java使用synchronized加锁

synchronized的作用：同步代码块

不使用synchronized

public class MyRunnable implements Runnable{

    private Integer id = 0;

    @Override
    public void run() {
            for (int i = 0; i < 100; i++) {
                System.out.println("我是 "+Thread.currentThread().getName()+" 的id: "+(++id));
            }
    }

}

public static void main(String[] args) {
    MyRunnable myRunnable = new MyRunnable();
    for (int i = 0; i < 50; i++) {
        new Thread(myRunnable).start();
    }
}

结果：

...
我是 Thread-10 的id: 4983
我是 Thread-10 的id: 4984
我是 Thread-10 的id: 4985
我是 Thread-10 的id: 4986
我是 Thread-10 的id: 4987

Process finished with exit code 0

使用synchronized

• 方式一：

  public class MyRunnable implements Runnable{
  
      private Integer id = 0;
  
      @Override
      public synchronized void run() {
              for (int i = 0; i < 100; i++) {
                  System.out.println("我是 "+Thread.currentThread().getName()+" 的id: "+(++id));
              }
      }
  
  }
  

  public static void main(String[] args) {
      MyRunnable myRunnable = new MyRunnable();
      for (int i = 0; i < 50; i++) {
          new Thread(myRunnable).start();
      }
  }
  

  结果：

  ...
  我是 Thread-15 的id: 4996
  我是 Thread-15 的id: 4997
  我是 Thread-15 的id: 4998
  我是 Thread-15 的id: 4999
  我是 Thread-15 的id: 5000
  
  Process finished with exit code 0
  

• 方式二：

  public class MyRunnable implements Runnable{
  
      private Integer id = 0;
      private Object obj;
  
      public MyRunnable(Object obj) {
          this.obj = obj;
      }
  
      @Override
      public void run() {
          synchronized (obj){
              for (int i = 0; i < 100; i++) {
                  System.out.println("我是 "+Thread.currentThread().getName()+" 的id: "+(++id));
              }
          }
  
  
      }
  
  }
  

  public static void main(String[] args) {
      Object o = new Object(); //加锁对象
      MyRunnable myRunnable = new MyRunnable(o);
      for (int i = 0; i < 50; i++) {
          new Thread(myRunnable).start();
      }
  }
  

  结果：

  ...
  我是 Thread-10 的id: 4997
  我是 Thread-10 的id: 4998
  我是 Thread-10 的id: 4999
  我是 Thread-10 的id: 5000
  
  Process finished with exit code 0
  

• 方式三：

  public class MyRunnable implements Runnable{
  
      private Integer id = 0;
      private static Object obj = new Object();
  
      @Override
      public void run() {
          synchronized (obj){
              for (int i = 0; i < 100; i++) {
                  System.out.println("我是 "+Thread.currentThread().getName()+" 的id: "+(++id));
              }
          }
  
      }
  
  }
  

  public static void main(String[] args) {
      MyRunnable myRunnable = new MyRunnable();
      for (int i = 0; i < 50; i++) {
          new Thread(myRunnable).start();
      }
  }
  

  结果：

  ...
  我是 Thread-5 的id: 4996
  我是 Thread-5 的id: 4997
  我是 Thread-5 的id: 4998
  我是 Thread-5 的id: 4999
  我是 Thread-5 的id: 5000
  
  Process finished with exit code 0