我最近遇到了一个非常奇怪的状态的多线程死锁。当一个线程实际上从未在其堆栈跟踪的任何方法中请求锁时，它似乎持有一个锁。这是必要的堆栈跟踪：
Found one Java-level deadlock:
=============================
"pool-1-thread-2":
waiting to lock monitor 0x000000000751fc20 (object 0x00000000f55e2960, a de.ntcomputer.minecraft.elevators2.plugin.implementation.extension.CraftElevatorsExtensionManager$ExtensionLoader),
which is held by "pool-1-thread-1"
"pool-1-thread-1":
waiting to lock monitor 0x000000000052ba78 (object 0x00000000f55dfc98, a de.ntcomputer.minecraft.elevators2.plugin.implementation.extension.CraftElevatorsExtensionManager),
which is held by "pool-1-thread-2"
Java stack information for the threads listed above:
===================================================
"pool-1-thread-2":
at java.lang.ClassLoader.checkCerts(ClassLoader.java:782)
- waiting to lock <0x00000000f55e2960> (a de.ntcomputer.minecraft.elevators2.plugin.implementation.extension.CraftElevatorsExtensionManager$ExtensionLoader)
at java.lang.ClassLoader.preDefineClass(ClassLoader.java:487)
at java.lang.ClassLoader.defineClassCond(ClassLoader.java:625)
at java.lang.ClassLoader.defineClass(ClassLoader.java:615)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at de.ntcomputer.minecraft.elevators2.plugin.implementation.extension.CraftElevatorsExtensionManager$ExtensionLoader.loadClassDirect(CraftElevatorsExtensionManager.java:149)
at de.ntcomputer.minecraft.elevators2.plugin.implementation.extension.CraftElevatorsExtensionManager$ExtensionLoader.access$4(CraftElevatorsExtensionManager.java:147)
at de.ntcomputer.minecraft.elevators2.plugin.implementation.extension.CraftElevatorsExtensionManager.findClass(CraftElevatorsExtensionManager.java:85)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
- locked <0x00000000f55dfc98> (a de.ntcomputer.minecraft.elevators2.plugin.implementation.extension.CraftElevatorsExtensionManager)
at java.lang.ClassLoader.loadClass(ClassLoader.java:295)
- locked <0x00000000f55dfc38> (a de.ntcomputer.minecraft.elevators2.plugin.implementation.extension.CraftElevatorsExtensionManager$ExtensionLoader)
at de.ntcomputer.minecraft.elevators2.plugin.implementation.extension.CraftElevatorsExtensionManager$ExtensionLoader.loadClass(CraftElevatorsExtensionManager.java:231)
- locked <0x00000000f55dfc98> (a de.ntcomputer.minecraft.elevators2.plugin.implementation.extension.CraftElevatorsExtensionManager)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
at java.lang.Class.getDeclaredMethods0(Native Method)
at java.lang.Class.privateGetDeclaredMethods(Class.java:2436)
at java.lang.Class.privateGetPublicMethods(Class.java:2556)
at java.lang.Class.getMethods(Class.java:1412)
at de.ntcomputer.minecraft.elevators2.plugin.implementation.elevator.CraftElevatorEventManager.register(CraftElevatorEventManager.java:147)
at de.ntcomputer.minecraft.elevators2.doorsext.implementation.elevator.CraftElevatorDoorsExtension.onEnable(CraftElevatorDoorsExtension.java:61)
- locked <0x00000000f55b8e58> (a de.ntcomputer.minecraft.elevators2.doorsext.implementation.elevator.CraftElevatorDoorsExtension)
at de.ntcomputer.minecraft.elevators2.plugin.api.elevator.extension.CraftElevatorExtension.initialize(CraftElevatorExtension.java:16)
at de.ntcomputer.minecraft.elevators2.plugin.implementation.elevator.CraftElevator.initialize(CraftElevator.java:72)
at de.ntcomputer.minecraft.elevators2.plugin.implementation.storage.CraftElevatorLoader.loadSync(CraftElevatorLoader.java:48)
- locked <0x00000000f55b8f08> (a de.ntcomputer.minecraft.elevators2.plugin.implementation.storage.CraftElevatorLoader)
at de.ntcomputer.minecraft.elevators2.plugin.implementation.storage.CraftElevatorLoader.access$0(CraftElevatorLoader.java:39)
at de.ntcomputer.minecraft.elevators2.plugin.implementation.storage.CraftElevatorLoader$1.run(CraftElevatorLoader.java:64)
at de.ntcomputer.minecraft.elevators2.plugin.implementation.scheduler.CraftElevatorTask.run(CraftElevatorTask.java:53)
at org.bukkit.craftbukkit.v1_6_R3.scheduler.CraftTask.run(CraftTask.java:53)
at org.bukkit.craftbukkit.v1_6_R3.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:53)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918)
