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Java Unleashed Second Edition

Chapter 37 -- Integrating Java and JavaScript

Chapter 37

Integrating Java and JavaScript

by Rick Darnell


CONTENTS


You've learned a lot so far about how to make Java a part of your Web pages. Standing alone, Java is a significant development because of its ability to stretch the behavior of your Web pages far beyond what was ever imagined for the World Wide Web.

Java can become even more powerful when harnessed with JavaScript. As you know from other chapters, although Java is powerful enough to add animation, sound, and other features to an applet, it's very cumbersome to directly interact with an HTML page. JavaScript isn't big or powerful enough to match Java's programming power, but it is uniquely suited to work directly with the elements that comprise an HTML document.

By combining the best features of both Java and JavaScript, your applet can interact with your Web page, offering a new level of interactivity.

Setting the Stage

For Java and JavaScript to interact on your Web pages, they both must be active and enabled in the user's browser.

To make sure that both features are active in Netscape Navigator when the user views Java applets, include these simple directions:

  1. Choose Options, Network Preferences from the menu bar. The Preferences dialog box appears.
  2. Select the Languages tab from the Preferences dialog box (see Figure 37.1).
  3. Both Java and JavaScript are enabled by default. If this has changed, make sure that both checkboxes are selected.

Figure 37.1: The Languages tab in the Network Preferences dialog box controls whether or not Java applets and JavaScript commands are processed for HTML documents.

The steps to include to make sure that both languages are active in Microsoft Internet Explorer are similar to the steps for Navigator:

  1. Choose View, Options from the menu bar. The Options dialog box appears.
  2. Select the Security tab from the Options dialog box (see Figure 37.2).
  3. Make sure that the Enable Java Programs checkbox is selected. The scripting languages available in Internet Explorer, JavaScript, and VBScript are automatically enabled; there is no way to disable them.

Figure 37.2: Internet Explorer controls which language features are enabled from the Security tab in the Options dialog box.

Note
Netscape Navigator also includes a Java Console for displaying applet-generated messages (see Figure 37.3). In addition to system messages such as errors and exceptions, the Java Console is where any messages generated by the applet using the java.lang.System package (including System.out.println) are displayed. To display the console, select Options, Show Java Console from the Netscape Navigator menu bar.
Microsoft Internet Explorer can show the results of system messages also, but not in real time as Navigator's Java Console can do. All messages are saved in javalog.txt in C:\Windows\Java. To make sure that this feature is active, select View, Options from the menu bar, select the Advanced tab in the Options dialog box, and make sure that the Java Logging checkbox is selected.

Figure 37.3: The Java Console displays any system messages generated by the applet.

Communicating with Java

The first and most commonly used feature of communication is to modify applet behavior from JavaScript. This is really quite easy to do with the right information, and it allows your applet to respond to events on the HTML page, including interacting with forms.

Java object syntax is very similar to other JavaScript object syntax, so if you're already familiar with this scripting language, adding Java control is an easy step.

Calling Java Methods

With Navigator 3.0, Netscape is providing a brand-new, never-before-seen object called Packages; the Packages object allows JavaScript to invoke native Java methods directly. This object is used in much the same way as the Document or Window objects are in regular JavaScript.

Note
As you'll recall from earlier discussions, groups of related classes are combined in a construct called a package. Classes from a package can be used by outside classes by using the import command.
Just to confuse things, that's not the case with the JavaScript version of Packages. In JavaScript, Packages is the parent object used to invoke native Java methods, such as Packages.System.out.println("Say Howdy").

Caution
Invoking native Java methods from JavaScript is possible only within Netscape Navigator 3.0 or later. Microsoft Internet Explorer doesn't include support for the JavaScript-to-Java packages in its 3.0 release, but may include its own version of this capability in later versions.
The source of the problem is that JavaScript is implemented individually for each browser; what Netscape includes for JavaScript isn't the same as what Microsoft includes. In the fast-paced world of browsers, however, you can expect Microsoft to catch up quickly.

Internet Explorer still includes support for all the now-standard features of JavaScript, including control and manipulation of windows, documents, and forms.

