
 // The compact hash table writer implementations
 //
-CompactHashtableWriter::CompactHashtableWriter(const char* table_name,
+CompactHashtableWriter::CompactHashtableWriter(int table_type,
                                                int num_entries,
                                                CompactHashtableStats* stats) {
   assert(DumpSharedSpaces, "dump-time only");
-  _table_name = table_name;
+  _type = table_type;
   _num_entries = num_entries;
   _num_buckets = number_of_buckets(_num_entries);
   _buckets = NEW_C_HEAP_ARRAY(Entry*, _num_buckets, mtSymbol);
@@ -99,7 +99,7 @@
                                           NumberSeq* summary) {
   int index;
   juint* compact_table = p;
-  // Find the start of the buckets, skip the compact_bucket_infos table
+  // Compute the start of the buckets, include the compact_bucket_infos table
   // and the table end offset.
   juint offset = _num_buckets + 1;
   *first_bucket = compact_table + offset;
@@ -130,10 +130,17 @@
 // Write the compact table's entries
 juint* CompactHashtableWriter::dump_buckets(juint* compact_table, juint* p,
                                             NumberSeq* summary) {
-  uintx base_address = uintx(MetaspaceShared::shared_rs()->base());
-  uintx max_delta    = uintx(MetaspaceShared::shared_rs()->size());
-  assert(max_delta <= 0x7fffffff, "range check");
+  uintx base_address = 0;
+  uintx max_delta = 0;
   int num_compact_buckets = 0;
+  if (_type == CompactHashtable<Symbol*, char>::_symbol_table) {
+    base_address = uintx(MetaspaceShared::shared_rs()->base());
+    max_delta    = uintx(MetaspaceShared::shared_rs()->size());
+    assert(max_delta <= 0x7fffffff, "range check");
+  } else {
+    assert((_type == CompactHashtable<oop, char>::_string_table), "unknown table");
+    assert(UseCompressedOops, "UseCompressedOops is required");
+  }
 
   assert(p != NULL, "sanity");
   for (int index = 0; index < _num_buckets; index++) {
@@ -148,12 +155,16 @@
     for (Entry* tent = _buckets[index]; tent;
          tent = tent->next()) {
       if (bucket_type == REGULAR_BUCKET_TYPE) {
-        *p++ = juint(tent->hash()); // write symbol hash
+        *p++ = juint(tent->hash()); // write entry hash
       }
-      uintx deltax = uintx(tent->value()) - base_address;
-      assert(deltax < max_delta, "range check");
-      juint delta = juint(deltax);
-      *p++ = delta; // write symbol offset
+      if (_type == CompactHashtable<Symbol*, char>::_symbol_table) {
+        uintx deltax = uintx(tent->value()) - base_address;
+        assert(deltax < max_delta, "range check");
+        juint delta = juint(deltax);
+        *p++ = delta; // write entry offset
+      } else {
+        *p++ = oopDesc::encode_heap_oop(tent->string());
+      }
       count ++;
     }
     assert(count == _bucket_sizes[index], "sanity");
@@ -174,6 +185,10 @@
 
   uintx base_address = uintx(MetaspaceShared::shared_rs()->base());
 
+  // Now write the following at the beginning of the table:
+  //      base_address (uintx)
+  //      num_entries  (juint)
+  //      num_buckets  (juint)
   *p++ = high(base_address);
   *p++ = low (base_address); // base address
   *p++ = _num_entries;  // number of entries in the table
@@ -191,7 +206,8 @@
     if (_num_entries > 0) {
       avg_cost = double(_required_bytes)/double(_num_entries);
     }
-    tty->print_cr("Shared %s table stats -------- base: " PTR_FORMAT, _table_name, (intptr_t)base_address);
+    tty->print_cr("Shared %s table stats -------- base: " PTR_FORMAT,
+                  table_name(), (intptr_t)base_address);
     tty->print_cr("Number of entries       : %9d", _num_entries);
     tty->print_cr("Total bytes used        : %9d", (int)((*top) - old_top));
     tty->print_cr("Average bytes per entry : %9.3f", avg_cost);
@@ -202,12 +218,24 @@
   }
 }
 
+const char* CompactHashtableWriter::table_name() {
+  switch (_type) {
+  case CompactHashtable<Symbol*, char>::_symbol_table: return "symbol";
+  case CompactHashtable<oop, char>::_string_table: return "string";
+  default:
+    ;
+  }
+  return "unknown";
+}
+
 /////////////////////////////////////////////////////////////
 //
 // The CompactHashtable implementation
 //
-template <class T, class N> const char* CompactHashtable<T, N>::init(const char* buffer) {
+template <class T, class N> const char* CompactHashtable<T, N>::init(
+                           CompactHashtableType type, const char* buffer) {
   assert(!DumpSharedSpaces, "run-time only");
+  _type = type;
   juint*p = (juint*)buffer;
   juint upper = *p++;
   juint lower = *p++;
@@ -245,8 +273,34 @@
   }
 }
 
+template <class T, class N> void CompactHashtable<T, N>::oops_do(OopClosure* f) {
+  assert(!DumpSharedSpaces, "run-time only");
+  assert(_type == _string_table || _bucket_count == 0, "sanity");
+  for (juint i = 0; i < _bucket_count; i ++) {
+    juint bucket_info = _buckets[i];
+    juint bucket_offset = BUCKET_OFFSET(bucket_info);
+    int   bucket_type = BUCKET_TYPE(bucket_info);
+    juint* bucket = _buckets + bucket_offset;
+    juint* bucket_end = _buckets;
+
+    narrowOop o;
+    if (bucket_type == COMPACT_BUCKET_TYPE) {
+      o = (narrowOop)bucket[0];
+      f->do_oop(&o);
+    } else {
+      bucket_end += BUCKET_OFFSET(_buckets[i + 1]);
+      while (bucket < bucket_end) {
+        o = (narrowOop)bucket[1];
+        f->do_oop(&o);
+        bucket += 2;
+      }
+    }
+  }
+}
+
 // Explicitly instantiate these types
 template class CompactHashtable<Symbol*, char>;
+template class CompactHashtable<oop, char>;
 
 #ifndef O_BINARY       // if defined (Win32) use binary files.
 #define O_BINARY 0     // otherwise do nothing.
@@ -273,6 +327,8 @@
   _p = _base;
   _end = _base + st.st_size;
   _filename = filename;
+  _prefix_type = Unknown;
+  _line_no = 1;
 }
 
 HashtableTextDump::~HashtableTextDump() {
@@ -286,9 +342,9 @@
   vm_exit_during_initialization(err, msg);
 }
 
-void HashtableTextDump::corrupted(const char *p) {
+void HashtableTextDump::corrupted(const char *p, const char* msg) {
   char info[60];
-  sprintf(info, "corrupted at pos %d", (int)(p - _base));
+  sprintf(info, "%s. Corrupted at line %d (file pos %d)", msg, _line_no, (int)(p - _base));
   quit(info, _filename);
 }
 
@@ -298,8 +354,9 @@
   } else if (_p[0] == '\n') {
     _p += 1;
   } else {
-    corrupted(_p);
+    corrupted(_p, "Unexpected character");
   }
+  _line_no ++;
   return true;
 }
 
@@ -328,26 +385,60 @@
   skip_newline();
 }
 
+void HashtableTextDump::scan_prefix_type() {
+  _p ++;
+  if (strncmp(_p, "SECTION: String", 15) == 0) {
+    _p += 15;
+    _prefix_type = StringPrefix;
+  } else if (strncmp(_p, "SECTION: Symbol", 15) == 0) {
+    _p += 15;
+    _prefix_type = SymbolPrefix;
+  } else {
+    _prefix_type = Unknown;
+  }
+  skip_newline();
+}
 
-int HashtableTextDump::scan_prefix() {
+int HashtableTextDump::scan_prefix(int* utf8_length) {
+  if (*_p == '@') {
+    scan_prefix_type();
+  }
+
+  switch (_prefix_type) {
+  case SymbolPrefix:
+    *utf8_length = scan_symbol_prefix(); break;
+  case StringPrefix:
+    *utf8_length = scan_string_prefix(); break;
+  default:
+    tty->print_cr("Shared input data type: Unknown.");
+    corrupted(_p, "Unknown data type");
+  }
+
+  return _prefix_type;
+}
+
+int HashtableTextDump::scan_string_prefix() {
   // Expect /[0-9]+: /
-  int utf8_length = get_num(':');
+  int utf8_length;
+  get_num(':', &utf8_length);
   if (*_p != ' ') {
-    corrupted(_p);
+    corrupted(_p, "Wrong prefix format for string");
   }
   _p++;
   return utf8_length;
 }
 
