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8262046: Clean up parallel class loading code and comments
Reviewed-by: lfoltan, iklam
  • Loading branch information
Coleen Phillimore committed Apr 1, 2021
1 parent 04f24fe commit 5e59d28def665b9eedbb40b214c4ddac7c509470

Large diffs are not rendered by default.

@@ -46,9 +46,7 @@
// be done concurrently, but only by different loaders.
//
// During loading a placeholder (name, loader) is temporarily placed in
// a side data structure, and is used to detect ClassCircularityErrors
// and to perform verification during GC. A GC can occur in the midst
// of class loading, as we call out to Java, have to take locks, etc.
// a side data structure, and is used to detect ClassCircularityErrors.
//
// When class loading is finished, a new entry is added to the dictionary
// of the class loader and the placeholder is removed. Note that the protection
@@ -58,9 +56,8 @@
// Clients of this class who are interested in finding if a class has
// been completely loaded -- not classes in the process of being loaded --
// can read the dictionary unlocked. This is safe because
// - entries are only deleted at safepoints
// - readers cannot come to a safepoint while actively examining
// an entry (an entry cannot be deleted from under a reader)
// - entries are only deleted when the class loader is not alive, when the
// entire dictionary is deleted.
// - entries must be fully formed before they are available to concurrent
// readers (we must ensure write ordering)
//
@@ -340,21 +337,21 @@ class SystemDictionary : AllStatic {
static Klass* resolve_array_class_or_null(Symbol* class_name,
Handle class_loader,
Handle protection_domain, TRAPS);
static InstanceKlass* handle_parallel_super_load(Symbol* class_name,
Symbol* supername,
Handle class_loader,
Handle protection_domain,
Handle lockObject, TRAPS);
// Wait on SystemDictionary_lock; unlocks lockObject before
// waiting; relocks lockObject with correct recursion count
// after waiting, but before reentering SystemDictionary_lock
// to preserve lock order semantics.
static void double_lock_wait(JavaThread* thread, Handle lockObject);
static InstanceKlass* handle_parallel_loading(JavaThread* current,
unsigned int name_hash,
Symbol* name,
ClassLoaderData* loader_data,
Handle lockObject,
bool* throw_circularity_error);

static void define_instance_class(InstanceKlass* k, Handle class_loader, TRAPS);
static InstanceKlass* find_or_define_helper(Symbol* class_name,
Handle class_loader,
InstanceKlass* k, TRAPS);
static InstanceKlass* load_instance_class(Symbol* class_name, Handle class_loader, TRAPS);
static InstanceKlass* load_instance_class_impl(Symbol* class_name, Handle class_loader, TRAPS);
static InstanceKlass* load_instance_class(unsigned int name_hash,
Symbol* class_name,
Handle class_loader, TRAPS);

static bool is_shared_class_visible(Symbol* class_name, InstanceKlass* ik,
PackageEntry* pkg_entry,
@@ -0,0 +1,56 @@
/*
* Copyright (c) 2021, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/

import jdk.internal.org.objectweb.asm.*;

public class AsmClasses implements Opcodes {

// class B extends A {} to generate ClassCircularityError
// Can't use jcod, jasm or InMemoryCompiler because the compiler will
// see that A extends B and fail.
public static byte[] dumpB() throws Exception {

ClassWriter classWriter = new ClassWriter(0);
FieldVisitor fieldVisitor;
RecordComponentVisitor recordComponentVisitor;
MethodVisitor methodVisitor;
AnnotationVisitor annotationVisitor0;

classWriter.visit(61, ACC_PUBLIC | ACC_SUPER, "B", null, "A", null);

classWriter.visitSource("B.java", null);
methodVisitor = classWriter.visitMethod(ACC_PUBLIC, "<init>", "()V", null, null);
methodVisitor.visitCode();
Label label0 = new Label();
methodVisitor.visitLabel(label0);
methodVisitor.visitLineNumber(24, label0);
methodVisitor.visitVarInsn(ALOAD, 0);
methodVisitor.visitMethodInsn(INVOKESPECIAL, "A", "<init>", "()V", false);
methodVisitor.visitInsn(RETURN);
methodVisitor.visitMaxs(1, 1);
methodVisitor.visitEnd();
classWriter.visitEnd();

return classWriter.toByteArray();
}
}
@@ -0,0 +1,56 @@
/*
* Copyright (c) 2021, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/

class ClassLoadingThread extends Thread {

private ClassLoader ldr = null;
private Object thread_sync = null;

public ClassLoadingThread(ClassLoader loader, Object sync) {
ldr = loader;
thread_sync = sync;
}

private boolean success = true;
public boolean report_success() { return success; }

public void run() {
try {
ThreadPrint.println("Starting...");
// Initiate class loading using specified type
Class<?> a = Class.forName("ClassInLoader", true, ldr);
Object obj = a.getConstructor().newInstance();

