java并发中锁的应用
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2022-07-10 10:47:14
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锁的理解
锁产生于多线程并发应用，其作用是解决共享对象的同步同时也可以控制线程的行为。我认为锁不仅仅限于synchronize,ReentrantLock,ReadWriteLock.同时也包括CountDownLack, FutureTask, Semaphore, CyclicBarrier, Exchanger这些平时接触不多的并发控制类。后者经常会用在控制线程的运行行为。
1.
CountDownLack 这种锁经常用来控制多个线程同时启动，并且能够及时感知这些线程是否全部运行结束。举例如下：
例子中m_begin用来为那10个线程发送启动指令，当m_begin.countDown()时，10个线程同时启动。同时m_end.await()这些线程结束的好消息。
/**
* @filename CountDownLaunchTest.java
* @date 2014-11-14
*/
package lock;
import java.util.concurrent.CountDownLatch;
public class CountDownLatchTest
{
public static void main(String[] args)
{
new CountDownLatchTest(10).runAll();
}
private int m_threadNum = 10;
private CountDownLatch m_begin = null;
private CountDownLatch m_end = null;
CountDownLatchTest(int threadNum)
{
if(threadNum>0)
m_threadNum = threadNum;
m_begin = new CountDownLatch(1);
m_end = new CountDownLatch(m_threadNum);
}
public void runAll()
{
for(int i=0; i<m_threadNum; i++)
{
new InnerThread(String.valueOf(i)).start();
}
m_begin.countDown();
try
{
m_end.await();
} catch (InterruptedException e)
{
e.printStackTrace();
}
System.out.println("all thread finished");
}
private class InnerThread extends Thread
{
private final String name;
InnerThread(String threadName)
{
name = threadName;
System.out.println(name+" inited");
}
@Override
public void run()
{
try
{
//!!!!!注意是await,不是wait
m_begin.await();
Thread.currentThread().sleep(3000);
} catch (InterruptedException e)
{
e.printStackTrace();
}
m_end.countDown();
System.out.println(name+" finished");
}
}
}
2.FutureTask 这种锁可以获取一个线程任务的运行结果，也就是说我们有个任务需要启动一个线程进行处理，同时也需要得到这个线程的返回结果时，用这个锁比较好用。
/**
* @filename FutureTaskTest.java
* @date 2014-11-14
*/
package lock;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;
public class FutureTaskTest
{
public static void main(String[] args)
{
new FutureTaskTest().test();
}
public void test()
{
FutureTask<Integer> futureTask = new FutureTask<Integer>(new InnerRunnable());
futureTask.run();
try
{
int result = futureTask.get();
System.out.println(result);
} catch (InterruptedException e)
{
e.printStackTrace();
} catch (ExecutionException e)
{
//所有异常（除InterruptedException）均会封装成ExecutionException异常而抛出
e.printStackTrace();
}
}
private class InnerRunnable implements Callable<Integer>
{
public Integer call() throws Exception
{
try
{
Thread.sleep(3000);
// throw new IllegalStateException();
} catch (InterruptedException e)
{
e.printStackTrace();
return -1;
}
return 0;
}
}
}
3.
Semaphore,信号量，其经常用在线程池数量的控制或者队列大小的控制上，根据预先设定好的数值，然后有线程来acquire()和release(),当申请的次数大于预设值时将阻塞，直到其他线程释放资源。
/**
* @filename SemaphoreTest.java
* @date 2014-11-14
*/
package lock;
import java.util.concurrent.ConcurrentSkipListSet;
import java.util.concurrent.Semaphore;
public class SemaphoreTest
{
public static void main(String[] args)
{
SemaphoreTest test = new SemaphoreTest();
test.init(10);
test.releaseNumToPool(1);
test.getNumFromPool();
test.releaseNumToPool(11);
}
//通常用在连接池里面，用来限制申请连接的数目。
private Semaphore sem = new Semaphore(0);
private final ConcurrentSkipListSet<Integer> m_numPool = new ConcurrentSkipListSet<Integer>();
public void init(int size)
{
for(int i=1; i<=size; i++)
releaseNumToPool(i);
}
public Integer getNumFromPool()
{
try
{
sem.acquire();
} catch (InterruptedException e)
{
e.printStackTrace();
};
if(m_numPool.isEmpty())
return 0;
return m_numPool.first();
}
public void releaseNumToPool(Integer num)
{
if(!m_numPool.contains(num))
{
m_numPool.add(num);
sem.release();
}
}
}
4.
