Java线程临界资源 Java多线程的临界资源问题解决方案

时间:2021-04-21 戈德里克山谷人气:0

这篇文章主要介绍了Java多线程的临界资源问题解决方案,文中通过示例代码介绍的非常详细，对大家的学习或者工作具有一定的参考学习价值,需要的朋友可以参考下

临界资源问题的原因：某一个线程在对临界资源进行访问时，还没来得及完全修改临界资源的值，临界资源就被其他线程拿去访问，导致多个线程访问同一资源。直观表现为打印结果顺序混乱。

解决方法：加锁

静态方法中用类锁，非静态方法中用对象锁。

1.同步代码段：synchronized（）{...}

2.同步方法：使用关键字synchronized修饰的方法

3.使用显式同步锁ReentrantLock

锁池描述的即为锁外等待的状态

方法一：同步代码段：synchronized（）{...}

public class SourceConflict {
  public static void main(String[] args) {
    //实例化4个售票员，用4个线程模拟4个售票员
    
    Runnable r = () -> {
      while (TicketCenter.restCount > 0) {
        synchronized(" ") {
          if (TicketCenter.restCount <= 0) {
            return;
          }
          System.out.println(Thread.currentThread().getName() + "卖出一张票，剩余" + --TicketCenter.restCount + "张票");
        }
      }
    };
    
    //用4个线程模拟4个售票员
    Thread thread1 = new Thread(r, "thread-1");
    Thread thread2 = new Thread(r, "thread-2");
    Thread thread3 = new Thread(r, "thread-3");
    Thread thread4 = new Thread(r, "thread-4");
    
    //开启线程
    thread1.start();
    thread2.start();
    thread3.start();
    thread4.start();
    
  }  
}

//实现四名售票员共同售票，资源共享，非独立
//Lambda表达式或匿名内部类内部捕获的局部变量必须显式的声明为 final 或实际效果的的 final 类型，而捕获实例或静态变量是没有限制的
class TicketCenter{
  public static int restCount = 100; 
}

方法二：同步方法，即使用关键字synchronized修饰的方法

public class SourceConflict2 {
  public static void main(String[] args) {
    //实例化4个售票员，用4个线程模拟4个售票员
    
    Runnable r = () -> {
      while (TicketCenter.restCount > 0) {
        sellTicket();
      }
    };
    
    //用4个线程模拟4个售票员
    Thread thread1 = new Thread(r, "thread-1");
    Thread thread2 = new Thread(r, "thread-2");
    Thread thread3 = new Thread(r, "thread-3");
    Thread thread4 = new Thread(r, "thread-4");
    
    //开启线程
    thread1.start();
    thread2.start();
    thread3.start();
    thread4.start();
    
  }
  
  private synchronized static void sellTicket() {  
    if (TicketCenter.restCount <= 0) {
      return;
    }
    System.out.println(Thread.currentThread().getName() + "卖出一张票，剩余" + --TicketCenter.restCount + "张票");
  }
}

class TicketCenter{
  public static int restCount = 100; 
}

方法三：使用显式同步锁ReentrantLock

import java.util.concurrent.locks.ReentrantLock;

public class SourceConflict3 {
  public static void main(String[] args) {
    //实例化4个售票员，用4个线程模拟4个售票员
    
    //显式锁
    ReentrantLock lock = new ReentrantLock();
    Runnable r = () -> {
      while (TicketCenter.restCount > 0) {
        lock.lock();
        if (TicketCenter.restCount <= 0) {
          return;
        }
        System.out.println(Thread.currentThread().getName() + "卖出一张票，剩余" + --TicketCenter.restCount + "张票");
        lock.unlock();
      }
    };
    
    //用4个线程模拟4个售票员
    Thread thread1 = new Thread(r, "thread-1");
    Thread thread2 = new Thread(r, "thread-2");
    Thread thread3 = new Thread(r, "thread-3");
    Thread thread4 = new Thread(r, "thread-4");
    
    //开启线程
    thread1.start();
    thread2.start();
    thread3.start();
    thread4.start();
    
  }  
}
class TicketCenter{
  public static int restCount = 100; 
}

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