ロック実行時デッドロック検出を実現

詳細
JAvaのマルチスレッドメカニズムは非常に強力です.その中でデッドロックはプログラムの絶え間ない待機を招き、システムの正常な運行に影響を与え、発生時にすぐに発見されるのではなく、発見されると、システムに明らかな機能性が現れ、システムがクラッシュすることもある.
次に,デッドロックの成因とデッドロックの検出方法について論じる.
1、まずシステムランタイムスレッドとロックの関係です.
1つ目は、lockがthreadを持って釈放するのを待っていることです.これらのlockはthreadが所有している、すなわちthreadがlockを取得し、他のthreadがlockを取得できない.
第2のケース:threadはlockの取得を待つ.このlockが他のtheadによって占有されている場合、threadは他のthreadが解放するのを待つしかありません.
2、デッドロックの発生
最も簡単なデッドロック生成の形態は、thread 1がlock 1を取得し、その後lock 2を取得し、thread 2がlock 2を取得し、同時にlock 1を取得すると、デッドロックが発生する.
少し複雑なデッドロックの形式:
thread 1:lock 1が取得されました
thread 2:lock 2,lock 3が取得されました.lock 1を取得したい
thread 3:lock 4が取得されました.lock 2を取得したい
thread 1がlock 4を取得する必要がある場合、デッドロックが発生する可能性があります.
3、デッドロック発生原因の分析:
デッドロックの発生から、thread 1をルートノードとすると、lock 1はそのサブノードであり、thread 2はlock 1のサブノードであり、lock 2、lock 3はthread 2のサブノードであり、同様にthread 3はlock 2のサブノードであり、lock 4はthread 3のサブノードであることが考えられる.このようにthreadとそれに関連するロックはツリー構造であり、最上位層は現在のthreadであり、その下はthreadとlockが交差する階層であることがわかる.このツリーは、threadが所有するすべてのlockを反応させ、その後、これらのlockにthreadを待機しているすべてのことを表します.このツリーの各lockは、直接的にも間接的にもルートthreadが最終的に解放されるのを待っています.このthreadがlock hard waitを必要とする場合、このツリーにループを生成することはできません.つまりデッドロックの原因です.
4、次のコードは、デッドロック検出を実現したクラスであり、デッドロックが発生したときにデッドロック異常を放出する.
注意:すべてのロックの取得はlock形式でなければなりません.synchrensedのロック形式がある場合、検出できません.
次の例では、デッドロックを簡単に検出する方法を示します.(もちろん効率はかなり低い)

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 deadlockLocksRegistry = new ArrayList();

    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 hardwaitingThreads = new ArrayList();

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

    private static synchronized void freeIfHardwait(List 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 getAllLocksOwned(Thread t) {
        DeadlockDetectingLock current;
        ArrayList results = new ArrayList();

        Iterator 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 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) {
    	//   t     lock list
        Iterator 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    l  ，           l      ，           
            if (current == l) 
            	return false;
            //  (             current )    
            Iterator 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.
    	//           
        if (isHeldByCurrentThread()) {
            if (debugging){
            	 System.out.println("Already Own Lock");
            }
            super.lock();
            //        wait list（             ）   ，           
            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.
        //             ，           wait list（             ）
        markAsHardwait(hardwaitingThreads, Thread.currentThread());
        
        //          this   
        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       deadlockLocksRegistry  
    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);
    }
}

以上のコードは:java Thread第3版第6章のコードで、詳しくは原始コードを見てください