lock实现运行时死锁检测

java的多线程机制非常强大。其中死锁会导致程序无止境的等待，影响系统的正常运行，并且不是发生时立即被发现，一旦被发现定然是，系统出现了明显的功能性无法正常运行，甚至是系统崩溃。
下面我们讨论死锁的成因以及死锁的检测方法。

1、首先是系统运行时线程和锁之间的关系：
第一种情况：lock等待持有他们的thread将其释放。这些lock是由thread所持有，即thread获得了lock，其他的thread不能再获取到lock。

第二种情况：thread等待获取lock。如果此lock被其他的thead占用，那么此thread便只有等待其他thread将其释放。

2、死锁的产生
最简单的死锁产生的形式是：thread1 已经获取lock1，然后再去获取lock2，如果此时有thread2 已经获取了lock2 ，并且同时去获取lock1，那么就会发生死锁。

稍复杂的死锁的形式：
thread1：已经获得lock1
thread2：已经获得lock2，lock3。希望获得lock1
thread3：已经获得lock4。希望获得lock2

如果此时thread1 有需要获取到lock4，那么就可能会产生死锁。
3、死锁产生原因的分析：
从死锁的产生上，大家可以考虑到：如果将thread1 当成根节点，lock1是其子节点，thread2 是lock1的子节点，lock2，lock3 是thread2的子节点，同理：thread3 是lock2 的子节点，lock4 是thread3的子节点。如此可以看出thread与其相关的锁就是一个树结构，最上层是当前thread，其下是thread与lock交叉的分层。此树反应了一个thread所拥有的所有lock，然后是所有正在等待这些lock上thread，如此表示下去无论是直接还是间接，此树上的每个lock都在等待根thread最终将其释放，如此如果此thread需要对某个lock hard wait，它就不能在此树上，如此会产生一个循环，也就是死锁的成因。

4、下面的代码是实现了一个死锁检测的类，当产生死锁时抛出死锁异常。
注意：所有的锁的获取必须是lock形式，如果有synchrensed的锁形式，将不能被检测到。

下面这个例子展示了如何简单的检测到一个死锁。（当然效率是相当的低）

import java.util.*;
import java.util.concurrent.*;
import java.util.concurrent.locks.*;

//
// This is a very very slow implementation of a ReentrantLock class and is not for
//   everyday usage. The purpose of this class is to test for deadlocks. The lock()
//   method now throws a DeadlockDetectedException, if a deadlock occurs.
//
public class DeadlockDetectingLock extends ReentrantLock {
    /**
	 * 
	 */
	private static final long serialVersionUID = 1L;
	// List of deadlock detecting locks.
    // This array is not thread safe, and must be externally synchronized
    //    by the class lock. Hence, it should only be called by static
    //    methods.
    private static List<DeadlockDetectingLock> deadlockLocksRegistry = new ArrayList<DeadlockDetectingLock>();

    private static synchronized void registerLock(DeadlockDetectingLock ddl) {
        if (!deadlockLocksRegistry.contains(ddl)){
        	deadlockLocksRegistry.add(ddl);
        }
             
    }

    private static synchronized void unregisterLock(DeadlockDetectingLock ddl) {
        if (deadlockLocksRegistry.contains(ddl)){
        	deadlockLocksRegistry.remove(ddl);
        }
             
    }

    // List of threads hard waiting for this lock.
    // This array is not thread safe, and must be externally synchronized
    //    by the class lock. Hence, it should only be called by static
    //    methods.
    private List<Thread> hardwaitingThreads = new ArrayList<Thread>();

    private static synchronized void markAsHardwait(List<Thread> l, Thread t) {
        if (!l.contains(t)) {
        	l.add(t);
        }
    }

    private static synchronized void freeIfHardwait(List<Thread> l, Thread t) {
        if (l.contains(t)) {
        	l.remove(t);
        }
    }

    //
    // Deadlock checking methods
    //
    // Given a thread, return all locks that are already owned
    // Must own class lock prior to calling this method
    private static Iterator<DeadlockDetectingLock> getAllLocksOwned(Thread t) {
        DeadlockDetectingLock current;
        ArrayList<DeadlockDetectingLock> results = new ArrayList<DeadlockDetectingLock>();

        Iterator<DeadlockDetectingLock> itr = deadlockLocksRegistry.iterator();
        while (itr.hasNext()) {
            current = (DeadlockDetectingLock) itr.next();
            if (current.getOwner() == t) {
            	results.add(current);
            }
        }
        return results.iterator(); 
    }

    // Given a lock, return all threads that are hard waiting for the lock
    // Must own class lock prior to calling this method
    private static Iterator<Thread> getAllThreadsHardwaiting(DeadlockDetectingLock l) {
        return l.hardwaitingThreads.iterator();
    }

