线程并发之Lock and Condition

Lock and Condition

 

Lock比传统线程模型中的synchronized方式更加面向对象，与生活中的锁类似，锁本身也应该是一个对象。两个线程执行的代码片段要实现同步互斥的效果，它们必须用同一个Lock对象。

l读写锁：分为读锁和写锁，多个读锁不互斥，读锁与写锁互斥，这是由jvm自己控制的，你只要上好相应的锁即可。如果你的代码只读数据，可以很多人同时读，但不能同时写，那就上读锁；如果你的代码修改数据，只能有一个人在写，且不能同时读取，那就上写锁。总之，读的时候上读锁，写的时候上写锁！

l在等待 Condition 时，允许发生“虚假唤醒”，这通常作为对基础平台语义的让步。对于大多数应用程序，这带来的实际影响很小，因为 Condition 应该总是在一个循环中被等待，并测试正被等待的状态声明。某个实现可以随意移除可能的虚假唤醒，但建议应用程序程序员总是假定这些虚假唤醒可能发生，因此总是在一个循环中等待。

l一个锁内部可以有多个Condition，即有多路等待和通知，可以参看jdk1.5提供的Lock与Condition实现的可阻塞队列的应用案例，从中除了要体味算法，还要体味面向对象的封装。在传统的线程机制中一个监视器对象上只能有一路等待和通知，要想实现多路等待和通知，必须嵌套使用多个同步监视器对象。（如果只用一个Condition，两个放的都在等，一旦一个放的进去了，那么它通知可能会导致另一个放接着往下走。）

 

 为什么要有读写锁：

提高读的效率，多个读可一起进行.. 读的时候不能进行写操作.,但读的时候可以有其他读操作.

 

-----------------锁的例子----------------------------------

package thread;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.locks.Lock;import java.util.concurrent.locks.ReentrantLock;public class LockTest {public static void main(String[] args) {final Business business = new Business();ExecutorService executor =  Executors.newFixedThreadPool(3);for(int i=0;i<3;i++){executor.execute(new Runnable(){public void run(){business.service();}});}executor.shutdown();}private static class Business{private int count;Lock lock = new ReentrantLock();public void service(){lock.lock();try {count++;try {Thread.sleep(1);} catch (InterruptedException e) {e.printStackTrace();}System.out.println(count);} catch (RuntimeException e) {e.printStackTrace();}finally{lock.unlock();}}} }

-----------------------读写锁的例子---------------------------

注意：刚开始用eclipse for jee自己的jdk，没有看到读锁可以并发的效果，后来换成sun的jdk，就看到了效果！

 

import java.util.concurrent.locks.ReentrantReadWriteLock;public class ReadWriteLockTest {public static void main(String[] args) {final Queue3 q3 = new Queue3();for(int i=0;i<3;i++){new Thread(){public void run(){while(true){q3.get();  }}}.start();}for(int i=0;i<3;i++){ new Thread(){public void run(){while(true){q3.put(new Random().nextInt(10000));}} }.start(); }}}class Queue3{private Object data = null;//共享数据，只能有一个线程能写该数据，但可以有多个线程同时读该数据。private ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();public void get(){rwl.readLock().lock();System.out.println(Thread.currentThread().getName() + " be ready to read data!");try {Thread.sleep((long)(Math.random()*1000));} catch (InterruptedException e) {e.printStackTrace();}System.out.println(Thread.currentThread().getName() + “have read data :“ + data);rwl.readLock().unlock();}public void put(Object data){rwl.writeLock().lock();System.out.println(Thread.currentThread().getName() + " be ready to write data!"); try {Thread.sleep((long)(Math.random()*1000));} catch (InterruptedException e) {e.printStackTrace();}this.data = data; System.out.println(Thread.currentThread().getName() + " have write data: “ + data);rwl.writeLock().unlock(); }}

------------------ Condition的例子1：实现两个线程交替执行-----------------------------

public class ConditionTest {public static void main(String[] args) {ExecutorService service = Executors.newSingleThreadExecutor();final Business2 business = new Business2();service.execute(new Runnable(){public void run() {for(int i=0;i<50;i++){business.sub();}}});for(int i=0;i<50;i++){business.main();}}}class Business2{Lock lock = new ReentrantLock();Condition condition = lock.newCondition();boolean bShouldSub = true;public void sub(){lock.lock();if(!bShouldSub)try {condition.await();} catch (InterruptedException e) {e.printStackTrace();}try{for(int i=0;i<10;i++){System.out.println(Thread.currentThread().getName() + " : " + i);}bShouldSub = false;condition.signal();}finally{lock.unlock();}}public void main(){lock.lock();if(bShouldSub)try {condition.await();} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();} try{for(int i=0;i<5;i++){System.out.println(Thread.currentThread().getName() + " : " + i);}bShouldSub = true;condition.signal(); }finally{lock.unlock();} }}