-int HashtableTextDump::scan_prefix2() {
+int HashtableTextDump::scan_symbol_prefix() {
   // Expect /[0-9]+ (-|)[0-9]+: /
-  int utf8_length = get_num(' ');
-  if (*_p == '-') {
-    _p++;
+  int utf8_length;
+  get_num(' ', &utf8_length);
+    if (*_p == '-') {
+     _p++;
   }
-  (void)get_num(':');
+  int ref_num;
+  (void)get_num(':', &ref_num);
   if (*_p != ' ') {
-    corrupted(_p);
+    corrupted(_p, "Wrong prefix format for symbol");
   }
   _p++;
   return utf8_length;
@@ -408,7 +499,7 @@
       case 'r':  *to++ = '\r'; break;
       case '\\': *to++ = '\\'; break;
       default:
-        ShouldNotReachHere();
+        corrupted(_p, "Unsupported character");
       }
     }
   }
--- a/hotspot/src/share/vm/classfile/compactHashtable.hpp	Fri Jun 12 12:55:32 2015 +0200
+++ b/hotspot/src/share/vm/classfile/compactHashtable.hpp	Fri Jun 12 17:29:14 2015 -0400
@@ -28,6 +28,7 @@
 #include "classfile/stringTable.hpp"
 #include "classfile/symbolTable.hpp"
 #include "memory/allocation.inline.hpp"
+#include "oops/oop.inline.hpp"
 #include "oops/symbol.hpp"
 #include "services/diagnosticCommand.hpp"
 #include "utilities/hashtable.hpp"
@@ -49,7 +50,7 @@
 // the compact table to the shared archive.
 //
 // At dump time, the CompactHashtableWriter obtains all entries from the
-// symbol table and adds them to a new temporary hash table. The hash
+// symbol/string table and adds them to a new temporary hash table. The hash
 // table size (number of buckets) is calculated using
 // '(num_entries + bucket_size - 1) / bucket_size'. The default bucket
 // size is 4 and can be changed by -XX:SharedSymbolTableBucketSize option.
@@ -57,14 +58,14 @@
 // faster lookup. It also has relatively small number of empty buckets and
 // good distribution of the entries.
 //
-// We use a simple hash function (symbol_hash % num_bucket) for the table.
+// We use a simple hash function (hash % num_bucket) for the table.
 // The new table is compacted when written out. Please see comments
 // above the CompactHashtable class for the table layout detail. The bucket
 // offsets are written to the archive as part of the compact table. The
 // bucket offset is encoded in the low 30-bit (0-29) and the bucket type
 // (regular or compact) are encoded in bit[31, 30]. For buckets with more
-// than one entry, both symbol hash and symbol offset are written to the
-// table. For buckets with only one entry, only the symbol offset is written
+// than one entry, both hash and entry offset are written to the
+// table. For buckets with only one entry, only the entry offset is written
 // to the table and the buckets are tagged as compact in their type bits.
 // Buckets without entry are skipped from the table. Their offsets are
 // still written out for faster lookup.
@@ -78,6 +79,7 @@
 
   public:
     Entry(unsigned int hash, Symbol *symbol) : _next(NULL), _hash(hash), _literal(symbol) {}
+    Entry(unsigned int hash, oop string)     : _next(NULL), _hash(hash), _literal(string) {}
 
     void *value() {
       return _literal;
@@ -85,6 +87,9 @@
     Symbol *symbol() {
       return (Symbol*)_literal;
     }
+    oop string() {
+      return (oop)_literal;
+    }
     unsigned int hash() {
       return _hash;
     }
@@ -95,7 +100,7 @@
 private:
   static int number_of_buckets(int num_entries);
 
-  const char* _table_name;
+  int _type;
   int _num_entries;
   int _num_buckets;
   juint* _bucket_sizes;
@@ -105,7 +110,7 @@
 
 public:
   // This is called at dump-time only
-  CompactHashtableWriter(const char* table_name, int num_entries, CompactHashtableStats* stats);
+  CompactHashtableWriter(int table_type, int num_entries, CompactHashtableStats* stats);
   ~CompactHashtableWriter();
 
   int get_required_bytes() {
@@ -116,6 +121,10 @@
     add(hash, new Entry(hash, symbol));
   }
 
+  void add(unsigned int hash, oop string) {
+    add(hash, new Entry(hash, string));
+  }
+
 private:
   void add(unsigned int hash, Entry* entry);
   juint* dump_table(juint* p, juint** first_bucket, NumberSeq* summary);
@@ -123,6 +132,7 @@
 
 public:
   void dump(char** top, char* end);
+  const char* table_name();
 };
 
 #define REGULAR_BUCKET_TYPE       0
@@ -136,23 +146,23 @@
 
 /////////////////////////////////////////////////////////////////////////////
 //
-// CompactHashtable is used to stored the CDS archive's symbol table. Used
+// CompactHashtable is used to stored the CDS archive's symbol/string table. Used
 // at runtime only to access the compact table from the archive.
 //
 // Because these tables are read-only (no entries can be added/deleted) at run-time
 // and tend to have large number of entries, we try to minimize the footprint
 // cost per entry.
 //
-// Layout of compact symbol table in the shared archive:
+// Layout of compact table in the shared archive:
 //
 //   uintx base_address;
-//   juint num_symbols;
+//   juint num_entries;
 //   juint num_buckets;
 //   juint bucket_infos[num_buckets+1]; // bit[31,30]: type; bit[29-0]: offset
 //   juint table[]
 //
 // -----------------------------------
-// | base_address  | num_symbols     |
+// | base_address  | num_entries     |
 // |---------------------------------|
 // | num_buckets   | bucket_info0    |
 // |---------------------------------|
@@ -177,9 +187,13 @@
 // compact buckets have '01' in their highest 2-bit, and regular buckets have
 // '00' in their highest 2-bit.
 //
-// For normal buckets, each symbol's entry is 8 bytes in the table[]:
-//   juint hash;    /* symbol hash */
-//   juint offset;  /* Symbol* sym = (Symbol*)(base_address + offset) */
+// For normal buckets, each entry is 8 bytes in the table[]:
+//   juint hash;    /* symbol/string hash */
+//   union {
+//     juint offset;  /* Symbol* sym = (Symbol*)(base_address + offset) */
+//     narrowOop str; /* String narrowOop encoding */
+//   }
+//
 //
 // For compact buckets, each entry has only the 4-byte 'offset' in the table[].
 //
@@ -189,19 +203,41 @@
 //
 template <class T, class N> class CompactHashtable VALUE_OBJ_CLASS_SPEC {
   friend class VMStructs;
+
+ public:
+  enum CompactHashtableType {
+    _symbol_table = 0,
+    _string_table = 1
+  };
+
+private:
+  CompactHashtableType _type;
   uintx  _base_address;
   juint  _entry_count;
   juint  _bucket_count;
   juint  _table_end_offset;
   juint* _buckets;
 
-  inline bool equals(T entry, const char* name, int len) {
-    if (entry->equals(name, len)) {
-      assert(entry->refcount() == -1, "must be shared");
-      return true;
-    } else {
-      return false;
+  inline Symbol* lookup_entry(CompactHashtable<Symbol*, char>* const t,
+                              juint* addr, const char* name, int len) {
+    Symbol* sym = (Symbol*)((void*)(_base_address + *addr));
+    if (sym->equals(name, len)) {
+      assert(sym->refcount() == -1, "must be shared");
+      return sym;
     }
+
+    return NULL;
+  }
+
+  inline oop lookup_entry(CompactHashtable<oop, char>* const t,
+                        juint* addr, const char* name, int len) {
+    narrowOop obj = (narrowOop)(*addr);
+    oop string = oopDesc::decode_heap_oop(obj);
+    if (java_lang_String::equals(string, (jchar*)name, len)) {
+      return string;
+    }
+
+    return NULL;
   }
 
 public:
@@ -211,7 +247,14 @@
     _table_end_offset = 0;
     _buckets = 0;
   }
-  const char* init(const char *buffer);
+  const char* init(CompactHashtableType type, const char *buffer);
+
+  void reset() {
+    _entry_count = 0;
+    _bucket_count = 0;
+    _table_end_offset = 0;
+    _buckets = 0;
+  }
 