Here is the syntax to call a Java package directly:

[Packages.]packageName.className.methodName

The object name is optional for the three default packages-java, sun, and netscape. These three can be referenced by their package name alone, as shown here:

java.className.methodName
sun.className.methodName
netscape.className.methodName

Together with the package name, the object and class names can result in some unwieldy and error-prone typing. This is why you can also create new variables using the Package product. The following code assigns a Java package to the variable System and then uses the System variable to call a method in the package:

var System = Package.java.lang.System;
System.out.println("Hello from Java in JavaScript.");

Controlling Java Applets

Controlling an applet with a script is a fairly easy matter, but it does require some knowledge of the applet you're working with. Any public variable, method, or property within the applet is accessible through JavaScript.

Tip
If you're changing the values of variables within an applet, the safest way to do so is to create a new method within the applet for the purpose. This method can accept the value from JavaScript, perform any error checking, and then pass the new value along to the rest of the applet. This arrangement helps prevent unexpected behavior or applet crashes.

You have to know which methods, properties, and variables in the applet are public. Only the public items in an applet are accessible to JavaScript.

Tip
Two public methods are common to all applets and you can always use them-start() and stop(). These methods provide a handy way to control when the applet is active and running.

There are five basic activities common to all applets, as opposed to one basic activity for applications. An applet has more activities to correspond to the major events in its life cycle on the user's browser. None of the activities have any definitions. You must override the methods with a subclass within your applet. Here are the five activities common to all applets:

With this information in hand, getting started begins with the applet tag. It helps to give a name to your applet to make JavaScript references to it easier to read. The following snippit of code shows the basic constructor for an HTML applet tag that sets the stage for JavaScript control of a Java applet. The tag is identical to the tags you used in previous chapters to add applets, except that a new attribute is included for a name:

<APPLET CODE="UnderConstruction" NAME="AppletConstruction" WIDTH=60 HEIGHT=60>
</APPLET>

Assigning a name to your applet isn't absolutely necessary because JavaScript creates an array of applets when the page is loaded. However, doing so makes for a much more readable page.

Caution
Like the JavaScript Packages object, the JavaScript applets array is currently available only in Netscape Navigator 3.0 or later. This doesn't leave Microsoft Internet Explorer completely out in the cold-JavaScript can still reference an applet in Explorer using the applet's name.

To use a method of the applet from JavaScript, use the following syntax:

document.appletName.methodOrProperty

Tip
Netscape Navigator 3.0 uses an applets array to reference all the applets on a page. The applets array is used according to the following syntax:
document.applets[index].methodOrProperty
document.applets[appletName].methodOrProperty
These two methods also identify the applet you want to control, but the method using the applet's name without the applets array is the easiest to read and requires the least amount of typing.
Like other arrays, one of the properties of applets is length, which returns how many applets are in the document.
This array of applets is not currently available in the Microsoft Internet Explorer 3.0 implementation of JavaScript.

One of the easy methods of controlling applet behavior is starting and stopping its execution. You start and stop an applet using the start() and stop() methods that are common to every applet. Use a form and two buttons to add the functions to your Web page (see Figure 37.4). The following code snippet is a basic example of the HTML code needed to add the buttons, with the name of the applet substituted for appletName.

Figure 37.4: One of the simplest methods of controlling an applet is to use buttons that start and stop it.

<FORM>
<INPUT TYPE="button" VALUE="Start" onClick="document.appletName.start()">
<INPUT TYPE="button" VALUE="Stop" onClick="document.appletName.stop()">
</FORM>

You can also call other methods, depending on their visibility to the world outside the applet. JavaScript can call any method or variable with a public declaration.

Tip
Any variable or method within the applet that doesn't include a specific declaration of scope is protected by default. If you don't see the public declaration, it's not.

The syntax to call applet methods from JavaScript is simple and can be integrated with browser events, such as the button code snippet just shown. The basic syntax for calling an applet method from Java is shown here:

document.appletName.methodName(arg1,...,argx)

To call the stop() method from the underConstructionApplet applet within an HTML page, the syntax is as follows (assuming that the applet is the first one listed on the page):

document.underConstructionApplet.stop();

Here's how you do it with Navigator (again, assuming that the applet is the first one listed on the page):

document.applets[0].stop();

Integrating the start() and stop() methods for this applet with the applet tag and button code snippet used earlier results in the following code:

<APPLET CODE="UnderConstruction" NAME="underConstructionApplet" WIDTH=60 ÂHEIGHT=60></APPLET>
<FORM>
<INPUT TYPE="button" VALUE="Start" ÂonClick="document.underConstructionApplet.start()">
<INPUT TYPE="button" VALUE="Stop" ÂonClick="document.underConstructionApplet.stop()">
</FORM>

Communicating with JavaScript

With the addition of a new set of classes provided with Netscape Navigator 3.0, Java can take a direct look at your HTML page through JavaScript objects. To implement this functionality, you must import the netscape.javascript.JSObject class when the applet is created.

