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同步的关键是多个线程对象竞争同一个共享对象，使无序的多个线程有序地排队。

Example 1:

public class Thread06_sync {	public static void main(String[] args) {//		new Thread06_sync().test01();		new Thread06_sync().new Test02(new Object()).test02();	}	// 同步失败，原因：不是同步同一个对象	public void test01() {		for (int i = 1; i <= 3; ++i) {			new Thread(new Runnable() {				public synchronized void run() {					for (int i = 1; i <= 5; ++i) {						System.out.println("id:"								+ Thread.currentThread().getId() + ",i:" + i);					}				}			}).start();		}	}	// 同步成功，原因：同步同一个对象	public class Test02 {		private Object thisThread;		public Test02(Object thisThread) {			this.thisThread = thisThread;		}		public void test02() {			for (int i = 1; i <= 3; ++i) {				new Thread(new Runnable() {					public void run() {						synchronized (thisThread) {							for (int i = 1; i <= 5; ++i) {								System.out.println("id:"										+ Thread.currentThread().getId() + ",i:" + i);							}						}					}				}).start();			}		}	}}

第一个例子，如果不同的synchronized object, 则不能正确lock住代码片段。

Example 2:

public class Thread04_static {	// volatile 只是可见一致性，非原子性（也就是无写锁），仍然有机会有重复uniqueNumber的输出	public volatile static long uniqueNumber = 0;		static int threadNumber = 5;	static AtomicInteger testNum = new AtomicInteger(0);	static List
   
     resultList = new ArrayList<>(300);	private final static ReentrantLock lock = new ReentrantLock();		public static void main(String[] args) {		testThread01();	}		// CountDownLatch	public static void testThread01() {		try {			CountDownLatch doneSignal = new CountDownLatch(50);			for (int i = 0; i < threadNumber; i++) {				Thread thread = new Thread(new Thread04_static().new TestThread01(doneSignal));				thread.setName("t_" + i);				thread.start();			}			// 由于thread的无序性，不加await的话, 有可能运行下面语句时thread还没运行，resultList有可能是空值			doneSignal.await();			int i = 1;			for (Long result : resultList) {				System.out.println("result_" + i + ":" + result);				++i;			}		} catch (Exception e) {			e.printStackTrace();		}	}		// 静态类及非静态同步、锁测试	public class TestThread01 implements Runnable {				private CountDownLatch doneSignal;		public TestThread01(CountDownLatch doneSignal) {			this.doneSignal = doneSignal;		}		public void run() {			try {				// 第一个同步block/ReentrantLock：这里的同步无预期效果（即有重复的uniqueNumber出现），因为getUniqueNum是类静态方法//				synchronized (this) {//				lock.lock();					for (int i = 0; i < 10; i++) {//						System.out.println(testNum.addAndGet(1));						// 有重复的testNum出现, 原因：testNum是原子的，但下面的语句不原子//						System.out.println("Thread:" + Thread.currentThread().getName() + ", testNum:" + testNum + ", i:" + i);						resultList.add(getUniqueNum(2));						doneSignal.countDown();					}					// 空行不一定每十条记录输出一次，因为它不在下面的静态同步block					System.out.println("");					Thread.sleep(1000);//				}			} catch (Exception e) {				e.printStackTrace();			} finally {//				lock.unlock();			}		}	}		public static Long getUniqueNum(int len) {		// 第二个同步block/ReentrantLock：静态类同步block/lock，实现无重复uniqueNumber输出//		synchronized (TestThread01.class) {		try{			lock.lock();			if (len < 1) {				return null;			}			if (uniqueNumber < (Math.pow(10, len) - 1)) {				uniqueNumber++;			}			System.out.println("Thread:" + Thread.currentThread().getName()					+ ", uniqueNumber:" + uniqueNumber);			return uniqueNumber;		} finally {			lock.unlock();		}//		}	}}
   

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