sundar@68: Java Technology
sundar@68: Copyright © 2013, Oracle and/or its affiliates. All rights reserved. sundar@68:
sundar@68:| sundar@68: sundar@68: sundar@68: sundar@68: | sundar@68:
The JavaTM Scripting API sundar@68: is a scripting language indepedent framework for using script sundar@68: engines from Java code. With the Java Scripting API, it is possible sundar@68: to write customizable/extendable applications in the Java language sundar@68: and leave the customization scripting language choice to the end sundar@68: user. The Java application developer need not choose the extension sundar@68: language during development. If you write your application with sundar@68: JSR-223 API, then your users can use any JSR-223 compliant sundar@68: scripting language.
sundar@68:The Java Scripting functionality is in the javax.script
sundar@68: package. This is a relatively small, simple API. The starting point
sundar@68: of the scripting API is the ScriptEngineManager class.
sundar@68: A ScriptEngineManager object can discover script engines through
sundar@68: the jar file service discovery mechanism. It can also instantiate
sundar@68: ScriptEngine objects that interpret scripts written in a specific
sundar@68: scripting language. The simplest way to use the scripting API is as
sundar@68: follows:
ScriptEngineManager
sundar@68: object.ScriptEngine object from the
sundar@68: manager.ScriptEngine's
sundar@68: eval methods.Now, it is time to look at some sample code. While it is sundar@68: not mandatory, it may be useful to know a bit of JavaScript to read sundar@68: these examples.
sundar@68:From the ScriptEngineManager instance, we
sundar@68: request a JavaScript engine instance using
sundar@68: getEngineByName method. On the script engine, the
sundar@68: eval method is called to execute a given String as
sundar@68: JavaScript code! For brevity, in this as well as in subsequent
sundar@68: examples, we have not shown exception handling. There are checked
sundar@68: and runtime exceptions thrown from javax.script API.
sundar@68: Needless to say, you have to handle the exceptions
sundar@68: appropriately.
sundar@68:
sundar@68: // EvalScript.java
sundar@68:
sundar@68: import javax.script.*;
sundar@68: public class EvalScript {
sundar@68: public static void main(String[] args) throws Exception {
sundar@68: // create a script engine manager
sundar@68: ScriptEngineManager factory = new ScriptEngineManager();
sundar@68: // create a JavaScript engine
sundar@68: ScriptEngine engine = factory.getEngineByName("nashorn");
sundar@68: // evaluate JavaScript code from String
sundar@68: engine.eval("print('Hello, World')");
sundar@68: }
sundar@68: }
sundar@68:
sundar@68:
sundar@68: In this example, we call the eval method that
sundar@68: accepts java.io.Reader for the input source. The
sundar@68: script read by the given reader is executed. This way it is
sundar@68: possible to execute scripts from files, URLs and resources by
sundar@68: wrapping the relevant input stream objects as readers.
sundar@68:
sundar@68: // EvalFile.java
sundar@68:
sundar@68: import javax.script.*;
sundar@68:
sundar@68: public class EvalFile {
sundar@68: public static void main(String[] args) throws Exception {
sundar@68: // create a script engine manager
sundar@68: ScriptEngineManager factory = new ScriptEngineManager();
sundar@68: // create JavaScript engine
sundar@68: ScriptEngine engine = factory.getEngineByName("nashorn");
sundar@68: // evaluate JavaScript code from given file - specified by first argument
sundar@68: engine.eval(new java.io.FileReader(args[0]));
sundar@68: }
sundar@68: }
sundar@68:
sundar@68:
sundar@68: Let us assume that we have the file named test.js with the
sundar@68: following text:
sundar@68:
sundar@68: print("This is hello from test.js");
sundar@68:
sundar@68:
sundar@68: java EvalFile test.js
sundar@68:
sundar@68: When you embed script engines and scripts with your Java
sundar@68: application, you may want to expose your application objects as
sundar@68: global variables to scripts. This example demonstrates how you can
sundar@68: expose your application objects as global variables to a script. We
sundar@68: create a java.io.File in the application and expose
sundar@68: the same as a global variable with the name "file". The script can
sundar@68: access the variable - for example, it can call public methods on
sundar@68: it. Note that the syntax to access Java objects, methods and fields
sundar@68: is dependent on the scripting language. JavaScript supports the
sundar@68: most "natural" Java-like syntax.
