线程锁技术

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笔记摘要

这里介绍了java5中的线程锁技术：Lock和Condition，实现线程间的通信，其中的读锁和写锁的使用通过一个缓存系统进行了演示，对于Condition的应用通过一个阻塞队列进行演示。

线程锁技术：Lock & Condition 实现线程同步通信所属包：java.util.concurrent.locks

线程锁 说明
Synchronized 同步方法，锁对象是this；静态同步方法，锁对象是字节码.class；同步代码块，锁对象是任意对象，但必须是同一个对象
Lock 同步锁接口
ReentrantLock lock()，unlock()，newCondition()
ReadWriteLock 读写锁接口
ReentrantReadWriteLock readLock()获取读锁，writeLock()获取写锁
Condition 线程间通信 await()等待 signal()唤醒

1. Lock

Lock比传统线程模型中的synchronized方式更加面向对象，相对于synchronized 方法和语句它具有更广泛的锁定操作，此实现允许更灵活的结构，可以具有差别很大的属性，可以支持多个相关的 Condition 对象。

于现实生活中类似，锁本身也是一个对象。两个线程执行的代码片段要实现同步互斥的结果，它们必须用同一个Lock对象，锁是上在代表要操作的资源的类的内部方法中，而不是线程代码中。

读写锁

分为读锁和写锁，多个读锁不互斥，读锁与写锁互斥，写锁与写锁互斥，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！

读写锁的使用情景：

  • 如果代码只读数据，就可以很多人共同读取，但不能同时写。
  • 如果代码修改数据，只能有一个人在写，且不能同时读数据。

API中ReentrantReadWriteLock类提供的一个读写锁缓存示例：

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class CachedData {  
     
　　Object data;  
    volatile boolean cacheValid;  
     
　　ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();  
  
　　void processCachedData() {  
       
　　      rwl.readLock().lock();  
      
　　      if (!cacheValid) {  
          
　　      // Must release read lock before acquiring write lock  
          
　　      rwl.readLock().unlock();  
　　      rwl.writeLock().lock();  
          
　　     // Recheck state because another thread might have acquired  
　    　 // write lock and changed state before we did.    
　　    if (!cacheValid) {  
            
　　         data = ...       
　　         cacheValid = true;  
          
　　     }  
　　    // Downgrade by acquiring read lock before releasing write lock  
             rwl.readLock().lock();  
          
　　      rwl.writeLock().unlock(); // Unlock write, still hold read  
       
　　    }  
　　       use(data);  
　　      rwl.readLock().unlock();  
     
　　}  
}

读写锁的应用：编写一个缓存系统

注解：为了避免线程的安全问题，synchronized和ReadWriteLock都可以，synchronized也防止了并发读取，性能较低有一个线程先进去，开始读取数据，进行判断，发现没有数据，其他线程就没有必要进去了，就释放读锁，加上写锁，去查找数据写入，为了避免写入的其他对象等待，再做一次判断，数据写入完成后，释放写锁，上读锁，防止写入，还原原来的状态。

两次判断：第一次为了写入数据，所以释放读锁，上写锁。第二次为了防止阻塞的线程重复写入

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import java.util.HashMap;  
import java.util.Map;  
import java.util.concurrent.locks.ReadWriteLock;  
import java.util.concurrent.locks.ReentrantReadWriteLock;  
  
public class CacheDemo {  
  
    //定义一个map用于缓存对象  
    private Map<String, Object> cache = new HashMap<String, Object>();  
          
    //获取一个读写锁对象  
    private ReadWriteLock rwl = new ReentrantReadWriteLock();  
      
    //带有缓存的获取指定值的方法  
    public  Object getData(String key){  
        rwl.readLock().lock();      //上读锁  
        Object value = null;  
        try{  
            value = cache.get(key); //获取要查询的值     
            if(value == null){  //线程出现安全问题的地方  
                  
                rwl.readLock().unlock();    //没有数据，释放读锁，上写锁
                // 多个线程去上写锁，第一个上成功后，其他线程阻塞，第一个线程开始执行下面的代码，最后
                // 释放写锁后，后面的线程继续上写锁，为了避免后面的线程重复写入，进行二次判断
                rwl.writeLock().lock();
                try{  
                    if(value==null){    //二次判断，防止其他线程重复写数据  
                            value = "aaaa"; //实际是去查询数据库  
                    }  
                }finally{  
                    rwl.writeLock().unlock();   //写完数据，释放写锁  
                }  
                rwl.readLock().lock();  //恢复读锁  
            }  
        }finally{  
            rwl.readLock().unlock();    //最终释放读锁  
        }  
        return value;   //返回获取到的值  
    }  
}

