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Core JavaScript Guide 1.5



Chapter 9   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.

This chapter contains the following sections:

For additional information on using LiveConnect, see the JavaScript technical notes on the DevEdge site.



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.


Table 9.1    The LiveConnect Objects

Object

Description

JavaArray  

A wrapped Java array, accessed from within JavaScript code.  

JavaClass  

A JavaScript reference to a Java class.  

JavaObject  

A wrapped Java object, accessed from within JavaScript code.  

JavaPackage  

A JavaScript reference to a Java package.  

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.

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


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:

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.

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
theClass = java.lang.Class.forName("java.lang.String")
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
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:

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:

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.

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

See "Exception Handling Statements" on page 78 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: 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. See the Core JavaScript Reference for more information about these classes.

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

For example, in Navigator 4. 0 for Windows NT, the classes are delivered in the java40.jar file in the Program\Java\Classes directory beneath the Navigator directory. 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:

D:\Navigator\Program\Java\Classes\java40.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:

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:

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:

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" on page 78 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

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:

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:


Java parameter type

Conversion rules

double  

  • The exact value is transferred to Java without rounding and without a loss of magnitude or sign.  

  • NaN is converted to NaN.  

java.lang.Double
java.lang.Object  

A new instance of java.lang.Double is created, and the exact value is transferred to Java without rounding and without a loss of magnitude or sign.  

float  

  • Values are rounded to float precision.

  • Values which are too large or small to be represented are rounded to +infinity or -infinity.

  • NaN is converted to NaN.

 

byte
char
int
long
short  

  • Values are rounded using round-to-negative-infinity mode.

  • Values which are too large or small to be represented result in a run-time error.

  • NaN can not be converted and results in a run-time error.
 

java.lang.String  

Values are converted to strings. For example,

  • 237 becomes "237"
 

boolean  

  • 0 and NaN values are converted to false.

  • Other values are converted to true.
 

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:




Java parameter type

Conversion rules

boolean  

All values are converted directly to the Java equivalents.  

lava.lang.Boolean
java.lang.Object  

A new instance of java.lang.Boolean is created. Each parameter creates a new instance, not one instance with the same primitive value.  

java.lang.String  

Values are converted to strings. For example:

  • true becomes "true"

  • false becomes "false"
 

byte
char
double
float
int
long
short  

  • true becomes 1

  • false becomes 0
 

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:




Java parameter type

Conversion rules

lava.lang.String
java.lang.Object  

JavaScript 1.4:

  • A JavaScript string is converted to an instance of java.lang.String with a Unicode value.

JavaScript 1.3 and earlier:

  • A JavaScript string is converted to an instance of java.lang.String with an ASCII value.
 

byte
double
float
int
long
short  

All values are converted to numbers as described in ECMA-262.

The JavaScript string value is converted to a number according to the rules described in ECMA-262  

char  

JavaScript 1.4:

  • One-character strings are converted to Unicode characters.

  • All other values are converted to numbers.

JavaScript 1.3 and earlier:

  • All values are converted to numbers.
 

boolean  

  • The empty string becomes false.

  • All other values become true.
 


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:




Java parameter type

Conversion rules

lava.lang.String
java.lang.Object  

The value is converted to an instance of java.lang.String whose value is the string "undefined".  

boolean  

The value becomes false.  

double
float  

The value becomes NaN.  

byte
char
int
long
short  

The value becomes 0.  

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:




Java parameter type

Conversion rules

Any class
Any interface type  

The value becomes null.  

byte
char
double
float
int
long
short  

The value becomes 0.  

boolean  

The value becomes false.  


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:




Java parameter type

Conversion rules

Any interface or class that is assignment-compatible with the unwrapped object.  

The object is unwrapped.  

java.lang.String  

The object is unwrapped, the toString method of the unwrapped Java object is called, and the result is returned as a new instance of java.lang.String.  

byte
char
double
float
int
long
short  

The object is unwrapped, and either of the following situations occur:

  • If the unwrapped Java object has a doubleValue method, the JavaArray or JavaObject is converted to the value returned by this method.

  • If the unwrapped Java object does not have a doubleValue method, an error occurs.
 

boolean  

In JavaScript 1.3 and later versions, the object is unwrapped and either of the following situations occur:

  • If the object is null, it is converted to false.

  • If the object has any other value, it is converted to true.

In JavaScript 1.2 and earlier versions, the object is unwrapped and either of the following situations occur:

  • If the unwrapped object has a booleanValue method, the source object is converted to the return value.

  • If the object does not have a booleanValue method, the conversion fails.
 

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:




Java parameter type

Conversion rules

java.lang.Class  

The object is unwrapped.  

java.lang.JSObject
java.lang.Object  

The JavaClass object is wrapped in a new instance of java.lang.JSObject.  

java.lang.String  

The object is unwrapped, the toString method of the unwrapped Java object is called, and the result is returned as a new instance of java.lang.String.  

boolean  

In JavaScript 1.3 and later versions, the object is unwrapped and either of the following situations occur:

  • If the object is null, it is converted to false.

  • If the object has any other value, it is converted to true.

In JavaScript 1.2 and earlier versions, the object is unwrapped and either of the following situations occur:

  • If the unwrapped object has a booleanValue method, the source object is converted to the return value.

  • If the object does not have a booleanValue method, the conversion fails.
 


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 parameter type

Conversion rules

java.lang.JSObject
java.lang.Object  

The object is wrapped in a new instance of java.lang.JSObject.  

java.lang.String  

The object is unwrapped, the toString method of the unwrapped object is called, and the result is returned as a new instance of java.lang.String.  

byte
char
double
float
int
long
short  

The object is converted to a value using the logic of the ToPrimitive operator described in ECMA-262. The PreferredType hint used with this operator is Number.  

boolean  

In JavaScript 1.3 and later versions, the object is unwrapped and either of the following situations occur:

  • If the object is null, it is converted to false.

  • If the object has any other value, it is converted to true.

In JavaScript 1.2 and earlier versions, the object is unwrapped and either of the following situations occur:

  • If the unwrapped object has a booleanValue method, the source object is converted to the return value.

  • If the object does not have a booleanValue method, the conversion fails.
 


Java to JavaScript Conversions

Values passed from Java to JavaScript are converted as follows:

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 + "";


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Last Updated September 28, 2000