sundar@377: Nashorn script engine pre-defines two global variables named "context" sundar@377: and "engine". The "context" variable is of type javax.script.ScriptContext sundar@377: and refers to the current ScriptContext instance passed to script engine's sundar@377: eval method. The "engine" variable is of type javax.script.ScriptEngine and sundar@377: refers to the current nashorn script engine instance evaluating the script. sundar@377: Both of these variables are non-writable, non-enumerable and non-configurable sundar@377: - which implies script code can not write overwrite the value, for..loop iteration sundar@377: on global object will not iterate these variables and these variables can not be sundar@377: deleted by script. sundar@68:
sundar@68: // ScriptVars.java
sundar@68:
sundar@68: import javax.script.*;
sundar@68: import java.io.*;
sundar@68:
sundar@68: public class ScriptVars {
sundar@68: public static void main(String[] args) throws Exception {
sundar@68: ScriptEngineManager manager = new ScriptEngineManager();
sundar@68: ScriptEngine engine = manager.getEngineByName("nashorn");
sundar@68:
sundar@68: File f = new File("test.txt");
sundar@68: // expose File object as variable to script
sundar@68: engine.put("file", f);
sundar@68:
sundar@68: // evaluate a script string. The script accesses "file"
sundar@68: // variable and calls method on it
sundar@68: engine.eval("print(file.getAbsolutePath())");
sundar@68: }
sundar@68: }
sundar@68:
sundar@68:
sundar@68: Sometimes you may want to call a specific scripting function sundar@68: repeatedly - for example, your application menu functionality might sundar@68: be implemented by a script. In your menu's action event handler you sundar@68: may want to call a specific script function. The following example sundar@68: demonstrates invoking a specific script function from Java sundar@68: code.
sundar@68:
sundar@68: // InvokeScriptFunction.java
sundar@68:
sundar@68: import javax.script.*;
sundar@68:
sundar@68: public class InvokeScriptFunction {
sundar@68: public static void main(String[] args) throws Exception {
sundar@68: ScriptEngineManager manager = new ScriptEngineManager();
sundar@68: ScriptEngine engine = manager.getEngineByName("nashorn");
sundar@68:
sundar@68: // JavaScript code in a String
sundar@68: String script = "function hello(name) { print('Hello, ' + name); }";
sundar@68: // evaluate script
sundar@68: engine.eval(script);
sundar@68:
sundar@68: // javax.script.Invocable is an optional interface.
sundar@68: // Check whether your script engine implements it or not!
sundar@68: // Note that the JavaScript engine implements Invocable interface.
sundar@68: Invocable inv = (Invocable) engine;
sundar@68:
sundar@68: // invoke the global function named "hello"
sundar@68: inv.invokeFunction("hello", "Scripting!!" );
sundar@68: }
sundar@68: }
sundar@68:
sundar@68:
sundar@68: If your scripting language is object based (like JavaScript) or sundar@68: object-oriented, then you can invoke a script method on a script sundar@68: object.
sundar@68:
sundar@68: // InvokeScriptMethod.java
sundar@68:
sundar@68: import javax.script.*;
sundar@68:
sundar@68: public class InvokeScriptMethod {
sundar@68: public static void main(String[] args) throws Exception {
sundar@68: ScriptEngineManager manager = new ScriptEngineManager();
sundar@68: ScriptEngine engine = manager.getEngineByName("nashorn");
sundar@68:
sundar@68: // JavaScript code in a String. This code defines a script object 'obj'
sundar@68: // with one method called 'hello'.
sundar@68: String script = "var obj = new Object(); obj.hello = function(name) { print('Hello, ' + name); }";
sundar@68: // evaluate script
sundar@68: engine.eval(script);
sundar@68:
sundar@68: // javax.script.Invocable is an optional interface.
sundar@68: // Check whether your script engine implements or not!
sundar@68: // Note that the JavaScript engine implements Invocable interface.
