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<div id="sharepage" class="smallpagetitle"><h1>Java Scripting Programmer's Guide</h1><div class="sharepage">		<div class="sharepagew1 share-mailto">		<table summary="" cellpadding="0" cellspacing="0"><tbody><tr>		<td id="share-mailto"><a href="mailto:?subject=Java%20Documentation%20Page:%20Java%20Scripting%20Programmer%27s%20Guide&amp;body=Check%20out%20this%20page:%20%0A%0Ahttp%3A%2F%2Fdocs.oracle.com%2Fjavase%2F6%2Fdocs%2Ftechnotes%2Fguides%2Fscripting%2Fprogrammer_guide%2Findex.html" class="sharelink mailto" title="Email this page to a friend"></a></td>		<td id="share-technorati"><a href="http://technorati.com/search/http%3A%2F%2Fdocs.oracle.com%2Fjavase%2F6%2Fdocs%2Ftechnotes%2Fguides%2Fscripting%2Fprogrammer_guide%2Findex.html" class="sharelink technorati" title="See who links to this page on Technorati"></a></td>		<td id="share-delicious"><a href="http://del.icio.us/post?v=4;url=http%3A%2F%2Fdocs.oracle.com%2Fjavase%2F6%2Fdocs%2Ftechnotes%2Fguides%2Fscripting%2Fprogrammer_guide%2Findex.html;title=Java%20Scripting%20Programmer%27s%20Guide" class="sharelink delicious" title="Bookmark this page in del.icio.us"></a></td>		<td id="share-digg"><a href="http://digg.com/submit?phase=2&amp;url=http%3A%2F%2Fdocs.oracle.com%2Fjavase%2F6%2Fdocs%2Ftechnotes%2Fguides%2Fscripting%2Fprogrammer_guide%2Findex.html&amp;title=Java%20Scripting%20Programmer%27s%20Guide" class="sharelink digg" title="Submit this page to Digg"></a></td>		<td id="share-slashdot"><a href="http://slashdot.org/bookmark.pl?title=Java%20Scripting%20Programmer%27s%20Guide&amp;url=http%3A%2F%2Fdocs.oracle.com%2Fjavase%2F6%2Fdocs%2Ftechnotes%2Fguides%2Fscripting%2Fprogrammer_guide%2Findex.html" class="sharelink slashdot" title="Submit this page to Slashdot"></a></td>		<td id="share-blank"> </td></tr></tbody></table></div></div></div>

