Java多线程-Lock锁的使用，以及生产者和消费者的实现

版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/yangqian201175/article/details/51337488

本文中将主要介绍Java多线程编程基础中的Lock锁对象的使用，以及如何一步一步实现Java代码的生产者与消费者；

1、Java中如何使用Lock锁以及死锁问题的描述
2、Java实现生产者与消费者的过程（一步一步优化的步骤）

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1、Java中如何使用Lock锁以及死锁问题的描述
LOCK锁的出现：为了更清晰的表达如何加锁和释放锁，JDK5以后提供了一个新的锁对象LOCK；
Lock锁中最重要的个方法：
void lock()
void unlock()
下面就是Lock锁的一个简单的演示Demo，之后的死锁问题也会使用Lock锁来进行表现；

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1）例子一：利用Lock对象实现售票机制：

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SellTicket.java

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

public class SellTicket implements Runnable {

    // 定义票
    private int tickets = 100;

    // 定义锁对象
    private Lock lock = new ReentrantLock();

    @Override
    public void run() {
        while (tickets > 0) {
            // 之所以不带catch是因为保证在出错的情况下保证锁的释放
            try{
                // 加锁
                lock.lock();

                if (tickets > 0) {
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }

                    System.out.println(Thread.currentThread().getName() + "正在出售第" + tickets-- + "张票" );
                }
            } finally {
                // 释放锁
                lock.unlock();
            }
        }
    }

}

SellTicketDemo.java

public class SellTicketDemo {

    public static void main(String[] args) {

        // 创建资源对象
        SellTicket st = new SellTicket();

        // 创建窗口
        Thread thread1 = new Thread(st, "窗口一");
        Thread thread2 = new Thread(st, "窗口二");
        Thread thread3 = new Thread(st, "窗口三");

        // 开启线程
        thread1.start();
        thread2.start();
        thread3.start();
    }   
}

运行效果：

可以看见线程在运行的时候，基本上都是成片运行的，并没有比较好的交叉运行

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2）例子二：利用Lock实现死锁机制

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MyLock.java

 /**
 * 创建两把锁可以直接调用
 * @author YQ
 *
 */
public class MyLock {
    //创建两把锁对象
    public static final Object objA = new Object();
    public static final Object objB = new Object();
}

DieLock.java

public class DieLock extends Thread {

    private boolean flag;

    public DieLock(boolean flag) {
        this.flag = flag;
    }

    @Override
    public void run() {
        if (flag) {
            synchronized (MyLock.objA) {
                System.out.println("if ObjA");
                synchronized (MyLock.objB) {
                    System.out.println("if ObjB");
                }
            }
        } else {
            synchronized (MyLock.objB) {
                System.out.println("else ObjB");
                synchronized (MyLock.objA) {
                    System.out.println("else ObjA");
                }
            }
        }
    }
}

DieLockDemo.java

/**
 * 同步的弊端：
 *  A:效率低
 *  B:容易产生死锁
 * 死锁：
 *  两个或者两个以上的线程在争夺资源的过程中，发生的一种互相等待的现象
 * @author YQ
 */
public class DieLockDemo {
    public static void main(String[] args) {
        DieLock dieLock1 = new DieLock(true);
        DieLock dieLock2 = new DieLock(false);
        dieLock1.start();
        dieLock2.start();
    }
}

运行效果：

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2、Java实现生产者与消费者的过程（一步一步优化的步骤）

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1）初始实现，定义一个Student的JavaBean的类，然后通过set保存Student的数据作为生产者，之后通过get取出Student的数据作为消费者，但是以下的实现仅仅只有一次！
Student.java

public class Student {

    private String name;
    private int age;

    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "Student [name=" + name + ", age=" + age + "]";
    }

}

SetThread.java

public class SetThread extends Thread{

    private Student student;

    public SetThread(Student student) {
        super();
        this.student = student;
    }

    @Override
    public void run() {
        student.setName("admin");
        student.setAge(24);
    }

}

GetThread.java

public class GetThread extends Thread {

    private Student student;

    public GetThread(Student student) {
        super();
        this.student = student;
    }