sundar@68: Invocable inv = (Invocable) engine;
sundar@68:
sundar@68: // get script object on which we want to call the method
sundar@68: Object obj = engine.get("obj");
sundar@68:
sundar@68: // invoke the method named "hello" on the script object "obj"
sundar@68: inv.invokeMethod(obj, "hello", "Script Method !!" );
sundar@68: }
sundar@68: }
sundar@68:
sundar@68:
sundar@68: Instead of calling specific script functions from Java,
sundar@68: sometimes it is convenient to implement a Java interface by script
sundar@68: functions or methods. Also, by using interfaces we can avoid having
sundar@68: to use the javax.script API in many places. We can get
sundar@68: an interface implementor object and pass it to various Java APIs.
sundar@68: The following example demonstrates implementing the
sundar@68: java.lang.Runnable interface with a script.
sundar@68: // RunnableImpl.java
sundar@68:
sundar@68: import javax.script.*;
sundar@68:
sundar@68: public class RunnableImpl {
sundar@68: public static void main(String[] args) throws Exception {
sundar@68: ScriptEngineManager manager = new ScriptEngineManager();
sundar@68: ScriptEngine engine = manager.getEngineByName("nashorn");
sundar@68:
sundar@68: // JavaScript code in a String
sundar@68: String script = "function run() { print('run called'); }";
sundar@68:
sundar@68: // evaluate script
sundar@68: engine.eval(script);
sundar@68:
sundar@68: Invocable inv = (Invocable) engine;
sundar@68:
sundar@68: // get Runnable interface object from engine. This interface methods
sundar@68: // are implemented by script functions with the matching name.
sundar@68: Runnable r = inv.getInterface(Runnable.class);
sundar@68:
sundar@68: // start a new thread that runs the script implemented
sundar@68: // runnable interface
sundar@68: Thread th = new Thread(r);
sundar@68: th.start();
sundar@68: th.join();
sundar@68: }
sundar@68: }
sundar@68:
sundar@68: If your scripting language is object-based or object-oriented, sundar@68: it is possible to implement a Java interface by script methods on sundar@68: script objects. This avoids having to call script global functions sundar@68: for interface methods. The script object can store the "state" sundar@68: associated with the interface implementor.
sundar@68:
sundar@68: // RunnableImplObject.java
sundar@68:
sundar@68: import javax.script.*;
sundar@68:
sundar@68: public class RunnableImplObject {
sundar@68: public static void main(String[] args) throws Exception {
sundar@68: ScriptEngineManager manager = new ScriptEngineManager();
sundar@68: ScriptEngine engine = manager.getEngineByName("nashorn");
sundar@68:
sundar@68: // JavaScript code in a String
sundar@68: String script = "var obj = new Object(); obj.run = function() { print('run method called'); }";
sundar@68:
sundar@68: // evaluate script
sundar@68: engine.eval(script);
sundar@68:
sundar@68: // get script object on which we want to implement the interface with
sundar@68: Object obj = engine.get("obj");
sundar@68:
sundar@68: Invocable inv = (Invocable) engine;
sundar@68:
sundar@68: // get Runnable interface object from engine. This interface methods
sundar@68: // are implemented by script methods of object 'obj'
sundar@68: Runnable r = inv.getInterface(obj, Runnable.class);
sundar@68:
sundar@68: // start a new thread that runs the script implemented
sundar@68: // runnable interface
sundar@68: Thread th = new Thread(r);
sundar@68: th.start();
sundar@68: th.join();
sundar@68: }
sundar@68: }
sundar@68:
sundar@68: In the script variables example, we
sundar@68: saw how to expose application objects as script global variables.
sundar@68: It is possible to expose multiple global "scopes" for scripts. A
sundar@68: single scope is an instance of javax.script.Bindings.
sundar@68: This interface is derived from java.util.Map<String,
sundar@68: Object>. A scope a set of name-value pairs where name is
sundar@68: any non-empty, non-null String.
sundar@68: javax.script.ScriptContext interface supports multiple
sundar@68: scopes with associated Bindings for each
sundar@68: scope. By default, every script engine has a default script
sundar@68: context. The default script context has atleast one scope called
sundar@68: "ENGINE_SCOPE". Various scopes supported by a script context are
sundar@68: available through getScopes method.