    // Check to see if a thread can perform a hard wait on a lock
    private static synchronized boolean canThreadWaitOnLock(Thread t, DeadlockDetectingLock l) {
    	//先获取t已经获取的lock锁list
        Iterator<DeadlockDetectingLock> locksOwned = getAllLocksOwned(t);
        while (locksOwned.hasNext()) {
            DeadlockDetectingLock current = locksOwned.next();

            // Thread can't wait if lock is already owned. This is the end condition
            //      for the recursive algorithm -- as the initial condition should be
            //      already tested for.
            // t 获取 l 锁，如果此锁已经被需要获得l锁的线程获取，那么就是产生死锁的条件
            if (current == l) 
            	return false;
            //获取(等待获取当前线程已经获得的current锁)的线程组
            Iterator<Thread> waitingThreads = getAllThreadsHardwaiting(current);
            while (waitingThreads.hasNext()) {
                Thread otherthread = (Thread) waitingThreads.next();

                // In order for the thread to safely wait on the lock, it can't
                //   own any locks that have waiting threads that already owns
                //   lock. etc. etc. etc. recursively etc.
                if (!canThreadWaitOnLock(otherthread, l)) {
                    return false;
                }
            }
        }
        return true;
    }

    //
    // Core Constructors
    //
    public DeadlockDetectingLock() {
        this(false, false);
    }

    public DeadlockDetectingLock(boolean fair) {
        this(fair, false);
    }

    private boolean debugging;
    public DeadlockDetectingLock(boolean fair, boolean debug) {
        super(fair);
        debugging = debug;
        registerLock(this);
    }

    //
    // Core Methods
    //
    public void lock() {
        // Note: Owner can't change if current thread is owner. It is
        //       not guaranteed otherwise. Other owners can change due to
        //       condition variables.
    	//当前线程是否获得当前锁
        if (isHeldByCurrentThread()) {
            if (debugging){
            	 System.out.println("Already Own Lock");
            }
            super.lock();
            //把当前线程从  wait list（无限等待获取该锁的线程列表）中移除，因为该线程已经获得该锁
            freeIfHardwait(hardwaitingThreads, Thread.currentThread());
            return;
        }

        // Note: The wait list must be marked before it is tested because
        //       there is a race condition between lock() method calls.
        //如果当前线程没有获得当前锁，那么把当前线程加入到 wait list（无限等待获取该锁的线程列表）
        markAsHardwait(hardwaitingThreads, Thread.currentThread());
        
        //测试当前线程能否获取this当前锁
        if (canThreadWaitOnLock(Thread.currentThread(), this)) {
            if (debugging) {
            	System.out.println("Waiting For Lock");
            }
            super.lock();
            freeIfHardwait(hardwaitingThreads, Thread.currentThread());
            if (debugging) {
            	System.out.println("Got New Lock");
            }
        } else {
            throw new DeadlockDetectedException("DEADLOCK");
        }
    }

    //
    // Note: It is debatable whether this is a hard or soft wait. Even if
    //       interruption is common, we don't know if the interrupting thread
    //       is also involved in the deadlock. As a compromise, we'll just
    //       not allow interrupts. This method is disabled.
    public void lockInterruptibly() throws InterruptedException {
        lock();
    }

    //
    // Note: It is not necessary to override the tryLock() methods. These
    //     methods perform a soft wait -- there is a limit to the wait. It
    //     not possible to deadlock when locks are not waiting indefinitely.
    //

    // Note 1: Deadlocks are possible with any hard wait -- this includes
    //      the reacquitition of the lock upon return from an await() method.
    //      As such, condition variables will mark for the future hard
    //      wait, prior to releasing the lock.
    // Note 2: There is no need to check for deadlock on this end because
    //      a deadlock can be created whether the condition variable owns the
    //      lock or is reacquiring it. Since we are marking *before* giving
    //      up ownership, the deadlock will be detected on the lock() side
    //      first. It is not possible to create a new deadlock just by releasing
    //      locks.
    public class DeadlockDetectingCondition implements Condition {
	Condition embedded;
        protected DeadlockDetectingCondition(ReentrantLock lock, Condition embedded) {
	    this.embedded = embedded;
        }

        // Note: The algorithm can detect a deadlock condition if the thead is
        //    either waiting for or already owns the lock, or both. This is why
        //    we have to mark for waiting *before* giving up the lock.
        public void await() throws InterruptedException {
            try {
                markAsHardwait(hardwaitingThreads, Thread.currentThread());
                embedded.await();
            } finally {
                freeIfHardwait(hardwaitingThreads, Thread.currentThread());
            }
        }

        public void awaitUninterruptibly() {
            markAsHardwait(hardwaitingThreads, Thread.currentThread());
            embedded.awaitUninterruptibly();
            freeIfHardwait(hardwaitingThreads, Thread.currentThread());
        }

