JavaScript: The Definitive Guide, Fourth Edition

Using Java Classes Directly

As described in the previous two sections, both Netscape and Internet Explorer allow JavaScript code to interact with Java applets and Java applets to interact with JavaScript. Netscape’s LiveConnect technology also allows JavaScript programs to instantiate their own Java objects and use them, even in the absence of any applets. Internet Explorer does not have any analogous capability.

In Netscape, the Packages object provides access to all the Java packages that Netscape knows about. The expression Packages.java.lang refers to the java.lang package, and the expression Packages.java.lang.System refers to the java.lang.System class. For convenience, java is a shortcut for Packages.java. In Netscape, JavaScript code might invoke a static method of this java.lang.System class as follows:

// Invoke the static Java method System.getProperty(  )
var javaVersion = java.lang.System.getProperty("java.version");

This use of LiveConnect is not limited to system classes, because LiveConnect allows us to use the JavaScript new operator to create new instances of Java classes (just as we would in Java). Example 22-2 shows JavaScript code that uses standard Java classes (the JavaScript code looks almost identical to Java code, in fact) to pop up a window and display some text. The result is shown in Figure 22-1.

Figure 22-1. A Java window created from JavaScript

Example 22-2. Scripting ...