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Java Scripting Programmer's Guide |
The JavaTM 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.
The Java Scripting functionality is in the javax.script
package. This is a relatively small, simple API. The starting point
of the scripting API is the ScriptEngineManager 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:
ScriptEngineManager
object.ScriptEngine object from the
manager.ScriptEngine's
eval methods.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.
From the ScriptEngineManager instance, we
request a JavaScript engine instance using
getEngineByName method. On the script engine, the
eval 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 javax.script API.
Needless to say, you have to handle the exceptions
appropriately.
import javax.script.*;
public class EvalScript {
public static void main(String[] args) throws Exception {
// create a script engine manager
ScriptEngineManager factory = new ScriptEngineManager();
// create a JavaScript engine
ScriptEngine engine = factory.getEngineByName("JavaScript");
// evaluate JavaScript code from String
engine.eval("print('Hello, World')");
}
}
In this example, we call the eval method that
accepts java.io.Reader 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.
import javax.script.*;
public class EvalFile {
public static void main(String[] args) throws Exception {
// create a script engine manager
ScriptEngineManager factory = new ScriptEngineManager();
// create JavaScript engine
ScriptEngine engine = factory.getEngineByName("JavaScript");
// evaluate JavaScript code from given file - specified by first argument
engine.eval(new java.io.FileReader(args[0]));
}
}
Let us assume that we have the file named "test.js" with the
following text:
println("This is hello from test.js");
We can run the above Java as
java EvalFile test.js
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 java.io.File 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.
public class ScriptVars {
public static void main(String[] args) throws Exception {
ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("JavaScript");
File f = new File("test.txt");
// expose File object as variable to script
engine.put("file", f);
// evaluate a script string. The script accesses "file"
// variable and calls method on it
engine.eval("print(file.getAbsolutePath())");
}
}
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.
import javax.script.*;
public class InvokeScriptFunction {
public static void main(String[] args) throws Exception {
ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("JavaScript");
// JavaScript code in a String
String script = "function hello(name) { print('Hello, ' + name); }";
// evaluate script
engine.eval(script);
// javax.script.Invocable is an optional interface.
// Check whether your script engine implements or not!
// Note that the JavaScript engine implements Invocable interface.
Invocable inv = (Invocable) engine;
// invoke the global function named "hello"
inv.invokeFunction("hello", "Scripting!!" );
}
}
If your scripting language is object based (like JavaScript) or object-oriented, then you can invoke a script method on a script object.
import javax.script.*;
public class InvokeScriptMethod {
public static void main(String[] args) throws Exception {
ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("JavaScript");
// 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);
// javax.script.Invocable is an optional interface.
// Check whether your script engine implements or not!
// Note that the JavaScript engine implements Invocable interface.
Invocable inv = (Invocable) engine;
// get script object on which we want to call the method
Object obj = engine.get("obj");
// invoke the method named "hello" on the script object "obj"
inv.invokeMethod(obj, "hello", "Script Method !!" );
}
}
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 javax.script API in many places. We can get
an interface implementor object and pass it to various Java APIs.
The following example demonstrates implementing the
java.lang.Runnable interface with a script.
import javax.script.*;
public class RunnableImpl {
public static void main(String[] args) throws Exception {
ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("JavaScript");
// JavaScript code in a String
String script = "function run() { println('run called'); }";
// evaluate script
engine.eval(script);
Invocable inv = (Invocable) engine;
// get Runnable interface object from engine. This interface methods
// are implemented by script functions with the matching name.
Runnable r = inv.getInterface(Runnable.class);
// start a new thread that runs the script implemented
// runnable interface
Thread th = new Thread(r);
th.start();
}
}
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.
import javax.script.*;
public class RunnableImplObject {
public static void main(String[] args) throws Exception {
ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("JavaScript");
// JavaScript code in a String
String script = "var obj = new Object(); obj.run = function() { println('run method called'); }";
// evaluate script
engine.eval(script);
// get script object on which we want to implement the interface with
Object obj = engine.get("obj");
Invocable inv = (Invocable) engine;
// get Runnable interface object from engine. This interface methods
// are implemented by script methods of object 'obj'
Runnable r = inv.getInterface(obj, Runnable.class);
// start a new thread that runs the script implemented
// runnable interface
Thread th = new Thread(r);
th.start();
}
}
In the script variables 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 javax.script.Bindings.
This interface is derived from java.util.Map<String,
Object>. A scope a set of name-value pairs where name is
any non-empty, non-null String. Multiple scopes are supported by
javax.script.ScriptContext interface. A script context
supports one or more 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 getScopes method.
import javax.script.*;
public class MultiScopes {
public static void main(String[] args) throws Exception {
ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("JavaScript");
engine.put("x", "hello");
// print global variable "x"
engine.eval("println(x);");
// the above line prints "hello"
// Now, pass a different script context
ScriptContext newContext = new SimpleScriptContext();
Bindings engineScope = newContext.getBindings(ScriptContext.ENGINE_SCOPE);
// add new variable "x" to the new engineScope
engineScope.put("x", "world");
// execute the same script - but this time pass a different script context
engine.eval("println(x);", newContext);
// the above line prints "world"
}
}
Sun's implementation of JDK 6 is co-bundled with the Mozilla Rhino based JavaScript script engine. This is based on Mozilla Rhino version 1.6R2. Most of the Rhino implementation is included. A few components have been excluded due to footprint and security reasons:
var v = new java.lang.Runnable() {
run: function() { print('hello'); }
}
v.run();
In most cases, JavaAdapter is used to implement aa single interface
with Java anonymizer class-like syntax. The uses of JavaAdapter to
extend a Java class or to implement multiple interfaces are very
rare.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. For more details, please refer to http://www.mozilla.org/rhino/scriptjava.html. 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.
The built-in functions importPackage and
importClass can be used to import Java packages and
classes.
// Import Java packages and classes
// like import package.*; in Java
importPackage(java.awt);
// like import java.awt.Frame in Java
importClass(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);
The Packages global variable can
be used to access Java packages. Examples:
Packages.java.util.Vector,
Packages.javax.swing.JFrame. 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.
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.
importPackage and importClass
functions "pollute" the global variable scope of JavaScript. To
avoid that, you may use JavaImporter.
// create JavaImporter with specific packages and classes to import
var SwingGui = new JavaImporter(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");
}
While creating a Java object is the same as in Java, to create Java arrays in JavaScript we need to use Java reflection explicitly. But once created the 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.
// create Java String array of 5 elements
var a = java.lang.reflect.Array.newInstance(java.lang.String, 5);
// Accessing elements and length access is by usual Java syntax
a[0] = "scripting is great!";
print(a.length);
A Java interface can be implemented in JavaScript by using a Java anonymous class-like syntax:
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();
When an interface with a single method is expected, you can pass a script function directly.(auto conversion)
function func() {
print("I am func!");
}
// pass script function for java.lang.Runnable argument
var th = new java.lang.Thread(func);
th.start();
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.
var out = java.lang.System.out;
// select a particular println function
out["println(java.lang.Object)"]("hello");
More details on JavaScript's Java method overload resolution is at http://www.mozilla.org/js/liveconnect/lc3_method_overloading.html
We will not cover implementation of JSR-223 compliant script
engines in detail. Minimally, you need to implement the
javax.script.ScriptEngine and
javax.script.ScriptEngineFactory interfaces. The
abstract class javax.script.AbstractScriptEngine
provides useful defaults for a few methods of the
ScriptEngine interface.
Before starting to implement a JSR-223 engine, you may want to check http://scripting.dev.java.net project. This project maintains JSR-223 implementations for many popular open source scripting languages.
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