死锁

死锁

• 多个线程各自占有一些共享资源 , 并且互相等待其他线程占有的资源才能运行 , 而导致两个或者多个线程都在等待对方释放资源 , 都停止执行的情形 . 某一个同步块 同时拥有 “ 两个以上对象的锁 ” 时 , 就可能会发生 “ 死锁 ” 的问题 .

• 鱼与熊掌不能兼得

  public class Demo04 {
      public static void main(String[] args) {
          Eat person1 = new Eat(0,"小王");
          Eat person2 = new Eat(1,"小李");
          person1.start();
          person2.start();
      }
  }
  
  class Fish{
      String mc = "吃鱼！";
  }
  
  class Bear{
      String mc = "吃熊掌！";
  }
  
  class Eat extends Thread{
      //需要的资源只有一份，用static来表示
      static Fish fish = new Fish();
      static Bear bear = new Bear();
  
      int choice;
      String name;
  
      public Eat(int choice, String name) {
          this.choice = choice;
          this.name = name;
      }
  
      @Override
      public void run() {
          try {
              eat();
          } catch (InterruptedException e) {
              e.printStackTrace();
          }
      }
      //吃饭，互相持有对方的锁，需要拿到对方的资源
      private void eat() throws InterruptedException {
          if (choice == 0){
              synchronized (fish){
                  System.out.println(this.name+fish.mc);
                  Thread.sleep(1000);
                  synchronized (bear){  //一秒钟后吃熊掌
                  System.out.println(this.name+bear.mc);
                  }
              }
  
          }else {
              synchronized (bear){
                  System.out.println(this.name+bear.mc);
                  Thread.sleep(1000);
                  synchronized (fish){  //一秒钟后吃鱼
                  System.out.println(this.name+fish.mc);
           	   }
              }
          }
      }
  }
  

  • 解决死锁：当小王想要抢小明手中熊掌的时候，必须等待对方释放锁后，才能拿得到，不在产生死锁

  public class Demo04 {
      public static void main(String[] args) {
          Eat person1 = new Eat(0,"小王");
          Eat person2 = new Eat(1,"小李");
          person1.start();
          person2.start();
      }
  }
  
  class Fish{
      String mc = "吃鱼！";
  }
  
  class Bear{
      String mc = "吃熊掌！";
  }
  
  class Eat extends Thread{
      //需要的资源只有一份，用static来表示
      static Fish fish = new Fish();
      static Bear bear = new Bear();
  
      int choice;
      String name;
  
      public Eat(int choice, String name) {
          this.choice = choice;
          this.name = name;
      }
  
      @Override
      public void run() {
          try {
              eat();
          } catch (InterruptedException e) {
              e.printStackTrace();
          }
      }
      //吃饭，互相持有对方的锁，需要拿到对方的资源
      private void eat() throws InterruptedException {
          if (choice == 0){
              synchronized (fish){
                  System.out.println(this.name+fish.mc);
                  Thread.sleep(1000);
              }
              synchronized (bear){  //一秒钟后吃熊掌
                  System.out.println(this.name+bear.mc);
              }
  
          }else {
              synchronized (bear){
                  System.out.println(this.name+bear.mc);
                  Thread.sleep(1000);
              }
              synchronized (fish){  //一秒钟后吃鱼
                  System.out.println(this.name+fish.mc);
              }
          }
      }
  }