   // Lookup an entry from the compact table
   inline T lookup(const N* name, unsigned int hash, int len) {
@@ -225,23 +268,22 @@
       juint* bucket_end = _buckets;
 
       if (bucket_type == COMPACT_BUCKET_TYPE) {
-        // the compact bucket has one entry with symbol offset only
-        T entry = (T)((void*)(_base_address + bucket[0]));
-        if (equals(entry, name, len)) {
-          return entry;
+        // the compact bucket has one entry with entry offset only
+        T res = lookup_entry(this, &bucket[0], name, len);
+        if (res != NULL) {
+          return res;
         }
       } else {
         // This is a regular bucket, which has more than one
-        // entries. Each entry is a pair of symbol (hash, offset).
+        // entries. Each entry is a pair of entry (hash, offset).
         // Seek until the end of the bucket.
         bucket_end += BUCKET_OFFSET(_buckets[index + 1]);
         while (bucket < bucket_end) {
           unsigned int h = (unsigned int)(bucket[0]);
           if (h == hash) {
-            juint offset = bucket[1];
-            T entry = (T)((void*)(_base_address + offset));
-            if (equals(entry, name, len)) {
-              return entry;
+            T res = lookup_entry(this, &bucket[1], name, len);
+            if (res != NULL) {
+              return res;
             }
           }
           bucket += 2;
@@ -253,12 +295,15 @@
 
   // iterate over symbols
   void symbols_do(SymbolClosure *cl);
+
+  // iterate over strings
+  void oops_do(OopClosure* f);
 };
 
 ////////////////////////////////////////////////////////////////////////
 //
 // Read/Write the contents of a hashtable textual dump (created by
-// SymbolTable::dump).
+// SymbolTable::dump and StringTable::dump).
 // Because the dump file may be big (hundred of MB in extreme cases),
 // we use mmap for fast access when reading it.
 //
@@ -269,21 +314,29 @@
   const char* _end;
   const char* _filename;
   size_t      _size;
+  int         _prefix_type;
+  int         _line_no;
 public:
   HashtableTextDump(const char* filename);
   ~HashtableTextDump();
 
+  enum {
+    SymbolPrefix = 1 << 0,
+    StringPrefix = 1 << 1,
+    Unknown = 1 << 2
+  };
+
   void quit(const char* err, const char* msg);
 
   inline int remain() {
     return (int)(_end - _p);
   }
 
-  void corrupted(const char *p);
+  void corrupted(const char *p, const char *msg);
 
   inline void corrupted_if(bool cond) {
     if (cond) {
-      corrupted(_p);
+      corrupted(_p, NULL);
     }
   }
 
@@ -292,7 +345,7 @@
   void skip_past(char c);
   void check_version(const char* ver);
 
-  inline int get_num(char delim) {
+  inline bool get_num(char delim, int *utf8_length) {
     const char* p   = _p;
     const char* end = _end;
     int num = 0;
@@ -303,18 +356,22 @@
         num = num * 10 + (c - '0');
       } else if (c == delim) {
         _p = p;
-        return num;
+        *utf8_length = num;
+        return true;
       } else {
-        corrupted(p-1);
+        // Not [0-9], not 'delim'
+        return false;
       }
     }
-    corrupted(_end);
+    corrupted(_end, "Incorrect format");
     ShouldNotReachHere();
-    return 0;
+    return false;
   }
 
-  int scan_prefix();
-  int scan_prefix2();
+  void scan_prefix_type();
+  int scan_prefix(int* utf8_length);
+  int scan_string_prefix();
+  int scan_symbol_prefix();
 
   jchar unescape(const char* from, const char* end, int count);
   void get_utf8(char* utf8_buffer, int utf8_length);
--- a/hotspot/src/share/vm/classfile/javaClasses.hpp	Fri Jun 12 12:55:32 2015 +0200
+++ b/hotspot/src/share/vm/classfile/javaClasses.hpp	Fri Jun 12 17:29:14 2015 -0400
@@ -118,6 +118,10 @@
     return hash_offset;
   }
 
+  static void set_value_raw(oop string, typeArrayOop buffer) {
+    assert(initialized, "Must be initialized");
+    string->obj_field_put_raw(value_offset, buffer);
+  }
   static void set_value(oop string, typeArrayOop buffer) {
     assert(initialized && (value_offset > 0), "Must be initialized");
     string->obj_field_put(value_offset, (oop)buffer);
@@ -210,6 +214,7 @@
   // Debugging
   static void print(oop java_string, outputStream* st);
   friend class JavaClasses;
+  friend class StringTable;
 };
 
 
--- a/hotspot/src/share/vm/classfile/stringTable.cpp	Fri Jun 12 12:55:32 2015 +0200
+++ b/hotspot/src/share/vm/classfile/stringTable.cpp	Fri Jun 12 17:29:14 2015 -0400
@@ -38,6 +38,7 @@
 #include "utilities/hashtable.inline.hpp"
 #include "utilities/macros.hpp"
 #if INCLUDE_ALL_GCS
+#include "gc/g1/g1CollectedHeap.hpp"
 #include "gc/g1/g1SATBCardTableModRefBS.hpp"
 #include "gc/g1/g1StringDedup.hpp"
 #endif
@@ -87,19 +88,28 @@
 
 // --------------------------------------------------------------------------
 StringTable* StringTable::_the_table = NULL;
-
+bool StringTable::_ignore_shared_strings = false;
 bool StringTable::_needs_rehashing = false;
 
 volatile int StringTable::_parallel_claimed_idx = 0;
 
+CompactHashtable<oop, char> StringTable::_shared_table;
+
 // Pick hashing algorithm
 unsigned int StringTable::hash_string(const jchar* s, int len) {
   return use_alternate_hashcode() ? AltHashing::murmur3_32(seed(), s, len) :
                                     java_lang_String::hash_code(s, len);
 }
 
-oop StringTable::lookup(int index, jchar* name,
-                        int len, unsigned int hash) {
+oop StringTable::lookup_shared(jchar* name, int len) {
+  // java_lang_String::hash_code() was used to compute hash values in the shared table. Don't
+  // use the hash value from StringTable::hash_string() as it might use alternate hashcode.
+  return _shared_table.lookup((const char*)name,
+                              java_lang_String::hash_code(name, len), len);
+}
+
+oop StringTable::lookup_in_main_table(int index, jchar* name,
+                                int len, unsigned int hash) {
   int count = 0;
   for (HashtableEntry<oop, mtSymbol>* l = bucket(index); l != NULL; l = l->next()) {
     count++;
@@ -140,7 +150,8 @@
   // Since look-up was done lock-free, we need to check if another
   // thread beat us in the race to insert the symbol.
 
-  oop test = lookup(index, name, len, hashValue); // calls lookup(u1*, int)
+  // No need to lookup the shared table from here since the caller (intern()) already did
+  oop test = lookup_in_main_table(index, name, len, hashValue); // calls lookup(u1*, int)
   if (test != NULL) {
     // Entry already added
     return test;
@@ -172,9 +183,14 @@
 }
 
 oop StringTable::lookup(jchar* name, int len) {
+  oop string = lookup_shared(name, len);
+  if (string != NULL) {
+    return string;
+  }
+
   unsigned int hash = hash_string(name, len);
   int index = the_table()->hash_to_index(hash);
-  oop string = the_table()->lookup(index, name, len, hash);
+  string = the_table()->lookup_in_main_table(index, name, len, hash);
 
   ensure_string_alive(string);
 
@@ -184,9 +200,14 @@
 
 oop StringTable::intern(Handle string_or_null, jchar* name,
                         int len, TRAPS) {
+  oop found_string = lookup_shared(name, len);
+  if (found_string != NULL) {
+    return found_string;
+  }
+
   unsigned int hashValue = hash_string(name, len);
   int index = the_table()->hash_to_index(hashValue);
-  oop found_string = the_table()->lookup(index, name, len, hashValue);
+  found_string = the_table()->lookup_in_main_table(index, name, len, hashValue);
 
