多线程-条件变量： 转自：http://huanyue.javaeye.com/blog/560975

条件变量是Java5线程中很重要的一个概念，顾名思义，条件变量就是表示条件的一种变量。但是必须说明，这里的条件是没有实际含义的，仅仅是个标记而已，并且条件的含义往往通过代码来赋予其含义。

这里的条件和普通意义上的条件表达式有着天壤之别。

条件变量都实现了java.util.concurrent.locks.Condition接口，条件变量的实例化是通过一个Lock对象上调用newCondition()方法来获取的，这样，条件就和一个锁对象绑定起来了。因此，Java中的条件变量只能和锁配合使用，来控制并发程序访问竞争资源的安全。

条件变量的出现是为了更精细控制线程等待与唤醒，在Java5之前，线程的等待与唤醒依靠的是Object对象的wait()和notify()/notifyAll()方法，这样的处理不够精细。

而在Java5中，一个锁可以有多个条件，每个条件上可以有多个线程等待，通过调用await()方法，可以让线程在该条件下等待。当调用signalAll()方法，又可以唤醒该条件下的等待的线程。有关Condition接口的API可以具体参考JavaAPI文档。

条件变量比较抽象，原因是他不是自然语言中的条件概念，而是程序控制的一种手段。

下面以一个银行存取款的模拟程序为例来揭盖Java多线程条件变量的神秘面纱：

有一个账户，多个用户（线程）在同时操作这个账户，有的存款有的取款，存款随便存，取款有限制，不能透支，任何试图透支的操作都将等待里面有足够存款才执行操作。
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

/** 
* Java线程：条件变量 
* 
* @author leizhimin 2009-11-5 10:57:29 
*/ 
public class Test { 
        public static void main(String[] args) { 
                //创建并发访问的账户 
                MyCount myCount = new MyCount("95599200901215522", 10000); 
                //创建一个线程池 
                ExecutorService pool = Executors.newFixedThreadPool(2); 
                Thread t1 = new SaveThread("张三", myCount, 2000); 
                Thread t2 = new SaveThread("李四", myCount, 3600); 
                Thread t3 = new DrawThread("王五", myCount, 2700); 
                Thread t4 = new SaveThread("老张", myCount, 600); 
                Thread t5 = new DrawThread("老牛", myCount, 1300); 
                Thread t6 = new DrawThread("胖子", myCount, 800); 
                //执行各个线程 
                pool.execute(t1); 
                pool.execute(t2); 
                pool.execute(t3); 
                pool.execute(t4); 
                pool.execute(t5); 
                pool.execute(t6); 
                //关闭线程池 
                pool.shutdown(); 
        } 
} 

/** 
* 存款线程类 
*/ 
class SaveThread extends Thread { 
        private String name;                //操作人 
        private MyCount myCount;        //账户 
        private int x;                            //存款金额 

        SaveThread(String name, MyCount myCount, int x) { 
                this.name = name; 
                this.myCount = myCount; 
                this.x = x; 
        } 

        public void run() { 
                myCount.saving(x, name); 
        } 
} 

/** 
* 取款线程类 
*/ 
class DrawThread extends Thread { 
        private String name;                //操作人 
        private MyCount myCount;        //账户 
        private int x;                            //存款金额 

        DrawThread(String name, MyCount myCount, int x) { 
                this.name = name; 
                this.myCount = myCount; 
                this.x = x; 
        } 

        public void run() { 
                myCount.drawing(x, name); 
        } 
} 


/** 
* 普通银行账户，不可透支 
*/ 
class MyCount { 
        private String oid;                         //账号 
        private int cash;                             //账户余额 
        private Lock lock = new ReentrantLock();                //账户锁 
        private Condition _save = lock.newCondition();    //存款条件 
        private Condition _draw = lock.newCondition();    //取款条件 

        MyCount(String oid, int cash) { 
                this.oid = oid; 
                this.cash = cash; 
        } 

