• CountDownLatch

CountDownLatch允许一个或多个线程等待其他线程完成操作。

CountDownLatch的构造函数接受一个int类型的参数作为计数器，如果你想等待N个点完成，这里就传入N。

当我们调用CountDownLatch的countDown方法时，N就会减1，CountDownLatch的await方法会阻塞当前线程，直到N编程零。此外还有一个带指定时间的await方法，这个方法等待指定时间后，就不会再阻塞当前线程。

package test;public class ThreadJoinTest {	public static void main(String[] args) throws InterruptedException {		Thread parser1 = new Thread(new Runnable(){			public void run() {				try {					Thread.sleep(2000);				} catch (InterruptedException e) {					e.printStackTrace();				}				System.out.println("parser1 finish");			}		});		Thread parser2 = new Thread(new Runnable(){			public void run() {				System.out.println("parser2 finish");			}		});		parser1.start();		parser2.start();		parser1.join();		parser2.join();		System.out.println("all finished");	}}---------parser2 finishparser1 finishall finishedpackage test;import java.util.concurrent.CountDownLatch;public class CountDownLatchTest {	static CountDownLatch c = new CountDownLatch(2);		public static void main(String[] args) throws InterruptedException {		new Thread(new Runnable(){			public void run() {				try {					Thread.sleep(2000);				} catch (InterruptedException e) {					e.printStackTrace();				}				System.out.println("parser1 finish");				c.countDown();			}		}).start();		new Thread(new Runnable(){			public void run() {				System.out.println("parser2 finish");				c.countDown();			}		}).start();		c.await();		System.out.println("all finished");	}}---------parser2 finishparser1 finishall finished

• CyclicBarrier
  CyclicBarrier让一组线程到达一个屏障（也可以叫同步点）时被阻塞，直到最后一个线程到达屏障时，所有被屏障拦截的线程才会继续运行。
  CyclicBarrier默认的构造方法是CyclicBarrier（int parties），其参数表示屏障拦截的线程数量，每个线程调用await方法告诉CyclicBarrier已经到达了屏障，然后当前线程被阻塞。
  CyclicBarrier还提供一个更高级的构造函数CyclicBarrier（int parties , Runnable barrierAction），用于在线程到达屏障是，优先执行barrierAction，方便处理更复杂的业务。

  package test;import java.util.concurrent.BrokenBarrierException;import java.util.concurrent.CyclicBarrier;public class CyclicBarrierTest {	static CyclicBarrier c = new CyclicBarrier(2);		public static void main(String args[]) throws InterruptedException, BrokenBarrierException{		new Thread(new Runnable(){			public void run() {				System.out.println("begin");				try {					Thread.sleep(1000);					c.await();				} catch (InterruptedException e) {					e.printStackTrace();				} catch (BrokenBarrierException e) {					e.printStackTrace();				}				System.out.println("end");			}		}).start();				System.out.println("current thread waiting..");		c.await();		System.out.println("all arrived");	}}-------current thread waiting..beginendall arrived

  package test;import java.util.concurrent.BrokenBarrierException;import java.util.concurrent.CyclicBarrier;public class CyclicBarrierTest2 {	static CyclicBarrier c = new CyclicBarrier(2,new A());		public static void main(String args[]) throws InterruptedException, BrokenBarrierException{		new Thread(new Runnable(){			public void run() {				System.out.println("begin");				try {					Thread.sleep(1000);					c.await();				} catch (InterruptedException e) {					e.printStackTrace();				} catch (BrokenBarrierException e) {					e.printStackTrace();				}				System.out.println("end");			}		}).start();				System.out.println("current thread waiting..");		c.await();		System.out.println("all arrived");	}		static class A implements Runnable{		public void run() {			System.out.println("Thread A Running..");		}	}}----------current thread waiting..beginThread A Running..endall arrived

   

• CyclicBarrier和CountDownLatch的区别
   CountDownLatch的计数只能使用一次，而CyclicBarrier的计数器可以使用reset（）方法重置，所以CyclicBarrier能处理更复杂的业务场景。
   CyclicBarrier还提供其他的方法，如getNumberWaiting方法可以获得CyclicBarrier阻塞的线程数量，isBroken()方法用来了解阻塞的线程是否被中断。
   
• Semaphore
  Semaphore（信号量）是用来控制同时访问特定资源的线程数量，他通过协调各个线程，以保证合理的使用公共资源。
  Semaphore可以用于做流量控制，特别是公共资源有限的应用场景，比如数据库连接。

  package test;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Semaphore;public class SemaphoreTest {	private static final int THREAD_COUNT = 10;	private static ExecutorService threadPool =  Executors.newFixedThreadPool(THREAD_COUNT);	private static Semaphore s = new Semaphore(5);	public static void main(String[] args) throws InterruptedException {			while(true){				threadPool.execute(new Runnable(){					public void run() {						try {							s.acquire();							System.out.println(Thread.currentThread().getName()+": save data...");							Thread.sleep(5000);							s.release();						} catch (InterruptedException e) {							e.printStackTrace();						}					}				});				System.out.println("availablePermits: "+s.availablePermits());				Thread.sleep(500);			}		//threadPool.shutdown();	}}----------------------------availablePermits: 5pool-1-thread-1: save data...availablePermits: 4pool-1-thread-2: save data...availablePermits: 3pool-1-thread-3: save data...availablePermits: 2pool-1-thread-4: save data...availablePermits: 1pool-1-thread-5: save data...availablePermits: 0availablePermits: 0availablePermits: 0availablePermits: 0availablePermits: 0pool-1-thread-6: save data...availablePermits: 0pool-1-thread-7: save data...availablePermits: 0pool-1-thread-8: save data...availablePermits: 0pool-1-thread-9: save data...availablePermits: 0pool-1-thread-10: save data...availablePermits: 0availablePermits: 0availablePermits: 0availablePermits: 0availablePermits: 0availablePermits: 0pool-1-thread-1: save data...availablePermits: 0pool-1-thread-7: save data...availablePermits: 0pool-1-thread-2: save data...availablePermits: 0pool-1-thread-9: save data...availablePermits: 0pool-1-thread-3: save data...availablePermits: 0availablePermits: 0availablePermits: 0

  在代码中，虽然有10个线程在执行，但是只允许5个并发执行。