# Scripting Java

This paper shows how to use Rhino to reach beyond JavaScript into Java [1]. Scripting Java has many uses. It allows us to write powerful scripts quickly by making use of the many Java libraries available. We can test Java classes by writing scripts. We can also aid our Java development by using scripting for exploratory programming. Exploratory programming is the process of learning about what a library or API can do by writing quick programs that use it. As we will see, scripting makes this process easier.

Note that the ECMA standard doesn't cover communication with Java (or with any external object system for that matter). All the functionality covered in this chapter should thus be considered an extension.

## Accessing Java packages and classes

Every piece of Java code is part of a class. Every Java class is part of a package. In JavaScript, however, scripts exist outside of any package hierarchy. How then, do we access classes in Java packages?

Rhino defines a top-level variable named `Packages`. The properties of the `Packages` variable are all the top-level Java packages, such as `java` and `com`. For example, we can access the value of the `java`package:

    js> Packages.java
    [JavaPackage java]

As a handy shortcut, Rhino defines a top-level variable `java` that is equivalent to `Packages.java`. So the previous example could be even shorter:

    js> java
    [JavaPackage java]

We can access Java classes simply by stepping down the package hierarchy:

    js> java.io.File
    [JavaClass java.io.File]

If your scripts access a lot of different Java classes it can get awkward to use the full package name of the class every time. Rhino provides a top-level function `importPackage` that serves the same purpose as Java's `import` declaration. For example, we could import all of the classes in the `java.io`package and access class `java.io.File` using just the name `File`:

    js> importPackage(java.io)
    js> File
    [JavaClass java.io.File]

Here `importPackage(java.io)` makes all the classes in the `java.io` package (such as `File`) available at the top level. It's equivalent in effect to the Java declaration `import java.io.*;`.

It's important to note that Java imports `java.lang.*` implicitly, while Rhino does not. The reason is that JavaScript has its own top-level objects `Boolean`, `Math`, `Number`, `Object`, and `String` that are different from the classes by those names defined in the `java.lang` package. Because of this conflict, it's a good idea not to use `importPackage` on the `java.lang` package.

One thing to be careful of is Rhino's handling of errors in specifying Java package or class names. If `java.MyClass` is accessed, Rhino attempts to load a class named `java.MyClass`. If that load fails, it assumes that `java.MyClass` is a package name, and no error is reported:

    js> java.MyClass
    [JavaPackage java.MyClass]

Only if you attempt to use this object as a class will an error be reported.

## Working with Java objects

Now that we can access Java classes, the next logical step is to create an object. This works just as in Java, with the use of the `new` operator:

    js> new java.util.Date()
    Thu Jan 24 16:18:17 EST 2002

If we store the new object in a JavaScript variable, we can then call methods on it:

    js> f = new java.io.File("test.txt")
    test.txt
    js> f.exists()
    true
    js> f.getName()
    test.txt

Static methods and fields can be accessed from the class object itself:

    js> java.lang.Math.PI
    3.141592653589793
    js> java.lang.Math.cos(0)
    1

In JavaScript, unlike Java, the method by itself is an object and can be evaluated as well as being called. If we just view the method object by itself we can see the various overloaded forms of the method:

    js> f.listFiles
    function listFiles() {/*
    java.io.File[] listFiles()
    java.io.File[] listFiles(java.io.FilenameFilter)
    java.io.File[] listFiles(java.io.FileFilter)
    */}

This output shows that the `File` class defines three overloaded methods `listFiles`: one that takes no arguments, another with a `FilenameFilter` argument, and a third with a `FileFilter` argument. All the methods return an array of `File` objects. Being able to view the parameters and return type of Java methods is particularly useful in exploratory programming where we might be investigating a method and are unsure of the parameter or return types.

Another useful feature for exploratory programming is the ability to see all the methods and fields defined for an object. Using the JavaScript `for..in` construct, we can print out all these values:

    js> for (i in f) { print(i) }
    exists
    parentFile
    mkdir
    toString
    wait
    [44 others]

Note that not only the methods of the `File` class are listed, but also the methods inherited from the base class `java.lang.Object` (like `wait`). This makes it easier to work with objects in deeply nested inheritance hierarchies since you can see all the methods that are available for that object.