Tip
In most applets, the java package is all you need. The netscape package includes methods and properties for Java to reach out to JavaScript and HTML (as covered later in this chapter). The last package, sun, includes platform-specific and system utility classes.

Tip
The netscape.javascript.JSObject class is included with the other class files in the Netscape folder in the java_30 file. In Windows, the complete location is \Program Files\Netscape\Navigator\Program\java\classes. To enable the Java compiler to find the packages, you'll have to create a new set of folders under \Java\Lib called \netscape\javascript and then copy java_30 to the new location. The netscape.javascript.JSObject class extends the standard Java Object class, so the newly created JSObject objects are treated as other Java objects.

To include the JSObject class as part of your applet, use the import command as you normally do to include any other class package:

import netscape.javascript.JSObject;

Another important addition is necessary in the applet tag-MAYSCRIPT. This security feature gives specific permission for the applet to access JavaScript objects. Here's how to include the tag:

<APPLET CODE="colorPreview.class" WIDTH=50 HEIGHT=50 NAME="Preview" MAYSCRIPT>

Without the MAYSCRIPT parameter, any attempt to access JavaScript from the applet results in an exception. If you want to exclude an applet from accessing the page, simply leave out the MAYSCRIPT parameter.

Java and JavaScript Values

The JSObject class gives Java the ability to look at and change objects defined through JavaScript. This requires certain assumptions, especially when passing or receiving values from Java. To ensure compatibility, every JavaScript value is assigned some form from java.lang.Object:

Looking at the JavaScript Window

To get a handle on JavaScript objects-including form items and frames-you must first create an object to hold the current Navigator window. The getWindow() method provides the means.

First, you have to create a new variable of type JSObject:

JSObject jsWin;

Then, using the JSObject class, assign the window to the variable:

jsWin = JSObject.getWindow(this);

Tip
This type of work is typically accomplished within the applet's init() method.

After you have a handle on the window, you can start to break it apart into its various components with getMember(). This method returns a specific object from the next level of precedence. For example, to get a handle on a form on a Web page with a form called response, you can use the following set of statements:

jsWin = JSObject.getWindow(this);
JSObject jsDoc = (JSObject) jsWin.getMember("document");
JSObject responseForm = (JSObject) jsDoc.getMember("response");

In JavaScript, this form is referred to as window.document.response. Note that each JavaScript object is assigned to its own variable in Java and is not a property of a parent object. The form in Java is contained in responseForm, not in jsWin.jsDoc.responseForm.

Note
All parts of an HTML document exist in JavaScript in set relationships to each other. This arrangement is called instance hierarchy because it works with specific items on the page, rather than general classes of items.
At the top of the pyramid is the window object. It is the parent of all other objects. Its children include document, location, and history, which share a precedence level. The document object's children include objects specific to the page, such as forms, links, anchors, and applets.
The Java netscape package recognizes and uses this hierarchy through its getWindow() and getMethod() methods. The first gets the window object (the highest object); the latter returns individual members of the next level.

So far, you've retrieved only broad objects, such as windows and forms. Getting a specific value from JavaScript follows the same principles, although now you need a Java variable of the proper type to hold the results instead of an instance of JSObject.

Tip
Don't forget about passing numbers between JavaScript and Java. All JavaScript numbers are converted to a float. You can cast it to another Java type if needed once the value is in the applet.

Using the form described earlier in this section, suppose that there is a text field (name), a number (idNum), and a checkbox (member). You can retrieve each of these values from JavaScript using the following commands:

jsWin = JSObject.getWindow(this);
JSObject jsDoc = (JSObject) jsWin.getMember("document");
JSObject responseForm = (JSObject) jsDoc.getMember("response");
JSObject nameField = (JSObject) responseForm.getMember("name");
JSOBject idNumField = (JSObject) responseForm.getMember("idNum");
JSOBject memberField = (JSObject) responseForm.getMember("memberField");
String nameValue = (String) nameField.getMember("value");
Float idNumValue = (Float) idNumField.getMember("value");
Boolean memberValue = (Boolean) memberField.getMember("checked");

This chunk of code becomes a bit unwieldy, especially when you need several values from JavaScript. If you need to access more than several elements on a page, it can help to create a new method to handle the process.

