java vm：如何记录 class 卸载

Question

I could not find here on SO and via web search working way for usage of parameters for Java VM (JVM) to log class unloading.

Here http://www.herongyang.com/JVM/ClassLoader-JVM-Option-verbose-class.html advised to call java -verbose:class -version , but it is said for loading and it gave log for loading only.

On Java HotSpot VM Options :

-XX:-TraceClassUnloading Trace unloading of classes. -XX:-TraceClassLoading Trace loading of classes.

java -XX:-TraceClassUnloading -version ouputs no info on classes:

[0.004s][warning][arguments] -XX:-TraceClassUnloading is deprecated. Will use -Xlog:class+unload=off instead.
java version "13" 2019-09-17
Java(TM) SE Runtime Environment (build 13+33)
Java HotSpot(TM) 64-Bit Server VM (build 13+33, mixed mode, sharing)

and:

java -XX:-TraceClassLoading -version
[0.002s][warning][arguments] -XX:-TraceClassLoading is deprecated. Will use -Xlog:class+load=off instead.
java version "13" 2019-09-17
Java(TM) SE Runtime Environment (build 13+33)
Java HotSpot(TM) 64-Bit Server VM (build 13+33, mixed mode, sharing)

Same thing.

Side question: deprecated word means discouraged, not no-longer-working, why such behaviour?

however for class loading java -Xlog:class+load -version outputs info eg

[0.037s][info][class,load] java.nio.CharBuffer source: shared objects file
[0.037s][info][class,load] java.nio.HeapCharBuffer source: shared objects file
[0.037s][info][class,load] java.nio.charset.CoderResult source: shared objects file

But java -Xlog:class+unload -version only version info.

How can I get class unloading log for Java VM HotSpot (and/or OpenJDK)?

Answer 1

You say that

java -XX:-TraceClassUnloading -version 

outputs no information about classes. But I don't know why would expect it to provide information.

You would only see logging if the JVM was started AND classes were loaded and unloaded:

• The documentation for the -version option says:

  -version Displays version information and then exits.

  It doesn't say that a JVM is started.

• Assuming that a JVM is started, and classes are loaded, classes are not unloaded in a normal Java application. Class unloading only occurs happens if:

  1. an additional classloader is created, and
  2. that classloader is used to load classes, and
  3. those classes and classloader all become unreachable ¹ , and
  4. the GC runs ² , and
  5. the GC detects that the classes are unreachable ² and unloads ⁴ them.

In short you are (were) most likely not seeing any unload log message because no classes are (were) being unloaded.

---

^{1 - There are two-way links between a class and its classloader, and a one-way link from any instance of a class to the class itself. One consequence is that the bootstrap classloader and application classloader will remain reachable for the lifetime of the JVM.}

^{2 - The GC is normally only run when necessary; ie when the heap utilization hits a certain threshold. It is not run automatically as the JVM exits.}

^{3 - A single run of the GC won't necessarily find all of the unreachable objects.}

^{4 - Some JVMs / GC's do not support class unloading, and this feature can be disabled (or may need to be enabled) via an command line option.}

Answer 2

If anyone is looking for how to log class unloading?

Add JVM argument -Xlog:class+load=info