sundar@68: // MultiScopes.java
sundar@68:
sundar@68: import javax.script.*;
sundar@68:
sundar@68: public class MultiScopes {
sundar@68: public static void main(String[] args) throws Exception {
sundar@68: ScriptEngineManager manager = new ScriptEngineManager();
sundar@68: ScriptEngine engine = manager.getEngineByName("nashorn");
sundar@68:
sundar@68: engine.put("x", "hello");
sundar@68: // print global variable "x"
sundar@68: engine.eval("print(x);");
sundar@68: // the above line prints "hello"
sundar@68:
sundar@68: // Now, pass a different script context
sundar@68: ScriptContext newContext = new SimpleScriptContext();
sundar@68: newContext.setBindings(engine.createBindings(), ScriptContext.ENGINE_SCOPE);
sundar@68: Bindings engineScope = newContext.getBindings(ScriptContext.ENGINE_SCOPE);
sundar@68:
sundar@68: // add new variable "x" to the new engineScope
sundar@68: engineScope.put("x", "world");
sundar@68:
sundar@68: // execute the same script - but this time pass a different script context
sundar@68: engine.eval("print(x);", newContext);
sundar@68: // the above line prints "world"
sundar@68: }
sundar@68: }
sundar@68:
sundar@68:
sundar@68: Oracle's implementation of JDK 8 is co-bundled with the Nashorn ECMAScript sundar@68: script engine. sundar@68:
For the most part, accessing Java classes, objects and methods sundar@68: is straightforward. In particular field and method access from sundar@68: JavaScript is the same as it is from Java. We highlight important sundar@68: aspects of JavaScript Java access here. sundar@68: The following examples are JavaScript snippets accessing Java. This sundar@68: section requires knowledge of JavaScript. This section can be sundar@68: skipped if you are planning to use some other JSR-223 scripting sundar@68: language rather than JavaScript.
sundar@68:
sundar@68:
sundar@68: // javatypes.js
sundar@68:
sundar@68: var arrayListType = Java.type("java.util.ArrayList")
sundar@68: var intType = Java.type("int")
sundar@68: var stringArrayType = Java.type("java.lang.String[]")
sundar@68: var int2DArrayType = Java.type("int[][]")
sundar@68:
sundar@68:
sundar@68:
attila@267: Note that the name of the type is always a string for a fully qualified name. You can use any of these expressions to create new instances, e.g.:
sundar@68:
sundar@68:
attila@267: var anArrayList = new (Java.type("java.util.ArrayList"))
sundar@68:
sundar@68: var ArrayList = Java.type("java.util.ArrayList")
sundar@68: var anArrayList = new ArrayList
sundar@68: var anArrayListWithSize = new ArrayList(16)
sundar@68:
sundar@68: var ftype = Java.type("java.awt.geom.Arc2D$Float")
sundar@68:
attila@409: var ftype = Java.type("java.awt.geom.Arc2D.Float")
attila@409:
sundar@68: var arctype = Java.type("java.awt.geom.Arc2D")
sundar@68: var ftype = arctype.Float
sundar@68: sundar@68: 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. sundar@68:
attila@267:
attila@267: In addition to creating new instances, the type objects returned from Java.type calls can also be used to access the
attila@267: static fields and methods of the classes:
attila@267:
attila@267: var File = Java.type("java.io.File")
attila@267: File.createTempFile("nashorn", ".tmp")
attila@267:
attila@267: Methods with names of the form isXxx(), getXxx(), and setXxx() can also be used as properties, for both instances and statics.
attila@267:
attila@267: A type object returned from Java.type is distinct from a java.lang.Class object. You can obtain one from the other using properties class and static on them.
