Scripting Java

出典: MDC

目次 1 Java でスクリプト 2 Java のパッケージとクラスにアクセスする 3 Java のオブジェクトと連携する 4 オーバーロードされたメソッドを呼び出す 5 Java のインターフェイスを実装する 6 JavaAdapter のコンストラクタ 7 Java のインターフェイスとしての JavaScript の関数 8 Java の配列を作成 9 Java の文字列と JavaScript の文字列

[編集] Java でスクリプト

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.

[編集] Java のパッケージとクラスにアクセスする

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 javapackage:

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.

[編集] Java のオブジェクトと連携する

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

[編集] オーバーロードされたメソッドを呼び出す

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.

[編集] Java のインターフェイスを実装する

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.

[編集] JavaAdapter のコンストラクタ

前のセクションで、Javaインターフェイスと共にnew演算子を使ってJavaへのアダプタを作成した。このやり方には限界がある。複数のインターフェイスを実装できなく、また、非アブストラクトクラスを継承できない。これらの理由から、JavaAdapterコンストラクタがある。

JavaAdapterコンストラクタの文法は

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

javaIntfOrClassは実装するインターフェイスか継承するクラスで、javaIntfは追加実装するインターフェイスである。javascriptObjectはJavaアダプタから呼ばれるメソッドを含むJavaScript のオブジェクトである。

実際には、JavaAdapterコンストラクタを呼ぶ必要性はあまりない。ほとんどの場合、new演算子を使う前の文法で十分である。

[編集] Java のインターフェイスとしての JavaScript の関数

前述の Runnable の例や様々なイベントリスナーの実装を提供する時のように、私たちはメソッドが一つだけのインターフェイスを実装する必要がしばしばあります。これを簡単にできるように、Rhinoは JavaScript の関数をそのような期待されているインターフェイスとして渡すことが出来るようになっています。その関数は、インターフェイスを実装されたメソッドとして呼ばれます。

ここに、簡単化された Runnable の例があります:

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

Rhino は、全てのメソッドが同じシグニチャを持つ場合、JavaScript の関数を Java のインターフェイスの実装としてメソッド以上の物として使うことが出来ます。関数を呼ぶ時は、Rhino はメソッド名を追加された引数として渡します。関数は、どのメソッドが呼ばれたかを識別するためにそれを使うことが出来ます:

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()...

[編集] Java の配列を作成

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

[編集] Java の文字列と JavaScript の文字列

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