   // Found
   if (found_string != NULL) {
@@ -611,3 +632,131 @@
     return 0;
   }
 }
+
+// Sharing
+bool StringTable::copy_shared_string(GrowableArray<MemRegion> *string_space,
+                                     CompactHashtableWriter* ch_table) {
+#if INCLUDE_CDS && INCLUDE_ALL_GCS && defined(_LP64) && !defined(_WINDOWS)
+  assert(UseG1GC, "Only support G1 GC");
+  assert(UseCompressedOops && UseCompressedClassPointers,
+         "Only support UseCompressedOops and UseCompressedClassPointers enabled");
+
+  Thread* THREAD = Thread::current();
+  G1CollectedHeap::heap()->begin_archive_alloc_range();
+  for (int i = 0; i < the_table()->table_size(); ++i) {
+    HashtableEntry<oop, mtSymbol>* bucket = the_table()->bucket(i);
+    for ( ; bucket != NULL; bucket = bucket->next()) {
+      oop s = bucket->literal();
+      unsigned int hash = java_lang_String::hash_code(s);
+      if (hash == 0) {
+        continue;
+      }
+
+      // allocate the new 'value' array first
+      typeArrayOop v = java_lang_String::value(s);
+      int v_len = v->size();
+      typeArrayOop new_v;
+      if (G1CollectedHeap::heap()->is_archive_alloc_too_large(v_len)) {
+        continue; // skip the current String. The 'value' array is too large to handle
+      } else {
+        new_v = (typeArrayOop)G1CollectedHeap::heap()->archive_mem_allocate(v_len);
+        if (new_v == NULL) {
+          return false; // allocation failed
+        }
+      }
+      // now allocate the new String object
+      int s_len = s->size();
+      oop new_s = (oop)G1CollectedHeap::heap()->archive_mem_allocate(s_len);
+      if (new_s == NULL) {
+        return false;
+      }
+
+      s->identity_hash();
+      v->identity_hash();
+
+      // copy the objects' data
+      Copy::aligned_disjoint_words((HeapWord*)s, (HeapWord*)new_s, s_len);
+      Copy::aligned_disjoint_words((HeapWord*)v, (HeapWord*)new_v, v_len);
+
+      // adjust the pointer to the 'value' field in the new String oop. Also pre-compute and set the
+      // 'hash' field. That avoids "write" to the shared strings at runtime by the deduplication process.
+      java_lang_String::set_value_raw(new_s, new_v);
+      if (java_lang_String::hash(new_s) == 0) {
+        java_lang_String::set_hash(new_s, hash);
+      }
+
+      // add to the compact table
+      ch_table->add(hash, new_s);
+    }
+  }
+
+  G1CollectedHeap::heap()->end_archive_alloc_range(string_space, os::vm_allocation_granularity());
+  assert(string_space->length() <= 2, "sanity");
+#endif
+  return true;
+}
+
+bool StringTable::copy_compact_table(char** top, char *end, GrowableArray<MemRegion> *string_space,
+                                     size_t* space_size) {
+#if INCLUDE_CDS && defined(_LP64) && !defined(_WINDOWS)
+  if (!(UseG1GC && UseCompressedOops && UseCompressedClassPointers)) {
+    if (PrintSharedSpaces) {
+      tty->print_cr("Shared strings are excluded from the archive as UseG1GC, "
+                    "UseCompressedOops and UseCompressedClassPointers are required.");
+    }
+    return true;
+  }
+
+  CompactHashtableWriter ch_table(CompactHashtable<oop, char>::_string_table,
+                                  the_table()->number_of_entries(),
+                                  &MetaspaceShared::stats()->string);
+
+  // Copy the interned strings into the "string space" within the java heap
+  if (!copy_shared_string(string_space, &ch_table)) {
+    return false;
+  }
+
+  for (int i = 0; i < string_space->length(); i++) {
+    *space_size += string_space->at(i).byte_size();
+  }
+
+  // Now dump the compact table
+  if (*top + ch_table.get_required_bytes() > end) {
+    // not enough space left
+    return false;
+  }
+  ch_table.dump(top, end);
+  *top = (char*)align_pointer_up(*top, sizeof(void*));
+
+#endif
+  return true;
+}
+
+void StringTable::shared_oops_do(OopClosure* f) {
+#if INCLUDE_CDS && defined(_LP64) && !defined(_WINDOWS)
+  _shared_table.oops_do(f);
+#endif
+}
+
+const char* StringTable::init_shared_table(FileMapInfo *mapinfo, char *buffer) {
+#if INCLUDE_CDS && defined(_LP64) && !defined(_WINDOWS)
+  if (mapinfo->space_capacity(MetaspaceShared::first_string) == 0) {
+    // no shared string data
+    return buffer;
+  }
+
+  // initialize the shared table
+  juint *p = (juint*)buffer;
+  const char* end = _shared_table.init(
+          CompactHashtable<oop, char>::_string_table, (char*)p);
+  const char* aligned_end = (const char*)align_pointer_up(end, sizeof(void*));
+
+  if (_ignore_shared_strings) {
+    _shared_table.reset();
+  }
+
+  return aligned_end;
+#endif
+
+  return buffer;
+}
--- a/hotspot/src/share/vm/classfile/stringTable.hpp	Fri Jun 12 12:55:32 2015 +0200
+++ b/hotspot/src/share/vm/classfile/stringTable.hpp	Fri Jun 12 17:29:14 2015 -0400
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1997, 2015, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -28,6 +28,10 @@
 #include "memory/allocation.inline.hpp"
 #include "utilities/hashtable.hpp"
 
+template <class T, class N> class CompactHashtable;
+class CompactHashtableWriter;
+class FileMapInfo;
+
 class StringTable : public RehashableHashtable<oop, mtSymbol> {
   friend class VMStructs;
   friend class Symbol;
@@ -36,6 +40,10 @@
   // The string table
   static StringTable* _the_table;
 
+  // Shared string table
+  static CompactHashtable<oop, char> _shared_table;
+  static bool _ignore_shared_strings;
+
   // Set if one bucket is out of balance due to hash algorithm deficiency
   static bool _needs_rehashing;
 
@@ -46,7 +54,8 @@
   oop basic_add(int index, Handle string_or_null, jchar* name, int len,
                 unsigned int hashValue, TRAPS);
 
-  oop lookup(int index, jchar* chars, int length, unsigned int hashValue);
+  oop lookup_in_main_table(int index, jchar* chars, int length, unsigned int hashValue);
+  static oop lookup_shared(jchar* name, int len);
 
   // Apply the give oop closure to the entries to the buckets
   // in the range [start_idx, end_idx).
@@ -141,12 +150,14 @@
   static int verify_and_compare_entries();
 
   // Sharing
-  static void copy_buckets(char** top, char*end) {
-    the_table()->Hashtable<oop, mtSymbol>::copy_buckets(top, end);
-  }
-  static void copy_table(char** top, char*end) {
-    the_table()->Hashtable<oop, mtSymbol>::copy_table(top, end);
-  }
+  static void ignore_shared_strings(bool v) { _ignore_shared_strings = v; }
+  static bool shared_string_ignored()       { return _ignore_shared_strings; }
+  static void shared_oops_do(OopClosure* f);
+  static bool copy_shared_string(GrowableArray<MemRegion> *string_space,
+                                 CompactHashtableWriter* ch_table);
+  static bool copy_compact_table(char** top, char* end, GrowableArray<MemRegion> *string_space,
+                                 size_t* space_size);
+  static const char* init_shared_table(FileMapInfo *mapinfo, char* buffer);
   static void reverse() {
     the_table()->Hashtable<oop, mtSymbol>::reverse();
   }
--- a/hotspot/src/share/vm/classfile/symbolTable.cpp	Fri Jun 12 12:55:32 2015 +0200
+++ b/hotspot/src/share/vm/classfile/symbolTable.cpp	Fri Jun 12 17:29:14 2015 -0400
@@ -539,7 +539,8 @@
 
 bool SymbolTable::copy_compact_table(char** top, char*end) {
 #if INCLUDE_CDS
-  CompactHashtableWriter ch_table("symbol", the_table()->number_of_entries(),
+  CompactHashtableWriter ch_table(CompactHashtable<Symbol*, char>::_symbol_table,
+                                  the_table()->number_of_entries(),
                                   &MetaspaceShared::stats()->symbol);
   if (*top + ch_table.get_required_bytes() > end) {
     // not enough space left
@@ -556,7 +557,6 @@
     }
   }
 
-  char* old_top = *top;
   ch_table.dump(top, end);
 
   *top = (char*)align_pointer_up(*top, sizeof(void*));
@@ -565,7 +565,8 @@
 }
 
 const char* SymbolTable::init_shared_table(const char* buffer) {
-  const char* end = _shared_table.init(buffer);
+  const char* end = _shared_table.init(
+          CompactHashtable<Symbol*, char>::_symbol_table, buffer);
   return (const char*)align_pointer_up(end, sizeof(void*));
 }
 
--- a/hotspot/src/share/vm/gc/g1/g1StringDedupThread.cpp	Fri Jun 12 12:55:32 2015 +0200
+++ b/hotspot/src/share/vm/gc/g1/g1StringDedupThread.cpp	Fri Jun 12 17:29:14 2015 -0400
@@ -23,12 +23,14 @@
  */
 
 #include "precompiled.hpp"
+#include "classfile/stringTable.hpp"
 #include "gc/g1/g1Log.hpp"
 #include "gc/g1/g1StringDedup.hpp"
 #include "gc/g1/g1StringDedupQueue.hpp"
 #include "gc/g1/g1StringDedupTable.hpp"
 #include "gc/g1/g1StringDedupThread.hpp"
 #include "gc/g1/suspendibleThreadSet.hpp"
+#include "oops/oop.inline.hpp"
 #include "runtime/atomic.inline.hpp"
 
