On 2/09/2021 10:05 pm, S A wrote:
Hi all,
bisection lead to:
https://github.com/openjdk/jdk16/commit/ba721f5f2fbbf08e22a9993dce0087f46b1f5552
<https://github.com/openjdk/jdk16/commit/ba721f5f2fbbf08e22a9993dce0087f46b1f5552>
I've opened: https://bugzilla.redhat.com/show_bug.cgi?id=2000557
<https://bugzilla.redhat.com/show_bug.cgi?id=2000557>
private void unloadClasses() throws Exception {
urlClassLoader.close();
urlClassLoader = null;
System.gc();
System.runFinalization();
}
The above is not sufficient to guarantee class unloading has occurred.
There are no specified ways to guarantee class unloading. It will depend
on the GC in use as to what will be sufficient to trigger class unloading.
Cheers,
David
Best regards,
Simeon
On Wed, 1 Sept 2021 at 16:12, S A <[email protected]
<mailto:[email protected]>> wrote:
Hi all,
my team is currently evaluating OpenJDK 17 (the latest early access
build) for a part of our application. I've been able to get all our
relevant tests to work, except for one that checks class unload
events sent over JDI.
Originally we added the test due to:
https://bugs.openjdk.java.net/browse/JDK-8227269
<https://bugs.openjdk.java.net/browse/JDK-8227269> (we wanted to be
sure no class unload events are lost despite the fix)
The test is Eclipse based (Eclipse has its own JDI implementation)
and expects to receive a certain set of class unload events under
controlled debugging conditions. The test has been running on
OpenJDK 8 and OpenJDK 11 for over an year now, without failures. On
OpenJDK 17 however, the test seems to fail nearly always (but not at
100% rate).
I've created a plain Java snippet and was able to reproduce the
problem with jdb, see instructions below.
In essence, I see all class unloading is done (I'm checking with "
-Xlog:class+load,class+unload") at the expected time. No amount of
waiting in the test or in the snippet (e.g. a loop with 50 ms sleeps
for several seconds after class unloading is triggered) helps with
receiving all expected class unload JDI events though. Sometimes all
of them are sent, sometimes none are sent and sometimes only a
partial set of events is sent.
Could someone knowledgeable take a look?
I'll try to find the changes that cause the problem (so far I've not
checked whether earlier OpenJDK releases are affected). I'll also
open a RHEL support case for this, but those take a while to get in
contact with OpenJDK developers.
Best regards and thanks,
Simeon
*Reproduction:*
*
*
First, I've made a change to OpenJDK code to check whether all class
unload events are being sent (in case the Eclipse JDI implementation
broke for some reason with OpenJDK 17). Its a printf in eventHandler.c :
diff --git a/src/jdk.jdwp.agent/share/native/libjdwp/eventHandler.c
b/src/jdk.jdwp.agent/share/native/libjdwp/eventHandler.c
index 2a5912f0777..19a91126e18 100644
--- a/src/jdk.jdwp.agent/share/native/libjdwp/eventHandler.c
+++ b/src/jdk.jdwp.agent/share/native/libjdwp/eventHandler.c
@@ -500,6 +500,7 @@ synthesizeUnloadEvent(void *signatureVoid, void
*envVoid)
char *durableSignature =
jvmtiAllocate((int)strlen(signature)+1);
(void)strcpy(durableSignature, signature);
+ printf("Sending class unload event for: %s\n",
durableSignature);
eventHelper_recordClassUnload(node->handlerID,
durableSignature,
eventBag);
The snippet I use to reproduce the problem loads a few classes, then
unloads them. It is:
import java.io.File;
import java.net.URL;
import java.net.URLClassLoader;
public class TestClassUnloadEvents {
public static void main(String[] args) throws Exception {
new TestClassUnloadEvents().execute();
}
private URLClassLoader urlClassLoader;
public void execute() throws Exception {
beforeClassLoading();
loadClasses();
afterClassLoading();
unloadClasses();
afterClassUnloading();
}
private void loadClasses() throws Exception {
int numberOfClassesToLoad = Integer.valueOf("10");
String libraryPath = "classes.jar";
URL[] urls = {
new File(libraryPath).toURI().toURL()
};
urlClassLoader = new URLClassLoader(urls, null);
for (int i = 0; i < numberOfClassesToLoad; ++i) {
String className = "sample.C" + i;
Class<?> loadClass = urlClassLoader.loadClass(className);
loadClass.newInstance();
}
}
private void unloadClasses() throws Exception {
urlClassLoader.close();
urlClassLoader = null;
System.gc();
System.runFinalization();
}
public static void beforeClassLoading() {
System.out.println("BEFORE CLASS LOADING");
}
public static void afterClassLoading() {
System.out.println("AFTER CLASS LOADING");
}
public static void afterClassUnloading() {
System.out.println("AFTER CLASS UNLOADING");
}
}
Create a jar file from the attached "classes.zip" (I would attach a
.jar directly but gmail is blocking jar attachments, even archived
ones; sorry about this). Place the "classes.jar" jar file in the
same folder as the snippet and from that folder run the snippet. E.g.:
/data/git/jdk17/build/linux-x86_64-server-release/jdk/bin/java
-Xlog:class+load,class+unload
-agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=8080
TestClassUnloadEvents.java
Then attach jdb with:
/data/git/jdk17/build/linux-x86_64-server-release/jdk/bin/jdb
-attach 8080
Input "resume", to let the application finish.
