Note: This Java deployment guide describes features released prior to the Java SE 6 update 10 release. See Java Rich Internet Applications Development and Deployment for the latest information.
java.lang.NullPointerException Thrown From a
Thread When Applet.start() or
Applet.stop() is CalledWhen running an applet in a browser using the Sun Java™
Runtime Environment (JRE™), a
java.lang.NullPointerException is thrown from a thread
when Applet.start() or Applet.stop() is
called. The same applet runs under the Microsoft Virtual Machine
(VM).
In the Microsoft VM, an applet may be visible immediately after
Applet.init() is called but before
Applet.start(). Similarly, an applet may still be
visible after Applet.stop() but before
Applet.destroy().
The Sun JRE implementation is slightly different in terms of applet visibility related to the lifecycle methods, so any applet that relies on the Microsoft VM event sequences may break.
Code defensively to ensure the applet is visible before drawing.
One way to do this is to test whether the Graphics
object is null. For example, suppose the applet has the following
methods:
private Thread
paintThread = null;
public void
start()
{
if (paintThread == null)
{
paintThread = new Thread(this);
paintThread.start();
}
}
public void
stop()
{
if (paintThread != null)
{
paintThread.interrupt(); // Assuming
the thread has sufficient
//
time to be interrupted and stop
//
drawing before destroy() is called.
paintThread = null;
}
}
public void
run()
{
while (Thread.currentThread().isInterrupted() ==
false)
{
Graphics g =
getGraphics();
g.drawString("Hello World", 100,
100); // java.lang.NullPointerException if
//
applet is not visible -
//g will be null !
}
}
The code for the
run method should be changed to the following:
public void
run()
{
while (Thread.currentThread().isInterrupted() ==
false)
{
Graphics g =
getGraphics();
if (g !=
null) //
Code defensively!
g.drawString("Hello World", 100,
100);
}
}
None.
