架构师养成记--14.重入锁ReentrantLock 和 读写锁 ReentrantReadWriteLock

ReentrantLock

有嗅探锁定和多路分支等功能，其实就是synchronized，wait，notify的升级。

this锁定当前对象不方便，于是就有了用new Object()来作为锁的解决方案，后面jdk干脆就提供了一个Lock类。

伪代码：

Lock lock = new ReentrantLock();//新建一个lock

Condition condition = lock.newCondition();//获取条件

method1(){

　　try{

　　　　lock.lock();

　　　　代码块；

　　　　lock.unlock();

　　　　后续代码块；

　　}

}

method2(){

　　try{

　　　　lock.lock();

　　　　代码块；

　　　　lock.signal();

　　}

}

单condition操作：

 import java.util.concurrent.locks.Condition;
 import java.util.concurrent.locks.Lock;
 import java.util.concurrent.locks.ReentrantLock;

 public class UseCondition {

     private Lock 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..");
             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 {
             lock.unlock();
         }
     }

     public static void main(String[] args) {

         final UseCondition uc = new UseCondition();
         Thread t1 = new Thread(new Runnable() {
             @Override
             public void run() {
                 uc.method1();
             }
         }, "t1");
         Thread t2 = new Thread(new Runnable() {
             @Override
             public void run() {
                 uc.method2();
             }
         }, "t2");
         t1.start();

         t2.start();
     }

 }

执行结果：

解释：线程1调用method1方法，线程2调用同一对象的method2方法，线程1先启动，线程1获取锁，进入method1的代码块，线程2也启动了，单是线程2被锁定，直到线程1发出lock.unlock()了，线程1才将锁释放，这时线程2获取锁，执行method2的代码块，线程2发出lock.signal(); 线程1才继续执行后续代码块。

多condition操作：

 import java.util.concurrent.locks.Condition;
 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

     }

 }

执行结果：

解释：t1、t2线程都是用第一个condition c1，t3线程用第二个condition c2，t4线程发出c1.signalAll() 唤醒t1和t2线程，t5发出c2.signal()唤醒t3线程。

ReentrantReadWriteLock

读写分离锁，读读共享、写写互斥、读写互斥。

 import java.util.concurrent.locks.ReentrantReadWriteLock;
 import java.util.concurrent.locks.ReentrantReadWriteLock.ReadLock;
 import java.util.concurrent.locks.ReentrantReadWriteLock.WriteLock;

 public class UseReentrantReadWriteLock {

     private ReentrantReadWriteLock rwLock = new ReentrantReadWriteLock();
     private ReadLock readLock = rwLock.readLock();
     private 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();
         }
     }

     public static void main(String[] args) {

         final UseReentrantReadWriteLock urrw = new UseReentrantReadWriteLock();

         Thread t1 = new Thread(new Runnable() {
             @Override
             public void run() {
                 urrw.read();
             }
         }, "t1");
         Thread t2 = new Thread(new Runnable() {
             @Override
             public void run() {
                 urrw.read();
             }
         }, "t2");
         Thread t3 = new Thread(new Runnable() {
             @Override
             public void run() {
                 urrw.write();
             }
         }, "t3");
         Thread t4 = new Thread(new Runnable() {
             @Override
             public void run() {
                 urrw.write();
             }
         }, "t4");        

         t1.start();
         t2.start();

 //        t1.start(); // R
 //        t3.start(); // W

 //        t3.start();
 //        t4.start();
     }
 }

解释：t1和t2都是读，是共享的，可同时获得readLock同时进入read方法执行。t1是读和t3是写，是互斥的，只有t1执行完t3才能执行。t3、t4都是写，是互斥的，只有等t3执行完t4才能继续。