Java并发程序设计(13)并发锁之读写锁

Posted 2020-09-06

1.1.1. 读写锁的应用之一读写分离

 

读写锁ReentrantReadWriteLock相对于ReentrantLock在特定情况下能提高同步性能，这是因为读写锁有以下特点：

(1)读和读可以同时进行。 这一点是ReentrantLock所没有的优点。

(2)读和写不能同时进行。

(3)写和写不能同时进行。

 

应用读写锁访问资源的代码如下所示。

class  MySharedResource {

 

private ReadWriteLock  lock = new ReentrantReadWriteLock();

 

public void  read(int taskid){

lock.readLock().lock();

try{

for(int i=0;i<10;i++){

x--;

System.out.println(taskid + "," + i + ": read called: x:" + x + " , "+ Thread.currentThread().getName());

Thread.sleep(1000);

}

} catch (InterruptedException e) {

e.printStackTrace();

}

finally{

lock.readLock().unlock();

}

}

 

 

public void  write(int taskid){

lock.writeLock().lock();

try{

for(int i=0;i<10;i++){

x++;

System.out.println(taskid + "," + i  + ": write called: x:" + x + " , "+ Thread.currentThread().getName());

Thread.sleep(1000);

}

} catch (InterruptedException e) {

e.printStackTrace();

}

finally{

lock.writeLock().unlock();

}

}

 

 

private   int  x = 0;

}

 

创建了一个线程池来提交5个读任务和5个写任务。

final MySharedResource  sr = new MySharedResource();

ExecutorService  executorService =  Executors.newCachedThreadPool();

int i;

 

for( i=0;i<10;i++) {

final int taskid = i;

executorService.execute(new Runnable(){

@Override

public void run() {

System.out.println("taskid:" + taskid);

if( 0 == (taskid % 2)){

sr.write(taskid);

}

else {

sr.read(taskid);

}

}

});

}

executorService.shutdown();

 

 

