ReentrantLock，Condition，ReentrantReadWriteLock.ReadLock，ReentrantReadWriteLock.WriterLock简介

ReentrantLock重入锁

    重入锁，在需要同步的代码部分进行锁定，使用完毕之后一定要释放锁。

实例方式：

ReentrantLock lock = new ReentrantLock(true) ;

后面有个 Boolean 类型的 fair 参数，是标识该锁是否公平的，默认 不公平，公平的意思就是 锁的给与基于调用顺序，先调用的，先获取，不公平则是遵循cpu调用规则，默认不公平效率更高。

举个栗子：

public class Main3 implements Callable<String> { private ReentrantLock lock = new ReentrantLock(true) ; private int i =10; @Override public String call() throws Exception { lock.lock(); System.out.println( Thread.currentThread().getName() + "==>" + --i ); lock.unlock(); return "" ; } public static void main(String[] args) throws Exception { ExecutorService pool = Executors.newFixedThreadPool(10); Main3 task = new Main3(); for (int i = 0; i < 10; i++) { pool.submit( task ); } pool.shutdown(); } }  

Condition，类似wait() 和 notify()

实例方式：

private ReentrantLock lock = new ReentrantLock(true) ; private Condition condition = lock.newCondition();

举个栗子：

import java.util.concurrent.locks.Condition; import java.util.concurrent.locks.ReentrantLock; public class Main4 { private ReentrantLock lock = new ReentrantLock(); private Condition condition = lock.newCondition(); public void method1(){ try { lock.lock(); System.out.println("当前线程：" + Thread.currentThread().getName() + "进入等待状态.."); Thread.sleep(3000); System.out.println("当前线程：" + Thread.currentThread().getName() + "释放锁.."); condition.await(); // Object wait System.out.println("当前线程：" + Thread.currentThread().getName() +"继续执行..."); } catch (Exception e) { e.printStackTrace(); } finally { lock.unlock(); } } public void method2(){ try { lock.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() + "发出唤醒.."); condition.signal(); //Object notify } catch (Exception e) { e.printStackTrace(); } finally { System.out.println("当前线程：" + Thread.currentThread().getName() + "执行完毕"); lock.unlock(); } } public static void main(String[] args) { final Main4 main4 = new Main4(); new Thread(new Runnable() { @Override public void run() { main4.method1(); } } , "t1").start(); new Thread(new Runnable() { @Override public void run() { main4.method2(); } } , "t2").start(); } }

执行结果:

当前线程：t1进入等待状态.. 当前线程：t1释放锁.. 当前线程：t2进入.. 当前线程：t2发出唤醒.. 当前线程：t2执行完毕 当前线程：t1继续执行...

分析：

t1线程先开始执行，获得重入锁，执行method1，输出第一句，然后休眠之后输出第二句，紧接着释放锁，然后 t2获得锁，执行method2，输出第三句，然后第二唤醒 其他的线程，此操作和 notify 一样不释放锁，所以 t2执行完毕之后，t1 才继续执行

 

Condition在一个程序里面可以获取多个，可以有多个 Condition

例如：

import java.util.concurrent.locks.Condition; import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock; public class UseManyCondition { private ReentrantLock lock = new ReentrantLock(); private Condition c1 = lock.newCondition(); private Condition c2 = lock.newCondition(); public void m1(){ try { lock.lock(); System.out.println("当前线程：" +Thread.currentThread().getName() + "进入方法m1等待.."); c1.await(); System.out.println("当前线程：" +Thread.currentThread().getName() + "方法m1继续.."); } catch (Exception e) { e.printStackTrace(); } finally { lock.unlock(); } } public void m2(){ try { lock.lock(); System.out.println("当前线程：" +Thread.currentThread().getName() + "进入方法m2等待.."); c1.await(); System.out.println("当前线程：" +Thread.currentThread().getName() + "方法m2继续.."); } catch (Exception e) { e.printStackTrace(); } finally { lock.unlock(); } } public void m3(){ try { lock.lock(); System.out.println("当前线程：" +Thread.currentThread().getName() + "进入方法m3等待.."); c2.await(); System.out.println("当前线程：" +Thread.currentThread().getName() + "方法m3继续.."); } catch (Exception e) { e.printStackTrace(); } finally { lock.unlock(); } } public void m4(){ try { lock.lock(); System.out.println("当前线程：" +Thread.currentThread().getName() + "唤醒.."); c1.signalAll(); } catch (Exception e) { e.printStackTrace(); } finally { lock.unlock(); } } public void m5(){ try { lock.lock(); System.out.println("当前线程：" +Thread.currentThread().getName() + "唤醒.."); c2.signal(); } catch (Exception e) { e.printStackTrace(); } finally { lock.unlock(); } } public static void main(String[] args) { final UseManyCondition umc = new UseManyCondition(); Thread t1 = new Thread(new Runnable() { @Override public void run() { umc.m1(); } },"t1"); Thread t2 = new Thread(new Runnable() { @Override public void run() { umc.m2(); } },"t2"); Thread t3 = new Thread(new Runnable() { @Override public void run() { umc.m3(); } },"t3"); Thread t4 = new Thread(new Runnable() { @Override public void run() { umc.m4(); } },"t4"); Thread t5 = new Thread(new Runnable() { @Override public void run() { umc.m5(); } },"t5"); t1.start(); // c1 t2.start(); // c1 t3.start(); // c2 try { Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } t4.start(); // c1 try { Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } t5.start(); // c2 } }

 

ReentrantReadWriteLock.ReadLock，ReentrantReadWriteLock.WriterLock读写锁

读写锁ReentrantReadWriteLock,其核心就是实现读写分离的锁。在高并发访问下，尤其是读多写少的情况下，性能要远高于重入锁。

之前学synchronized、ReentrantLock时，我们知道，同一时间内，只能有-个线程进行访问被锁定的代码，那么读写锁则不同，其本质是分成两个锁，即读锁、写锁。在读锁下，多个线程可以并发的进行访问，但是在写锁的时候，只能一个一个的顺序访问。

口诀:读读共享，写写互际，读写互斥。 实例方式:

private ReentrantReadWriteLock rwLock = new ReentrantReadWriteLock(); private ReadLock readLock = rwLock.readLock(); private WriteLock writeLock = rwLock.writeLock();

实例：使用读锁

public class Main4 { private ReentrantLock lock = new ReentrantLock(); private Condition condition = lock.newCondition(); private ReentrantReadWriteLock readWriteLock = new ReentrantReadWriteLock(); private ReentrantReadWriteLock.ReadLock readLock = readWriteLock.readLock(); public void read(){ readLock.lock(); try{ 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){ }finally { readLock.unlock(); } } public static void main(String[] args) { final Main4 main4 = new Main4(); new Thread(new Runnable() { @Override public void run() { main4.read(); } } , "t1").start(); new Thread(new Runnable() { @Override public void run() { main4.read(); } } , "t2").start(); new Thread(new Runnable() { @Override public void run() { main4.read(); } } , "t3").start(); } }

执行结果：

当前线程:t1进入... 当前线程:t2进入... 当前线程:t3进入... 当前线程:t1退出... 当前线程:t2退出... 当前线程:t3退出...

可以看到，读锁，其实是并发执行的，同时进入同时退出，若果换成 重入锁则就是 进入出来，进入出来这样了

实例：使用写锁

代码跟上面一个，只是把 read 里面的 readLock 改为 writeLock,效果和 重入锁一样，所以读锁适用于 写多读少操作