Learning Java - Chapter 12 : Java : Supplements

Before the Sun Java plug-in became available, the JVM could vary considerably from browser to browser and version to version. This made it very difficult to insure consistent look and behaviour.

This can still be a problem if you want your applets to be used the widest possible audience. In particular, many MS Windows users are still saddled with the Java version 1.1 level JVM that has been the default for the Internet Explorer for many years. Asking your users to got to java.com and download the plug-in may not be an attractive idea for people still using slow dial-up lines.

Challenges for Java Applets include:

Different browser types having inconsistent look and behaviour to same Java applet.
They run on different hosts platforms with different "Look & Feel" graphical interfaces.
Many versions of the same browser type exist concurrently.
Different JVM features such as pre-emptive threading in some and cooperative threading in others.

To insure that your applet will run and look the way you expect, regardless of the browser or host, the only solution is to test your applet on as many browsers and hosts as you can. "Write once, test everywhere!"

One option is to find out what JVM is running your applet. The security system in the browser's JVM blocks the applet from finding many of the system properties values. (See Chapter 14 for an introduction to the SecurityManager and Chapter 23 for a discussion of System Properties.) The applet code below gets around this by finding the JVM type via the name of the security manager:

BrowserTestApplet.java

// Derived from TechTip in Java-Pro, Feb.1999

/** A utility to find the browser or plug-in name. **/
public class BrowserTestApplet extends
                     java.applet.Applet
{
  String fBrowserName = "";
public void init () {
    int browser = getPlatform ();
    if (browser == 0)
       fBrowserName = "Application running";
    else if (browser == 1)
       fBrowserName = "Netscape browser";
    else if ( browser == 2)
       fBrowserName = "MS Internet Explorer";
    else if ( browser == 3)
       fBrowserName = "Sun Plugin";
    else if (browser == 4)
       fBrowserName = "Sun Appletviewer";
    else
       fBrowserName = "Unknown Platform ";
    showStatus ("Applet runs on " + fBrowserName);
}
  /** Use the security manager name to id the platform.**/
public int getPlatform (){
    SecurityManager sm = System.getSecurityManager ();
    if (sm == null) return 0;
    String str = sm.toString ().toLowerCase ();
    if (str.startsWith ("netscape"))return 1;
    if (str.startsWith ("com.ms.")) return 2;
    if (str.startsWith ("sun.plugin.")) return 3;
    if (str.startsWith ("sun.applet.")) return 4;
    return -1;
}

public void paint (Graphics g){
    g.drawString (browserName, 20, 20);
}
} // class BrowserTestApplet

Some other browser tips:

Use yield() liberally in threads with intense processing to prevent the applet from dominating the processing on non-preemptive threaded systems.
Note that some broswers call Start()/Stop() even for scrolling and resizing, so you cannot rely on them to indicate page unloading/loading. (One alternative is to use Javascripts to call into a Java routine using the Javascript onload/onunload signals.)

Latest update: Dec. 6, 2004