CyclicBarrier, 与CountDownLack和FutureTask不通的是，这种锁可以循环的使用，其用来规范线程任务的运行后行为，也就是当设置此锁有，多个线程运行完后，均会等在此锁上，当最后一个线程运行到此锁时，大家才继续运行。这个锁目前我能想到应用场景的地方不多，或许可以用在某些需要拼装工作同时拼装的场景里面吧。
/**
* @filename CyclicBarrierTest.java
* @date 2014-11-14
*/
package lock;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
public class CyclicBarrierTest
{
public static void main(String[] args)
{
CyclicBarrierTest test = new CyclicBarrierTest(10);
for(int i=0; i<5; i++)
test.startOnce();
}
private CyclicBarrier m_barrier;
private final Object lock = new Object();
public CyclicBarrierTest(int barrierSize)
{
m_barrier = new CyclicBarrier(barrierSize, new Runnable()
{
public void run()
{
synchronized (lock)
{
lock.notify();
}
System.out.println("all thread run completed");
}
});
}
public void startOnce()
{
for(int i=1,size=m_barrier.getParties(); i<=size; i++)
{
InnerThread thread = new InnerThread(i*1000, String.valueOf(i));
thread.start();
}
synchronized (lock)
{
try
{
lock.wait();
} catch (InterruptedException e)
{
e.printStackTrace();
}
}
}
private class InnerThread extends Thread
{
private long m_waitTime = 0;
private String m_name;
public InnerThread(long waitTime, String threadName)
{
if(waitTime>0)
m_waitTime = waitTime;
m_name = threadName;
}
@Override
public void run()
{
try
{
Thread.sleep(m_waitTime);
System.out.println(m_name+" waited "+m_waitTime);
m_barrier.await();
} catch (InterruptedException e)
{
e.printStackTrace();
} catch (BrokenBarrierException e)
{
e.printStackTrace();
}
System.out.println(m_name+" completed");
}
}
}
5.
Exchanger,其与SynchronizeQueue有相似之处，SynchronizeQueue是单向的，Exchanger是双向的。也就是说当两个线程运行到Exchanger时，双方均会将自己的数据交换给对方。目前我也没有想到很好的应用。
/**
* @filename ExchangerTest.java
* @date 2014-11-14
*/
package lock;
import java.util.concurrent.Exchanger;
public class ExchangerTest
{
public static void main(String[] args)
{
new ExchangerTest().start();
}
private Exchanger<String> exchanger = new Exchanger<String>();
public void start()
{
new cargoThread().start();
new moneyThread().start();
}
private class cargoThread extends Thread
{
String message = "cargo";
public void run()
{
System.out.println("cargo thread is producing cargo");
try
{
Thread.sleep(5000);
} catch (InterruptedException e1)
{
e1.printStackTrace();
}
System.out.println("cargo thread begin to wait exchange");
try
{
message = exchanger.exchange(message);
} catch (InterruptedException e)
{
e.printStackTrace();
}
System.out.println("cargo thread exchange message "+message);
}
}
private class moneyThread extends Thread
{
String message = "money";
public void run()
{
System.out.println("money thread begin to wait exchange");
try
{
message = exchanger.exchange(message);
} catch (InterruptedException e)
{
e.printStackTrace();
}
System.out.println("money thread exchange message "+message);
}
}
}