一个可能的测试结果如下：

taskid:0

taskid:2

taskid:1

taskid:3

taskid:4

taskid:5

0,0: write called: x:1 , pool-1-thread-1

taskid:6

taskid:8

taskid:7

taskid:9

0,1: write called: x:2 , pool-1-thread-1

0,2: write called: x:3 , pool-1-thread-1

0,3: write called: x:4 , pool-1-thread-1

0,4: write called: x:5 , pool-1-thread-1

0,5: write called: x:6 , pool-1-thread-1

0,6: write called: x:7 , pool-1-thread-1

0,7: write called: x:8 , pool-1-thread-1

0,8: write called: x:9 , pool-1-thread-1

0,9: write called: x:10 , pool-1-thread-1

2,0: write called: x:11 , pool-1-thread-3

2,1: write called: x:12 , pool-1-thread-3

2,2: write called: x:13 , pool-1-thread-3

2,3: write called: x:14 , pool-1-thread-3

2,4: write called: x:15 , pool-1-thread-3

2,5: write called: x:16 , pool-1-thread-3

2,6: write called: x:17 , pool-1-thread-3

2,7: write called: x:18 , pool-1-thread-3

2,8: write called: x:19 , pool-1-thread-3

2,9: write called: x:20 , pool-1-thread-3

1,0: read called: x:19 , pool-1-thread-2

3,0: read called: x:18 , pool-1-thread-4

3,1: read called: x:16 , pool-1-thread-4

1,1: read called: x:16 , pool-1-thread-2

1,2: read called: x:15 , pool-1-thread-2

3,2: read called: x:14 , pool-1-thread-4

1,3: read called: x:12 , pool-1-thread-2

3,3: read called: x:12 , pool-1-thread-4

1,4: read called: x:11 , pool-1-thread-2

3,4: read called: x:10 , pool-1-thread-4

1,5: read called: x:9 , pool-1-thread-2

3,5: read called: x:9 , pool-1-thread-4

3,6: read called: x:8 , pool-1-thread-4

1,6: read called: x:7 , pool-1-thread-2

3,7: read called: x:6 , pool-1-thread-4

1,7: read called: x:6 , pool-1-thread-2

1,8: read called: x:5 , pool-1-thread-2

3,8: read called: x:4 , pool-1-thread-4

1,9: read called: x:3 , pool-1-thread-2

3,9: read called: x:3 , pool-1-thread-4

4,0: write called: x:4 , pool-1-thread-5

4,1: write called: x:5 , pool-1-thread-5

4,2: write called: x:6 , pool-1-thread-5

4,3: write called: x:7 , pool-1-thread-5

4,4: write called: x:8 , pool-1-thread-5

4,5: write called: x:9 , pool-1-thread-5

4,6: write called: x:10 , pool-1-thread-5

4,7: write called: x:11 , pool-1-thread-5

4,8: write called: x:12 , pool-1-thread-5

4,9: write called: x:13 , pool-1-thread-5

5,0: read called: x:12 , pool-1-thread-6

5,1: read called: x:11 , pool-1-thread-6

5,2: read called: x:10 , pool-1-thread-6

5,3: read called: x:9 , pool-1-thread-6

5,4: read called: x:8 , pool-1-thread-6

5,5: read called: x:7 , pool-1-thread-6

5,6: read called: x:6 , pool-1-thread-6

5,7: read called: x:5 , pool-1-thread-6

5,8: read called: x:4 , pool-1-thread-6

5,9: read called: x:3 , pool-1-thread-6

6,0: write called: x:4 , pool-1-thread-7

6,1: write called: x:5 , pool-1-thread-7

6,2: write called: x:6 , pool-1-thread-7

6,3: write called: x:7 , pool-1-thread-7

6,4: write called: x:8 , pool-1-thread-7

6,5: write called: x:9 , pool-1-thread-7

6,6: write called: x:10 , pool-1-thread-7

6,7: write called: x:11 , pool-1-thread-7

6,8: write called: x:12 , pool-1-thread-7

6,9: write called: x:13 , pool-1-thread-7

8,0: write called: x:14 , pool-1-thread-9

8,1: write called: x:15 , pool-1-thread-9

8,2: write called: x:16 , pool-1-thread-9

8,3: write called: x:17 , pool-1-thread-9

8,4: write called: x:18 , pool-1-thread-9

8,5: write called: x:19 , pool-1-thread-9

8,6: write called: x:20 , pool-1-thread-9

8,7: write called: x:21 , pool-1-thread-9

8,8: write called: x:22 , pool-1-thread-9

8,9: write called: x:23 , pool-1-thread-9

7,0: read called: x:22 , pool-1-thread-8

9,0: read called: x:21 , pool-1-thread-10

7,1: read called: x:20 , pool-1-thread-8

9,1: read called: x:20 , pool-1-thread-10

9,2: read called: x:19 , pool-1-thread-10

7,2: read called: x:19 , pool-1-thread-8

9,3: read called: x:17 , pool-1-thread-10

7,3: read called: x:17 , pool-1-thread-8

7,4: read called: x:16 , pool-1-thread-8

9,4: read called: x:16 , pool-1-thread-10

7,5: read called: x:14 , pool-1-thread-8

9,5: read called: x:14 , pool-1-thread-10

9,6: read called: x:13 , pool-1-thread-10

7,6: read called: x:12 , pool-1-thread-8

7,7: read called: x:10 , pool-1-thread-8

9,7: read called: x:10 , pool-1-thread-10

7,8: read called: x:9 , pool-1-thread-8

9,8: read called: x:9 , pool-1-thread-10

7,9: read called: x:7 , pool-1-thread-8

9,9: read called: x:7 , pool-1-thread-10

 

可以看到，当一个线程在持有写锁的时候，没有其他线程持有读锁，而存在线程8和线程10同时成功持有读锁。