attila@267:
attila@267: var ArrayList = Java.type("java.util.ArrayList")
attila@267: var a = new ArrayList
attila@267:
attila@267: // All of the following print true:
attila@267: print("Type acts as target of instanceof: " + (a instanceof ArrayList))
attila@267: print("Class doesn't act as target of instanceof: " + !(a instanceof a.getClass()))
attila@267: print("Type is not same as instance's getClass(): " + (a.getClass() !== ArrayList))
attila@267: print("Type's `class` property is same as instance getClass(): " + (a.getClass() === ArrayList.class))
attila@267: print("Type is same as instance getClass()'s `static` property: " + (a.getClass().static === ArrayList))
attila@267:
attila@267: You can think of the type object as similar to the class names as used in Java source code: you use them as the
attila@267: arguments to the new and instanceof operators and as the namespace for the static fields
attila@267: and methods, but they are different than the runtime Class objects returned by getClass() calls.
attila@267: Syntactically and semantically, this separation produces code that is most similar to Java code, where a distinction
attila@267: between compile-time class expressions and runtime class objects also exists. (Also, Java can't have the equivalent of static
attila@267: property on a Class object since compile-time class expressions are never reified as objects).
attila@267:
The built-in functions importPackage (in compatibility script) and
sundar@68: importClass can be used to import Java packages and
sundar@68: classes.
sundar@68:
sundar@68: // importpackageclass.js
sundar@68:
sundar@68: // load compatibility script
sundar@68: load("nashorn:mozilla_compat.js");
sundar@68: // Import Java packages and classes
sundar@68: // like import package.*; in Java
sundar@68: importPackage(java.awt);
sundar@68: // like import java.awt.Frame in Java
sundar@68: importClass(java.awt.Frame);
sundar@68: // Create Java Objects by "new ClassName"
sundar@68: var frame = new java.awt.Frame("hello");
sundar@68: // Call Java public methods from script
sundar@68: frame.setVisible(true);
sundar@68: // Access "JavaBean" properties like "fields"
sundar@68: print(frame.title);
sundar@68:
sundar@68: The Packages global variable can
sundar@68: be used to access Java packages. Examples:
sundar@68: Packages.java.util.Vector,
sundar@68: Packages.javax.swing.JFrame. Please note that "java"
sundar@68: is a shortcut for "Packages.java". There are equivalent shortcuts
sundar@68: for javax, org, edu, com, net prefixes, so pratically all JDK
sundar@68: platform classes can be accessed without the "Packages" prefix.
Note that java.lang is not imported by default (unlike Java) sundar@68: because that would result in conflicts with JavaScript's built-in sundar@68: Object, Boolean, Math and so on.
sundar@68:importPackage and importClass
sundar@68: functions "pollute" the global variable scope of JavaScript. To
sundar@68: avoid that, you may use JavaImporter.
sundar@68:
sundar@68: // javaimporter.js
sundar@68:
sundar@68: // create JavaImporter with specific packages and classes to import
sundar@68:
sundar@68: var SwingGui = new JavaImporter(javax.swing,
sundar@68: javax.swing.event,
sundar@68: javax.swing.border,
sundar@68: java.awt.event);
sundar@68: with (SwingGui) {
sundar@68: // within this 'with' statement, we can access Swing and AWT
sundar@68: // classes by unqualified (simple) names.
sundar@68:
sundar@68: var mybutton = new JButton("test");
sundar@68: var myframe = new JFrame("test");
sundar@68: }
sundar@68:
sundar@68:
sundar@68: attila@267: Array element access or length access is the same as in Java.
sundar@68:
sundar@68: // javaarray.js
sundar@68:
sundar@68: // create Java String array of 5 elements
sundar@136: var StringArray = Java.type("java.lang.String[]");
sundar@136: var a = new StringArray(5);
sundar@68:
sundar@68: // Accessing elements and length access is by usual Java syntax
sundar@68: a[0] = "scripting is great!";
sundar@68: print(a.length);
sundar@68: print(a[0]);
sundar@68:
sundar@68:
sundar@68: It is also possible to convert between JavaScript and Java arrays.
attila@279: Given a JavaScript array and a Java type, Java.to returns a Java array with the same initial contents, and with the specified array type.