虚假唤醒：用while代替if

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Lock lock = new ReentrantLock();
try {
    lock.lock();
    //需要加锁的代码
}finally {
    lock.unlock();
}

读写锁测试

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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();
            new Thread(){
                public void run(){
                    while(true){
                        q3.put(new Random().nextInt(10000));
                    }
                }
            }.start();
        }
    }
}
class Queue3{
    private Object data = null;
ReadWriteLock rwl = new ReentrantReadWriteLock
();
    public void get(){
        rwl.readLock().lock();
        try {
            System.out.println(Thread.currentThread().getName() + " be ready to read data!");
            Thread.sleep((long)(Math.random()*1000));
            System.out.println(Thread.currentThread().getName() + "have read data :" + data);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }finally{
            rwl.readLock().unlock();
        }
    }
    public void put(Object data){
        rwl.writeLock().lock();
        try {
            System.out.println(Thread.currentThread().getName() + " be ready to write data!");
            Thread.sleep((long)(Math.random()*1000));
            this.data = data;
            System.out.println(Thread.currentThread().getName() + " have write data: " + data);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }finally{
            rwl.writeLock().unlock();
        }
    }
}

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Thread-0 be ready to read data!
Thread-2 be ready to read data!
Thread-4 be ready to read data!
Thread-0have read data :null
Thread-2have read data :null
Thread-4have read data :null
Thread-5 be ready to write data!
Thread-5 have write data: 7975
Thread-5 be ready to write data!
Thread-5 have write data: 9832
Thread-3 be ready to write data!
Thread-3 have write data: 2813
Thread-3 be ready to write data!
Thread-3 have write data: 7998
Thread-1 be ready to write data!
Thread-1 have write data: 6737
Thread-1 be ready to write data!
...

2. Condition

Condition 将 Object 监视器方法（wait、notify 和 notifyAll）分解成截然不同的对象，以便通过将这些对象与任意 Lock 实现组合使用，为每个对象提供多个等待 set（wait-set）。其中，Lock 替代了 synchronized 方法和语句的使用，Condition 替代了 Object 监视器方法wait和notify的使用

一个锁内部可以有多个Condition，即有多路等待通知，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。使用一个监视器往往会产生顾此失彼的情况。

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。

Condition的应用：阻塞队列（使用了两个监视器）

说明：该应用是 java.util.concurrent.locks包中Condition接口中的示例代码。使用了两个Condition分别用于管理取数据的线程，和存数据的线程，这样就可以明确的唤醒需要的一类线程，如果使用一个Condition，当队列满了之后，唤醒的并不一定就是取数据的线程

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class BoundedBuffer {  
  
  final Lock lock = new ReentrantLock();  
  final Condition notFull  = lock.newCondition();   
  final Condition notEmpty = lock.newCondition();   
  
  final Object[] items = new Object[100];  
  int putptr, takeptr, count;  
  
  public void put(Object x) throws InterruptedException {  
    lock.lock();  
    try {  
      while (count == items.length) //循环判断队列是否已存满  
        notFull.await();    //如果队列存满了，则要存入数据的线程等待  
      items[putptr] = x;   
      if (++putptr == items.length) putptr = 0;//当队列放满，指针回到0  
      ++count;      //添加了一个数据  
      notEmpty.signal();    //队列中有数据了，所以就唤醒取数据的线程  
    } finally {  
      lock.unlock();  
    }  
  }  
  
  public Object take() throws InterruptedException {  
    lock.lock();  
    try {  
      while (count == 0)    //循环判断，队列是否有空位  
        notEmpty.await();   //要取的线程等待  
      Object x = items[takeptr];   
      if (++takeptr == items.length) takeptr = 0;  
      --count;  //取走一个，说明队列有空闲的位置，  
      notFull.signal(); //所以通知存入的线程  
      return x;  
    } finally {  
      lock.unlock();  
    }  
  }   
}