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<ul>
<li><span><a href="#who">Who is the Java Scripting API
For?</a></span></li>
<li><span><a href="#package">Scripting Package</a></span></li>
<li><span><a href="#examples">Examples</a></span>
<ul>
<li><span><a href="#helloworld">"Hello, World"</a></span></li>
<li><span><a href="#evalfile">Evaluating a Script
File</a></span></li>
<li><span><a href="#scriptvars">Script Variables</a></span></li>
<li><span><a href="#invoke">Invoking Script Functions and
Methods</a></span></li>
<li><span><a href="#interfaces">Implementing Java Interfaces by
Scripts</a></span></li>
<li><span><a href="#scopes">Multiple Scopes for
Scripts</a></span></li>
</ul>
</li>
<li><span><a href="#jsengine">JavaScript Script
Engine</a></span></li>
<li><span><a href="#jstojava">JavaScript to Java
Communication</a></span>
<ul>
<li><span><a href="#jsjavaclass">Accessing Java
Classes</a></span></li>
<li><span><a href="#jsimport">Importing Java Packages,
Classes</a></span></li>
<li><span><a href="#jsarrays">Creating, Converting and Using Java
Arrays</a></span></li>
<li><span><a href="#jsimplement">Implementing Java
Interfaces</a></span></li>
<li><span><a href="#jsextend">Extending Java classes
</a></span></li>
<li><span><a href="#jsoverload">Overload Resolution</a></span></li>
</ul>
</li>
<li><span><a href="#engineimpl">Implementing Your Own Script
Engine</a></span></li>
<li><span><a href="#refs">References</a></span></li>
</ul>
<span><a name="who" id="who"></a></span>
<h2><span>Who is the Java Scripting API For?</span></h2>
<span>Some useful characteristics of scripting languages
are:</span>
<ul>
<li><span><b>Convenience</b>: Most scripting languages are
dynamically typed. You can usually create new variables without
declaring the variable type, and you can reuse variables to store
objects of different types. Also, scripting languages tend to
perform many type conversions automatically, for example,
converting the number 10 to the text "10" as necessary.</span></li>
<li><span><b>Developing rapid prototypes</b>: You can avoid the
edit-compile-run cycle and just use edit-run!</span></li>
<li><span><b>Application extension/customization</b>: You can
"externalize" parts of your application - like configuration
scripts, business logic/rules and math expressions for financial
applications.</span></li>
<li><span><b>"Command line" shells for applications</b> -for
debugging, runtime/deploy time configuration etc. Most applications
have a web-based GUI configuaration tool these days. But
sysadmins/deployers frequently prefer command line tools. Instead
of inventing ad-hoc scripting language for that purpose, a
"standard" scripting language can be used.</span></li>
</ul>
<p><span>The Java<font size="-1"><sup>TM</sup></font> Scripting API
is a scripting language indepedent framework for using script
engines from Java code. With the Java Scripting API, it is possible
to write customizable/extendable applications in the Java language
and leave the customization scripting language choice to the end
user. The Java application developer need not choose the extension
language during development. If you write your application with
JSR-223 API, then your users can use any JSR-223 compliant
scripting language.</span></p>
<hr>
<span><a name="package" id="package"></a></span>
<h2><span>Scripting Package</span></h2>
<p><span>The Java Scripting functionality is in the <code><a href="http://docs.oracle.com/javase/6/docs/api/javax/script/package-summary.html">javax.script</a></code>
package. This is a relatively small, simple API. The starting point
of the scripting API is the <code>ScriptEngineManager</code> class.
A ScriptEngineManager object can discover script engines through
the jar file service discovery mechanism. It can also instantiate
ScriptEngine objects that interpret scripts written in a specific
scripting language. The simplest way to use the scripting API is as
follows:</span></p>
<ol>
<li><span>Create a <code>ScriptEngineManager</code>
object.</span></li>
<li><span>Get a <code>ScriptEngine</code> object from the
manager.</span></li>
<li><span>Evaluate script using the <code>ScriptEngine</code>'s
<code>eval</code> methods.</span></li>
</ol>
<p><span>Now, it is time to look at some sample code. While it is
not mandatory, it may be useful to know a bit of JavaScript to read
these examples.</span></p>
<hr>
<span><a name="examples" id="examples"></a></span>
<h2><span>Examples</span></h2>
<span><a name="helloworld" id="helloworld"></a></span>
<h3><span>"Hello, World"</span></h3>
<p><span>From the <code>ScriptEngineManager</code> instance, we
request a JavaScript engine instance using
<code>getEngineByName</code> method. On the script engine, the
<code>eval</code> method is called to execute a given String as
JavaScript code! For brevity, in this as well as in subsequent
examples, we have not shown exception handling. There are checked
and runtime exceptions thrown from <code>javax.script</code> API.
Needless to say, you have to handle the exceptions
appropriately.</span></p>
<pre>
<span><code>
// <a href="source/EvalScript.java">EvalScript.java</a>

import javax.script.*;
public class EvalScript {
    public static void main(String[] args) throws Exception {
        // create a script engine manager
        <span class="classref">ScriptEngineManager</span> factory = new ScriptEngineManager();
        // create a JavaScript engine
        <span class="classref">ScriptEngine</span> engine = factory.<span class="methodref">getEngineByName</span>("nashorn");
        // evaluate JavaScript code from String
        engine.<span class="methodref">eval</span>("print('Hello, World')");
    }
}
</code></span>
</pre>
<hr>
<a name="evalfile" id="evalfile"></a>
<h3>Evaluating a Script File</h3>
<p>In this example, we call the <code>eval</code> method that
accepts <code>java.io.Reader</code> for the input source. The
script read by the given reader is executed. This way it is
possible to execute scripts from files, URLs and resources by
wrapping the relevant input stream objects as readers.</p>
<pre>
<code>
// <a href="source/EvalFile.java">EvalFile.java</a>