 G1StringDedupThread* G1StringDedupThread::_thread = NULL;
@@ -55,11 +57,36 @@
   return _thread;
 }
 
+class G1StringDedupSharedClosure: public OopClosure {
+ private:
+  G1StringDedupStat& _stat;
+
+ public:
+  G1StringDedupSharedClosure(G1StringDedupStat& stat) : _stat(stat) {}
+
+  virtual void do_oop(oop* p) { ShouldNotReachHere(); }
+  virtual void do_oop(narrowOop* p) {
+    oop java_string = oopDesc::load_decode_heap_oop(p);
+    G1StringDedupTable::deduplicate(java_string, _stat);
+  }
+};
+
+// The CDS archive does not include the string dedupication table. Only the string
+// table is saved in the archive. The shared strings from CDS archive need to be
+// added to the string dedupication table before deduplication occurs. That is
+// done in the begining of the G1StringDedupThread (see G1StringDedupThread::run()
+// below).
+void G1StringDedupThread::deduplicate_shared_strings(G1StringDedupStat& stat) {
+  G1StringDedupSharedClosure sharedStringDedup(stat);
+  StringTable::shared_oops_do(&sharedStringDedup);
+}
+
 void G1StringDedupThread::run() {
   G1StringDedupStat total_stat;
 
   initialize_in_thread();
   wait_for_universe_init();
+  deduplicate_shared_strings(total_stat);
 
   // Main loop
   for (;;) {
--- a/hotspot/src/share/vm/gc/g1/g1StringDedupThread.hpp	Fri Jun 12 12:55:32 2015 +0200
+++ b/hotspot/src/share/vm/gc/g1/g1StringDedupThread.hpp	Fri Jun 12 17:29:14 2015 -0400
@@ -52,6 +52,8 @@
   static G1StringDedupThread* thread();
 
   virtual void run();
+
+  void deduplicate_shared_strings(G1StringDedupStat& stat);
 };
 
 #endif // SHARE_VM_GC_G1_G1STRINGDEDUPTHREAD_HPP
--- a/hotspot/src/share/vm/memory/filemap.cpp	Fri Jun 12 12:55:32 2015 +0200
+++ b/hotspot/src/share/vm/memory/filemap.cpp	Fri Jun 12 17:29:14 2015 -0400
@@ -28,6 +28,9 @@
 #include "classfile/symbolTable.hpp"
 #include "classfile/systemDictionaryShared.hpp"
 #include "classfile/altHashing.hpp"
+#if INCLUDE_ALL_GCS
+#include "gc/g1/g1CollectedHeap.hpp"
+#endif
 #include "memory/filemap.hpp"
 #include "memory/metadataFactory.hpp"
 #include "memory/oopFactory.hpp"
@@ -166,6 +169,9 @@
   _version = _current_version;
   _alignment = alignment;
   _obj_alignment = ObjectAlignmentInBytes;
+  _narrow_oop_mode = Universe::narrow_oop_mode();
+  _narrow_oop_shift = Universe::narrow_oop_shift();
+  _max_heap_size = MaxHeapSize;
   _classpath_entry_table_size = mapinfo->_classpath_entry_table_size;
   _classpath_entry_table = mapinfo->_classpath_entry_table;
   _classpath_entry_size = mapinfo->_classpath_entry_size;
@@ -441,7 +447,16 @@
   } else {
     si->_file_offset = _file_offset;
   }
-  si->_base = base;
+  if (MetaspaceShared::is_string_region(region)) {
+    assert((base - (char*)Universe::narrow_oop_base()) % HeapWordSize == 0, "Sanity");
+    if (base != NULL) {
+      si->_addr._offset = (intx)oopDesc::encode_heap_oop_not_null((oop)base);
+    } else {
+      si->_addr._offset = 0;
+    }
+  } else {
+    si->_addr._base = base;
+  }
   si->_used = size;
   si->_capacity = capacity;
   si->_read_only = read_only;
@@ -450,6 +465,38 @@
   write_bytes_aligned(base, (int)size);
 }
 
+// Write the string space. The string space contains one or multiple GC(G1) regions.
+// When the total string space size is smaller than one GC region of the dump time,
+// only one string region is used for shared strings.
+//
+// If the total string space size is bigger than one GC region, there would be more
+// than one GC regions allocated for shared strings. The first/bottom GC region might
+// be a partial GC region with the empty portion at the higher address within that region.
+// The non-empty portion of the first region is written into the archive as one string
+// region. The rest are consecutive full GC regions if they exist, which can be written
+// out in one chunk as another string region.
+void FileMapInfo::write_string_regions(GrowableArray<MemRegion> *regions) {
+  for (int i = MetaspaceShared::first_string;
+           i < MetaspaceShared::first_string + MetaspaceShared::max_strings; i++) {
+    char* start = NULL;
+    size_t size = 0;
+    if (regions->is_nonempty()) {
+      if (i == MetaspaceShared::first_string) {
+        MemRegion first = regions->first();
+        start = (char*)first.start();
+        size = first.byte_size();
+      } else {
+        int len = regions->length();
+        if (len > 1) {
+          start = (char*)regions->at(1).start();
+          size = (char*)regions->at(len - 1).end() - start;
+        }
+      }
+    }
+    write_region(i, start, size, size, false, false);
+  }
+}
+
 
 // Dump bytes to file -- at the current file position.
 
@@ -514,7 +561,8 @@
 // JVM/TI RedefineClasses() support:
 // Remap the shared readonly space to shared readwrite, private.
 bool FileMapInfo::remap_shared_readonly_as_readwrite() {
-  struct FileMapInfo::FileMapHeader::space_info* si = &_header->_space[0];
+  int idx = 0;
+  struct FileMapInfo::FileMapHeader::space_info* si = &_header->_space[idx];
   if (!si->_read_only) {
     // the space is already readwrite so we are done
     return true;
@@ -524,15 +572,16 @@
   if (!open_for_read()) {
     return false;
   }
+  char *addr = _header->region_addr(idx);
   char *base = os::remap_memory(_fd, _full_path, si->_file_offset,
-                                si->_base, size, false /* !read_only */,
+                                addr, size, false /* !read_only */,
                                 si->_allow_exec);
   close();
   if (base == NULL) {
     fail_continue("Unable to remap shared readonly space (errno=%d).", errno);
     return false;
   }
-  if (base != si->_base) {
+  if (base != addr) {
     fail_continue("Unable to remap shared readonly space at required address.");
     return false;
   }
@@ -543,7 +592,7 @@
 // Map the whole region at once, assumed to be allocated contiguously.
 ReservedSpace FileMapInfo::reserve_shared_memory() {
   struct FileMapInfo::FileMapHeader::space_info* si = &_header->_space[0];
-  char* requested_addr = si->_base;
+  char* requested_addr = _header->region_addr(0);
 
   size_t size = FileMapInfo::shared_spaces_size();
 
@@ -561,14 +610,16 @@
 }
 
 // Memory map a region in the address space.
-static const char* shared_region_name[] = { "ReadOnly", "ReadWrite", "MiscData", "MiscCode"};
+static const char* shared_region_name[] = { "ReadOnly", "ReadWrite", "MiscData", "MiscCode",
+                                            "String1", "String2" };
 
 char* FileMapInfo::map_region(int i) {
+  assert(!MetaspaceShared::is_string_region(i), "sanity");
   struct FileMapInfo::FileMapHeader::space_info* si = &_header->_space[i];
   size_t used = si->_used;
   size_t alignment = os::vm_allocation_granularity();
   size_t size = align_size_up(used, alignment);
-  char *requested_addr = si->_base;
+  char *requested_addr = _header->region_addr(i);
   bool read_only;
 
   // If a tool agent is in use (debugging enabled), we must map the address space RW
@@ -583,7 +634,7 @@
   char *base = os::map_memory(_fd, _full_path, si->_file_offset,
                               requested_addr, size, read_only,
                               si->_allow_exec);
-  if (base == NULL || base != si->_base) {
+  if (base == NULL || base != requested_addr) {
     fail_continue("Unable to map %s shared space at required address.", shared_region_name[i]);
     return NULL;
   }
@@ -592,15 +643,119 @@
   // in method FileMapInfo::reserve_shared_memory(), which is not called on Windows.
   MemTracker::record_virtual_memory_type((address)base, mtClassShared);
 #endif
+
   return base;
 }
 