Expected output that I sometimes see (with Java 11 I always see all
expected prints from the change, where event order is not relevant):
...
AFTER CLASS LOADING
[6.312s][info][class,load] java.io.RandomAccessFile$1 source:
/data/git/jdk17/build/linux-x86_64-server-release/jdk/modules/java.base
[6.315s][info][class,unload] unloading class sample.C9
0x00000008001c5c30
[6.315s][info][class,unload] unloading class sample.C8
0x00000008001c5a28
[6.315s][info][class,unload] unloading class sample.C7
0x00000008001c5820
[6.315s][info][class,unload] unloading class sample.C6
0x00000008001c5618
[6.315s][info][class,unload] unloading class sample.C5
0x00000008001c5410
[6.315s][info][class,unload] unloading class sample.C4
0x00000008001c5208
[6.315s][info][class,unload] unloading class sample.C3
0x00000008001c5000
[6.315s][info][class,unload] unloading class sample.C2
0x00000008001c4c10
[6.315s][info][class,unload] unloading class sample.C1
0x00000008001c4a08
[6.315s][info][class,unload] unloading class sample.C0
0x00000008001c4800
[6.325s][info][class,load ] java.lang.ref.Finalizer$2 source:
/data/git/jdk17/build/linux-x86_64-server-release/jdk/modules/java.base
Sending class unload event for: Lsample/C9;
Sending class unload event for: Lsample/C7;
Sending class unload event for: Lsample/C5;
Sending class unload event for: Lsample/C3;
Sending class unload event for: Lsample/C1;
Sending class unload event for: Lsample/C0;
Sending class unload event for: Lsample/C8;
Sending class unload event for: Lsample/C6;
Sending class unload event for: Lsample/C4;
Sending class unload event for: Lsample/C2;
[6.325s][info][class,load ] java.lang.ref.Finalizer$1 source:
/data/git/jdk17/build/linux-x86_64-server-release/jdk/modules/java.base
AFTER CLASS UNLOADING
...
Example output that I (often) see, which is unexpected:
...
AFTER CLASS LOADING
[4.863s][info][class,load] java.io.RandomAccessFile$1 source:
/data/git/jdk17/build/linux-x86_64-server-release/jdk/modules/java.base
[4.867s][info][class,unload] unloading class sample.C9
0x00000008001c5c30
[4.867s][info][class,unload] unloading class sample.C8
0x00000008001c5a28
[4.867s][info][class,unload] unloading class sample.C7
0x00000008001c5820
[4.867s][info][class,unload] unloading class sample.C6
0x00000008001c5618
[4.867s][info][class,unload] unloading class sample.C5
0x00000008001c5410
[4.867s][info][class,unload] unloading class sample.C4
0x00000008001c5208
[4.867s][info][class,unload] unloading class sample.C3
0x00000008001c5000
[4.867s][info][class,unload] unloading class sample.C2
0x00000008001c4c10
[4.867s][info][class,unload] unloading class sample.C1
0x00000008001c4a08
[4.867s][info][class,unload] unloading class sample.C0
0x00000008001c4800
[4.876s][info][class,load ] java.lang.ref.Finalizer$2 source:
/data/git/jdk17/build/linux-x86_64-server-release/jdk/modules/java.base
Sending class unload event for: Lsample/C9;
Sending class unload event for: Lsample/C7;
Sending class unload event for: Lsample/C5;
Sending class unload event for: Lsample/C3;
[4.876s][info][class,load ] java.lang.ref.Finalizer$1 source:
/data/git/jdk17/build/linux-x86_64-server-release/jdk/modules/java.base
AFTER CLASS UNLOADING
...