import javax.script.*;

public class EvalFile {
    public static void main(String[] args) throws Exception {
        // create a script engine manager
        <span class="classref">ScriptEngineManager</span> factory = new ScriptEngineManager();
        // create JavaScript engine
        <span class="classref">ScriptEngine</span> engine = factory.<span class="methodref">getEngineByName</span>("nashorn");
        // evaluate JavaScript code from given file - specified by first argument
        engine.<span class="methodref">eval</span>(new java.io.FileReader(args[0]));
    }
}
</code>
</pre>
Let us assume that we have the file named <a href="source/test.js">test.js</a> with the
following text:
<pre><code>
print("This is hello from test.js");
</code>
</pre>
We can run the above Java as
<pre><code>
java EvalFile test.js
</code>
</pre>
<hr>
<a name="scriptvars" id="scriptvars"></a>
<h3>Script Variables</h3>
<p>When you embed script engines and scripts with your Java
application, you may want to expose your application objects as
global variables to scripts. This example demonstrates how you can
expose your application objects as global variables to a script. We
create a <code>java.io.File</code> in the application and expose
the same as a global variable with the name "file". The script can
access the variable - for example, it can call public methods on
it. Note that the syntax to access Java objects, methods and fields
is dependent on the scripting language. JavaScript supports the
most "natural" Java-like syntax.</p>
<pre><code>
// <a href="source/ScriptVars.java">ScriptVars.java</a>

import javax.script.*;
import java.io.*;

public class ScriptVars { 
    public static void main(String[] args) throws Exception {
        ScriptEngineManager manager = new ScriptEngineManager();
        ScriptEngine engine = manager.getEngineByName("nashorn");

        File f = new File("test.txt");
        // expose File object as variable to script
        engine.<span class="methodref">put</span>("file", f);

        // evaluate a script string. The script accesses "file" 
        // variable and calls method on it
        engine.eval("print(file.getAbsolutePath())");
    }
}

</code>
</pre>
<hr>
<a name="invoke" id="invoke"></a>
<h3>Invoking Script Functions and Methods</h3>
<p>Sometimes you may want to call a specific scripting function
repeatedly - for example, your application menu functionality might
be implemented by a script. In your menu's action event handler you
may want to call a specific script function. The following example
demonstrates invoking a specific script function from Java
code.</p>
<pre><code>
// <a href="source/InvokeScriptFunction.java">InvokeScriptFunction.java</a>

import javax.script.*;

public class InvokeScriptFunction {
    public static void main(String[] args) throws Exception {
        ScriptEngineManager manager = new ScriptEngineManager();
        ScriptEngine engine = manager.getEngineByName("nashorn");

        // JavaScript code in a String
        String script = "function hello(name) { print('Hello, ' + name); }";
        // evaluate script
        engine.eval(script);

        // <code>javax.script.Invocable</code> is an optional interface.
        // Check whether your script engine implements it or not!
        // Note that the JavaScript engine implements Invocable interface.
        <span class="classref">Invocable</span> inv = (Invocable) engine;

        // invoke the global function named "hello"
        inv.<span class="methodref">invokeFunction</span>("hello", "Scripting!!" );
    }
}

</code>
</pre>
<p>If your scripting language is object based (like JavaScript) or
object-oriented, then you can invoke a script method on a script
object.</p>
<pre><code>
// <a href="source/InvokeScriptMethod.java">InvokeScriptMethod.java</a>

import javax.script.*;

public class InvokeScriptMethod {
    public static void main(String[] args) throws Exception {
        ScriptEngineManager manager = new ScriptEngineManager();
        ScriptEngine engine = manager.getEngineByName("nashorn");

        // JavaScript code in a String. This code defines a script object 'obj'
        // with one method called 'hello'.        
        String script = "var obj = new Object(); obj.hello = function(name) { print('Hello, ' + name); }";
        // evaluate script
        engine.eval(script);