+MemRegion *string_ranges = NULL;
+int num_ranges = 0;
+bool FileMapInfo::map_string_regions() {
+#if INCLUDE_ALL_GCS
+  if (UseG1GC && UseCompressedOops && UseCompressedClassPointers) {
+    if (narrow_oop_mode() == Universe::narrow_oop_mode() &&
+        narrow_oop_shift() == Universe::narrow_oop_shift()) {
+      string_ranges = new MemRegion[MetaspaceShared::max_strings];
+      struct FileMapInfo::FileMapHeader::space_info* si;
+
+      for (int i = MetaspaceShared::first_string;
+               i < MetaspaceShared::first_string + MetaspaceShared::max_strings; i++) {
+        si = &_header->_space[i];
+        size_t used = si->_used;
+        if (used > 0) {
+          size_t size = used;
+          char* requested_addr = (char*)((void*)oopDesc::decode_heap_oop_not_null(
+                                                 (narrowOop)si->_addr._offset));
+          string_ranges[num_ranges] = MemRegion((HeapWord*)requested_addr, size / HeapWordSize);
+          num_ranges ++;
+        }
+      }
+
+      if (num_ranges == 0) {
+        return true; // no shared string data
+      }
+
+      // Check that ranges are within the java heap
+      if (!G1CollectedHeap::heap()->check_archive_addresses(string_ranges, num_ranges)) {
+        fail_continue("Unable to allocate shared string space: range is not "
+                      "within java heap.");
+        return false;
+      }
+
+      // allocate from java heap
+      if (!G1CollectedHeap::heap()->alloc_archive_regions(string_ranges, num_ranges)) {
+        fail_continue("Unable to allocate shared string space: range is "
+                      "already in use.");
+        return false;
+      }
+
+      // Map the string data. No need to call MemTracker::record_virtual_memory_type()
+      // for mapped string regions as they are part of the reserved java heap, which
+      // is already recorded.
+      for (int i = 0; i < num_ranges; i++) {
+        si = &_header->_space[MetaspaceShared::first_string + i];
+        char* addr = (char*)string_ranges[i].start();
+        char* base = os::map_memory(_fd, _full_path, si->_file_offset,
+                                    addr, string_ranges[i].byte_size(), si->_read_only,
+                                    si->_allow_exec);
+        if (base == NULL || base != addr) {
+          fail_continue("Unable to map shared string space at required address.");
+          return false;
+        }
+      }
+      return true; // the shared string data is mapped successfuly
+    } else {
+      //  narrow oop encoding differ, the shared string data are not used
+      if (PrintSharedSpaces && _header->_space[MetaspaceShared::first_string]._used > 0) {
+        tty->print_cr("Shared string data from the CDS archive is being ignored. "
+                     "The current CompressedOops encoding differs from that archived "
+                     "due to heap size change. The archive was dumped using max heap "
+                     "size %dM.", max_heap_size() >> 20);
+      }
+    }
+  } else {
+    if (PrintSharedSpaces && _header->_space[MetaspaceShared::first_string]._used > 0) {
+      tty->print_cr("Shared string data from the CDS archive is being ignored. UseG1GC, "
+                    "UseCompressedOops and UseCompressedClassPointers are required.");
+    }
+  }
+
+  // if we get here, the shared string data is not mapped
+  assert(string_ranges == NULL && num_ranges == 0, "sanity");
+  StringTable::ignore_shared_strings(true);
+#endif
+  return true;
+}
+
+bool FileMapInfo::verify_string_regions() {
+  for (int i = MetaspaceShared::first_string;
+           i < MetaspaceShared::first_string + MetaspaceShared::max_strings; i++) {
+    if (!verify_region_checksum(i)) {
+      return false;
+    }
+  }
+  return true;
+}
+
+void FileMapInfo::fixup_string_regions() {
+  if (string_ranges != NULL) {
+    G1CollectedHeap::heap()->fill_archive_regions(string_ranges, num_ranges);
+  }
+}
+
 bool FileMapInfo::verify_region_checksum(int i) {
   if (!VerifySharedSpaces) {
     return true;
   }
-  const char* buf = _header->_space[i]._base;
+
   size_t sz = _header->_space[i]._used;
+
+  if (sz == 0) {
+    return true; // no data
+  }
+  if (MetaspaceShared::is_string_region(i) && StringTable::shared_string_ignored()) {
+    return true; // shared string data are not mapped
+  }
+  const char* buf = _header->region_addr(i);
   int crc = ClassLoader::crc32(0, buf, (jint)sz);
   if (crc != _header->_space[i]._crc) {
     fail_continue("Checksum verification failed.");
@@ -612,14 +767,36 @@
 // Unmap a memory region in the address space.
 
 void FileMapInfo::unmap_region(int i) {
+  assert(!MetaspaceShared::is_string_region(i), "sanity");
   struct FileMapInfo::FileMapHeader::space_info* si = &_header->_space[i];
   size_t used = si->_used;
   size_t size = align_size_up(used, os::vm_allocation_granularity());
-  if (!os::unmap_memory(si->_base, size)) {
+
+  if (used == 0) {
+    return;
+  }
+
+  char* addr = _header->region_addr(i);
+  if (!os::unmap_memory(addr, size)) {
     fail_stop("Unable to unmap shared space.");
   }
 }
 
+void FileMapInfo::unmap_string_regions() {
+  for (int i = MetaspaceShared::first_string;
+           i < MetaspaceShared::first_string + MetaspaceShared::max_strings; i++) {
+    struct FileMapInfo::FileMapHeader::space_info* si = &_header->_space[i];
+    size_t used = si->_used;
+    if (used > 0) {
+      size_t size = align_size_up(used, os::vm_allocation_granularity());
+      char* addr = (char*)((void*)oopDesc::decode_heap_oop_not_null(
+                                             (narrowOop)si->_addr._offset));
+      if (!os::unmap_memory(addr, size)) {
+        fail_stop("Unable to unmap shared space.");
+      }
+    }
+  }
+}
 
 void FileMapInfo::assert_mark(bool check) {
   if (!check) {
@@ -663,6 +840,15 @@
   return true;
 }
 
+char* FileMapInfo::FileMapHeader::region_addr(int idx) {
+  if (MetaspaceShared::is_string_region(idx)) {
+    return (char*)((void*)oopDesc::decode_heap_oop_not_null(
+              (narrowOop)_space[idx]._addr._offset));
+  } else {
+    return _space[idx]._addr._base;
+  }
+}
+
 int FileMapInfo::FileMapHeader::compute_crc() {
   char* header = data();
   // start computing from the field after _crc
@@ -734,8 +920,12 @@
 // True if the p is within the mapped shared space, otherwise, false.
 bool FileMapInfo::is_in_shared_space(const void* p) {
   for (int i = 0; i < MetaspaceShared::n_regions; i++) {
-    if (p >= _header->_space[i]._base &&
-        p < _header->_space[i]._base + _header->_space[i]._used) {
+    char *base;
+    if (MetaspaceShared::is_string_region(i) && _header->_space[i]._used == 0) {
+      continue;
+    }
+    base = _header->region_addr(i);
+    if (p >= base && p < base + _header->_space[i]._used) {
       return true;
     }
   }
@@ -747,9 +937,10 @@
   gclog_or_tty->print_cr("Shared Spaces:");
   for (int i = 0; i < MetaspaceShared::n_regions; i++) {
     struct FileMapInfo::FileMapHeader::space_info* si = &_header->_space[i];
+    char *base = _header->region_addr(i);
     gclog_or_tty->print("  %s " INTPTR_FORMAT "-" INTPTR_FORMAT,
                         shared_region_name[i],
-                        si->_base, si->_base + si->_used);
+                        base, base + si->_used);
   }
 }
 
@@ -758,12 +949,14 @@
   FileMapInfo *map_info = FileMapInfo::current_info();
   if (map_info) {
     map_info->fail_continue("%s", msg);
-    for (int i = 0; i < MetaspaceShared::n_regions; i++) {
-      if (map_info->_header->_space[i]._base != NULL) {
+    for (int i = 0; i < MetaspaceShared::num_non_strings; i++) {
+      char *addr = map_info->_header->region_addr(i);
+      if (addr != NULL && !MetaspaceShared::is_string_region(i)) {
         map_info->unmap_region(i);
-        map_info->_header->_space[i]._base = NULL;
+        map_info->_header->_space[i]._addr._base = NULL;
       }
     }
+    map_info->unmap_string_regions();
   } else if (DumpSharedSpaces) {
     fail_stop("%s", msg);
   }
--- a/hotspot/src/share/vm/memory/filemap.hpp	Fri Jun 12 12:55:32 2015 +0200
+++ b/hotspot/src/share/vm/memory/filemap.hpp	Fri Jun 12 17:29:14 2015 -0400
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2003, 2014, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2003, 2015, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -94,11 +94,18 @@
     int    _version;                  // (from enum, above.)
     size_t _alignment;                // how shared archive should be aligned
     int    _obj_alignment;            // value of ObjectAlignmentInBytes
+    int    _narrow_oop_shift;         // compressed oop encoding shift
+    uintx  _max_heap_size;            // java max heap size during dumping
+    Universe::NARROW_OOP_MODE _narrow_oop_mode; // compressed oop encoding mode
 
     struct space_info {
       int    _crc;           // crc checksum of the current space
       size_t _file_offset;   // sizeof(this) rounded to vm page size
-      char*  _base;          // copy-on-write base address
+      union {
+        char*  _base;        // copy-on-write base address
+        intx   _offset;      // offset from the compressed oop encoding base, only used
+                             // by string space
+      } _addr;
       size_t _capacity;      // for validity checking
       size_t _used;          // for setting space top on read
       bool   _read_only;     // read only space?
@@ -138,6 +145,8 @@
     size_t _classpath_entry_size;
     SharedClassPathEntry* _classpath_entry_table;
 