        public long awaitNanos(long nanosTimeout) throws InterruptedException {
            try {
                markAsHardwait(hardwaitingThreads, Thread.currentThread());
                return embedded.awaitNanos(nanosTimeout);
            } finally {
                freeIfHardwait(hardwaitingThreads, Thread.currentThread());
            }
        }

        public boolean await(long time, TimeUnit unit) throws InterruptedException {
            try {
                markAsHardwait(hardwaitingThreads, Thread.currentThread());
                return embedded.await(time, unit);
            } finally {
                freeIfHardwait(hardwaitingThreads, Thread.currentThread());
            }
        }

        public boolean awaitUntil(Date deadline) throws InterruptedException {
            try {
                markAsHardwait(hardwaitingThreads, Thread.currentThread());
                return embedded.awaitUntil(deadline);
            } finally {
                freeIfHardwait(hardwaitingThreads, Thread.currentThread());
            }
        }

	public void signal() {
	    embedded.signal();
	}

	public void signalAll() {
	    embedded.signalAll();
	}
    }

    // Return a condition variable that support detection of deadlocks
    public Condition newCondition() {
        return new DeadlockDetectingCondition(this, super.newCondition());
    }

    //
    // Testing routines here
    //
    // These are very simple tests -- more tests will have to be written a b c 都被注册到 deadlockLocksRegistry 中
    private static Lock a = new DeadlockDetectingLock(false, true);
    private static Lock b = new DeadlockDetectingLock(false, true);
    private static Lock c = new DeadlockDetectingLock(false, true);
    private static Condition wa = a.newCondition();
    private static Condition wb = b.newCondition();
    private static Condition wc = c.newCondition();

    private static void delaySeconds(int seconds) {
        try {
             Thread.sleep(seconds * 1000);
        } catch (InterruptedException ex) {
        }
    }

    private static void awaitSeconds(Condition c, int seconds) {
        try {
             c.await(seconds, TimeUnit.SECONDS);
        } catch (InterruptedException ex) {
        }
    }

    private static void testOne() {
         new Thread(new Runnable() {
             public void run() {
                 System.out.println("thread one grab a");
                 a.lock();
                 delaySeconds(2);
                 System.out.println("thread one grab b");
                 b.lock();
                 delaySeconds(2);
                 a.unlock(); b.unlock();
             }
         }).start();

         new Thread(new Runnable() {
             public void run() {
                 System.out.println("thread two grab b");
                 b.lock();
                 delaySeconds(2);
                 System.out.println("thread two grab a");
                 a.lock();
                 delaySeconds(2);
                 a.unlock(); b.unlock();
             }
         }).start();
    }

    private static void testTwo() {
         new Thread(new Runnable() {
             public void run() {
                 System.out.println("thread one grab a");
                 a.lock();
                 delaySeconds(2);
                 System.out.println("thread one grab b");
                 b.lock();
                 delaySeconds(10);
                 a.unlock(); b.unlock();
             }
         }).start();

         new Thread(new Runnable() {
             public void run() {
                 System.out.println("thread two grab b");
                 b.lock();
                 delaySeconds(2);
                 System.out.println("thread two grab c");
                 c.lock();
                 delaySeconds(10);
                 b.unlock(); c.unlock();
             }
         }).start();

         new Thread(new Runnable() {
             public void run() {
                 System.out.println("thread three grab c");
                 c.lock();
                 delaySeconds(4);
                 System.out.println("thread three grab a");
                 a.lock();
                 delaySeconds(10);
                 c.unlock(); a.unlock();
             }
         }).start();
    }
 
    private static void testThree() {
         new Thread(new Runnable() {
             public void run() {
                 System.out.println("thread one grab b");
                 b.lock();
                 System.out.println("thread one grab a");
                 a.lock();
                 delaySeconds(2);
                 System.out.println("thread one waits on b");
                 awaitSeconds(wb, 10);                 
                 a.unlock(); b.unlock();
             }
         }).start();

         new Thread(new Runnable() {
             public void run() {
                 delaySeconds(1);
                 System.out.println("thread two grab b");
                 b.lock();
                 System.out.println("thread two grab a");
                 a.lock();
                 delaySeconds(10);
                 b.unlock(); c.unlock();
             }
         }).start();

    }

    public static void main(String args[]) {
        int test = 1;
	if (args.length > 0)
	    test = Integer.parseInt(args[0]);
	switch(test) {
	    case 1:
                testOne();    // 2 threads deadlocking on grabbing 2 locks
		break;
	    case 2:
        	testTwo();    // 3 threads deadlocking on grabbing 2 out of 3 locks
		break;
	    case 3:
                testThree();  // 2 threads deadlocking on 2 locks with CV wait 
		break;
	    default:
	        System.err.println("usage: java DeadlockDetectingLock [ test# ]");
	}
        delaySeconds(60);
        System.out.println("--- End Program ---");
        System.exit(0);
    }
}

以上代码是：java Thread 第三版 第六章的代码，详细请查看原始代码