读写锁：核心思想实现读写分离，高并发下特别适合 读多写少的场景。

synchronized关键字和ReentrantLock 同一时间只能有一个线程进行访问被锁定的代码，读写锁的机制则不是，本质上分为两把锁，读锁和写锁，在读锁情况下，多个线程可以并发访问资源，只有当是写锁时只能一个一个的顺序执行。

口诀：读读共享，写写互斥，读写互斥。

private ReentrantReadWriteLock rwLock = new ReentrantReadWriteLock(); 
private ReentrantReadWriteLock.ReadLock readLock = rwLock.readLock(); 
private ReentrantReadWriteLock.WriteLock writeLock = rwLock.writeLock(); 
public void read() { 
    try { 
      readLock.lock(); 
      System.out.println("当前线程:" + Thread.currentThread().getName() + "进入..."); 
      Thread.sleep(3000); 
      System.out.println("当前线程:" + Thread.currentThread().getName() + "退出..."); 
    } catch (Exception e) { 
      e.printStackTrace(); 
    } finally { 
      readLock.unlock(); 
    } 
} 
public void write() { 
     try { 
        writeLock.lock(); 
        System.out.println("当前线程:" + Thread.currentThread().getName() + "进入..."); 
        Thread.sleep(3000); 
        System.out.println("当前线程:" + Thread.currentThread().getName() + "退出..."); 
     } catch (Exception e) { 
        e.printStackTrace();
     } finally {
       writeLock.unlock();
     } 
}