sundar@68:
sundar@68: var anArray = [1, "13", false]
attila@279: var javaIntArray = Java.to(anArray, "int[]")
sundar@68: print(javaIntArray[0]) // prints 1
sundar@68: print(javaIntArray[1]) // prints 13, as string "13" was converted to number 13 as per ECMAScript ToNumber conversion
sundar@68: print(javaIntArray[2]) // prints 0, as boolean false was converted to number 0 as per ECMAScript ToNumber conversion
sundar@68:
attila@279: You can use either a string or a type object returned from Java.type() to specify the type of the array.
attila@267: You can also omit the array type, in which case a Object[] will be created.
attila@267:
attila@279: Given a Java array or Collection, Java.from 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.
sundar@68:
sundar@68: var File = Java.type("java.io.File");
sundar@68: var listCurDir = new File(".").listFiles();
attila@279: var jsList = Java.from(listCurDir);
sundar@68: print(jsList);
sundar@68: A Java interface can be implemented in JavaScript by using a sundar@68: Java anonymous class-like syntax:
sundar@68:
sundar@68: // runnable.js
sundar@68:
sundar@68: var r = new java.lang.Runnable() {
sundar@68: run: function() {
sundar@68: print("running...\n");
sundar@68: }
sundar@68: };
sundar@68:
sundar@68: // "r" can be passed to Java methods that expect java.lang.Runnable
sundar@68: var th = new java.lang.Thread(r);
sundar@68: th.start();
sundar@68: th.join();
sundar@68:
sundar@68: When an interface with a single method is expected, you can pass sundar@68: a script function directly.(auto conversion)
sundar@68:
sundar@68: // samfunc.js
sundar@68:
sundar@68: function func() {
sundar@68: print("I am func!");
sundar@68: }
sundar@68:
sundar@68: // pass script function for java.lang.Runnable argument
sundar@68: var th = new java.lang.Thread(func);
sundar@68: th.start();
sundar@68: th.join();
sundar@68:
sundar@68: sundar@68: 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.: sundar@68:
sundar@68: sundar@68:
sundar@68: var TimerTask = Java.type("java.util.TimerTask")
sundar@68: var task = new TimerTask({ run: function() { print("Hello World!") } })
sundar@68:
sundar@68: var task = new TimerTask {
sundar@68: run: function() {
sundar@68: print("Hello World!")
sundar@68: }
sundar@68: }
sundar@68:
sundar@68: var task = new TimerTask(function() { print("Hello World!") })
sundar@68: sundar@68: 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. sundar@68:
sundar@68:sundar@68: 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: sundar@68:
sundar@68:
sundar@68: Java.type("java.util.Timer")
sundar@68: timer.schedule(function() { print("Hello World!") })
sundar@68: Timer.schedule() expects a TimerTask 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.
sundar@68:
attila@267:
sundar@68: To extend a concrete Java class, you have to use Java.extend function.
sundar@68: Java.extend 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.
sundar@68:
sundar@68: // javaextend.js
sundar@68:
sundar@68: var ArrayList = Java.type("java.util.ArrayList")
sundar@68: var ArrayListExtender = Java.extend(ArrayList)
sundar@68: var printSizeInvokedArrayList = new ArrayListExtender() {
sundar@68: size: function() { print("size invoked!"); }
sundar@68: }
sundar@68: var printAddInvokedArrayList = new ArrayListExtender() {
sundar@68: add: function(x, y) {
sundar@68: if(typeof(y) === "undefined") {
sundar@68: print("add(e) invoked!");
sundar@68: } else {
sundar@68: print("add(i, e) invoked!");
sundar@68: }
sundar@68: }
sundar@68: };
sundar@68: printSizeInvokedArrayList.size();
sundar@68: printAddInvokedArrayList.add(33, 33);
sundar@68:
attila@267: The reason you must use Java.extend() with concrete classes is that with concrete classes, there can be a
attila@267: syntactic ambiguity if you just invoke their constructor. Consider this example:
attila@267:
attila@267: var t = new java.lang.Thread({ run: function() { print("Hello!") } })
attila@267:
attila@267: If we allowed subclassing of concrete classes with constructor syntax, Nashorn couldn't tell if you're creating a new
attila@267: Thread and passing it a Runnable at this point, or you are subclassing Thread and
attila@267: passing it a new implementation for its own run() method.
