Java基础之线程新特性条件变量

Posted 2020-08-06 潇湘剑雨

     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

package unit_fifteen;

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 
* 
*/ 
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
                } 
        } 
}

 

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：

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

/** 
* Java线程：不用条件变量
* 
*/ 
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
        } 
}

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：

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

/** 
* Java线程：改为同步代码块
* 
*/ 
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
        } 
}