iveConnect enables communication between JavaScript and Java applets in a page and between JavaScript and plug-ins loaded on a page.
iveConnect enables communication between JavaScript and Java applets in a page and between JavaScript and plug-ins loaded on a page.
For the Netscape packages reference, see Chapter 12, "The Netscape packages."This chapter explains how to use LiveConnect in Netscape Navigator. It assumes you are familiar with Java programming. For the HTML syntax required to add applets and plug-ins to your web page, see "Applet" and "Plugin".
public void init() {
System.out.println("Hello, world!")
}
You can use the Java Console to present messages to users, or to trace the values of variables at different places in a program's execution. To display the Java Console, the user chooses Show Java Console from the Options menu. Note that most users probably do not display the Java Console.
java_30 file that includes the following Java packages:
classes.zip. These packages have been tested by Sun, and similar security enhancements will be implemented in future releases of the Sun JDK. Use these packages until the Sun JDK provides these security enhancements.
The file java_30 contains the following netscape packages:
In addition, java_30 contains some other netscape packages:
java_30, place the file in the classpath of the JDK compiler in either of the following ways:
CLASSPATH environment variable to specify the paths and names of java_30 and classes.zip.
java_30 when you compile by using the -classpath command line parameter.
java_30 file is delivered 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:\JDK\java\lib\classes.zip;D:\Navigator\Program\java\classes\java_30See the Sun JDK documentation for more information about CLASSPATH.
System.out.println to display a message on the Java Console.
In JavaScript, Java packages and classes are properties of the Packages object. Use Java syntax to reference Java objects in JavaScript, with the name of the Packages object optionally prepended:
[Packages.]packageName.className.methodNameThe name
Packages is optional for java, sun, and netscape packages; in code, java, sun, and netscape are aliases for Packages.java, Packages.sun, and Packages.netscape. For example, you can refer to the Java class java.lang.System as either Packages.java.lang.System or as java.lang.System in your code. The name Packages is required for other packages.
Access fields and methods in a class with the same syntax that you use in Java. For example, the following JavaScript code prints a message to the Java Console:
var System = java.lang.SystemThe first line in this example makes the JavaScript variable System refer to the class java.lang.System. The second line invokes the println method of the static variable err in the Java System class. Because println expects a java.lang.String argument, the JavaScript string is automatically converted to a java.lang.String. You can even use Java class constructors in JavaScript. For example, the following JavaScript code creates a Java Date object and prints it to the Java Console.
System.err.println("Greetings from JavaScript")
var mydate = new java.util.Date()
System.out.println(myDate)
document.appletName, where appletName is the value of the NAME attribute of the <APPLET> tag. The applets array also contains all the applets in a page; you can reference elements of the array through the applet name (as in an associative array) or by the ordinal number of the applet on the page (starting from zero).
For example, consider the basic "Hello World" applet in Java:
import java.applet.Applet;The following HTML runs and displays the applet, and names it "HelloWorld" (with the NAME attribute):
import java.awt.Graphics;
public class HelloWorld extends Applet {
public void paint(Graphics g) {
g.drawString("Hello world!", 50, 25);
}
}
<APPLET CODE="HelloWorld.class" NAME="HelloWorld" WIDTH=150 HEIGHT=25>You can reference this applet in JavaScript in either of the following ways:
</APPLET>
document.HelloWorldIf this is the first applet in the document (topmost on the page), you could also reference it as:
document.applets["HelloWorld"]
document.applets[0]The applets array has a length property,
document.applets.length, that indicates the number of applets in the document.
All public variables declared in an applet, and its ancestor classes and packages are available in JavaScript. Static methods and properties declared in an applet are available to JavaScript as methods and properties of the Applet object. You can get and set property values, and you can call methods that return string, numeric, and boolean values.
java.awt.Component).
import java.applet.Applet;Making the message string a variable allows you to modify it from JavaScript. Now modify the HTML file as follows:
import java.awt.Graphics;
public class HelloWorld extends Applet {
String myString;
public void init() {
myString = new String("Hello, world!");
}
public void paint(Graphics g) {
g.drawString(myString, 25, 20);
}
public void setString(String aString) {
myString = aString;
repaint();
}
}
<APPLET CODE="HelloWorld1.class" NAME="Hello" WIDTH=150 HEIGHT=25>When you compile the HelloWorld applet, and load the HTML page into Navigator, you initially see "Hello, World!" displayed in the gray applet panel. However, you can now change it by entering text in the text field and clicking on the button. This demonstrates controlling an applet from JavaScript.
</APPLET>
<FORM NAME="form1">
<INPUT TYPE="button" VALUE="Set String"
onClick="document.HelloWorld.setString(document.form1.str.value)">
<BR>
<INPUT TYPE="text" SIZE="20" NAME="str">
</FORM>
<APPLET CODE="colors.class" WIDTH=500 HEIGHT=60 NAME="colorApp">This applet uses the public method setString to specify the text for the flashing string that appears. In the HTML form, the onClick event handler of the button lets a user change the "Hello, world!" string that the applet initially displays by calling the setString method as follows:
</APPLET>
<FORM NAME=colorText>
<P>Enter new text for the flashing display:
<INPUT TYPE="text"
NAME="textBox"
LENGTH=50>
<P>Click the button to change the display:
<INPUT TYPE="button"
VALUE="Change Text"
onClick="document.colorApp.setString(document.colorText.textBox.value)">
</FORM>
<INPUT TYPE="button"In this code, colorText is the name of the HTML form and textBox is the name of the text field. The event handler passes the value that a user enters in the text field to the setString method in the Java applet.
