synchronized 实例

synchronized Java语言的关键字，当它用来修饰一个方法或者一个代码块的时候，能够保证在同一时刻最多只有一个线程执行该段代码。

先看几个例子：

实例一：

import java.util.ArrayList;
import java.util.List;

public class MyThread {
public void printNum(List list) {
  synchronized (list) {
   if (null != list) {
    for (int i = 0; i < list.size(); i++) {
     System.out.println(Thread.currentThread().getName() + ":"+ list.get(i));
     try {
      Thread.sleep(100);
     } catch (InterruptedException e) {
      e.printStackTrace();
     }
    }
   }
  }
}

public static List getList() {
  List list = new ArrayList();
  for (int i = 1; i <= 5; i++) {
   list.add(i);
  }
  return list;
}

public static void main(String[] args) {
  final MyThread mt = new MyThread();
  final MyThread mt1 = new MyThread();
  final List list = mt.getList();

  Thread t1 = new Thread(new Runnable(){
   public void run() {
    mt.printNum(list);
   }
  }, "t1");

  Thread t2 = new Thread(new Runnable(){
   public void run() {
    mt1.printNum(list);
   }
  }, "t2");
  t1.start();
  t2.start();

}

输出结果：

t1:1
t1:2
t1:3
t1:4
t1:5
t2:1
t2:2
t2:3
t2:4
t2:5

实例二：

import java.util.ArrayList;
import java.util.List;

public static List getList() {
  List list = new ArrayList();
  for (int i = 1; i <= 5; i++) {
   list.add(i);
  }
  return list;
}

public static void main(String[] args) {
  final MyThread mt = new MyThread();
  final MyThread mt1 = new MyThread();
  final List list = mt.getList();
  final List list1 = new ArrayList();
  for (int i = 1; i <= 5; i++) {
   list1.add(i);
  }
  Thread t1 = new Thread(new Runnable(){
   public void run() {
    mt.printNum(list);
   }
  }, "t1");

  Thread t2 = new Thread(new Runnable(){
   public void run() {
    mt1.printNum(list1);
   }
  }, "t2");
  t1.start();
  t2.start();

}

输出结果：

t1:1
t2:1
t1:2
t2:2
t2:3
t1:3
t1:4
t2:4
t2:5
t1:5

实例三：

import java.util.ArrayList;
import java.util.List;

public class MyThread {
public void printNum(List list) {
  synchronized (this) {
   if (null != list) {
    for (int i = 0; i < list.size(); i++) {
     System.out.println(Thread.currentThread().getName() + ":"+ list.get(i));
     try {
      Thread.sleep(100);
     } catch (InterruptedException e) {
      e.printStackTrace();
     }
    }
   }
  }
}

}

输出结果：

t1:1
t2:1
t1:2
t2:2
t1:3
t2:3
t1:4
t2:4
t1:5
t2:5

实例四：

import java.util.ArrayList;
import java.util.List;

public static List getList() {
  List list = new ArrayList();
  for (int i = 1; i <= 5; i++) {
   list.add(i);
  }
  return list;
}

public static void main(String[] args) {
  final MyThread mt = new MyThread();
  final MyThread mt1 = new MyThread();
  final List list = mt.getList();
  final List list1 = new ArrayList();
  for (int i = 1; i <= 5; i++) {
   list1.add(i);
  }
  Thread t1 = new Thread(new Runnable(){
   public void run() {
    mt.printNum(list);
   }
  }, "t1");

  Thread t2 = new Thread(new Runnable(){
   public void run() {
    mt.printNum(list1);
   }
  }, "t2");
  t1.start();
  t2.start();

}

输出结果：

t1:1
t1:2
t1:3
t1:4
t1:5
t2:1
t2:2
t2:3
t2:4
t2:5

实例五：

public static List getList() {
  List list = new ArrayList();
  for (int i = 1; i <= 5; i++) {
   list.add(i);
  }
  return list;
}

public static void main(String[] args) {
  final MyThread mt = new MyThread();
  final MyThread mt1 = new MyThread();
  final List list = mt.getList();
  final List list1 = new ArrayList();
  for (int i = 1; i <= 5; i++) {
   list1.add(i);
  }
  Thread t1 = new Thread(new Runnable(){
   public void run() {
    mt.printNum(list);
   }
  }, "t1");

  Thread t2 = new Thread(new Runnable(){
   public void run() {
    mt.printNum(list1);
   }
  }, "t2");
  t1.start();
  t2.start();

}

输出结果：

t1:1
t2:1
t1:2
t2:2
t2:3
t1:3
t1:4
t2:4
t2:5
t1:5

由以上实例可以得出，不同的线程调用的必须是同一个对象，并且该对象与synchronized 锁的对象是同一个类时，才能够保证在同一时刻最多只有一个线程执行该段代码。

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