最近在看Java编程思想，遇到一个问题：//: concurrency/CriticalSection.java// Synchronizing blocks instead of entire methods. Also// demonstrates protection of a non-thread-safe class// with a thread-safe one.package concurrency;import java.util.concurrent.*;import java.util.concurrent.atomic.*;import java.util.*;class Pair { // Not thread-safe private int x, y; public Pair(int x, int y) { this.x = x; this.y = y; } public Pair() { this(0, 0); } public int getX() { return x; } public int getY() { return y; } public void incrementX() { x++; } public void incrementY() { y++; } public String toString() { return "x: " + x + ", y: " + y; } public class PairValuesNotEqualException extends RuntimeException { public PairValuesNotEqualException() { super("Pair values not equal: " + Pair.this); } } // Arbitrary invariant -- both variables must be equal: public void checkState() { if(x != y) throw new PairValuesNotEqualException(); }}// Protect a Pair inside a thread-safe class:abstract class PairManager { AtomicInteger checkCounter = new AtomicInteger(0); protected Pair p = new Pair(); private List<Pair> storage = Collections.synchronizedList(new ArrayList<Pair>()); public synchronized Pair getPair() { // Make a copy to keep the original safe: return new Pair(p.getX(), p.getY()); } // Assume this is a time consuming operation protected void store(Pair p) { storage.add(p); try { TimeUnit.MILLISECONDS.sleep(50); } catch(InterruptedException ignore) {} } public abstract void increment();}// Synchronize the entire method:class PairManager1 extends PairManager { public synchronized void increment() { p.incrementX(); p.incrementY(); store(getPair()); }}// Use a critical section:class PairManager2 extends PairManager { public void increment() { Pair temp; synchronized(this) { p.incrementX(); p.incrementY(); temp = getPair(); } store(temp); }}class PairManipulator implements Runnable { private PairManager pm; public PairManipulator(PairManager pm) { this.pm = pm; } public void run() { while(true) pm.increment(); } public String toString() { return "Pair: " + pm.getPair() + " checkCounter = " + pm.checkCounter.get(); }}class PairChecker implements Runnable { private PairManager pm; public PairChecker(PairManager pm) { this.pm = pm; } public void run() { while(true) { pm.checkCounter.incrementAndGet(); pm.getPair().checkState(); } }}public class CriticalSection { // Test the two different approaches: static void testApproaches(PairManager pman1, PairManager pman2) { ExecutorService exec = Executors.newCachedThreadPool(); PairManipulator pm1 = new PairManipulator(pman1), pm2 = new PairManipulator(pman2); PairChecker pcheck1 = new PairChecker(pman1), pcheck2 = new PairChecker(pman2); exec.execute(pm1); exec.execute(pm2); exec.execute(pcheck1); exec.execute(pcheck2); try { TimeUnit.MILLISECONDS.sleep(500); } catch(InterruptedException e) { System.out.println("Sleep interrupted"); } System.out.println("pm1: " + pm1 + "\npm2: " + pm2); System.exit(0); } public static void main(String[] args) { PairManager pman1 = new PairManager1(), pman2 = new PairManager2(); testApproaches(pman1, pman2); }} /* Output: (Sample)pm1: Pair: x: 15, y: 15 checkCounter = 272565pm2: Pair: x: 16, y: 16 checkCounter = 3956974*///:~这是其并发部分的一个同步示例，这个没问题，但另外一种用Lock的就有问题了//: concurrency/ExplicitCriticalSection.java// Using explicit Lock objects to create critical sections.package concurrency;import java.util.concurrent.locks.*;// Synchronize the entire method:class ExplicitPairManager1 extends PairManager { private Lock lock = new ReentrantLock(); public synchronized void increment() { lock.lock(); try { p.incrementX(); p.incrementY(); store(getPair()); } finally { lock.unlock(); } }}// Use a critical section:class ExplicitPairManager2 extends PairManager { private Lock lock = new ReentrantLock(); public void increment() { Pair temp; lock.lock(); try { p.incrementX(); p.incrementY(); temp = getPair(); } finally { lock.unlock(); } store(temp); }}public class ExplicitCriticalSection { public static void main(String[] args) throws Exception { PairManager pman1 = new ExplicitPairManager1(), pman2 = new ExplicitPairManager2(); CriticalSection.testApproaches(pman1, pman2); }} /* Output: (Sample)pm1: Pair: x: 15, y: 15 checkCounter = 174035pm2: Pair: x: 16, y: 16 checkCounter = 2608588*///:~这样就在checkState的时候出问题了，在pman2的检查会抛出之前定义的异常，在增的过程中两个值不相等的时候作了check，貌似ReentrantLock没起作用，不知道是什么原因
转载于:https://www.cnblogs.com/mylordbin/p/8993519.html
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