VALUE="Change Text"
onClick="document.colorApp.setString(document.colorText.textBox.value)">
<EMBED SRC=myavi.avi NAME="myEmbed" WIDTH=320 HEIGHT=200>If this HTML defines the first plug-in in a document, you can access it in any of the following ways:
document.embeds[0]If the plug-in is associated with the Java class netscape.plugin.Plugin, you can access its static variables and methods the way you access an applet's variables and methods. The embeds array has a length property,
document.embeds["myEmbed"]
document.myEmbed
document.embeds.length, that indicates the number of plug-ins embedded in the document. See "Determining installed plug-ins" for more information about plug-ins.The Plug-in Developer's Guide contains information on:
(String) window.getMember("name");
(JSObject) window.getMember("document");.
import netscape.javascript.*The package netscape.javascript defines the JSObject class and the JSException exception object. The author of an HTML page must permit an applet to access JavaScript by specifying the MAYSCRIPT attribute of the <APPLET> tag. This prevents an applet from accessing JavaScript on a page without the knowledge of the page author. Attempting to access JavaScript from an applet that does not have the MAYSCRIPT attribute generates an exception. The MAYSCRIPT tag is needed only for Java to access JavaScript; it is not needed for JavaScript to access Java.
public class myApplet extends Applet {
public void init() {
JSObject win = JSObject.getWindow(this);
}
}
public void init() {
win = JSObject.getWindow(this);
myForm=win.eval("document.testForm")
}
Note that you could use the following lines in place of myForm=win.eval("document.testForm"):
JSObject doc = (JSObject) win.getMember("document");
JSObject myForm = (JSObject) doc.getMember("testForm");
Notice that JavaScript objects appear as instances of the class netscape.javascript.JSObject in Java. Values passed between Java and JavaScript are converted as described in "netscape.javascript.JSObject".
If the JavaScript object document.testForm.jazz is a checkbox, the following Java code allows you to access its checked property:
public void init() {
win = JSObject.getWindow(this);
JSObject doc = (JSObject) win.getMember("document");
JSObject myForm = (JSObject) doc.getMember("testForm");
JSObject check = (JSObject) myForm.getMember("jazz");
Boolean isChecked = (Boolean) check.getMember("checked");
} Calling JavaScript methods
The eval method in the class netscape.javascript.JSObject let you evaluate an arbitrary JavaScript expression. Use getWindow to get a handle for the JavaScript window, then use eval to access a JavaScript method.
Use the following syntax to call JavaScript methods:
JSObject.getWindow().eval("expression")
expression is a JavaScript expression that evaluates to a JavaScript method call.
For example, the following Java code uses eval to call the JavaScript alert method when a mouseUp event occurs:
public void init() {
JSObject win = JSObject.getWindow(this);
}public boolean mouseUp(Event e, int x, int y) {
Another way to call JavaScript methods is with the call method of JSObject. Use the following to call a JavaScript method from Java when you want to pass Java objects as arguments:
win.eval("alert(\"Hello world!\");");
return true;
}JSObject.call(methodName, argArray)
where argArray is an Array of Java objects used to pass arguments to the JavaScript method.
If you want to pass primitive values to a JavaScript method, you must use the Java object wrappers (such as Integer, Float, and Boolean), and then populate an Array with such objects.
Example: Hello World
Returning to the HelloWorld example, modify the paint method in the Java code so that it calls the JavaScript alert method (with the message "Painting!") as follows:
public void paint(Graphics g) {
Then add the MAYSCRIPT attribute to the <APPLET> tag in the HTML page, recompile the applet, and try it. Each time the applet is painted (when it is initalized, when you enter a new text value, and when the page is reloaded) a JavaScript alert box is displayed. This is a simple illustration of calling JavaScript from Java.
This same effect could be achieved with the following:
g.drawString(myString, 25, 20);
JSObject win = JSObject.getWindow(this);
String args[] = {"Painting!"};
win.call("alert", args);
}public void paint(Graphics g) {
g.drawString(myString, 25, 20);
JSObject win = JSObject.getWindow(this);
win.eval("alert(`Painting')");
}Note
You may have to reload the HTML page by choosing Open File from the File
menu instead of clicking the Reload button, to ensure that the applet is re-initialized.
Calling user-defined functions
You can also call user-defined functions from a Java applet. For example, add the following function to the <HEAD> of the HTML page with the HelloWorld applet:
<SCRIPT>
This simple function displays an alert dialog box containing the name and version of the client software being used. Then modify the init method in your Java code similarly to the way you modified paint:
function test() {
alert("You are using " + navigator.appName + " " +
navigator.appVersion)
}
</SCRIPT>public void init() {
Notice that args2 is declared as an array with no elements, even though the method does not take any arguments. When you recompile the applet and reload the HTML page (and re-initialize the applet), a JavaScript alert dialog box will display the version of Navigator you are running. This is a simple illustration of calling a user-defined function from Java.
myString = new String("Hello, world!")
JSObject win = JSObject.getWindow(this)
String args2[] = {""}
win.call("test", args2)
} Data type conversion
Values passed from Java to JavaScript are converted as follows:
arrayName[index] (where index is an integer), and determine its length with arrayName.length.