        /** 
         * 存款 
         * 
         * @param x        操作金额 
         * @param name 操作人 
         */ 
        public void saving(int x, String name) { 
                lock.lock();                        //获取锁 
                if (x > 0) { 
                        cash += x;                    //存款 
                        System.out.println(name + "存款" + x + "，当前余额为" + cash); 
                } 
                _draw.signalAll();            //唤醒所有等待线程。 
                lock.unlock();                    //释放锁 
        } 

        /** 
         * 取款 
         * 
         * @param x        操作金额 
         * @param name 操作人 
         */ 
        public void drawing(int x, String name) { 
                lock.lock();                                 //获取锁 
                try { 
                        if (cash - x < 0) { 
                                _draw.await();             //阻塞取款操作 
                        } else { 
                                cash -= x;                     //取款 
                                System.out.println(name + "取款" + x + "，当前余额为" + cash); 
                        } 
                        _save.signalAll();             //唤醒所有存款操作 
                } catch (InterruptedException e) { 
                        e.printStackTrace(); 
                } finally { 
                        lock.unlock();                     //释放锁 
                } 
        } 
}

李四存款3600，当前余额为13600 
张三存款2000，当前余额为15600 
老张存款600，当前余额为16200 
老牛取款1300，当前余额为14900 
胖子取款800，当前余额为14100 
王五取款2700，当前余额为11400 

Process finished with exit code 0

假如我们不用锁和条件变量，如何实现此功能呢？下面是实现代码：

import java.util.concurrent.ExecutorService; 
import java.util.concurrent.Executors; 

/** 
* Java线程：不用条件变量 
* 
* @author leizhimin 2009-11-5 10:57:29 
*/ 
public class Test { 
        public static void main(String[] args) { 
                //创建并发访问的账户 
                MyCount myCount = new MyCount("95599200901215522", 10000); 
                //创建一个线程池 
                ExecutorService pool = Executors.newFixedThreadPool(2); 
                Thread t1 = new SaveThread("张三", myCount, 2000); 
                Thread t2 = new SaveThread("李四", myCount, 3600); 
                Thread t3 = new DrawThread("王五", myCount, 2700); 
                Thread t4 = new SaveThread("老张", myCount, 600); 
                Thread t5 = new DrawThread("老牛", myCount, 1300); 
                Thread t6 = new DrawThread("胖子", myCount, 800); 
                //执行各个线程 
                pool.execute(t1); 
                pool.execute(t2); 
                pool.execute(t3); 
                pool.execute(t4); 
                pool.execute(t5); 
                pool.execute(t6); 
                //关闭线程池 
                pool.shutdown(); 
        } 
} 

/** 
* 存款线程类 
*/ 
class SaveThread extends Thread { 
        private String name;                //操作人 
        private MyCount myCount;        //账户 
        private int x;                            //存款金额 

        SaveThread(String name, MyCount myCount, int x) { 
                this.name = name; 
                this.myCount = myCount; 
                this.x = x; 
        } 

        public void run() { 
                myCount.saving(x, name); 
        } 
} 

/** 
* 取款线程类 
*/ 
class DrawThread extends Thread { 
        private String name;                //操作人 
        private MyCount myCount;        //账户 
        private int x;                            //存款金额 

        DrawThread(String name, MyCount myCount, int x) { 
                this.name = name; 
                this.myCount = myCount; 
                this.x = x; 
        } 

        public void run() { 
                myCount.drawing(x, name); 
        } 
} 


/** 
* 普通银行账户，不可透支 
*/ 
class MyCount { 
        private String oid;                         //账号 
        private int cash;                             //账户余额 

        MyCount(String oid, int cash) { 
                this.oid = oid; 
                this.cash = cash; 
        } 

        /** 
         * 存款 
         * 
         * @param x        操作金额 
         * @param name 操作人 
         */ 
        public synchronized void saving(int x, String name) { 
                if (x > 0) { 
                        cash += x;                    //存款 
                        System.out.println(name + "存款" + x + "，当前余额为" + cash); 
                } 
                notifyAll();            //唤醒所有等待线程。 
        } 

        /** 
         * 取款 
         * 
         * @param x        操作金额 
         * @param name 操作人 
         */ 
        public synchronized void drawing(int x, String name) { 
                if (cash - x < 0) { 
                        try { 
                                wait(); 
                        } catch (InterruptedException e1) { 
                                e1.printStackTrace(); 
                        } 
                } else { 
                        cash -= x;                     //取款 
                        System.out.println(name + "取款" + x + "，当前余额为" + cash); 
                } 
                notifyAll();             //唤醒所有存款操作 
        } 
}

