On 9/26/17 5:34 PM, David Holmes wrote:
Hi Mandy,
On 27/09/2017 3:27 AM, mandy chung wrote:
On 9/25/17 11:37 PM, Kim Barrett wrote:
Thanks for dealing with this.
I'm all for eliminating the finalizers in the JDK. Looking forward
to having a finalizer-free JDK.
==============================================================================
src/java.base/share/native/libjava/ClassLoader.c
415
Java_java_lang_ClassLoader_00024NativeLibrary_00024Unloader_unload
416 (JNIEnv *env, jobject this, jstring name, jboolean isBuiltin,
jlong address)
With this change, the "this" argument is no longer used.
Because of this, the native function could be a static member function
of the new Unloader class, or could (I think) still be a (now static)
member function of NativeLibrary. The latter would not require a name
change, only a signature change. But I don't really care which class
has the method.
Yes it can be a static method.
==============================================================================
src/java.base/share/classes/java/lang/ClassLoader.java
2394 public void register() {
[...]
2406 // register the class loader for cleanup when
unloaded
2407 if (loader != getBuiltinPlatformClassLoader() &&
2408 loader != getBuiltinAppClassLoader()) {
2409 CleanerFactory.cleaner()
2410 .register(loader, new Unloader(name,
handle, isBuiltin));
2411 }
Can anything before the cleanup registration throw?
No within the register method. The builtin loaders are created early
during startup. I shall make sure that System::loadLibrary is not
called during init phase 1.
If so, do we leak
because we haven't registered the cleanup yet? And what if the
registration itself fails to complete?
If Cleaner::register fails, it should throw an exception. Otherwise,
the registration should complete.
==============================================================================
src/hotspot/share/prims/jni.cpp
405 // Special handling to make sure JNI_OnLoad are executed
in the correct class context.
s/are executed/is executed/
Will fix it.
I'm considering to separate the JNI_FindClass change to target 18.3
and provide a flag to restore the current behavior so that it may
help existing code to identify its dependency on the current behavior
and give time to migrate. Then target the finalizer to Cleaner
change in 18.9.
It's unknown to us how many existing native libraries are impacted by
this change (calling FindClass from JNI_OnUnload to load classes
visible the class loader being unloaded). I suspect it's rare. If
FindClass is called when the native library is being unloaded and
fail to find the class due to this change, it is not hard to find out
but the code might crash if it does not handle the class not found
case properly.
Any opinion?
Yes :) I agree with being conservative here. We don't know how this
may be being used. But I'm still not completely clear how the
FindClass change is tied to the switch to Cleaner ??
It is not tied with the Cleaner change. Instead, the FindClass bug
blocks the finalizer to Cleaner change.
FindClass bug is uncovered when I implemented the change from finalizer
to Cleaner (or phantom reference). There is a test calling FindClass
to look for a class defined by the class loader being unloaded, say L.
L is not Gc'ed and so FindClass successfully finds the class (which
resurrect the class loader which was marked finalizable).
Is that clearer?
Mandy
