Xử lý Deadlock trong Java

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Miêu tả vấn đề

Cách xử lý Deadlock bởi sử dụng thread trong Java?

Giải pháp

Ví dụ sau minh họa cách xử lý Deadlock bởi sử dụng khái niệm thread trong Java.

import java.util.*;
import java.util.concurrent.*;
import java.util.concurrent.locks.*;public class DeadlockDetectingLock extends ReentrantLock {
   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);
   }
   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);
}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();
}private static Iterator 
getAllThreadsHardwaiting(DeadlockDetectingLock l) {
   return l.hardwaitingThreads.iterator();
}private static synchronized boolean canThreadWaitOnLock
(Thread t,DeadlockDetectingLock l) {
   Iterator locksOwned = getAllLocksOwned(t);
   while (locksOwned.hasNext()) {
      DeadlockDetectingLock current 
      = (DeadlockDetectingLock) locksOwned.next();
      if (current == l)
      return false;
      Iterator waitingThreads 
      = getAllThreadsHardwaiting(current);
      while (waitingThreads.hasNext()) {
         Thread otherthread = (Thread) waitingThreads.next();
         if (!canThreadWaitOnLock(otherthread, l)) {
            return false;
         }
      }
   }
   return true;
}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);
}public void lock() {
   if (isHeldByCurrentThread()) {
      if (debugging)
      System.out.println("Already Own Lock");
      super.lock();
      freeIfHardwait(hardwaitingThreads, 
      Thread.currentThread());
      return;
   }
   markAsHardwait(hardwaitingThreads, 
   Thread.currentThread());
   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");
   }
}public void lockInterruptibly() throws InterruptedException {
   lock();
}locks.
public class DeadlockDetectingCondition implements Condition {
   Condition embedded;
   protected DeadlockDetectingCondition(ReentrantLock lock,
   Condition embedded) {
      this.embedded = embedded;
   }   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();
   }
}public Condition newCondition() {
   return new DeadlockDetectingCondition(this, 
   super.newCondition());
}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();
      break;
      case 2:
      testTwo(); 
      break; 
      case 3:
      testThree();
      break;
      default:
      System.err.println("usage: java 
      DeadlockDetectingLock [ test# ]");
   }
   delaySeconds(60);
   System.out.println("--- End Program ---");
   System.exit(0);
   }
}class DeadlockDetectedException extends RuntimeException {
   public DeadlockDetectedException(String s) {
      super(s);
   }
}

Kết quả

Code trên sẽ cho kết quả sau:

thread one grab a
Waiting For Lock
Got New Lock
thread two grab b
Waiting For Lock
Got New Lock
thread one grab b
Waiting For Lock
thread two grab a
Exception in thread "Thread-1" 
DeadlockDetectedException:DEADLOCK
		at DeadlockDetectingLock.
        lock(DealockDetectingLock.java:152)
		at java.lang.Thread.run(Thread.java:595)

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