Rhino provides another convenience by allowing properties of JavaBeans to be accessed directly by their property names. A JavaBean property `foo` is defined by the methods `getFoo` and `setFoo`. Additionally, a boolean property of the same name can be defined by an `isFoo` method [2]. For example, the following code actually calls the `File` object's `getName` and `isDirectory` methods.

    js> f.name
    test.txt
    js> f.directory
    false

## Calling overloaded methods

The process of choosing a method to call based upon the types of the arguments is called overload resolution. In Java, overload resolution is performed at compile time, while in Rhino it occurs at runtime. This difference is inevitable given JavaScript's use of dynamic typing as was discussed in Chapter 2: since the type of a variable is not known until runtime, only then can overload resolution occur.

As an example, consider the following Java class that defines a number of overloaded methods and calls them.

    public class Overload {

        public String f(Object o) { return "f(Object)"; }
        public String f(String s) { return "f(String)"; }
        public String f(int i)    { return "f(int)"; }

        public String g(String s, int i) { return "g(String,int)"; }
        public String g(int i, String s) { return "g(int,String)"; }

        public static void main(String[] args) {
            Overload o = new Overload();
            Object[] a = new Object[] { new Integer(3), "hi", Overload.class };
            for (int i = 0; i != a.length; ++i)
                System.out.println(o.f(a[i]));
        }
    }

When we compile and execute the program, it produces the output

    f(Object)
    f(Object)
    f(Object)

However, if we write a similar script

    var o = new Packages.Overload();
    var a = [ 3, "hi", Packages.Overload ];
    for (var i = 0; i != a.length; ++i)
        print(o.f(a[i]));

and execute it, we get the output

    f(int)
    f(String)
    f(Object)

Because Rhino selects an overloaded method at runtime, it calls the more specific type that matches the argument. Meanwhile Java selects the overloaded method purely on the type of the argument at compile time.

Although this has the benefit of selecting a method that may be a better match for each call, it does have an impact on performance since more work is done at each call. In practice this performance cost hasn't been noticeable in real applications.

Because overload resolution occurs at runtime, it can fail at runtime. For example, if we call `Overload`'s method `g` with two integers we get an error because neither form of the method is closer to the argument types than the other:

    js> o.g(3,4)
    js:"<stdin>", line 2: The choice of Java method Overload.g
    matching JavaScript argument types (number,number) is ambiguous;
    candidate methods are:
    class java.lang.String g(java.lang.String,int)
    class java.lang.String g(int,java.lang.String)

A more precise definition of overloading semantics can be found at http://www.mozilla.org/js/liveconnect/lc3_method_overloading.html.

## Implementing Java interfaces

Now that we can access Java classes, create Java objects, and access fields, methods, and properties of those objects, we have a great deal of power at our fingertips. However, there are a few instances where that is not enough: many APIs in Java work by providing interfaces that clients must implement. One example of this is the `Thread` class: its constructor takes a `Runnable` that contains a single method `run` that will be called when the new thread is started.

To address this need, Rhino provides the ability to create new Java objects that implement interfaces. First we must define a JavaScript object with function properties whose names match the methods required by the Java interface. To implement a `Runnable`, we need only define a single method `run` with no parameters. If you remember from Chapter 3, it is possible to define a JavaScript object with the `{propertyName: value}` notation. We can use that syntax here in combination with a function expression to define a JavaScript object with a `run` method:

    js> obj = { run: function () { print("\nrunning"); } }
    [object Object]
    js> obj.run()

    running

Now we can create an object implementing the `Runnable` interface by constructing a `Runnable`:

    js> r = new java.lang.Runnable(obj);
    [object JavaObject]

In Java it is not possible to use the `new` operator on an interface because there is no implementation available. Here Rhino gets the implementation from the JavaScript object `obj`. Now that we have an object implementing `Runnable`, we can create a `Thread` and run it. The function we defined for `run` will be called on a new thread.

    js> t = new java.lang.Thread(r)
    Thread[Thread-2,5,main]
    js> t.start()
    js>

    running

The final `js` prompt and the output from the new thread may appear in either order, depending on thread scheduling.

