Java如何使用线程解决死锁？

在Java编程中，如何使用线程解决死锁？

以下示例演示如何使用线程的概念解决死锁问题。

// from W w w .Y I I b  AI.c  o  M
package com.yiibai;

import java.util.*; import java.util.concurrent.*; import java.util.concurrent.locks.*; public class SolvingDeadlock extends ReentrantLock { private static List deadlockLocksRegistry = new ArrayList(); private static synchronized void registerLock(SolvingDeadlock ddl) { if (!deadlockLocksRegistry.contains(ddl)) deadlockLocksRegistry.add(ddl); } private static synchronized void unregisterLock(SolvingDeadlock 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) { SolvingDeadlock current; ArrayList results = new ArrayList(); Iterator itr = deadlockLocksRegistry.iterator(); while (itr.hasNext()) { current = (SolvingDeadlock) itr.next(); if (current.getOwner() == t) results.add(current); } return results.iterator(); } private static Iterator getAllThreadsHardwaiting(SolvingDeadlock l) { return l.hardwaitingThreads.iterator(); } private static synchronized boolean canThreadWaitOnLock(Thread t, SolvingDeadlock l) { Iterator locksOwned = getAllLocksOwned(t); while (locksOwned.hasNext()) { SolvingDeadlock current = (SolvingDeadlock) 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 SolvingDeadlock() { this(false, false); } public SolvingDeadlock(boolean fair) { this(fair, false); } private boolean debugging; public SolvingDeadlock(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(); } 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 SolvingDeadlock(false, true); private static Lock b = new SolvingDeadlock(false, true); private static Lock c = new SolvingDeadlock(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); } } 

Java

上述代码示例将产生以下结果 -

thread one grab a
Waiting For Lock
thread two grab b
Got New Lock
Waiting For Lock
Got New Lock
thread one grab b
Exception in thread "Thread-1" thread two grab a
Waiting For Lock
com.yiibai.DeadlockDetectedException: DEADLOCK
    at com.yiibai.SolvingDeadlock.lock(SolvingDeadlock.java:102)
    at com.yiibai.SolvingDeadlock$2.run(SolvingDeadlock.java:213)
    at java.lang.Thread.run(Unknown Source)