Java concurrency Programming (15) Concurrent Lock read/write Lock (cont.) Write lock downgrade

1.1.1. three write lock downgrade for read-write lock application

Reentrantreadwritelock also features write-lock demotion, which has some relationship to reentrant sex.

(1) you can downgrade to read lock when holding a write lock.

(2) You cannot upgrade to a write lock while holding a read lock.

Reentrantreadwritelock and reentrantlock are similar in that they have a feature that can be re-entered. Reentrant means that the thread that has acquired the lock can acquire the lock again, ensuring that the lock and unlock are the same number of times.

 Packagecom.test.concurrence;ImportJava.util.Random;ImportJava.util.concurrent.ExecutorService;Importjava.util.concurrent.Executors;ImportJava.util.concurrent.locks.ReadWriteLock;ImportJava.util.concurrent.locks.ReentrantReadWriteLock; classMySharedResource3 {//Constructor: Fair lock. PrivateReadwritelock lock =NewReentrantreadwritelock (true);  Public voidReadintTaskID) {Lock.readlock (). Lock ();Try{ for(inti=0;i<10;i++) {x--; System.out.println (TaskID+ "," + i + ": Read called:x:" + x + "," +Thread.CurrentThread (). GetName ()); Thread.Sleep (100);}} Catch(interruptedexception e) {e.printstacktrace ();}finally{lock.readlock (). Unlock ();}}  Public voidDowngrade (intTaskID) {//Hold write lockLock.writelock (). Lock (); System.out.println (TaskID+ "Writelock locked."); Try{     for(inti=0;i<10;i++) {x++; System.out.println (TaskID+ "," + i + ": Write called:x:" + x + "," +Thread.CurrentThread (). GetName ()); Thread.Sleep (100);} //continue to hold the read lock in the case of a write lock already held. Lock.readlock (). Lock (); System.out.println (TaskID+ "Writelock-readlock locked."); //after the write lock is released, it is downgraded to read lock. lock.writelock (). Unlock (); System.out.println (TaskID+ "Writelock unlocked.");  for(inti=0;i<10;i++) {x++; System.out.println (TaskID+ "," + i + ": Read in Write:x:" + x + "," +Thread.CurrentThread (). GetName ()); Thread.Sleep (1000);} //release the read lock. lock.readlock (). Unlock (); System.out.println (TaskID+ "Readlock locked."); } Catch(interruptedexception e) {e.printstacktrace ();}finally{//Release the lock. Lock.readlock (). Lock (); Lock.writelock (). Unlock ();}} Private   intx = 0;}  Public classReadWriteLockTest3 { Public Static voidMain (string[] args) {FinalMySharedResource3 sr =NewMySharedResource3 (); Executorservice Executorservice=Executors.newcachedthreadpool ();inti; for(i=0;i<10;i++) {Final intTaskID =I;executorservice.execute (NewRunnable () {@Override Public voidrun () {System.out.println ("TaskID:" +taskid); while(true){if(0 = = (taskid% 2) {sr.downgrade (taskid);}Else{sr.read (TaskID);}}});} Executorservice.shutdown ();}} 

observing the results of the operation, you can see that in the downgrade () function, when a thread executes a block of code after the write lock has been downgraded to a read lock, the other thread can obtain a read lock.

One possible result of the operation is as follows:

taskid:0

Taskid:4

Taskid:3

Taskid:1

Taskid:2

0 Writelock locked.

Taskid:5

0,0:write Called:x:1, Pool-1-thread-1

Taskid:6

Taskid:9

Taskid:8

Taskid:7

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

0 writelock, Readlock locked.