+    char* region_addr(int idx);
+
     virtual bool validate();
     virtual void populate(FileMapInfo* info, size_t alignment);
     int compute_crc();
@@ -166,8 +175,10 @@
   void   invalidate();
   int    version()                    { return _header->_version; }
   size_t alignment()                  { return _header->_alignment; }
+  Universe::NARROW_OOP_MODE narrow_oop_mode() { return _header->_narrow_oop_mode; }
+  int    narrow_oop_shift()           { return _header->_narrow_oop_shift; }
+  uintx  max_heap_size()              { return _header->_max_heap_size; }
   size_t space_capacity(int i)        { return _header->_space[i]._capacity; }
-  char*  region_base(int i)           { return _header->_space[i]._base; }
   struct FileMapHeader* header()      { return _header; }
 
   static FileMapInfo* current_info() {
@@ -185,10 +196,15 @@
   void  write_space(int i, Metaspace* space, bool read_only);
   void  write_region(int region, char* base, size_t size,
                      size_t capacity, bool read_only, bool allow_exec);
+  void  write_string_regions(GrowableArray<MemRegion> *regions);
   void  write_bytes(const void* buffer, int count);
   void  write_bytes_aligned(const void* buffer, int count);
   char* map_region(int i);
+  bool  map_string_regions();
+  bool  verify_string_regions();
+  void  fixup_string_regions();
   void  unmap_region(int i);
+  void  unmap_string_regions();
   bool  verify_region_checksum(int i);
   void  close();
   bool  is_open() { return _file_open; }
--- a/hotspot/src/share/vm/memory/metaspace.cpp	Fri Jun 12 12:55:32 2015 +0200
+++ b/hotspot/src/share/vm/memory/metaspace.cpp	Fri Jun 12 17:29:14 2015 -0400
@@ -3307,7 +3307,7 @@
       // Map in spaces now also
       if (mapinfo->initialize() && MetaspaceShared::map_shared_spaces(mapinfo)) {
         cds_total = FileMapInfo::shared_spaces_size();
-        cds_address = (address)mapinfo->region_base(0);
+        cds_address = (address)mapinfo->header()->region_addr(0);
       } else {
         assert(!mapinfo->is_open() && !UseSharedSpaces,
                "archive file not closed or shared spaces not disabled.");
--- a/hotspot/src/share/vm/memory/metaspaceShared.cpp	Fri Jun 12 12:55:32 2015 +0200
+++ b/hotspot/src/share/vm/memory/metaspaceShared.cpp	Fri Jun 12 17:29:14 2015 -0400
@@ -422,6 +422,8 @@
   GrowableArray<Klass*> *_class_promote_order;
   VirtualSpace _md_vs;
   VirtualSpace _mc_vs;
+  CompactHashtableWriter* _string_cht;
+  GrowableArray<MemRegion> *_string_regions;
 
 public:
   VM_PopulateDumpSharedSpace(ClassLoaderData* loader_data,
@@ -540,7 +542,7 @@
 
   NOT_PRODUCT(SystemDictionary::verify();)
 
-  // Copy the the symbol table, and the system dictionary to the shared
+  // Copy the the symbol table, string table, and the system dictionary to the shared
   // space in usable form.  Copy the hashtable
   // buckets first [read-write], then copy the linked lists of entries
   // [read-only].
@@ -548,6 +550,15 @@
   NOT_PRODUCT(SymbolTable::verify());
   handle_misc_data_space_failure(SymbolTable::copy_compact_table(&md_top, md_end));
 
+  size_t ss_bytes = 0;
+  char* ss_low;
+  // The string space has maximum two regions. See FileMapInfo::write_string_regions() for details.
+  _string_regions = new GrowableArray<MemRegion>(2);
+  NOT_PRODUCT(StringTable::verify());
+  handle_misc_data_space_failure(StringTable::copy_compact_table(&md_top, md_end, _string_regions,
+                                                                 &ss_bytes));
+  ss_low = _string_regions->is_empty() ? NULL : (char*)_string_regions->first().start();
+
   SystemDictionary::reverse();
   SystemDictionary::copy_buckets(&md_top, md_end);
 
@@ -576,7 +587,8 @@
   const size_t rw_alloced = rw_space->capacity_bytes_slow(Metaspace::NonClassType);
   const size_t md_alloced = md_end-md_low;
   const size_t mc_alloced = mc_end-mc_low;
-  const size_t total_alloced = ro_alloced + rw_alloced + md_alloced + mc_alloced;
+  const size_t total_alloced = ro_alloced + rw_alloced + md_alloced + mc_alloced
+                             + ss_bytes;
 
   // Occupied size of each space.
   const size_t ro_bytes = ro_space->used_bytes_slow(Metaspace::NonClassType);
@@ -585,11 +597,12 @@
   const size_t mc_bytes = size_t(mc_top - mc_low);
 
   // Percent of total size
-  const size_t total_bytes = ro_bytes + rw_bytes + md_bytes + mc_bytes;
+  const size_t total_bytes = ro_bytes + rw_bytes + md_bytes + mc_bytes + ss_bytes;
   const double ro_t_perc = ro_bytes / double(total_bytes) * 100.0;
   const double rw_t_perc = rw_bytes / double(total_bytes) * 100.0;
   const double md_t_perc = md_bytes / double(total_bytes) * 100.0;
   const double mc_t_perc = mc_bytes / double(total_bytes) * 100.0;
+  const double ss_t_perc = ss_bytes / double(total_bytes) * 100.0;
 
   // Percent of fullness of each space
   const double ro_u_perc = ro_bytes / double(ro_alloced) * 100.0;
@@ -602,6 +615,7 @@
   tty->print_cr(fmt_space, "rw", rw_bytes, rw_t_perc, rw_alloced, rw_u_perc, rw_space->bottom());
   tty->print_cr(fmt_space, "md", md_bytes, md_t_perc, md_alloced, md_u_perc, md_low);
   tty->print_cr(fmt_space, "mc", mc_bytes, mc_t_perc, mc_alloced, mc_u_perc, mc_low);
+  tty->print_cr(fmt_space, "st", ss_bytes, ss_t_perc, ss_bytes, 100.0, ss_low);
   tty->print_cr("total   : %9d [100.0%% of total] out of %9d bytes [%4.1f%% used]",
                  total_bytes, total_alloced, total_u_perc);
 
@@ -631,6 +645,7 @@
                         pointer_delta(mc_top, _mc_vs.low(), sizeof(char)),
                         SharedMiscCodeSize,
                         true, true);
+  mapinfo->write_string_regions(_string_regions);
 
   // Pass 2 - write data.
   mapinfo->open_for_write();
@@ -646,6 +661,8 @@
                         pointer_delta(mc_top, _mc_vs.low(), sizeof(char)),
                         SharedMiscCodeSize,
                         true, true);
+  mapinfo->write_string_regions(_string_regions);
+
   mapinfo->close();
 
   memmove(vtbl_list, saved_vtbl, vtbl_list_size * sizeof(void*));
@@ -942,6 +959,11 @@
   return UseSharedSpaces && FileMapInfo::current_info()->is_in_shared_space(p);
 }
 
+bool MetaspaceShared::is_string_region(int idx) {
+  return (idx >= MetaspaceShared::first_string &&
+          idx < MetaspaceShared::first_string + MetaspaceShared::max_strings);
+}
+
 void MetaspaceShared::print_shared_spaces() {
   if (UseSharedSpaces) {
     FileMapInfo::current_info()->print_shared_spaces();
@@ -972,13 +994,15 @@
 
