How to track when class is loaded and destroyed in jvm?

Question

How do I keep track when class is loaded and destroyed in jvm? Is there any callback method that is exposed by the jvm?

You're asking two questions in one here. What exactly are you trying to learn, or trying to accomplish? Once you've figured that out, accept the answer that's helped you the most with that. — Frans, Dec 11 '14 at 07:05

score 25 · Answer 1 · answered Mar 29 '12 at 14:43

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If you're using a Sun/Oracle JVM, you could use the TraceClassLoading and TraceClassUnloading options. Use the following to see what options your JVM supports:

java -XX:+AggressiveOpts -XX:+UnlockDiagnosticVMOptions -XX:+UnlockExperimentalVMOptions -XX:+PrintFlagsFinal -version

If these options are supported, run your Java application using -XX:+TraceClassLoading -XX:+TraceClassUnloading. You should see messages like:

[Loaded ... from ...]
[Unloading class ...]

answered Mar 29 '12 at 14:43

Go Dan

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`TraceClassLoading` was "expired" in JDK 17 unfortunately: https://chriswhocodes.com/hotspot_options_openjdk8.html?s=TraceClassLoading and the alternative seems to be `-verbose:class` or `-Xlog:class+load -Xlog:class+unload` – Guillaume Aug 22 '22 at 16:29

score 10 · Answer 2 · answered Mar 29 '12 at 07:57

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You can add the command line option -verbose:class to your Java process, this will display information about each class loaded.

answered Mar 29 '12 at 07:57

rsp

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Pau Kiat Wee · Answer 3 · 2012-03-29T08:01:09.667

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You can add Java Opts to see which class is loaded via:

java -verbose:class

About destroyed class, I am not sure.

edited Mar 29 '12 at 08:01

answered Mar 29 '12 at 07:55

Pau Kiat Wee

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score 4 · Answer 4 · answered May 22 '19 at 18:27

From Java 9 onward, you can use -Xlog. e.g.:

java -Xlog:class+load -Xlog:class+unload ...

This will print entries like:

[0.296s][info][class,load] java.lang.Shutdown source: jrt:/java.base
[0.296s][info][class,load] java.lang.Shutdown$Lock source: jrt:/java.base

You can also use -Xlog:help to get more information on the option.

score 2 · Answer 5 · answered Mar 29 '12 at 08:14

If are ok with using JRockit JVM, you can make use of the APIs below, which will give you callback methods when a class is loaded and class is unloaded.
Have a look at JVM class from which we are supposed to use the getClassLibrary() method.
On the classLibrary object we can register listeners for classloading events which gives the class names etc.

Luca · Answer 6 · 2012-03-29T08:10:19.187

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You can use static block to detect class loading but you can not detect class unloading. In java all classes loaded through the system classloader will never be unloaded and all classes loaded through other classloaders will be unloaded only when the classloader is unloaded.

static{ 

        //execute when the class will be loaded

    }

edited Mar 29 '12 at 08:10

answered Mar 29 '12 at 07:54

Luca

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score 0 · Answer 7 · edited May 23 '17 at 12:10

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Do you want this information in your application or do you just want to analyse that from the outside? In the latter case you maybe can use VisualVM for that. Maybe your question is related to this one: Loaded classes in VisualVM.

edited May 23 '17 at 12:10

Community

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answered Mar 29 '12 at 07:56

Kai

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score 0 · Answer 8 · edited May 23 '17 at 12:18

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You can track class's creation in static constructor. And you cannot track it's destruction as far as I'm concerned. Classes are only unloaded when gc collects the classloader which was used to load the classes.
You might also be interested in reading this: another question on classloaders at StackOverflow

edited May 23 '17 at 12:18

Community

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answered Mar 29 '12 at 08:02

Sergey Savenko

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How to track when class is loaded and destroyed in jvm?

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