} catch (Throwable e) {
ThreadPrint.println("Exception is caught: " + e);
e.printStackTrace();
success = false;
} finally {
ThreadPrint.println("Finished");
// Wake up the second thread
synchronized (thread_sync) {
thread_sync.notify();
}
}
}
}
@@ -0,0 +1,151 @@
/*
* Copyright (c) 2021, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/

import java.io.*;
import jdk.test.lib.classloader.ClassUnloadCommon;

// This class loader will deadlock where one thread has a lock for A, trying to get a lock for B
// and the other thread has a lock for B, trying to get a lock for A in the case of
// A extends B extends A. It should throw ClassCircularityError from both threads.

class MyLoader extends ClassLoader {
static {
registerAsParallelCapable();
}

private static boolean first = true;

public Class loadClass(String name) throws ClassNotFoundException {
// Wait before getting B lock.
if (name.equals("B") && first) {
first = false;
makeThreadWait();
}
synchronized(getClassLoadingLock(name)) {
Class<?> c = findLoadedClass(name);
if (c != null) return c;

if (name.equals("B") && !first) {
wakeUpThread();
}

byte[] b = loadClassData(name);
if (b != null) {
return defineClass(name, b, 0, b.length);
} else {
return super.loadClass(name);
}
}
}

private static boolean parallel = false;
private Object sync = new Object();
private Object thread_sync = new Object();

private void makeThreadWait() {
if (!parallel) { return; }

// Wake up the second thread here.
synchronized (thread_sync) {
thread_sync.notify();
}
if (isRegisteredAsParallelCapable()) {
synchronized(sync) {
try {
ThreadPrint.println("t1 waits parallelCapable loader");
sync.wait(); // Give up lock before request to load B
} catch (InterruptedException e) {}
}
} else {
try {
ThreadPrint.println("t1 waits non-parallelCapable loader");
wait(); // Give up lock before request to load B
} catch (InterruptedException e) {}
}
}

// Parallel capable loader should wake up the first thread.
// Non-parallelCapable class loader thread will be woken up by the jvm.
private void wakeUpThread() {
if (isRegisteredAsParallelCapable()) {
synchronized(sync) {
sync.notify();
}
}
}

private byte[] loadClassData(String name) {
if (name.equals("A")) {
ThreadPrint.println("loading A extends B");
return ClassUnloadCommon.getClassData("A");
} else if (name.equals("B")) {
ThreadPrint.println("loading B extends A");
try {
return AsmClasses.dumpB();
} catch (Exception e) {
e.printStackTrace();
return null;
}
} else if (!name.startsWith("java")) {
return ClassUnloadCommon.getClassData(name);
}
return null;
}


ClassLoadingThread[] threads = new ClassLoadingThread[2];
private boolean success = true;

public boolean report_success() {
for (int i = 0; i < 2; i++) {
try {
threads[i].join();
if (!threads[i].report_success()) success = false;
} catch (InterruptedException e) {}
}
return success;
}

void startLoading() {

for (int i = 0; i < 2; i++) {
threads[i] = new ClassLoadingThread(this, thread_sync);
threads[i].setName("Loading Thread #" + (i + 1));
threads[i].start();
System.out.println("Thread " + (i + 1) + " was started...");
// wait to start the second thread if not concurrent
if (i == 0) {
synchronized(thread_sync) {
try {
ThreadPrint.println("t2 waits");
thread_sync.wait();
} catch (InterruptedException e) {}
}
}
}
}

MyLoader(boolean load_in_parallel) {
parallel = load_in_parallel;
}
}
@@ -0,0 +1,30 @@
/*
* Copyright (c) 2021, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/

class MyNonParallelLoader extends MyLoader {
// This loader isn't parallel capable because it's not registered in the static
// initializer as such. parallelCapable is not an inheritable attribute.
MyNonParallelLoader(boolean load_in_parallel) {
super(load_in_parallel);
}
}

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