attila@267:
attila@267: Java.extend can in fact take a list of multiple types. At most one of the types can be a class, and the rest must
attila@267: be interfaces (the class doesn't have to be the first in the list). You will get back an object that extends the class and
attila@267: implements all the interfaces. (Obviously, if you only specify interfaces and no class, the object will extend java.lang.Object).
attila@267:
attila@267: The methods shown so far for extending Java classes and implementing interfaces – passing an implementation JavaScript object
attila@267: or function to a constructor, or using Java.extend with new – all produce classes that take an
attila@267: extra JavaScript object parameter in their constructors that specifies the implementation. The implementation is therefore always bound
attila@267: to the actual instance being created with new, and not to the whole class. This has some advantages, for example in the
attila@267: memory footprint of the runtime, as Nashorn can just create a single "universal adapter" for every combination of types being implemented.
attila@267: In reality, the below code shows that different instantiations of, say, Runnable have the same class regardless of them having
attila@267: different JavaScript implementation objects:
attila@267:
attila@267: var Runnable = java.lang.Runnable;
attila@267: var r1 = new Runnable(function() { print("I'm runnable 1!") })
attila@267: var r2 = new Runnable(function() { print("I'm runnable 2!") })
attila@267: r1.run()
attila@267: r2.run()
attila@267: print("We share the same class: " + (r1.class === r2.class))
attila@267: attila@267: prints: attila@267:
attila@267:
attila@267: I'm runnable 1!
attila@267: I'm runnable 2!
attila@267: We share the same class: true
attila@267: attila@267: Sometimes, however, you'll want to extend a Java class or implement an interface with implementation bound to the class, not to attila@267: its instances. Such a need arises, for example, when you need to pass the class for instantiation to an external API; prime example attila@267: of this is the JavaFX framework where you need to pass an Application class to the FX API and let it instantiate it. attila@267:
attila@267:
attila@267: Fortunately, there's a solution for that: Java.extend() – aside from being able to take any number of type parameters
attila@267: denoting a class to extend and interfaces to implement – can also take one last argument that has to be a JavaScript object
attila@267: that serves as the implementation for the methods. In this case, Java.extend() will create a class that has the same
attila@267: constructors as the original class had, as they don't need to take an an extra implementation object parameter. The example below
attila@267: shows how you can create class-bound implementations, and shows that in this case, the implementation classes for different invocations
attila@267: are indeed different:
attila@267:
attila@267: var RunnableImpl1 = Java.extend(java.lang.Runnable, function() { print("I'm runnable 1!") })
attila@267: var RunnableImpl2 = Java.extend(java.lang.Runnable, function() { print("I'm runnable 2!") })
attila@267: var r1 = new RunnableImpl1()
attila@267: var r2 = new RunnableImpl2()
attila@267: r1.run()
attila@267: r2.run()
attila@267: print("We share the same class: " + (r1.class === r2.class))
attila@267: attila@267: prints: attila@267:
attila@267:
attila@267: I'm runnable 1!
attila@267: I'm runnable 2!
attila@267: We share the same class: false
attila@267:
attila@267: As you can see, the major difference here is that we moved the implementation object into the invocation of Java.extend
attila@267: from the constructor invocations – indeed the constructor invocations now don't even need to take an extra parameter! Since
attila@267: the implementations are bound to a class, the two classes obviously can't be the same, and we indeed see that the two runnables no
attila@267: longer share the same class – every invocation of Java.extend() with a class-specific implementation object triggers
attila@267: the creation of a new Java adapter class.
attila@267:
attila@267: Finally, the adapter classes with class-bound implementations can still take an additional constructor parameter to further
attila@267: override the behavior on a per-instance basis. Thus, you can even combine the two approaches: you can provide part of the implementation
attila@267: in a class-based JavaScript implementation object passed to Java.extend, and part in another object passed to the constructor.
attila@267: Whatever functions are provided by the constructor-passed object will override the functions in the class-bound object.