李四存款3600，当前余额为13600 
王五取款2700，当前余额为10900 
老张存款600，当前余额为11500 
老牛取款1300，当前余额为10200 
胖子取款800，当前余额为9400 
张三存款2000，当前余额为11400 

Process finished with exit code 0

结合先前同步代码知识，举一反三，将此例改为同步代码块来实现，代码如下：

import java.util.concurrent.ExecutorService; 
import java.util.concurrent.Executors; 

/** 
* Java线程：改为同步代码块 
* 
* @author leizhimin 2009-11-5 10:57:29 
*/ 
public class Test { 
        public static void main(String[] args) { 
                //创建并发访问的账户 
                MyCount myCount = new MyCount("95599200901215522", 10000); 
                //创建一个线程池 
                ExecutorService pool = Executors.newFixedThreadPool(2); 
                Thread t1 = new SaveThread("张三", myCount, 2000); 
                Thread t2 = new SaveThread("李四", myCount, 3600); 
                Thread t3 = new DrawThread("王五", myCount, 2700); 
                Thread t4 = new SaveThread("老张", myCount, 600); 
                Thread t5 = new DrawThread("老牛", myCount, 1300); 
                Thread t6 = new DrawThread("胖子", myCount, 800); 
                //执行各个线程 
                pool.execute(t1); 
                pool.execute(t2); 
                pool.execute(t3); 
                pool.execute(t4); 
                pool.execute(t5); 
                pool.execute(t6); 
                //关闭线程池 
                pool.shutdown(); 
        } 
} 

/** 
* 存款线程类 
*/ 
class SaveThread extends Thread { 
        private String name;                //操作人 
        private MyCount myCount;        //账户 
        private int x;                            //存款金额 

        SaveThread(String name, MyCount myCount, int x) { 
                this.name = name; 
                this.myCount = myCount; 
                this.x = x; 
        } 

        public void run() { 
                myCount.saving(x, name); 
        } 
} 

/** 
* 取款线程类 
*/ 
class DrawThread extends Thread { 
        private String name;                //操作人 
        private MyCount myCount;        //账户 
        private int x;                            //存款金额 

        DrawThread(String name, MyCount myCount, int x) { 
                this.name = name; 
                this.myCount = myCount; 
                this.x = x; 
        } 

        public void run() { 
                myCount.drawing(x, name); 
        } 
} 


/** 
* 普通银行账户，不可透支 
*/ 
class MyCount { 
        private String oid;                         //账号 
        private int cash;                             //账户余额 

        MyCount(String oid, int cash) { 
                this.oid = oid; 
                this.cash = cash; 
        } 

        /** 
         * 存款 
         * 
         * @param x        操作金额 
         * @param name 操作人 
         */ 
        public void saving(int x, String name) { 
                if (x > 0) { 
                        synchronized (this) { 
                                cash += x;                    //存款 
                                System.out.println(name + "存款" + x + "，当前余额为" + cash); 
                                notifyAll();            //唤醒所有等待线程。 
                        } 
                } 
        } 

        /** 
         * 取款 
         * 
         * @param x        操作金额 
         * @param name 操作人 
         */ 
        public synchronized void drawing(int x, String name) { 
                synchronized (this) { 
                        if (cash - x < 0) { 
                                try { 
                                        wait(); 
                                } catch (InterruptedException e1) { 
                                        e1.printStackTrace(); 
                                } 
                        } else { 
                                cash -= x;                     //取款 
                                System.out.println(name + "取款" + x + "，当前余额为" + cash); 
                        } 
                } 
                notifyAll();             //唤醒所有存款操作 
        } 
}

李四存款3600，当前余额为13600 
王五取款2700，当前余额为10900 
老张存款600，当前余额为11500 
老牛取款1300，当前余额为10200 
胖子取款800，当前余额为9400 
张三存款2000，当前余额为11400 

Process finished with exit code 0