--------- Condition的例子2：实现三个线程交替运行的效果--------------------------

import java.util.concurrent.locks.Condition;import java.util.concurrent.locks.Lock;import java.util.concurrent.locks.ReentrantLock;public class SignalTest2 {public static void main(String[] args) {new SignalTest2().init();}private void init(){final Business b = new Business();new Thread(){public void run(){for(int i=0;i<50;i++)b.main();}}.start();new Thread(){public void run(){for(int i=0;i<50;i++)  b.sub();} }.start();new Thread(){public void run(){for(int i=0;i<50;i++)  b.sub2();} }.start(); }private class Business{int status = 1;Lock lock = new ReentrantLock();Condition cond1 = lock.newCondition();Condition cond2 = lock.newCondition();Condition cond3 = lock.newCondition(); public  void main(){lock.lock();while(status != 1){try{cond1.await();}catch(Exception e){}}for(int i=1;i<=5;i++){  try{Thread.sleep(200);}catch(Exception e){}  System.out.println(Thread.currentThread().getName() + ":" + i);}status = 2;cond2.signal();lock.unlock();}public  void sub(){lock.lock(); while(status != 2){try{cond2.await();}catch(Exception e){}}for(int i=1;i<=10;i++){  try{Thread.sleep(200);}catch(Exception e){}  System.out.println(Thread.currentThread().getName() + ":" + i);}status = 3;cond3.signal();lock.unlock();}public  void sub2(){lock.lock(); while(status != 3){try{cond3.await();}catch(Exception e){}}for(int i=1;i<=10;i++){  try{Thread.sleep(200);}catch(Exception e){}  System.out.println(Thread.currentThread().getName() + ":" + i);}status = 1;cond1.signal();lock.unlock();} }}

 

Semaphore

Semaphore可以维护当前访问自身的线程个数，并提供了同步机制。使用Semaphore可以控制同时访问资源的线程个数，例如，实现一个文件允许的并发访问数。

单个信号量的Semaphore对象可以实现互斥锁的功能，并且可以是由一个线程获得了“锁”，再由另一个线程释放“锁”，这可应用于死锁恢复的一些场合。

 

package cn.itcast.thread;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Semaphore;public class SemaphoreTest { public static void main(String[] args) {  ExecutorService service = Executors.newCachedThreadPool();  final  Semaphore sp = new Semaphore(3);  for(int i=0;i<10;i++){   Runnable runnable = new Runnable(){     public void run(){     try {      sp.acquire();     } catch (InterruptedException e1) {      e1.printStackTrace();     }     System.out.println("线程" + Thread.currentThread().getName() +        "进入，当前已有" + (3-sp.availablePermits()) + "个并发");     try {      Thread.sleep((long)(Math.random()*10000));     } catch (InterruptedException e) {      e.printStackTrace();     }     System.out.println("线程" + Thread.currentThread().getName() +        "即将离开");          sp.release();     //下面代码有时候执行不准确，因为其没有和上面的代码合成原子单元     System.out.println("线程" + Thread.currentThread().getName() +        "已离开，当前已有" + (3-sp.availablePermits()) + "个并发");         }   };   service.execute(runnable);     } }}

 

CyclicBarrier

Ø表示大家彼此等待，大家集合好后才开始出发，分散活动后又在指定地点集合碰面，这就好比整个公司的人员利用周末时间集体郊游一样，先各自从家出发到公司集合后，再同时出发到公园游玩，在指定地点集合后再同时开始就餐，…。

lCountDownLatch

Ø犹如倒计时计数器，调用CountDownLatch对象的countDown方法就将计数器减1，当计数到达0时，则所有等待者或单个等待者开始执行。这直接通过代码来说明CountDownLatch的作用，这样学员的理解效果更直接。

Ø可以实现一个人（也可以是多个人）等待其他所有人都来通知他，这犹如一个计划需要多个领导都签字后才能继续向下实施。还可以实现一个人通知多个人的效果，类似裁判一声口令，运动员同时开始奔跑。用这个功能做百米赛跑的游戏程序不错哦！

lExchanger

Ø用于实现两个人之间的数据交换，每个人在完成一定的事务后想与对方交换数据，第一个先拿出数据的人将一直等待第二个人拿着数据到来时，才能彼此交换数据。

 