   // Map each shared region
   if ((_ro_base = mapinfo->map_region(ro)) != NULL &&
-       mapinfo->verify_region_checksum(ro) &&
+      mapinfo->verify_region_checksum(ro) &&
       (_rw_base = mapinfo->map_region(rw)) != NULL &&
-       mapinfo->verify_region_checksum(rw) &&
+      mapinfo->verify_region_checksum(rw) &&
       (_md_base = mapinfo->map_region(md)) != NULL &&
-       mapinfo->verify_region_checksum(md) &&
+      mapinfo->verify_region_checksum(md) &&
       (_mc_base = mapinfo->map_region(mc)) != NULL &&
-       mapinfo->verify_region_checksum(mc) &&
+      mapinfo->verify_region_checksum(mc) &&
+      mapinfo->map_string_regions() &&
+      mapinfo->verify_string_regions() &&
       (image_alignment == (size_t)max_alignment()) &&
       mapinfo->validate_classpath_entry_table()) {
     // Success (no need to do anything)
@@ -990,6 +1014,7 @@
     if (_rw_base != NULL) mapinfo->unmap_region(rw);
     if (_md_base != NULL) mapinfo->unmap_region(md);
     if (_mc_base != NULL) mapinfo->unmap_region(mc);
+    mapinfo->unmap_string_regions();
 #ifndef _WINDOWS
     // Release the entire mapped region
     shared_rs.release();
@@ -1011,7 +1036,7 @@
 void MetaspaceShared::initialize_shared_spaces() {
   FileMapInfo *mapinfo = FileMapInfo::current_info();
 
-  char* buffer = mapinfo->region_base(md);
+  char* buffer = mapinfo->header()->region_addr(md);
 
   // Skip over (reserve space for) a list of addresses of C++ vtables
   // for Klass objects.  They get filled in later.
@@ -1027,13 +1052,16 @@
   buffer += sizeof(intptr_t);
   buffer += vtable_size;
 
-  // Create the shared symbol table using the bucket array at this spot in the
+  // Create the shared symbol table using the compact table at this spot in the
   // misc data space. (Todo: move this to read-only space. Currently
   // this is mapped copy-on-write but will never be written into).
 
   buffer = (char*)SymbolTable::init_shared_table(buffer);
   SymbolTable::create_table();
 
+  // Create the shared string table using the compact table
+  buffer = (char*)StringTable::init_shared_table(mapinfo, buffer);
+
   // Create the shared dictionary using the bucket array at this spot in
   // the misc data space.  Since the shared dictionary table is never
   // modified, this region (of mapped pages) will be (effectively, if
@@ -1100,6 +1128,11 @@
   }
 }
 
+void MetaspaceShared::fixup_shared_string_regions() {
+  FileMapInfo *mapinfo = FileMapInfo::current_info();
+  mapinfo->fixup_string_regions();
+}
+
 // JVM/TI RedefineClasses() support:
 bool MetaspaceShared::remap_shared_readonly_as_readwrite() {
   assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
--- a/hotspot/src/share/vm/memory/metaspaceShared.hpp	Fri Jun 12 12:55:32 2015 +0200
+++ b/hotspot/src/share/vm/memory/metaspaceShared.hpp	Fri Jun 12 17:29:14 2015 -0400
@@ -53,6 +53,7 @@
     memset(this, 0, sizeof(*this));
   }
   CompactHashtableStats symbol;
+  CompactHashtableStats string;
 };
 
 // Class Data Sharing Support
@@ -90,7 +91,10 @@
     rw = 1,  // read-write shared space in the heap
     md = 2,  // miscellaneous data for initializing tables, etc.
     mc = 3,  // miscellaneous code - vtable replacement.
-    n_regions = 4
+    max_strings = 2, // max number of string regions in string space
+    num_non_strings = 4, // number of non-string regions
+    first_string = num_non_strings, // index of first string region
+    n_regions = max_strings + num_non_strings // total number of regions
   };
 
   // Accessor functions to save shared space created for metadata, which has
@@ -124,10 +128,13 @@
   }
   static bool map_shared_spaces(FileMapInfo* mapinfo) NOT_CDS_RETURN_(false);
   static void initialize_shared_spaces() NOT_CDS_RETURN;
+  static void fixup_shared_string_regions() NOT_CDS_RETURN;
 
   // Return true if given address is in the mapped shared space.
   static bool is_in_shared_space(const void* p) NOT_CDS_RETURN_(false);
 
+  static bool is_string_region(int idx) NOT_CDS_RETURN_(false);
+
   static void generate_vtable_methods(void** vtbl_list,
                                       void** vtable,
                                       char** md_top, char* md_end,
--- a/hotspot/src/share/vm/memory/universe.cpp	Fri Jun 12 12:55:32 2015 +0200
+++ b/hotspot/src/share/vm/memory/universe.cpp	Fri Jun 12 17:29:14 2015 -0400
@@ -311,6 +311,7 @@
              SystemDictionary::Cloneable_klass(), "u3");
       assert(_the_array_interfaces_array->at(1) ==
              SystemDictionary::Serializable_klass(), "u3");
+      MetaspaceShared::fixup_shared_string_regions();
     } else {
       // Set up shared interfaces array.  (Do this before supers are set up.)
       _the_array_interfaces_array->at_put(0, SystemDictionary::Cloneable_klass());
--- a/hotspot/src/share/vm/prims/whitebox.cpp	Fri Jun 12 12:55:32 2015 +0200
+++ b/hotspot/src/share/vm/prims/whitebox.cpp	Fri Jun 12 17:29:14 2015 -0400
@@ -31,6 +31,7 @@
 #include "code/codeCache.hpp"
 #include "jvmtifiles/jvmtiEnv.hpp"
 #include "memory/metadataFactory.hpp"
+#include "memory/metaspaceShared.hpp"
 #include "memory/universe.hpp"
 #include "oops/oop.inline.hpp"
 #include "prims/wbtestmethods/parserTests.hpp"
@@ -1207,6 +1208,11 @@
   return NULL;
 WB_END
 
+WB_ENTRY(jboolean, WB_IsShared(JNIEnv* env, jobject wb, jobject obj))
+  oop obj_oop = JNIHandles::resolve(obj);
+  return MetaspaceShared::is_in_shared_space((void*)obj_oop);
+WB_END
+
 //Some convenience methods to deal with objects from java
 int WhiteBox::offset_for_field(const char* field_name, oop object,
     Symbol* signature_symbol) {
@@ -1431,6 +1437,7 @@
   {CC"getMethodStringOption",
       CC"(Ljava/lang/reflect/Executable;Ljava/lang/String;)Ljava/lang/String;",
                                                       (void*)&WB_GetMethodStringOption},
+  {CC"isShared",           CC"(Ljava/lang/Object;)Z", (void*)&WB_IsShared },
 };
 
 #undef CC
--- a/hotspot/src/share/vm/runtime/synchronizer.cpp	Fri Jun 12 12:55:32 2015 +0200
+++ b/hotspot/src/share/vm/runtime/synchronizer.cpp	Fri Jun 12 17:29:14 2015 -0400
@@ -24,6 +24,7 @@
 
 #include "precompiled.hpp"
 #include "classfile/vmSymbols.hpp"
+#include "memory/metaspaceShared.hpp"
 #include "memory/padded.hpp"
 #include "memory/resourceArea.hpp"
 #include "oops/markOop.hpp"
@@ -638,11 +639,11 @@
 
   // hashCode() is a heap mutator ...
   // Relaxing assertion for bug 6320749.
-  assert(Universe::verify_in_progress() ||
+  assert(Universe::verify_in_progress() || DumpSharedSpaces ||
          !SafepointSynchronize::is_at_safepoint(), "invariant");
-  assert(Universe::verify_in_progress() ||
+  assert(Universe::verify_in_progress() || DumpSharedSpaces ||
          Self->is_Java_thread() , "invariant");
-  assert(Universe::verify_in_progress() ||
+  assert(Universe::verify_in_progress() || DumpSharedSpaces ||
          ((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
 
   ObjectMonitor* monitor = NULL;
--- a/hotspot/src/share/vm/services/virtualMemoryTracker.cpp	Fri Jun 12 12:55:32 2015 +0200
+++ b/hotspot/src/share/vm/services/virtualMemoryTracker.cpp	Fri Jun 12 17:29:14 2015 -0400
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2013, 2014, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2013, 2015, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -347,6 +347,13 @@
         return true;
       }
 
+      // Mapped CDS string region.
+      // The string region(s) is part of the java heap.
+      if (reserved_rgn->flag() == mtJavaHeap) {
+        assert(reserved_rgn->contain_region(base_addr, size), "Reserved heap region should contain this mapping region");
+        return true;
+      }
+
       ShouldNotReachHere();
       return false;
     }