Condition测试

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public class ConditionCommunication {
    public static void main(String[] args) {
        final Business business = new Business();
        new Thread(
                new Runnable() {
                    @Override
                    public void run() {
                        for(int i=1;i<=5;i++){
                            business.sub(i);
                        }
                    }
                }
        ).start();
        for(int i=1;i<=5;i++){
            business.main(i);
        }
    }
    	class Business {
        Lock lock = new ReentrantLock();
        Condition condition = lock.newCondition();
        private boolean bShouldSub = true;
        public  void sub(int i){
            lock.lock();
            try{
                while(!bShouldSub){
                    try {
                        condition.await();
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
                for(int j=1;j<=2;j++){
                  System.out.println("sub thread sequence of " + j + ",loop of " + i);
                }
                bShouldSub = false;
                condition.signal();
            }finally{
                lock.unlock();
            }
        }
        public  void main(int i){
            lock.lock();
            try{
                while(bShouldSub){
                    try {
                        condition.await();
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
                for(int j=1;j<=4;j++){
                 System.out.println("main thread sequence of " + j + ",loop of " + i);
                }
                bShouldSub = true;
                condition.signal();
            }finally{
                lock.unlock();
            }
        }
    }
}

输出结果

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sub thread sequence of 1,loop of 1
sub thread sequence of 2,loop of 1
main thread sequence of 1,loop of 1
main thread sequence of 2,loop of 1
main thread sequence of 3,loop of 1
main thread sequence of 4,loop of 1
sub thread sequence of 1,loop of 2
sub thread sequence of 2,loop of 2
main thread sequence of 1,loop of 2
main thread sequence of 2,loop of 2
main thread sequence of 3,loop of 2
main thread sequence of 4,loop of 2
sub thread sequence of 1,loop of 3
sub thread sequence of 2,loop of 3
main thread sequence of 1,loop of 3
main thread sequence of 2,loop of 3
main thread sequence of 3,loop of 3
main thread sequence of 4,loop of 3
sub thread sequence of 1,loop of 4
sub thread sequence of 2,loop of 4
main thread sequence of 1,loop of 4
main thread sequence of 2,loop of 4
main thread sequence of 3,loop of 4
main thread sequence of 4,loop of 4
sub thread sequence of 1,loop of 5
sub thread sequence of 2,loop of 5
main thread sequence of 1,loop of 5
main thread sequence of 2,loop of 5
main thread sequence of 3,loop of 5
main thread sequence of 4,loop of 5

3. Condition练习

一共有3个线程，两个子线程先后循环2次，接着主线程循环3次，接着又回到两 个子线程先后循环2次，再回到主线程又循环3次，如此循环5次。

思路：老二先执行，执行完唤醒老三，老三执行完唤醒老大，老大执行完唤醒老二，以此循环，所以定义3个Condition对象和一个执行标识即可

示例出现的问题：两个文件中有同名类的情况

解决方案：可以将一个文件中的那个同名外部类放进类中，但是静态不能创建内部类的实例对象，所以需要加上static，这样两个类的名称就不一样了。 一个是原来的类名，一个是在自己类名前面加上外部类的类名。

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import java.util.concurrent.locks.Condition;  
import java.util.concurrent.locks.Lock;  
import java.util.concurrent.locks.ReentrantLock;  
  
public class ThreeConditionCommunication {  
    public static void main(String[] args){  
          
        final Business business = new Business();  
          
　　	   //创建并启动子线程老二  
        new Thread(new Runnable(){  
            @Override  
            public void run() {  
                for(int i=1;i<=5;i++){  
                    business.sub2(i);  
                }  
            }  
        }).start();  
                  
　　     //创建并启动子线程老三  
        new Thread(new Runnable(){  
            @Override  
            public void run() {  
                for(int i=1;i<=5;i++){  
                    business.sub3(i);  
                }  
            }  
        }).start();  
  
            //主线程  
            for(int i=1;i<=5;i++){  
                business.main(i);  
          }  
       }  
      
    static class Business{  
          
        Lock lock = new ReentrantLock();   
        Condition condition1 = lock.newCondition();  
        Condition condition2 = lock.newCondition();  
        Condition condition3 = lock.newCondition();  
          
