读锁与写锁

记住这3条：“读锁-读锁可以共存，读锁-写锁不可共存，写锁-写锁不可共存”。

用多线程来展示读锁和写锁的使用

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public class ReadWriteLockDemo {       private static volatile Map<Integer, Object> map = new ConcurrentHashMap<>();     private static ReentrantReadWriteLock lock = new ReentrantReadWriteLock();       public void get(Integer key) {         Lock readLock = null;         try {             readLock = lock.readLock();             readLock.lock();             System.out.println(Thread.currentThread().getName() + "\t 开始读取key:" + key);             Object value = map.get(key);             System.out.println(Thread.currentThread().getName() + "\t 结束读取key:" + key + ", value:" + value);         } catch (Exception e) {             e.printStackTrace();         } finally {             if (readLock != null) {                 readLock.unlock();             }         }     }       private void put(Integer key, Object value) {         Lock writeLock = null;         try {             writeLock = lock.writeLock();             writeLock.lock();             System.out.println(Thread.currentThread().getName() + "\t 开始写入key:" + key + ", value:" + value);             map.put(key, value);             System.out.println(Thread.currentThread().getName() + "\t 结束写入key:" + key + ", value:" + value);         } catch (Exception e) {             e.printStackTrace();         } finally {             writeLock.unlock();         }     }       public static void main(String[] args) throws InterruptedException {         System.out.println("------写锁不可与写锁共存-----");         for (int i = 0; i < 5; i++) {             final int finalI = i;             new Thread(() -> {                 ReadWriteLockDemo readWriteLockDemo = new ReadWriteLockDemo();                 readWriteLockDemo.put(finalI, finalI);             }, "A").start();         }           TimeUnit.SECONDS.sleep(3);           System.out.println("-----读锁与读锁可以共存-----");         for (int i = 0; i < 5; i++) {             final int finalI = i;             new Thread(() -> {                 ReadWriteLockDemo readWriteLockDemo = new ReadWriteLockDemo();                 readWriteLockDemo.get(finalI);             }, "B").start();         }     } }

控制台输出如下：

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------写锁不可与写锁共存----- A    开始写入key:0, value:0 A    结束写入key:0, value:0 A    开始写入key:3, value:3 A    结束写入key:3, value:3 A    开始写入key:4, value:4 A    结束写入key:4, value:4 A    开始写入key:1, value:1 A    结束写入key:1, value:1 A    开始写入key:2, value:2 A    结束写入key:2, value:2 -----读锁与读锁可以共存----- B    开始读取key:0 B    结束读取key:0, value:0 B    开始读取key:2 B    结束读取key:2, value:2 B    开始读取key:1 B    结束读取key:1, value:1