        // <code>javax.script.Invocable</code> is an optional interface.
        // Check whether your script engine implements or not!
        // Note that the JavaScript engine implements Invocable interface.
        <span class="classref">Invocable</span> inv = (Invocable) engine;

        // get script object on which we want to call the method
        Object obj = engine.<span class="methodref">get</span>("obj");

        // invoke the method named "hello" on the script object "obj"
        inv.<span class="methodref">invokeMethod</span>(obj, "hello", "Script Method !!" );
    }
}

</code>
</pre>
<hr>
<a name="interfaces" id="interfaces"></a>
<h3>Implementing Java Interfaces by Scripts</h3>
<p>Instead of calling specific script functions from Java,
sometimes it is convenient to implement a Java interface by script
functions or methods. Also, by using interfaces we can avoid having
to use the <code>javax.script</code> API in many places. We can get
an interface implementor object and pass it to various Java APIs.
The following example demonstrates implementing the
<code>java.lang.Runnable</code> interface with a script.</p>
<pre><code>
// <a href="source/RunnableImpl.java">RunnableImpl.java</a>

import javax.script.*;

public class RunnableImpl {
    public static void main(String[] args) throws Exception {
        ScriptEngineManager manager = new ScriptEngineManager();
        ScriptEngine engine = manager.getEngineByName("nashorn");

        // JavaScript code in a String
        String script = "function run() { print('run called'); }";

        // evaluate script
        engine.eval(script);

        <span class="classref">Invocable</span> inv = (Invocable) engine;

        // get Runnable interface object from engine. This interface methods
        // are implemented by script functions with the matching name.
        Runnable r = inv.<span class="methodref">getInterface</span>(Runnable.class);

        // start a new thread that runs the script implemented
        // runnable interface
        Thread th = new Thread(r);
        th.start();
        th.join();
    }
}
</code>
</pre>
<p>If your scripting language is object-based or object-oriented,
it is possible to implement a Java interface by script methods on
script objects. This avoids having to call script global functions
for interface methods. The script object can store the "state"
associated with the interface implementor.</p>
<pre><code>
// <a href="source/RunnableImplObject.java">RunnableImplObject.java</a>

import javax.script.*;

public class RunnableImplObject {
    public static void main(String[] args) throws Exception {
        ScriptEngineManager manager = new ScriptEngineManager();
        ScriptEngine engine = manager.getEngineByName("nashorn");

        // JavaScript code in a String
        String script = "var obj = new Object(); obj.run = function() { print('run method called'); }";

        // evaluate script
        engine.eval(script);

        // get script object on which we want to implement the interface with
        Object obj = engine.<span class="methodref">get</span>("obj");

        <span class="classref">Invocable</span> inv = (Invocable) engine;

        // get Runnable interface object from engine. This interface methods
        // are implemented by script methods of object 'obj'
        Runnable r = inv.<span class="methodref">getInterface</span>(obj, Runnable.class);

        // start a new thread that runs the script implemented
        // runnable interface
        Thread th = new Thread(r);
        th.start();
        th.join();
    }
}
</code>
</pre>
<hr>
<a name="scopes" id="scopes"></a>
<h3>Multiple Scopes for Scripts</h3>
<p>In the <a href="#scriptvars">script variables</a> example, we
saw how to expose application objects as script global variables.
It is possible to expose multiple global "scopes" for scripts. A
single scope is an instance of <code>javax.script.Bindings</code>.
This interface is derived from <code>java.util.Map&lt;String,
Object&gt;</code>. A scope a set of name-value pairs where name is
any non-empty, non-null String.
<code>javax.script.ScriptContext</code> interface supports multiple
scopes with associated Bindings for each
scope. By default, every script engine has a default script
context. The default script context has atleast one scope called
"ENGINE_SCOPE". Various scopes supported by a script context are
available through <code>getScopes</code> method.</p>
<pre><code>
// <a href="source/MultiScopes.java">MultiScopes.java</a>

import javax.script.*;

public class MultiScopes {
    public static void main(String[] args) throws Exception {
        ScriptEngineManager manager = new ScriptEngineManager();
        ScriptEngine engine = manager.getEngineByName("nashorn");

        engine.put("x", "hello");
        // print global variable "x"
        engine.eval("print(x);");
        // the above line prints "hello"