attila@267:
attila@267: var RunnableImpl = Java.extend(java.lang.Runnable, function() { print("I'm runnable 1!") })
attila@267: var r1 = new RunnableImpl()
attila@267: var r2 = new RunnableImpl(function() { print("I'm runnable 2!") })
attila@267: r1.run()
attila@267: r2.run()
attila@267: print("We share the same class: " + (r1.class === r2.class))
attila@267: attila@267: prints: attila@267:
attila@267:
attila@267: I'm runnable 1!
attila@267: I'm runnable 2!
attila@267: We share the same class: true
attila@267: Java methods can be overloaded by argument types. In Java, sundar@68: overload resolution occurs at compile time (performed by javac). attila@267: When calling Java methods from Nashorn, the appropriate method will be attila@267: selected based on the argument types at invocation time. You do not need attila@267: to do anything special – the correct Java method overload variant attila@267: is selected based automatically. You still have the option of explicitly attila@267: specifying a particular overload variant. Reasons for this include attila@267: either running into a genuine ambiguity with actual argument types, or attila@267: rarely reasons of performance – if you specify the actual overload attila@267: then the engine doesn't have to perform resolution during invocation. attila@267: Individual overloads of a Java methods are exposed as special properties attila@267: with the name of the method followed with its signature in parentheses. attila@267: You can invoke them like this:
sundar@68:
sundar@68: // overload.js
sundar@68:
sundar@68: var out = java.lang.System.out;
sundar@68:
sundar@68: // select a particular print function
attila@267: out["println(Object)"]("hello");
sundar@68:
sundar@68: attila@267: Note that you normally don't even have to use qualified class names in attila@267: the signatures as long as the unqualified name of the type is sufficient attila@267: for uniquely identifying the signature. In practice this means that only attila@267: in the extremely unlikely case that two overloads only differ in attila@267: parameter types that have identical unqualified names but come from attila@267: different packages would you need to use the fully qualified name of the attila@267: class. attila@267:
attila@267:attila@267: We have previously shown some of the data type mappings between Java and JavaScript. attila@267: We saw that arrays need to be explicitly converted. We have also shown that JavaScript functions attila@267: are automatically converted to SAM types when passed as parameters to Java methods. Most other attila@267: conversions work as you would expect. attila@267:
attila@267:
attila@267: Every JavaScript object is also a java.util.Map so APIs receiving maps will receive them directly.
attila@267:
attila@267: When numbers are passed to a Java API, they will be converted to the expected target numeric type, either boxed or
attila@267: primitive, but if the target type is less specific, say Number or Object, you can only
attila@267: count on them being a Number, and have to test specifically for whether it's a boxed Double,
attila@267: Integer, Long, etc. – it can be any of these due to internal optimizations. Also, you
attila@267: can pass any JavaScript value to a Java API expecting either a boxed or primitive number; the JavaScript specification's
attila@267: ToNumber conversion algorithm will be applied to the value.
attila@267:
attila@267: In a similar vein, if a Java method expects a String or a Boolean, the values will be
attila@267: converted using all conversions allowed by the JavaScript specification's ToString and ToBoolean
attila@267: conversions.
attila@267:
attila@267: Finally, a word of caution about strings. Due to internal performance optimizations of string operations, JavaScript strings are
attila@267: not always necessarily of type java.lang.String, but they will always be of type java.lang.CharSequence.
attila@267: If you pass them to a Java method that expects a java.lang.String parameter, then you will naturally receive a Java
attila@267: String, but if the signature of your method is more generic, i.e. it receives a java.lang.Object parameter, you can
attila@267: end up with an object of private engine implementation class that implements CharSequence but is not a Java String.
attila@267:
We will not cover implementation of JSR-223 compliant script
sundar@68: engines in detail. Minimally, you need to implement the
sundar@68: javax.script.ScriptEngine and
sundar@68: javax.script.ScriptEngineFactory interfaces. The
sundar@68: abstract class javax.script.AbstractScriptEngine
sundar@68: provides useful defaults for a few methods of the
sundar@68: ScriptEngine interface.
Before starting to implement a JSR-223 engine, you may want to sundar@68: check http://java.net/projects/Scripting sundar@68: project. This project maintains JSR-223 implementations for many sundar@68: popular open source scripting languages.
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