0 Writelock Unlocked.

0,0:read in Write:x:11, pool-1-thread-1

3,0:read Called:x:10, Pool-1-thread-4

3,1:read Called:x:9, Pool-1-thread-4

3,2:read Called:x:8, Pool-1-thread-4

3,3:read Called:x:7, Pool-1-thread-4

3,4:read Called:x:6, Pool-1-thread-4

3,5:read Called:x:5, Pool-1-thread-4

3,6:read Called:x:4, Pool-1-thread-4

3,7:read Called:x:3, Pool-1-thread-4

3,8:read Called:x:2, Pool-1-thread-4

3,9:read Called:x:1, Pool-1-thread-4

0,1:read in Write:x:2, pool-1-thread-1

0,2:read in Write:x:3, pool-1-thread-1

0,3:read in Write:x:4, pool-1-thread-1

0,4:read in Write:x:5, pool-1-thread-1

0,5:read in Write:x:6, pool-1-thread-1

0,6:read in Write:x:7, pool-1-thread-1

0,7:read in Write:x:8, pool-1-thread-1

0,8:read in Write:x:9, pool-1-thread-1

0,9:read in Write:x:10, pool-1-thread-1

0 Readlock locked.

4 Writelock locked.

4,0:write Called:x:11, pool-1-thread-5

4,1:write Called:x:12, pool-1-thread-5

4,2:write Called:x:13, pool-1-thread-5

4,3:write called:x:14, pool-1-thread-5

4,4:write called:x:15, pool-1-thread-5

4,5:write Called:x:16, pool-1-thread-5

4,6:write Called:x:17, pool-1-thread-5

4,7:write called:x:18, pool-1-thread-5

4,8:write called:x:19, pool-1-thread-5

4,9:write called:x:20, pool-1-thread-5

4 Writelock, Readlock locked.

4 Writelock Unlocked.

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

4,0:read in Write:x:20, pool-1-thread-5

1,1:read called:x:19, Pool-1-thread-2

1,2:read called:x:18, Pool-1-thread-2

1,3:read Called:x:17, Pool-1-thread-2

1,4:read Called:x:16, Pool-1-thread-2

1,5:read called:x:15, Pool-1-thread-2

1,6:read called:x:14, Pool-1-thread-2

1,7:read Called:x:13, Pool-1-thread-2

1,8:read Called:x:12, Pool-1-thread-2

1,9:read Called:x:11, Pool-1-thread-2

4,1:read in Write:x:12, pool-1-thread-5

4,2:read in Write:x:13, pool-1-thread-5

4,3:read in Write:x:14, pool-1-thread-5

4,4:read in Write:x:15, pool-1-thread-5

4,5:read in Write:x:16, pool-1-thread-5

4,6:read in Write:x:17, pool-1-thread-5

4,7:read in Write:x:18, pool-1-thread-5

4,8:read in Write:x:19, pool-1-thread-5

4,9:read in Write:x:20, pool-1-thread-5

4 Readlock locked.

2 writelock locked.

2,0:write called:x:21, pool-1-thread-3

2,1:write called:x:22, pool-1-thread-3

2,2:write called:x:23, pool-1-thread-3

2,3:write called:x:24, pool-1-thread-3

2,4:write called:x:25, pool-1-thread-3

2,5:write called:x:26, pool-1-thread-3

2,6:write called:x:27, pool-1-thread-3

2,7:write called:x:28, pool-1-thread-3

2,8:write called:x:29, pool-1-thread-3

2,9:write called:x:30, pool-1-thread-3

2 Writelock, Readlock locked.

2 Writelock Unlocked.

5,0:read called:x:29, pool-1-thread-6

2,0:read in Write:x:30, pool-1-thread-3

5,1:read called:x:29, pool-1-thread-6

5,2:read called:x:28, pool-1-thread-6

5,3:read called:x:27, pool-1-thread-6

5,4:read called:x:26, pool-1-thread-6

5,5:read called:x:25, pool-1-thread-6

5,6:read called:x:24, pool-1-thread-6

5,7:read called:x:23, pool-1-thread-6

5,8:read called:x:22, pool-1-thread-6

5,9:read called:x:21, pool-1-thread-6

2,1:read in Write:x:22, pool-1-thread-3

2,2:read in Write:x:23, pool-1-thread-3

2,3:read in Write:x:24, pool-1-thread-3

2,4:read in Write:x:25, pool-1-thread-3

2,5:read in Write:x:26, pool-1-thread-3

2,6:read in Write:x:27, pool-1-thread-3

2,7:read in Write:x:28, pool-1-thread-3

2,8:read in write:x:29, pool-1-thread-3

2,9:read in Write:x:30, pool-1-thread-3

2 readlock locked.

6 Writelock locked.

6,0:write called:x:31, pool-1-thread-7

6,1:write called:x:32, pool-1-thread-7

6,2:write called:x:33, pool-1-thread-7

6,3:write called:x:34, pool-1-thread-7

6,4:write called:x:35, pool-1-thread-7

6,5:write called:x:36, pool-1-thread-7

6,6:write called:x:37, pool-1-thread-7

6,7:write called:x:38, pool-1-thread-7

6,8:write called:x:39, pool-1-thread-7

6,9:write called:x:40, pool-1-thread-7

6 Writelock, Readlock locked.

6 Writelock Unlocked.

6,0:read in write:x:41, pool-1-thread-7

9,0:read called:x:40, pool-1-thread-10

9,1:read called:x:39, pool-1-thread-10

9,2:read called:x:38, pool-1-thread-10

9,3:read called:x:37, pool-1-thread-10

9,4:read called:x:36, pool-1-thread-10

9,5:read called:x:35, pool-1-thread-10

9,6:read called:x:34, pool-1-thread-10

9,7:read called:x:33, pool-1-thread-10

9,8:read called:x:32, pool-1-thread-10

9,9:read called:x:31, pool-1-thread-10

6,1:read in Write:x:32, pool-1-thread-7

6,2:read in Write:x:33, pool-1-thread-7

6,3:read in Write:x:34, pool-1-thread-7

6,4:read in Write:x:35, pool-1-thread-7

6,5:read in write:x:36, pool-1-thread-7

The API Doc is about reentrant and degraded.

Reentrancy

This lock allows both readers and writers to reacquire read or write locks in the style of a reentrantlock. Non-reentrant readers is not allowed until all write locks held by the writing thread has been released.

Additionally, a writer can acquire the read lock, but not vice-versa. Among other applications, reentrancy can is useful when write locks is held during calls or callbacks to methods that per Form reads under read locks. If a reader tries to acquire the write lock it'll never succeed.

Chinese

Re-entry Accessibility

this lock (Reentrantreadwritelock) allow read threads and write threads to follow Reentrantlock the model to get again to read or write the lock. Non-reentrant reads are not allowed until all write locks that have been held by the write thread are freed.

In addition, the write thread is able to acquire a read lock, but not in turn. In other applications, when a read lock is required in a call or callback method, it is useful to have a write lock in case it is already held. If a read thread attempts to acquire a write lock, it will not succeed.

Lock downgrading

Reentrancy also allows downgrading from the write lock to a read lock, by acquiring the write lock, then the read lock and Then releasing the write lock. However, upgrading from a read lock to the write lock are not possible.

Chinese

Lock downgrade

By acquiring a write lock, acquiring a read lock, and then releasing the write lock, Reentrant also allows the downgrade from a write lock to a read lock. However, it is not possible to upgrade from a read lock to a write lock.

Java concurrency Programming (15) Concurrent Lock read/write Lock (cont.) Write lock downgrade