Small problem with status() function

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mbraendle

Small problem with status() function

in Programming Questions • 2 years ago

Hello all,

I have a slight problem with the status() function in my applet. In the applet,

status() updates an object-specific URL if an object is hovered, otherwise it is reset to an empty string.

The code that resets the browser status is

if (hovered_object == -1 && online) {
status("");
}

So far so good, everything works as expected as long as the applet runs.

However, the funny (or slightly disturbing, depending on one's perspective) thing happens after the applet is quit, e.g. by loading another web page. The Java console outputs the following error (note that the error occurs after the applet had been quit):

basic: Stopping applet…
basic: Removed progress listener: sun.plugin.util.GrayBoxPainter$GrayBoxProgressListener@4dd03ed2
basic: Destroying applet…
basic: Disposing applet…
basic: Joining applet thread…
basic: Joined applet thread…
basic: Finding information…
basic: Releasing classloader: sun.plugin.ClassLoaderInfo@552c8fa8, refcount=0
basic: Caching classloader: sun.plugin.ClassLoaderInfo@552c8fa8
basic: Current classloader cache size: 1
basic: Done…
basic: Quitting applet…
Exception in thread "Animation Thread" java.lang.NullPointerException
at java.applet.Applet.showStatus(Applet.java:224)
at processing.core.PApplet.status(PApplet.java:2352)
at library_space.draw(library_space.java:596)
at processing.core.PApplet.handleDraw(PApplet.java:1606)
at processing.core.PApplet.run(PApplet.java:1503)
at java.lang.Thread.run(Thread.java:680)

Line number 596 corresponds exactly to the code above.

If I take a slightly modified example of the processing reference for status():

void draw() { if(mouseX < 50) { status("left"); } else { status(""); } }

everything works fine, no NullPointerException upon applet quit.

What is going on? My assumption is that there is either a timing problem or a memory problem upon stopping the threads. The complete applet code is pretty long, consisting of about 3600 lines of code and using several libraries.

Replies(4)

quarks

Re: Small problem with status() function

2 years ago

It probably means that the Applet is using resources that have not been released when the applet is destroyed.

An example is the Minim sound library where you must release the sound resources before quitting the applet. I suspect that you need to add a stop method e.g.

void stop(){
// release resources
}

This is a standard Applet method that is automatically called when the applet quits.

mbraendle

Re: Small problem with status() function

2 years ago

Thank you, quarks, for the hint

As far as I have understood from reading articles about Java garbage collection, HashMaps or ArrayLists should be removed or cleared. However, I'm not sure if and how to do with arrays - am I right that Java garbage collection takes automatically care of those when the applet is stopped?

quarks

Re: Re: Small problem with status() function

2 years ago

am I right that Java garbage collection takes automatically care of those when the applet is stopped?

Yes

Reading your original post you meantion threads, if you have created addition threads in your program it might be a good idea to terminate them on shutdown i.e. halt the threads in the stop() method.

mbraendle

Re: Small problem with status() function

2 years ago

if you have created addition threads in your program

Not that I knew about such. But in the Java Console i see that several threads are mentioned, especially network cleanup threads and an applet thread when the applet is destroyed.

I think I have to use some monitoring tools to find out what is going on:

http://java.sun.com/developer/technicalArticles/J2SE/monitoring/

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