        //定义一个变量来决定线程的执行权  
        private int ShouldSub = 1;  
          
        public  void  sub2(int i){  
            //上锁，不让其他线程执行  
            lock.lock();  
            try{  
            if(ShouldSub != 2){ //如果不该老二执行，就等待  
                try {  
                    condition2.await();  
                } catch (InterruptedException e) {  
                    e.printStackTrace();  
                }  
            }  
              
            for(int j=1;j<=2;j++){  
                System.out.println("sub thread sequence of"+i+",loop of "+j);  
            }  
            ShouldSub = 3;  //准备让老三执行  
            condition3.signal();        //唤醒老三  
            }finally{  
                lock.unlock();  
            }     
        }  
          
        public  void  sub3(int i){  
              
            lock.lock();  
            try{  
            if(ShouldSub != 3){  
                try {  
                    condition3.await();  
                } catch (InterruptedException e) {  
                    e.printStackTrace();  
                }  
            }  
              
            for(int j=1;j<=2;j++){  
                System.out.println("sub2 thread sequence of"+i+",loop of "+j);  
            }  
                ShouldSub = 1;  //准备让老大执行  
                condition1.signal();        //唤醒老大  
            }finally{  
                lock.unlock();  
            }  
        }  
          
　　    //主线程  
        public  void main(int i){  
              
            lock.lock();  
            try{  
            if(ShouldSub!=1){  
                try {  
                    condition1.await();  
                } catch (InterruptedException e) {  
                    e.printStackTrace();  
                }  
            }  
            for(int j=1;j<=3;j++){  
                System.out.println("main thread sequence of"+i+", loop of "+j);  
            }     
            ShouldSub = 2;  //准备让老二执行  
            condition2.signal();        //唤醒老二  
            }finally{  
                lock.unlock();  
            }     
        }     
    }  
   }

输出结果

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main thread sequence of1, loop of 1
main thread sequence of1, loop of 2
main thread sequence of1, loop of 3
sub thread sequence of1,loop of 1
sub thread sequence of1,loop of 2
sub2 thread sequence of1,loop of 1
sub2 thread sequence of1,loop of 2
main thread sequence of2, loop of 1
main thread sequence of2, loop of 2
main thread sequence of2, loop of 3
sub thread sequence of2,loop of 1
sub thread sequence of2,loop of 2
sub2 thread sequence of2,loop of 1
sub2 thread sequence of2,loop of 2
main thread sequence of3, loop of 1
main thread sequence of3, loop of 2
main thread sequence of3, loop of 3
sub thread sequence of3,loop of 1
sub thread sequence of3,loop of 2
sub2 thread sequence of3,loop of 1
sub2 thread sequence of3,loop of 2
main thread sequence of4, loop of 1
main thread sequence of4, loop of 2
main thread sequence of4, loop of 3
sub thread sequence of4,loop of 1
sub thread sequence of4,loop of 2
sub2 thread sequence of4,loop of 1
sub2 thread sequence of4,loop of 2
main thread sequence of5, loop of 1
main thread sequence of5, loop of 2
main thread sequence of5, loop of 3
sub thread sequence of5,loop of 1
sub thread sequence of5,loop of 2
sub2 thread sequence of5,loop of 1
sub2 thread sequence of5,loop of 2

多路等待和通知

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class BoundedBuffer {
    final Lock lock = new ReentrantLock();
    final Condition notFull  = lock.newCondition();
    final Condition notEmpty = lock.newCondition();
    final Object[] items = new Object[100];
    int putptr, takeptr, count;
    public void put(Object x) throws InterruptedException {
        lock.lock();
        try {
            while (count == items.length)
                notFull.await();
            items[putptr] = x;
            if (++putptr == items.length) putptr = 0;
            ++count;
            notEmpty.signal();
        } finally {
            lock.unlock();
        }
    }
    public Object take() throws InterruptedException {
        lock.lock();
        try {
            while (count == 0)
                notEmpty.await();
            Object x = items[takeptr];
            if (++takeptr == items.length) takeptr = 0;
            --count;
            notFull.signal();
            return x;
        } finally {
            lock.unlock();
        }
    }
}
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