 

-----------------CyclicBarrier的代码：---------------------------------

package cn.itcast.day3.thread;import java.util.concurrent.CyclicBarrier;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Semaphore;public class CyclicBarrierTest {public static void main(String[] args) {ExecutorService service = Executors.newCachedThreadPool();final  CyclicBarrier cb = new CyclicBarrier(3);for(int i=0;i<3;i++){Runnable runnable = new Runnable(){public void run(){try {  Thread.sleep((long)(Math.random()*10000)); System.out.println("线程" + Thread.currentThread().getName() + "即将到达集合地点1，当前已有" + cb.getNumberWaiting() + "个已经到达，正在等候");  cb.await();  Thread.sleep((long)(Math.random()*10000)); System.out.println("线程" + Thread.currentThread().getName() + "即将到达集合地点2，当前已有" + cb.getNumberWaiting() + "个已经到达，正在等候");  cb.await();   Thread.sleep((long)(Math.random()*10000)); System.out.println("线程" + Thread.currentThread().getName() + "即将到达集合地点3，当前已有" + cb.getNumberWaiting() + "个已经到达，正在等候");  cb.await();  } catch (Exception e) {e.printStackTrace();}  }};service.execute(runnable);}service.shutdown();}}

-----------------CountdownLatch的代码：---------------------------------

 

package cn.itcast.day3.thread;import java.util.concurrent.CountDownLatch;import java.util.concurrent.CyclicBarrier;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;public class CountdownLatchTest {public static void main(String[] args) {ExecutorService service = Executors.newCachedThreadPool();final CountDownLatch cdOrder = new CountDownLatch(1);final CountDownLatch cdAnswer = new CountDownLatch(3); for(int i=0;i<3;i++){Runnable runnable = new Runnable(){public void run(){try {System.out.println("线程" + Thread.currentThread().getName() + "正准备接受命令"); cdOrder.await();System.out.println("线程" + Thread.currentThread().getName() + "已接受命令");    Thread.sleep((long)(Math.random()*10000)); System.out.println("线程" + Thread.currentThread().getName() + "回应命令处理结果"); cdAnswer.countDown();  } catch (Exception e) {e.printStackTrace();}  }};service.execute(runnable);} try {Thread.sleep((long)(Math.random()*10000));System.out.println("线程" + Thread.currentThread().getName() + "即将发布命令");  cdOrder.countDown();System.out.println("线程" + Thread.currentThread().getName() + "已发送命令，正在等待结果"); cdAnswer.await();System.out.println("线程" + Thread.currentThread().getName() + "已收到所有响应结果"); } catch (Exception e) {e.printStackTrace();} service.shutdown();}}

---------------------------ExchangerTest-------------------------

讲解Exchanger的比喻：好比两个毒贩要进行交易，一手交钱、一手交货，不管谁先来到接头地点后，就处于等待状态了，当另外一方也到达了接头地点（所谓到达接头地点，也就是到到达了准备接头的状态）时，两者的数据就立即交换了，然后就又可以各忙各的了。

exchange方法就相当于两手高高举着待交换物，等待人家前来交换，一旦人家到来（即人家也执行到exchange方法），则两者立马完成数据的交换。

 

package cn.itcast.day3.thread;import java.util.concurrent.Exchanger;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;public class ExchangerTest {public static void main(String[] args) {ExecutorService service = Executors.newCachedThreadPool();final Exchanger exchanger = new Exchanger();service.execute(new Runnable(){public void run() {try { Thread.sleep((long)(Math.random()*10000));String data1 = "zxx";System.out.println("线程" + Thread.currentThread().getName() + "正在把数据" + data1 +"换出去");String data2 = (String)exchanger.exchange(data1);System.out.println("线程" + Thread.currentThread().getName() + "换回的数据为" + data2);}catch(Exception e){}} });service.execute(new Runnable(){public void run() {try { Thread.sleep((long)(Math.random()*10000));String data1 = "lhm";System.out.println("线程" + Thread.currentThread().getName() + "正在把数据" + data1 +"换出去");String data2 = (String)exchanger.exchange(data1);System.out.println("线程" + Thread.currentThread().getName() + "换回的数据为" + data2);}catch(Exception e){} } }); }}

 

Java5中提供了如下一些同步集合类：

Ø通过看java.util.concurrent包下的介绍可以知道有哪些并发集合

ØConcurrentHashMap

ØCopyOnWriteArrayList

ØCopyOnWriteArraySet