at java.lang.Thread.run(Thread.java:662)
"pool-1-thread-1":
at de.ntcomputer.minecraft.elevators2.plugin.implementation.extension.CraftElevatorsExtensionManager$ExtensionLoader.loadClass(CraftElevatorsExtensionManager.java:231)
- waiting to lock <0x00000000f55dfc98> (a de.ntcomputer.minecraft.elevators2.plugin.implementation.extension.CraftElevatorsExtensionManager)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
at de.ntcomputer.minecraft.elevators2.extfloors.implementation.elevator.CraftInteractiveElevatorFloorsExtension.deserializeFloor(CraftInteractiveElevatorFloorsExtension.java:116)
at de.ntcomputer.minecraft.elevators2.extfloors.implementation.elevator.CraftElevatorFloorsExtension$2.deserialize(CraftElevatorFloorsExtension.java:84)
at de.ntcomputer.minecraft.elevators2.extfloors.implementation.elevator.CraftElevatorFloorsExtension$2.deserialize(CraftElevatorFloorsExtension.java:1)
at de.ntcomputer.minecraft.elevators2.plugin.implementation.storagefile.CraftStorageSectionReader.deserializeObject(CraftStorageSectionReader.java:151)
at de.ntcomputer.minecraft.elevators2.plugin.implementation.storagefile.CraftStorageSectionReader.deserializeCoreObject(CraftStorageSectionReader.java:199)
at de.ntcomputer.minecraft.elevators2.plugin.implementation.storagefile.CraftStorageSectionReader.inflateKeyMapEmpty(CraftStorageSectionReader.java:530)
at de.ntcomputer.minecraft.elevators2.plugin.implementation.storagefile.CraftStorageSectionReader.inflateMapValuesEmpty(CraftStorageSectionReader.java:996)
at de.ntcomputer.minecraft.elevators2.extfloors.implementation.elevator.CraftElevatorFloorsExtension.onEnable(CraftElevatorFloorsExtension.java:76)
at de.ntcomputer.minecraft.elevators2.extfloors.implementation.elevator.CraftInteractiveElevatorFloorsExtension.onEnable(CraftInteractiveElevatorFloorsExtension.java:70)
at de.ntcomputer.minecraft.elevators2.plugin.api.elevator.extension.CraftElevatorExtension.initialize(CraftElevatorExtension.java:16)
at de.ntcomputer.minecraft.elevators2.plugin.implementation.elevator.CraftElevator.initialize(CraftElevator.java:72)
at de.ntcomputer.minecraft.elevators2.plugin.implementation.storage.CraftElevatorLoader.loadSync(CraftElevatorLoader.java:48)
- locked <0x00000000f5601ac8> (a de.ntcomputer.minecraft.elevators2.plugin.implementation.storage.CraftElevatorLoader)
at de.ntcomputer.minecraft.elevators2.plugin.implementation.storage.CraftElevatorLoader.access$0(CraftElevatorLoader.java:39)
at de.ntcomputer.minecraft.elevators2.plugin.implementation.storage.CraftElevatorLoader$1.run(CraftElevatorLoader.java:64)
at de.ntcomputer.minecraft.elevators2.plugin.implementation.scheduler.CraftElevatorTask.run(CraftElevatorTask.java:53)
at org.bukkit.craftbukkit.v1_6_R3.scheduler.CraftTask.run(CraftTask.java:53)
at org.bukkit.craftbukkit.v1_6_R3.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:53)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918)
at java.lang.Thread.run(Thread.java:662)
Found 1 deadlock.
如您所见，线程 2 尝试锁定显然由线程 1 持有的 <0x00000000f55e2960>，但线程 1 从未以任何方法锁定 <0x00000000f55e2960>！这怎么可能？
在此先感谢 DevCybran
编辑：添加了使用的类加载器代码：
public class CraftElevatorsExtensionManager extends ClassLoader implements FilenameFilter, ElevatorsExtensionManager {
private Map<String,ExtensionLoader> loaders = null;
...
@Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
for(ExtensionLoader loader: loaders.values()) {
try {
return loader.loadClassDirect(name); // line 85
} catch(ClassNotFoundException e) { }
}
throw new ClassNotFoundException();
}
...
private class ExtensionLoader extends URLClassLoader {
...
private Class<?> loadClassDirect(String name) throws ClassNotFoundException {
Class<?> loadedClass = this.findLoadedClass(name);
if(loadedClass==null) return this.findClass(name); // line 149
return loadedClass;
}
...
@Override
protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
// synchronized parent container first - should prevent deadlocks
synchronized(CraftElevatorsExtensionManager.this) {
return super.loadClass(name, resolve); // line 231
}
}
}
}
使用 Java 6 u45。