LiveConnect Overview

This chapter describes using LiveConnect technology to let Java and JavaScript code communicate with each other. The chapter assumes you are familiar with Java programming.

Working with Wrappers

In JavaScript, a wrapper is an object of the target language data type that encloses an object of the source language. When programming in JavaScript, you can use a wrapper object to access methods and fields of the Java object; calling a method or accessing a property on the wrapper results in a call on the Java object. On the Java side, JavaScript objects are wrapped in an instance of the class netscape.javascript.JSObject and passed to Java.

When a JavaScript object is sent to Java, the runtime engine creates a Java wrapper of type JSObject; when a JSObject is sent from Java to JavaScript, the runtime engine unwraps it to its original JavaScript object type. The JSObject class provides an interface for invoking JavaScript methods and examining JavaScript properties.

JavaScript to Java Communication

When you refer to a Java package or class, or work with a Java object or array, you use one of the special LiveConnect objects. All JavaScript access to Java takes place with these objects, which are summarized in the following table.

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 Conversion for complete information.

In some ways, the existence of the LiveConnect objects is transparent, because you interact with Java in a fairly intuitive way. For example, you can create a Java String object and assign it to the JavaScript variable myString by using the new operator with the Java constructor, as follows:

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

In the previous example, the variable myString is a JavaObject because it holds an instance of the Java object String. As a JavaObject, myString has access to the public instance methods of java.lang.String and its superclass, java.lang.Object. These Java methods are available in JavaScript as methods of the JavaObject, and you can call them as follows:

myString.length(); // returns 11

Static members can be called directly on the JavaClass object.

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

The Packages Object

If a Java class is not part of the java, sun, or netscape packages, you access it with the Packages object. For example, suppose the Redwood corporation uses a Java package called redwood to contain various Java classes that it implements. To create an instance of the HelloWorld class in redwood, you access the constructor of the class as follows:

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

You can also access classes in the default package (that is, classes that don't explicitly name a package). For example, if the HelloWorld class is directly in the CLASSPATH and not in a package, you can access it as follows:

var red = new Packages.HelloWorld();

The LiveConnect java, sun, and netscape objects provide shortcuts for commonly used Java packages. For example, you can use the following:

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

instead of the longer version:

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

Working with Java Arrays

When any Java method creates an array and you reference that array in JavaScript, you are working with a JavaArray. For example, the following code creates the JavaArray x with ten elements of type int:

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

Like the JavaScript Array object, JavaArray has a length property which returns the number of elements in the array. Unlike Array.length, JavaArray.length is a read-only property, because the number of elements in a Java array are fixed at the time of creation.

Package and Class References

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 + "";