    @Override
    public void run() {
        System.out.println(student.getName() + "====" + student.getAge());
    }
}

StudentDemo.java

public class StudentDemo {

    public static void main(String[] args) {

        // 创建两个线程的共用资源
        Student student = new Student();
        // 创建生产者和消费者
        SetThread thread1 = new SetThread(student);
        GetThread thread2 = new GetThread(student);

        //开启生产者和消费者线程
        thread1.start();
        thread2.start();
    }

}

运行效果：

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2）进一步的循环实现多次生产多次消费，使用的是同步的方式
Student.java代码同上！

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GetThread.java

public class GetThread extends Thread {

    private Student student;

    public GetThread(Student student) {
        super();
        this.student = student;
    }

    @Override
    public void run() {
        while (true) {
            synchronized (student) {
                System.out.println(student.getName() + "====" + student.getAge());
            }
        }
    }
}

SetThread.java

public class SetThread extends Thread{

    private Student student;

    private int x = 0;

    public SetThread(Student student) {
        super();
        this.student = student;
    }

    @Override
    public void run() {
        while (true) {
            //必须是相同的同一把锁
            synchronized (student) {
                if (x%2 == 0) {
                    student.setName("admin");
                    student.setAge(24);
                } else {
                    student.setName("manager");
                    student.setAge(28);
                }
                x++;
            }       
        } 
    }
}

StudentDemo.java

public class StudentDemo {

    public static void main(String[] args) {

        // 创建两个线程的共用资源
        Student student = new Student();
        // 创建生产者和消费者
        SetThread thread1 = new SetThread(student);
        GetThread thread2 = new GetThread(student);

        //开启生产者和消费者线程
        thread1.start();
        thread2.start();
    }

}

运行效果（停止的时候出现的ad没有完整与线程的实现没有关系，主要是因为命令窗口的缓冲没有完整的原因）：

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3）真正实现生产者与消费者：生产者生产后消费者才可以消费，消费者消费之后生产者才可以生产如此往复：

---

Student.java

public class Student {

    private String name;
    private int age;

    //默认情况下不存在数据
    private boolean flag;

    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }

    public boolean isFlag() {
        return flag;
    }
    public void setFlag(boolean flag) {
        this.flag = flag;
    }
    @Override
    public String toString() {
        return "Student [name=" + name + ", age=" + age + "]";
    }

}

SetThread.java

public class SetThread extends Thread{

    private Student student;

    private int x = 0;

    public SetThread(Student student) {
        super();
        this.student = student;
    }

    @Override
    public void run() {
        while (true) {
            // 必须是相同的同一把锁
            synchronized (student) {

                // 判断
                if (student.isFlag()) {
                    try {
                        student.wait();
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }

                if (x%2 == 0) {
                    student.setName("admin");
                    student.setAge(24);
                } else {
                    student.setName("manager");
                    student.setAge(28);
                }
                x++;

                // 修改标记
                student.setFlag(true);
                student.notify();
            }

        } 
    }

}

GetThread.java

public class GetThread extends Thread {

    private Student student;

    public GetThread(Student student) {
        super();
        this.student = student;
    }

    @Override
    public void run() {
        while (true) {
            synchronized (student) {

                if (!student.isFlag()) {
                    try {
                        student.wait();
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }

                System.out.println(student.getName() + "====" + student.getAge());

                student.setFlag(false);
                student.notify();
            }
        }
    }
}

运行效果：
StudentDemo.java

/**
 * 等待唤醒：
 *      Object类中提供了三个方法：
 *      wait():等待;
 *      notify():唤醒单个线程;
 *      notifyAll():唤醒所有线程;
 * @author YQ
 *
 */
public class StudentDemo {

    public static void main(String[] args) {

        // 创建两个线程的共用资源
        Student student = new Student();
        // 创建生产者和消费者
        SetThread thread1 = new SetThread(student);
        GetThread thread2 = new GetThread(student);

        //开启生产者和消费者线程
        thread1.start();
        thread2.start();
    }
}

运行效果：