        // Now, pass a different script context
        <span class="classref">ScriptContext</span> newContext = new <span class="classref">SimpleScriptContext</span>();
        newContext.setBindings(engine.createBindings(), ScriptContext.ENGINE_SCOPE);
        <span class="classref">Bindings</span> engineScope = newContext.<span class="methodref">getBindings</span>(ScriptContext.ENGINE_SCOPE);

        // add new variable "x" to the new engineScope        
        engineScope.<span class="methodref">put</span>("x", "world");

        // execute the same script - but this time pass a different script context
        engine.eval("print(x);", newContext);
        // the above line prints "world"
    }
}

</code>
</pre>
<hr>
<a name="jsengine" id="jsengine"></a>
<h2>JavaScript Script Engine</h2>
<p>Oracle's implementation of JDK 8 is co-bundled with the Nashorn ECMAScript
script engine.
<hr>
<a name="jstojava" id="jstojava"></a>
<h2>JavaScript to Java Communication</h2>
<p>For the most part, accessing Java classes, objects and methods
is straightforward. In particular field and method access from
JavaScript is the same as it is from Java. We highlight important
aspects of JavaScript Java access here. 
The following examples are JavaScript snippets accessing Java. This
section requires knowledge of JavaScript. This section can be
skipped if you are planning to use some other JSR-223 scripting
language rather than JavaScript.</p>
<hr>
<a name="jsjavaclass" id=jsjavalass"></a>
<h3>Accessing Java Classes</h3>
<pre>
<code>
// <a href="source/javatypes.js">javatypes.js</a>

 var arrayListType = Java.type("java.util.ArrayList")
 var intType = Java.type("int")
 var stringArrayType = Java.type("java.lang.String[]")
 var int2DArrayType = Java.type("int[][]")
</code>
</pre> 

Note that the name of the type is always a string for a fully qualified name. You can use any of these types to create new instances, e.g.:

<pre><code>
 var anArrayList = new Java.type("java.util.ArrayList")
</code></pre> 

or

<pre><code>
 var ArrayList = Java.type("java.util.ArrayList")
 var anArrayList = new ArrayList
 var anArrayListWithSize = new ArrayList(16)
</code></pre> 

In the special case of inner classes, you need to use the JVM fully qualified name, meaning using $ sign in the class name:

<pre><code>
 var ftype = Java.type("java.awt.geom.Arc2D$Float")
</code></pre> 
 

However, once you retrieved the outer class, you can access the inner class as a property on it:

<pre><code>
 var arctype = Java.type("java.awt.geom.Arc2D")
 var ftype = arctype.Float
</code></pre> 
<p>
You can access both static and non-static inner classes. If you want to create an instance of a non-static inner class, remember to pass an instance of its outer class as the first argument to the constructor.
</p>
<hr>
<a name="jsimport" id="jsimport"></a>
<h3>Importing Java Packages, Classes</h3>
<p>The built-in functions <code>importPackage</code> (in compatibility script) and
<code>importClass</code> can be used to import Java packages and
classes.</p>
<pre><code>

// <a href="source/importpackageclass.js">importpackageclass.js</a>

// load compatibility script
load("nashorn:mozilla_compat.js");
// Import Java packages and classes 
// like import package.*; in Java
<span class="functionref">importPackage</span>(java.awt);
// like import java.awt.Frame in Java
<span class="functionref">importClass</span>(java.awt.Frame);
// Create Java Objects by "new ClassName"
var frame = new java.awt.Frame("hello");
// Call Java public methods from script
frame.setVisible(true);
// Access "JavaBean" properties like "fields"
print(frame.title);
</code>
</pre>
<p>The <span class="objectref">Packages</span> global variable can
be used to access Java packages. Examples:
<code>Packages.java.util.Vector</code>,
<code>Packages.javax.swing.JFrame</code>. Please note that "java"
is a shortcut for "Packages.java". There are equivalent shortcuts
for javax, org, edu, com, net prefixes, so pratically all JDK
platform classes can be accessed without the "Packages" prefix.</p>
<p>Note that java.lang is not imported by default (unlike Java)
because that would result in conflicts with JavaScript's built-in
Object, Boolean, Math and so on.</p>
<p><code>importPackage</code> and <code>importClass</code>
functions "pollute" the global variable scope of JavaScript. To
avoid that, you may use <span class="functionref">JavaImporter</span>.</p>
<pre><code>

