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:

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:

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

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:

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:

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:

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

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

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:

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:

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.

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.

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

However, if we write a similar script

and execute it, we get the output

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:

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:

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

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.

The final js prompt and the output from the new thread may appear in either order, depending on thread scheduling. Rhino also allows the JavaScript object literal and the interface creation expression to be combined for a more convenient syntax:

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

Each javaIntfOrClass is an interface to implement or a class to extend. (Only one javaIntfOrClass may be a class, of course.) 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.

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:

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:

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

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:

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: