LiveConnect概述

这一章描述了使用 LiveConnect  技术来使JavaScript和Java能够互相通信。本章假设你熟悉Java编程。

使用包装器(Working with Wrappers)

在JavaScript中，包装器(wrapper)是一个目标语言数据类型的对象，它包装了一个源语言的对象。当使用JavaScript编程的时候，你可以使用包装器(wrapper)对象来访问Java对象的方法和字段;调用包装器(wrapper)的方法或者访问它的属性都会作用在Java对象上。在Java端，JavaScript对象被包装在类netscape.javascript.JSObject的实例中，然后传递给Java。

当Javascript对象被传递给Java，运行时引擎会创建一个JSObject类型的Java包装器(wrapper);当JSObject从Java传递到JavaScript, 运行时引擎将它解包成它原来的JavaScript对象类型。JSObject类提供了一个接口用来调用JavaScript的方法和检测JavaScript的属性。

JavaScript到Java通信

当你引用一个Java包或者类，或者使用Java的对象和数组，你会使用某一个特别的LiveConnect对象。所有访问Java的JavaScript使用下面表格中总结的对象：

注意: 因为Java是强类型语言，而JavaScript是弱类型的语言，当你使用LiveConnect的时候，JavaScript运行时引擎会将参数值转换为其他语言中的合适数据类型。请参考 Data Type Conversion 来获取完整信息。

在某些情形下，LiveConnect对象的存在是透明的，因为你凭借一定的直觉来和Java交互。例如，你能够通过使用new操作符跟上Java构造器来像下面一样创建一个Java字符串(String)对象，并且将它赋值给JavaScript变量myString：

var myString = new java.lang.String("Hello world");

在上面的例子中，变量myString是一个JavaObject，因为它持有一个Java的String对象。作为JavaObject, myString可以获取java.lang.String及其父类java.lang.Object的公有实例方法。这些Java方法在JavaScript中作为JavaObject的方法使用，你可以像下面这样来使用它们：

myString.length(); // returns 11

静态成员可以通过JavaClass对象来调用。

alert(java.lang.Integer.MAX_VALUE); //alerts 2147483647

包对象

如果一个Java类不是java, sun, 或者netscape包的一部分，你需要使用Packages对象来访问它。例如，假设Redwood公司使用一个叫redwood的java包来包含各种它实现的Java类。为了创建一个redwood中HelloWorld类的实例，你像下面这样访问类的构造器：

var red = new Packages.redwood.HelloWorld();

你也可以使用默认包中的类(就是说，类没有显式的命名包)。例如，如果HelloWorld类在CLASSPATH中，并且不在包中，你可以像下面这样使用它：

var red = new Packages.HelloWorld();

LiveConnect中的java, sun, 和netscape对象提供了常用Java包的快捷方式。例如，你可以像下面这样使用：

var myString = new java.lang.String("Hello world");

来替换下面长版本:

var myString = new Packages.java.lang.String("Hello world");

使用Java数组

当任何Java方法创建了一个数组，并且你在JavaScript中引用那个数组，你就会使用到JavaArray。例如，下面的代码创建了有十个int类型元素的JavaArray x对象：

var x = java.lang.reflect.Array.newInstance(java.lang.Integer, 10);

跟JavaScript的Array对象类似， JavaArray有一个length属性用来返回数组元素的长度。不像Array.length, JavaArray.length是一个只读属性, 因为Java数组的元素长度在创建的时候就固定了。

包和类的引用

Simple references to Java packages and classes from JavaScript create the JavaPackage and JavaClass objects. In the earlier example about the Redwood corporation, for example, the reference Packages.redwood is a JavaPackage object. Similarly, a reference such as java.lang.String is a JavaClass object.

Most of the time, you don't have to worry about the JavaPackage and JavaClass objects—you just work with Java packages and classes, and LiveConnect creates these objects transparently. There are cases where LiveConnect will fail to load a class, and you will need to manually load it like this:

var Widgetry = java.lang.Thread.currentThread().getContextClassLoader().loadClass("org.mywidgets.Widgetry");

In JavaScript 1.3 and earlier, JavaClass objects are not automatically converted to instances of java.lang.Class when you pass them as parameters to Java methods—you must create a wrapper around an instance of java.lang.Class. In the following example, the forName method creates a wrapper object theClass, which is then passed to the newInstance method to create an array.

// JavaScript 1.3
var theClass = java.lang.Class.forName("java.lang.String");
var theArray = java.lang.reflect.Array.newInstance(theClass, 5);

In JavaScript 1.4 and later, you can pass a JavaClass object directly to a method, as shown in the following example:

// JavaScript 1.4
var theArray = java.lang.reflect.Array.newInstance(java.lang.String, 5);

Arguments of Type char

In JavaScript 1.4 and later, you can pass a one-character string to a Java method which requires an argument of type char. For example, you can pass the string "H" to the Character constructor as follows:

var c = new java.lang.Character("H");

In JavaScript 1.3 and earlier, you must pass such methods an integer which corresponds to the Unicode value of the character. For example, the following code also assigns the value "H" to the variable c:

var c = new java.lang.Character(72);

Handling Java Exceptions in JavaScript

When Java code fails at run time, it throws an exception. If your JavaScript code accesses a Java data member or method and fails, the Java exception is passed on to JavaScript for you to handle. Beginning with JavaScript 1.4, you can catch this exception in a try...catch statement. (Although this functionality (along with some others) had been broken in Gecko 1.9 (see bug 391642) as the Mozilla-specific LiveConnect code had not been maintained inside Mozilla, with Java 6 update 11 and 12 building support for reliance on Mozilla's implementation of the generic (and cross-browser) NPAPI plugin code, this has again been fixed.)

For example, suppose you are using the Java forName method to assign the name of a Java class to a variable called theClass. The forName method throws an exception if the value you pass it does not evaluate to the name of a Java class. Place the forName assignment statement in a try block to handle the exception, as follows:

function getClass(javaClassName) {
   try {
      var theClass = java.lang.Class.forName(javaClassName);
   } catch (e) {
      return ("The Java exception is " + e);
   }
   return theClass;
}

In this example, if javaClassName evaluates to a legal class name, such as "java.lang.String", the assignment succeeds. If javaClassName evaluates to an invalid class name, such as "String", the getClass function catches the exception and returns something similar to the following:

The Java exception is java.lang.ClassNotFoundException: String

For specialized handling based on the exception type, use the instanceof operator:

try {
  // ...
} catch (e) {
  if (e instanceof java.io.FileNotFound) {
     // handling for FileNotFound
  } else {
    throw e;
  }
}

See Exception Handling Statements for more information about JavaScript exceptions.

Java to JavaScript Communication

If you want to use JavaScript objects in Java, you must import the netscape.javascript package into your Java file. This package defines the following classes:

• netscape.javascript.JSObject allows Java code to access JavaScript methods and properties.
• netscape.javascript.JSException allows Java code to handle JavaScript errors.

See the JavaScript Reference for more information about these classes.

Locating the LiveConnect classes

In older versions of the Netscape browser, these classes were distributed along with the browser. Starting with JavaScript 1.2, these classes are delivered in a .jar file; in previous versions of JavaScript, these classes are delivered in a .zip file. For example, with Netscape Navigator 4 for Windows NT, the classes are delivered in the java40.jar file in the Program\Java\Classes directory beneath the Navigator directory.

More recently, the classes have been distributed with Sun's Java Runtime; initially in the file "jaws.jar" in the "jre/lib" directory of the runtime distribution (for JRE 1.3), then in "plugin.jar" in the same location (JRE 1.4 and up).

Using the LiveConnect classes with the JDK

To access the LiveConnect classes, place the .jar or .zip file in the CLASSPATH of the JDK compiler in either of the following ways:

• Create a CLASSPATH environment variable to specify the path and name of .jar or .zip file.
• Specify the location of .jar or .zip file when you compile by using the -classpath command line parameter.

You can specify an environment variable in Windows NT by double-clicking the System icon in the Control Panel and creating a user environment variable called CLASSPATH with a value similar to the following:

C:\Program Files\Java\jre1.4.1\lib\plugin.jar

See the Sun JDK documentation for more information about CLASSPATH.

Note: Because Java is a strongly typed language and JavaScript is weakly typed, the JavaScript runtime engine converts argument values into the appropriate data types for the other language when you use LiveConnect. See Data Type Conversions for complete information.

Using the LiveConnect Classes

All JavaScript objects appear within Java code as instances of netscape.javascript.JSObject. When you call a method in your Java code, you can pass it a JavaScript object as one of its argument. To do so, you must define the corresponding formal parameter of the method to be of type JSObject.

Also, any time you use JavaScript objects in your Java code, you should put the call to the JavaScript object inside a try...catch statement which handles exceptions of type netscape.javascript.JSException. This allows your Java code to handle errors in JavaScript code execution which appear in Java as exceptions of type JSException.

Accessing JavaScript with JSObject

For example, suppose you are working with the Java class called JavaDog. As shown in the following code, the JavaDog constructor takes the JavaScript object jsDog, which is defined as type JSObject, as an argument:

import netscape.javascript.*;

public class JavaDog{
    public String dogBreed;
    public String dogColor;
    public String dogSex;

    // define the class constructor
    public JavaDog(JSObject jsDog){
        // use try...catch to handle JSExceptions here
        this.dogBreed = (String)jsDog.getMember("breed");
        this.dogColor = (String)jsDog.getMember("color");
        this.dogSex = (String)jsDog.getMember("sex");
    }
}

Notice that the getMember method of JSObject is used to access the properties of the JavaScript object. The previous example uses getMember to assign the value of the JavaScript property jsDog.breed to the Java data member JavaDog.dogBreed.

Note: A more realistic example would place the call to getMember inside a try...catch statement to handle errors of type JSException. See Handling JavaScript Exceptions in Java for more information.

To get a better sense of how getMember works, look at the definition of the custom JavaScript object Dog:

function Dog(breed,color,sex){
   this.breed = breed;
   this.color = color;
   this.sex = sex;
}

You can create a JavaScript instance of Dog called gabby as follows:

var gabby = new Dog("lab", "chocolate", "female");

If you evaluate gabby.color, you can see that it has the value "chocolate". Now suppose you create an instance of JavaDog in your JavaScript code by passing the gabby object to the constructor as follows:

var javaDog = new Packages.JavaDog(gabby);

If you evaluate javaDog.dogColor, you can see that it also has the value "chocolate", because the getMember method in the Java constructor assigns dogColor the value of gabby.color.

Handling JavaScript Exceptions in Java

When JavaScript code called from Java fails at run time, it throws an exception. If you are calling the JavaScript code from Java, you can catch this exception in a try...catch statement. The JavaScript exception is available to your Java code as an instance of netscape.javascript.JSException.

JSException is a Java wrapper around any exception type thrown by JavaScript, similar to the way that instances of JSObject are wrappers for JavaScript objects. Use JSException when you are evaluating JavaScript code in Java.

When you are evaluating JavaScript code in Java, the following situations can cause run-time errors:

• The JavaScript code is not evaluated, either due to a JavaScript compilation error or to some other error that occurred at run time. The JavaScript interpreter generates an error message that is converted into an instance of JSException.
• Java successfully evaluates the JavaScript code, but the JavaScript code executes an unhandled throw statement. JavaScript throws an exception that is wrapped as an instance of JSException. Use the getWrappedException method of JSException to unwrap this exception in Java.

For example, suppose the Java object eTest evaluates the string jsCode that you pass to it. You can respond to either type of run-time error the evaluation causes by implementing an exception handler such as the following:

import netscape.javascript.JSObject;
import netscape.javascript.JSException;

public class eTest {
    public static Object doit(JSObject obj, String jsCode) {
        try {
            obj.eval(jsCode);
        } catch (JSException e) {
            if (e.getWrappedException() == null)
                return e;
            return e.getWrappedException();
        }
        return null;
    }
}

In this example, the code in the try block attempts to evaluate the string jsCode that you pass to it. Let's say you pass the string "myFunction()" as the value of jsCode. If myFunction is not defined as a JavaScript function, the JavaScript interpreter cannot evaluate jsCode. The interpreter generates an error message, the Java handler catches the message, and the doit method returns an instance of netscape.javascript.JSException.

However, suppose myFunction is defined in JavaScript as follows:

function myFunction() {
   try {
      if (theCondition == true) {
         return "Everything's ok";
      } else {
         throw "JavaScript error occurred";
      }
   } catch (e) {
      if (canHandle == true) {
         handleIt();
      } else {
         throw e;
      }
   }
}

If theCondition is false, the function throws an exception. The exception is caught in the JavaScript code, and if canHandle is true, JavaScript handles it. If canHandle is false, the exception is rethrown, the Java handler catches it, and the doit method returns a Java string:

JavaScript error occurred

See Exception Handling Statements for complete information about JavaScript exceptions.

Backward Compatibility

In JavaScript 1.3 and earlier versions, the JSException class had three public constructors which optionally took a string argument, specifying the detail message or other information for the exception. The getWrappedException method was not available.

Use a try...catch statement such as the following to handle LiveConnect exceptions in JavaScript 1.3 and earlier versions:

try {
   global.eval("foo.bar = 999;");
} catch (Exception e) {
   if (e instanceof JSException) {
      jsCodeFailed();
   } else {
      otherCodeFailed();
   }
}

In this example, the eval statement fails if foo is not defined. The catch block executes the jsCodeFailed method if the eval statement in the try block throws a JSException; the otherCodeFailed method executes if the try block throws any other error.

Data Type Conversions

Because Java is a strongly typed language and JavaScript is weakly typed, the JavaScript runtime engine converts argument values into the appropriate data types for the other language when you use LiveConnect. These conversions are described in the following sections:

• JavaScript to Java Conversions
• Java to JavaScript Conversions

JavaScript to Java Conversions

When you call a Java method and pass it parameters from JavaScript, the data types of the parameters you pass in are converted according to the rules described in the following sections:

• Number Values
• Boolean Values
• String Values
• Undefined Values
• Null Values
• JavaArray and JavaObject objects
• JavaClass objects
• Other JavaScript objects

The return values of methods of netscape.javascript.JSObject are always converted to instances of java.lang.Object. The rules for converting these return values are also described in these sections.

For example, if JSObject.eval returns a JavaScript number, you can find the rules for converting this number to an instance of java.lang.Object in Number Values.

Number Values

When you pass JavaScript number types as parameters to Java methods, Java converts the values according to the rules described in the following table:

When a JavaScript number is passed as a parameter to a Java method which expects an instance of java.lang.String, the number is converted to a string. Use the equals() method to compare the result of this conversion with other string values.

Boolean Values

When you pass JavaScript Boolean types as parameters to Java methods, Java converts the values according to the rules described in the following table:

When a JavaScript Boolean is passed as a parameter to a Java method which expects an instance of java.lang.String, the Boolean is converted to a string. Use the == operator to compare the result of this conversion with other string values.

String Values

When you pass JavaScript string types as parameters to Java methods, Java converts the values according to the rules described in the following table:

Undefined Values

When you pass undefined JavaScript values as parameters to Java methods, Java converts the values according to the rules described in the following table:

The undefined value conversion is possible in JavaScript 1.3 and later versions only. Earlier versions of JavaScript do not support undefined values.

When a JavaScript undefined value is passed as a parameter to a Java method which expects an instance of java.lang.String, the undefined value is converted to a string. Use the == operator to compare the result of this conversion with other string values.

Null Values

When you pass null JavaScript values as parameters to Java methods, Java converts the values according to the rules described in the following table:

JavaArray and JavaObject objects

In most situations, when you pass a JavaScript JavaArray or JavaObject as a parameter to a Java method, Java simply unwraps the object; in a few situations, the object is coerced into another data type according to the rules described in the following table:

An interface or class is assignment-compatible with an unwrapped object if the unwrapped object is an instance of the Java parameter type. That is, the following statement must return true:

unwrappedObject instanceof parameterType;

JavaClass objects

When you pass a JavaScript JavaClass object as a parameter to a Java method, Java converts the object according to the rules described in the following table:

Other JavaScript objects

When you pass any other JavaScript object as a parameter to a Java method, Java converts the object according to the rules described in the following table:

Java to JavaScript Conversions

Values passed from Java to JavaScript are converted as follows:

• Java byte, char, short, int, long, float, and double are converted to JavaScript numbers.
• A Java boolean is converted to a JavaScript boolean.
• An object of class netscape.javascript.JSObject is converted to the original JavaScript object.
• Java arrays are converted to a JavaScript pseudo-Array object; this object behaves just like a JavaScript Array object: you can access it with the syntax arrayName[index] (where index is an integer), and determine its length with arrayName.length.
• A Java object of any other class is converted to a JavaScript wrapper, which can be used to access methods and fields of the Java object:
  • Converting this wrapper to a string calls the toString method on the original object.
  • Converting to a number calls the doubleValue method, if possible, and fails otherwise.
  • Converting to a boolean in JavaScript 1.3 and later versions returns false if the object is null, and true otherwise.
  • Converting to a boolean in JavaScript 1.2 and earlier versions calls the booleanValue method, if possible, and fails otherwise.

Note that instances of java.lang.Double and java.lang.Integer are converted to JavaScript objects, not to JavaScript numbers. Similarly, instances of java.lang.String are also converted to JavaScript objects, not to JavaScript strings.

Java String objects also correspond to JavaScript wrappers. If you call a JavaScript method that requires a JavaScript string and pass it this wrapper, you'll get an error. Instead, convert the wrapper to a JavaScript string by appending the empty string to it, as shown here:

var JavaString = JavaObj.methodThatReturnsAString();
var JavaScriptString = JavaString + "";