// <a href="source/javaimporter.js">javaimporter.js</a>

// create JavaImporter with specific packages and classes to import

var SwingGui = new <span class="functionref">JavaImporter</span>(javax.swing,
                            javax.swing.event,
                            javax.swing.border,
                            java.awt.event);
with (SwingGui) {
    // within this 'with' statement, we can access Swing and AWT
    // classes by unqualified (simple) names.

    var mybutton = new JButton("test");
    var myframe = new JFrame("test");
}

</code>
</pre>
<hr>
<a name="jsarrays" id="jsarrays"></a>
<h3>Creating, Converting and Using Java Arrays</h3>
<p>
Array element access or length access is
the same as in Java. Also, a script array can be used when a Java
method expects a Java array (auto conversion). So in most cases we
don't have to create Java arrays explicitly.</p>
<pre><code>
// <a href="source/javaarray.js">javaarray.js</a>

// create Java String array of 5 elements
var StringArray = Java.type("java.lang.String[]");
var a = new StringArray(5);

// Accessing elements and length access is by usual Java syntax
a[0] = "scripting is great!";
print(a.length);
print(a[0]);
</code>
</pre>
<p>
It is also possible to convert between JavaScript and Java arrays.
Given a JavaScript array and a Java type, <code>Java.toJavaArray</code> returns a Java array with the same initial contents, and with the specified component type. 
</p>
<pre><code>
 var anArray = [1, "13", false]
 var javaIntArray = Java.toJavaArray(anArray, "int")
 print(javaIntArray[0]) // prints 1
 print(javaIntArray[1]) // prints 13, as string "13" was converted to number 13 as per ECMAScript ToNumber conversion
 print(javaIntArray[2]) // prints 0, as boolean false was converted to number 0 as per ECMAScript ToNumber conversion
</code></pre>
<p>
Given a Java array or Collection, <code>Java.toJavaScriptArray</code> returns a JavaScript array with a shallow copy of its contents. Note that in most cases, you can use Java arrays and lists natively in Nashorn; in cases where for some reason you need to have an actual JavaScript native array (e.g. to work with the array comprehensions functions), you will want to use this method.i
</p>
<pre><code>
var File = Java.type("java.io.File");
var listCurDir = new File(".").listFiles();
var jsList = Java.toJavaScriptArray(listCurDir);
print(jsList);
</code></pre>
<hr>
<a name="jsimplement" id="jsimplement"></a>
<h3>Implementing Java Interfaces</h3>
<p>A Java interface can be implemented in JavaScript by using a
Java anonymous class-like syntax:</p>
<pre><code>
// <a href="source/runnable.js">runnable.js</a>

var r  = new java.lang.Runnable() {
    run: function() {
        print("running...\n");
    }
};

// "r" can be passed to Java methods that expect java.lang.Runnable
var th = new java.lang.Thread(r);
th.start();
th.join();
</code>
</pre>
<p>When an interface with a single method is expected, you can pass
a script function directly.(auto conversion)</p>
<pre><code>
// <a href="source/samfunc.js">samfunc.js</a>

function func() {
     print("I am func!");
}

// pass script function for java.lang.Runnable argument
var th = new java.lang.Thread(func);
th.start();
th.join();
</code>
</pre>
<hr>
<a name="jsextend" id="jsextend"></a>
<h3>Extending Java classes</h3>
<p>
If a Java class is abstract, you can instantiate an anonymous subclass of it using an argument list that is applicable to any of its public or protected constructors, but inserting a JavaScript object with functions properties that provide JavaScript implementations of the abstract methods. If method names are overloaded, the JavaScript function will provide implementation for all overloads. E.g.:
</p>

<pre><code>
 var TimerTask =  Java.type("java.util.TimerTask")
 var task = new TimerTask({ run: function() { print("Hello World!") } })
</code></pre>

Nashorn supports a syntactic extension where a "new" expression followed by an argument is identical to invoking the constructor and passing the argument to it, so you can write the above example also as:

<pre><code>
 var task = new TimerTask {
     run: function() {
       print("Hello World!")
     }
 }
</code></pre>

which is very similar to Java anonymous inner class definition. On the other hand, if the type is an abstract type with a single abstract method (commonly referred to as a "SAM type") or all abstract methods it has share the same overloaded name), then instead of an object, you can just pass a function, so the above example can become even more simplified to:

<pre><code>
 var task = new TimerTask(function() { print("Hello World!") })
</code></pre>

<p>
Note that in every one of these cases if you are trying to instantiate an abstract class that has constructors that take some arguments, you can invoke those simply by specifying the arguments after the initial implementation object or function.
</p>
<p>
The use of functions can be taken even further; if you are invoking a Java method that takes a SAM type, you can just pass in a function object, and Nashorn will know what you meant:
</p>
<code><pre>
 Java.type("java.util.Timer")
 timer.schedule(function() { print("Hello World!") })
</code></pre>

Here, <code>Timer.schedule()</code> expects a <code>TimerTask</code> as its argument, so Nashorn creates an instance of a TimerTask subclass and uses the passed function to implement its only abstract method, run(). In this usage though, you can't use non-default constructors; the type must be either an interface, or must have a protected or public no-arg constructor.

<p>
To extend a concrete Java class, you have to use <code>Java.extend</code> function.
<code>Java.extend</code> returns a type object for a subclass of the specified Java class (or implementation of the specified interface) that acts as a script-to-Java adapter for it.  
</p>
<pre><code>
// <a href="source/javaextend.js">javaextend.js</a>

var ArrayList = Java.type("java.util.ArrayList")
var ArrayListExtender = Java.extend(ArrayList)
var printSizeInvokedArrayList = new ArrayListExtender() {
    size: function() { print("size invoked!"); }
}
var printAddInvokedArrayList = new ArrayListExtender() {
    add: function(x, y) {
        if(typeof(y) === "undefined") {
            print("add(e) invoked!");
        } else {
            print("add(i, e) invoked!");
        }
    }
};
printSizeInvokedArrayList.size();
printAddInvokedArrayList.add(33, 33);
</code></pre>
<hr>
<a name="jsoverload" id="jsoverload"></a>
<h3>Overload Resolution</h3>
<p>Java methods can be overloaded by argument types. In Java,
overload resolution occurs at compile time (performed by javac).
When calling Java methods from a script, the script
interpreter/compiler needs to select the appropriate method. With
the JavaScript engine, you do not need to do anything special - the
correct Java method overload variant is selected based on the
argument types. But, sometimes you may want (or have) to explicitly
select a particular overload variant.</p>
<pre><code>
// <a href="source/overload.js">overload.js</a>

var out = java.lang.System.out;

// select a particular print function 
out["println(java.lang.Object)"]("hello");
</code>
</pre>
<hr>
<a name="engineimpl" id="engineimpl"></a>
<h2>Implementing Your Own Script Engine</h2>
<p>We will not cover implementation of JSR-223 compliant script
engines in detail. Minimally, you need to implement the
<code>javax.script.ScriptEngine</code> and
<code>javax.script.ScriptEngineFactory</code> interfaces. The
abstract class <code>javax.script.AbstractScriptEngine</code>
provides useful defaults for a few methods of the
<code>ScriptEngine</code> interface.</p>
<p>Before starting to implement a JSR-223 engine, you may want to
check <a href="http://java.net/projects/Scripting">http://java.net/projects/Scripting</a>
project. This project maintains JSR-223 implementations for many
popular open source scripting languages.</p>
<hr>
<a name="refs" id="refs"></a>
<h2>References</h2>
<ul>
<li><a href="http://jcp.org/en/jsr/detail?id=223">JSR-223 Scripting
for the Java Platform</a></li>
<li><a href="http://java.net/projects/Scripting">http://java.net/projects/Scripting
</a></li>
</ul>



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