The getElement() method defined in the next code snippet accepts the name of a form and element on an HTML page as arguments and returns a JSObject that contains it:

protected JSObject getElement(String formName, String elementName) {
    JSObject jsDoc = (JSObject) JSObject.getWindow().getMember("document");
    JSObject jsForm = (JSObject) jsDoc.getMember(formName);
    JSObject jsElement = (JSObject) jsElement.getMember(elementName);
    return jsElement;
}

This simple method creates the intervening JSObject objects needed to get to the form element, making the retrieval as easy as knowing the form and element name. If the form or form element is not included on the page, the method throws an exception and halts the applet.

To change a JavaScript value, use the JSObject setMember() method in Java. The syntax is setMember(name, value), with the name of the JavaScript object and its new value. The following snippet uses the getElement() method just defined to get the name element from the response form, and then uses the JSObject method setMember() to set its value to Your Name Here. This is equivalent to the this.name = newValue statement in JavaScript.

JSObject nameField = getElement("response","name");
nameField.setMember("name","Your Name Here");

The getWindow() and getMember() methods just described are the basic methods used when interfacing with JavaScript. Together, they make receiving values from an HTML page with JavaScript a straightforward task-even if the process is a little cumbersome in the number of statements required.

Getting Values Using Indexes

If your applet is designed to work with a variety of HTML pages that may contain different names for forms and elements, you can use the JavaScript arrays with the JSObject slot methods. If the desired form is always the first to appear on the document and the element is the third, then the form name is forms[0] and the element is elements[2].

After retrieving the document object using getWindow() and getMember(), use getSlot(index) to return a value within it. For example, in an HTML document containing three forms, the second is retrieved into Java using the following commands:

JSOBject jsWin = JSObject.getWindow(this);
JSObject jsDoc = (JSObject) jsWin.getMember("document");
JSObject jsForms = (JSObject) jsDoc.getMember("forms");
JSObject jsForm1 = (JSObject) jsForms.getSlot(1);

Using setSlot(), the same process is used to load a value into an array. The syntax is shown here:

JSObject.setSlot(index,value);

In this syntax, the index is an integer and the value is a String, boolean, or float.

Tip
The one rule that must stand firm is the placement of the form and the elements within it. When the applet is used with more than one document, the forms and elements must be in the same relative place every time to avoid exceptions and unpredictable results.

Using JavaScript Methods in Java

The netscape class package provides two methods to call JavaScript methods from within an applet: call() and eval(). The syntax differs slightly for the two methods, but the outcome is the same.

Tip
You need a handle for the JavaScript window before you can use the call() and eval() methods.

There are two ways to invoke these methods. The first uses a specific window instance; the second uses getWindow() to create a JavaScript window just for the expression:

jsWin.callOrEval(arguments)
JSOBject.getWindow().callOrEval(arguments)

The call() method separates the method from its arguments. This is useful for passing Java values to the JavaScript method. The syntax is call("method", args), where method is the name of the method you want to call and the arguments you want to pass are contained in an array.

The eval() method, on the other hand, uses a string that appears identical to the way a method is called within JavaScript. The syntax is eval("expression"), where expression is a complete method name and its arguments, such as document.writeln("Your name here.'"). Here's what it looks like when you include the string with the eval() expression:

eval("document.writeln(\"Your name here.\");")

Tip
To pass quotation marks as quotation marks to JavaScript within a Java string, use the backslash character before each occurrence.

Summary

Now you have a whole set of tools to get from JavaScript to Java and back again. The marriage of these two Web technologies can open up a whole new world of how to interact with your users. By using simple statements and definitions-already a part of both languages-you can make a previously static Web page communicate with an applet embedded in it; in return, the Web page can react to the output of the applet.

JavaScript-to-Java communication is a simple extension of JavaScript's functionality. As long as your applets have names, any applet's public method is accessible to your HTML page. And using the new Netscape package netscape.javascript makes the process a two-way street, allowing your Java applet to invoke JavaScript functions and look at the structure of your Web page as represented by the JavaScript document object.

This combination is one more set of capabilities in your toolbox that you can use to meet your users' needs.