Java多线程问题

[MagiSu]

初学Java，有一个线程同步的问题请教：

/*
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* and open the template in the editor.
*/
package threadtest;

import java.util.logging.Level;
import java.util.logging.Logger;

/**
*
* @author magi
*/
public class Main {

/**
* @param args the command line arguments
*/
public static void main(String[] args) {
// TODO code application logic here
Thread t1 = new Thread(new ThreadTest("Me"));
Thread t2 = new Thread(new ThreadTest("NotMe"));
t1.start();
t2.start();
}
}

class ThreadTest implements Runnable {

public ThreadTest(String init) {
caller = init;
}

void runThread() {
Object O = new Object();

synchronized (O) {
for (int i = 0; i < 5; i++) {
try {
System.out.println(caller + ": The size of Mimi is=" + mimi.toString());
Thread.sleep(10);
mimi++;
Thread.sleep(10);
System.out.println(caller + ": The size of mimi is=" + mimi.toString());
} catch (InterruptedException ex) {
Logger.getLogger(ThreadTest.class.getName()).log(Level.SEVERE, null, ex);
}
}
}
}

public void run() {
runThread();
}
public String caller;
static public Integer mimi =
10;
}

这个程序输出是：
Me: The size of Mimi is=10
NotMe: The size of Mimi is=10
Me: The size of mimi is=12
Me: The size of Mimi is=12
NotMe: The size of mimi is=12
NotMe: The size of Mimi is=12
Me: The size of mimi is=14
Me: The size of Mimi is=14
NotMe: The size of mimi is=14
NotMe: The size of Mimi is=14
Me: The size of mimi is=16
Me: The size of Mimi is=16
NotMe: The size of mimi is=16
NotMe: The size of Mimi is=16
Me: The size of mimi is=17
Me: The size of Mimi is=17
NotMe: The size of mimi is=19
NotMe: The size of Mimi is=19
Me: The size of mimi is=19
NotMe: The size of mimi is=20

而我看到书上讲的，加入synchronized修饰符，就相当于所谓的“atomic”特性，应当是原子操作，也就是不能中断的。请问结果为什么不对？

[vlinux]

Object O = new Object();

因为上面这句！

因为这样一来，每个线程都有自己的Object对象，各自锁各自的，也就谈不上同步了

需要把O对象放出到线程外，确保该对象只能被生成一次，这样就能所有的线程都共享该对象了。

BTW，在Java中，一般用一个byte[0]的数组来替代一个Object

[MagiSu]

谢谢！

[老紫竹]

楼上正确。
synchronized 虽让强制拿到了锁，但可惜这个锁只属于这个线程而已
package test.thread;

import java.util.logging.Level;
import java.util.logging.Logger;

/**
 * @author magi
 */
public class Test3 {
  /**
   * @param args the command line arguments
   */
  public static void main(String[] args) {
    // TODO code application logic here
    Thread t1 = new Thread(new ThreadTest("Me"));
    Thread t2 = new Thread(new ThreadTest("NotMe"));
    t1.start();
    t2.start();
  }
}

class ThreadTest implements Runnable {
  public ThreadTest(String init) {
    caller = init;
  }

  static Object O = new Object(); // 把锁拿到这里看看吧

  void runThread() {
    synchronized (O) {
      for (int i = 0; i < 5; i++) {
        try {
          System.out.println(caller + ":   The   size   of   Mimi   is=" + mimi.toString());
          Thread.sleep(10);
          mimi++;
          Thread.sleep(10);
          System.out.println(caller + ":   The   size   of   mimi   is=" + mimi.toString());
        } catch (Exception ex) {
          Logger.getLogger(ThreadTest.class.getName()).log(Level.SEVERE, null, ex);
        }
      }
    }
  }

  public void run() {
    runThread();
  }

  public String caller;

  static public Integer mimi = 10;
}