Behind the scenes, Rhino generates the bytecode for a new Java class that implements `Runnable` and forwards all calls to its `run` method over to an associated JavaScript object. The object that implements this class is called a Java adapter. Because the forwarding to JavaScript occurs at runtime, it is possible to delay defining the methods implementing an interface until they are called. While omitting a required method is bad practice for programming in the large, it's useful for small scripts and for exploratory programming.

## The `JavaAdapter` constructor

In the previous section we created Java adapters using the `new` operator with Java interfaces. This approach has its limitations: it's not possible to implement multiple interfaces, nor can we extend non-abstract classes. For these reasons there is a `JavaAdapter` constructor.

The syntax of the `JavaAdapter` constructor is

    new JavaAdapter(javaIntfOrClass, [javaIntf, ..., javaIntf,] javascriptObject)

Here `javaIntfOrClass` is an interface to implement or a class to extend and `javaIntf` are aditional interfaces to implement. The `javascriptObject` is the JavaScript object containing the methods that will be called from the Java adapter.

In practice there's little need to call the `JavaAdapter` constructor directly. Most of the time the previous syntaxes using the `new` operator will be sufficient.

## JavaScript functions as Java interfaces

Often we need to implement an interface with only one method, like in the previous `Runnable` example or when providing various event listener implementations. To facilitate this Rhino allows to pass JavaScript function when such interface is expected. The function is called as the implementation of interface method.

Here is the simplified `Runnable` example:

    js> t = java.lang.Thread(function () { print("\nrunning"); });
    Thread[Thread-0,5,main]
    js> t.start()
    js>
    running

Rhino also allows to use JavaScript function as implementation of Java interface with more then method if all the methods has the same signature. When calling the function, Rhino passes method's name as the additional argument. Function can use it to distinguish on behalf of which method it was called:

    js> var frame = new Packages.javax.swing.JFrame();
    js> frame.addWindowListener(function(event, methodName) {
    	if (methodName == "windowClosing") {
                print("Calling System.exit()..."); java.lang.System.exit(0);
    	}
        });
    js> frame.setSize(100, 100);
    js> frame.visible = true;
    true
    js> Calling System.exit()...

## Creating Java arrays

Rhino provides no special syntax for creating Java arrays. You must use the `java.lang.reflect.Array` class for this purpose. To create an array of five Java strings you would make the following call:

    js> a = java.lang.reflect.Array.newInstance(java.lang.String, 5);
    [Ljava.lang.String;@7ffe01

To create an array of primitive types, we must use the special TYPE field defined in the associated object class in the `java.lang` package. For example, to create an array of bytes, we must use the special field `java.lang.Byte.TYPE`:

    js> a = java.lang.reflect.Array.newInstance(java.lang.Character.TYPE, 2);
    [C@7a84e4

The resulting value can then be used anywhere a Java array of that type is allowed.

    js> a[0] = 104
    104
    js> a[1] = 105
    105
    js> new java.lang.String(a)
    hi

## Java strings and JavaScript strings

It's important to keep in mind that Java strings and JavaScript strings are not the same. Java strings are instances of the type `java.lang.String` and have all the methods defined by that class. JavaScript strings have methods defined by `String.prototype`. The most common stumbling block is `length`, which is a method of Java strings and a dynamic property of JavaScript strings:

    js> javaString = new java.lang.String("Java")
    Java
    js> jsString = "JavaScript"
    JavaScript
    js> javaString.length()
    4
    js> jsString.length
    10

Rhino provides some help in reducing the differences between the two types. First, you can pass a JavaScript string to a Java method that requires a Java string and Rhino will perform the conversion. We actually saw this feature in action on the call to the `java.lang.String` constructor in the preceding example.

Rhino also makes the JavaScript methods available to Java strings if the java.lang.String class doesn't already define them. For example:

    js> javaString.match(/a.*/)
    ava

[1] The ability to call Java from JavaScript was first implemented as part of a Netscape browser technology called LiveConnect. However, since that technology also encompassed communication with browser plugins, and since the way of calling JavaScript from Java in Rhino is entirely different, that term won't be used in this paper.

[2] For more information on JavaBeans, see Developing Java Beans by Robert Englander.