【Java】线程安全

当多个线程访问同一个资源时,导致了数据有问题。CPU在多个线程之间高速切换导致。

示例

public class MyTicket implements Runnable{
    //定义100张票
    int ticket = 100;

    @Override
    public void run() {
        while(true){
            if (ticket>0){
                System.out.println(Thread.currentThread().getName()+"买了第"+ticket+"张票");
                ticket--;
            }
        }
    }
}

public class Test01 {
    public static void main(String[] args) {
        MyTicket myTicket = new MyTicket();

        Thread t1 = new Thread(myTicket, "赵四");
        Thread t2 = new Thread(myTicket, "刘能");
        Thread t3 = new Thread(myTicket, "广坤");

        t1.start();
        t2.start();
        t3.start();
    }
}

方式一：同步代码块

基本流程：

1. 格式:

synchronized(任意对象){
    线程可能出现不安全的代码
}

2. 任意对象:就是我们的锁对象
3. 执行:
   • 一个线程拿到锁之后,会进入到同步代码块中执行,在此期间,其他线程拿不到锁,就进不去同步代码块,需要在同步代码块外面等待排队
   • 需要等着执行的线程执行完毕,出了同步代码块,相当于释放锁了,等待的线程才能抢到锁,才能进入到同步代码块中执行

public class MyTicket implements Runnable{
    //定义100张票
    int ticket = 100;

    //任意new一个对象
    Object obj = new Object();

    @Override
    public void run() {
        while(true){
            try {
                Thread.sleep(100L);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
            synchronized (obj){
                if (ticket>0){
                    System.out.println(Thread.currentThread().getName()+"买了第"+ticket+"张票");
                    ticket--;
                }
            }

        }
    }
}

public class Test01 {
    public static void main(String[] args) {
        MyTicket myTicket = new MyTicket();

        Thread t1 = new Thread(myTicket, "赵四");
        Thread t2 = new Thread(myTicket, "刘能");
        Thread t3 = new Thread(myTicket, "广坤");

        t1.start();
        t2.start();
        t3.start();
    }
}

方法二：同步方法

普通同步方法

基本流程：

1. 格式:

修饰符 synchronized 返回值类型 方法名(参数){
    方法体
    return 结果
}

2. 默认锁:this

public class MyTicket implements Runnable{
    //定义100张票
    int ticket = 100;

    @Override
    public void run() {
        while(true){
            try {
                Thread.sleep(100L);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
           //method01();
            method02();
        }
    }

   /* 
   public synchronized void method01(){
        if (ticket>0){
            System.out.println(Thread.currentThread().getName()+"买了第"+ticket+"张票");
            ticket--;
        }
    }
    */
   public void method02(){  //
       synchronized(this){
           System.out.println(this+"..........");
           if (ticket>0){
               System.out.println(Thread.currentThread().getName()+"买了第"+ticket+"张票");
               ticket--;
           }
       }
   }
}

public class Test01 {
    public static void main(String[] args) {
        MyTicket myTicket = new MyTicket();
        System.out.println(myTicket);

        Thread t1 = new Thread(myTicket, "赵四");
        Thread t2 = new Thread(myTicket, "刘能");
        Thread t3 = new Thread(myTicket, "广坤");

        t1.start();
        t2.start();
        t3.start();
    }
}

静态同步方法

基本流程：

1. 格式:

修饰符 static synchronized 返回值类型 方法名(参数){
    方法体
    return 结果
}

2. 默认锁:class对象

public class MyTicket implements Runnable{
    //定义100张票
    static int ticket = 100;

    @Override
    public void run() {
        while(true){
            try {
                Thread.sleep(100L);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
           //method01();
            method02();
        }
    }

    /*public static synchronized void method01(){
        if (ticket>0){
            System.out.println(Thread.currentThread().getName()+"买了第"+ticket+"张票");
            ticket--;
        }
    }*/
   public static void method02(){
       synchronized(MyTicket.class){
           if (ticket>0){
               System.out.println(Thread.currentThread().getName()+"买了第"+ticket+"张票");
               ticket--;
           }
       }

   }
}

public class Test01 {
    public static void main(String[] args) {
        MyTicket myTicket = new MyTicket();

        Thread t1 = new Thread(myTicket, "赵四");
        Thread t2 = new Thread(myTicket, "刘能");
        Thread t3 = new Thread(myTicket, "广坤");

        t1.start();
        t2.start();
        t3.start();
    }
}