java解決多線程併發安全問題的三種方式

java併發線程帶來的安全問題

場景模擬

創建三個線程模擬三個售票窗口,來同時出售一百張票

代碼

package com.shemuel;

/**
 * @Author: dengshaoxiang
 * @Date: 2019/6/11 15:46
 * @Description:
 */
public class ThreadDemo {
    public static void main(String[] args) {
        MyTicket ticket = new MyTicket();
        new Thread(ticket,"一號窗口").start();
        new Thread(ticket,"二號窗口").start();
        new Thread(ticket,"三號窗口").start();
    }
}
class MyTicket implements Runnable{
    // 一百張票
    private int ticket = 100;
    @Override
    public void run() {
        while (true) {
            if (ticket > 0) {
                try {
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName() + "售票完成,餘票 : " + --ticket);
            }
        }
    }
}

運行程序,結果:

可以看到結果數據產生了問題.

解決方法

方法一 :同步代碼塊

代碼:

package com.shemuel;

/**
 * @Author: dengshaoxiang
 * @Date: 2019/6/11 15:46
 * @Description:
 */
public class ThreadDemo {
    public static void main(String[] args) {
        MyTicket ticket = new MyTicket();
        new Thread(ticket,"一號窗口").start();
        new Thread(ticket,"二號窗口").start();
        new Thread(ticket,"三號窗口").start();
    }
}
class MyTicket implements Runnable{
    // 一百張票
    private int ticket = 100;
    @Override
    public void run() {
        sellTicket();
    }
    public synchronized void sellTicket() {
        while (true) {
            if (ticket > 0) {
                try {
                    Thread.sleep(50);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName() + "售票完成,餘票 : " + --ticket);
            }
        }
    }
}

結果:

方法二 : 同步方法

代碼:

package com.shemuel;

/**
 * @Author: dengshaoxiang
 * @Date: 2019/6/11 15:46
 * @Description:
 */
public class ThreadDemo {
    public static void main(String[] args) {
        MyTicket ticket = new MyTicket();
        new Thread(ticket,"一號窗口").start();
        new Thread(ticket,"二號窗口").start();
        new Thread(ticket,"三號窗口").start();
    }
}
class MyTicket implements Runnable{
    // 一百張票
    private int ticket = 100;
    @Override
    public void run() {
        synchronized (this){
            while (true) {
                if (ticket > 0) {
                    try {
                        Thread.sleep(50);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println(Thread.currentThread().getName() + "售票完成,餘票 : " + --ticket);
                }
            }
        }
    }
}

結果:

方法三 : Lock同步鎖

代碼:

package com.shemuel;

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

/**
 * @Author: dengshaoxiang
 * @Date: 2019/6/11 15:46
 * @Description:
 */
public class ThreadDemo {
    public static void main(String[] args) {
        MyTicket ticket = new MyTicket();
        new Thread(ticket,"一號窗口").start();
        new Thread(ticket,"二號窗口").start();
        new Thread(ticket,"三號窗口").start();
    }
}
class MyTicket implements Runnable{
    private Lock lock = new ReentrantLock();
    // 一百張票
    private int ticket = 100;
    @Override
    public void run() {
        while (true) {
            lock.lock();
            try {
                if (ticket > 0) {
                    try {
                        Thread.sleep(50);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println(Thread.currentThread().getName() + "售票完成,餘票 : " + --ticket);
                }
            }finally {
                lock.unlock();
            }
        }
    }
}

結果:

注意:lock 一定要在finally代碼塊裏解鎖.
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