java中每个对象都会有一个对象锁，而synchronized就是得到这个锁，看下面这个例子
import java.util.Random; public class MyData{ public synchronized void increment() { for (int i = 0; i < 10; i++) { try { Thread.sleep(new Random().nextInt(200)); } catch (Exception e) { e.printStackTrace(); } System.out.println(Thread.currentThread().getName() + ":" +i); } } public synchronized void decrement() { for (int i = 0; i < 10; i++) { try { Thread.sleep(new Random().nextInt(200)); } catch (Exception e) { e.printStackTrace(); } System.out.println(Thread.currentThread().getName() + ":" +i); } } public static void main(String[] args) { final MyData myData1 = new MyData(); // final MyData myData2 = new MyData(); new Thread(new Runnable() { @Override public void run() { myData1.increment(); } }).start(); new Thread(new Runnable() { @Override public void run() { myData1.decrement(); } }).start(); } }
无论执行多少次都是有序的，两个线程操作的是同一个对象，第一个执行的线程得到了锁，第二个线程只能等第一个线程执行完了才能拿到锁，进入方法。
再看下面这个例子
import java.util.Random; public class MyData{ public synchronized void increment() { for (int i = 0; i < 10; i++) { try { Thread.sleep(new Random().nextInt(200)); } catch (Exception e) { e.printStackTrace(); } System.out.println(Thread.currentThread().getName() + ":" +i); } } public synchronized void decrement() { for (int i = 0; i < 10; i++) { try { Thread.sleep(new Random().nextInt(200)); } catch (Exception e) { e.printStackTrace(); } System.out.println(Thread.currentThread().getName() + ":" +i); } } public static void main(String[] args) { final MyData myData1 = new MyData(); final MyData myData2 = new MyData(); new Thread(new Runnable() { @Override public void run() { myData1.increment(); } }).start(); new Thread(new Runnable() { @Override public void run() { myData2.decrement(); } }).start(); } }
执行的结果是无序的，两个对象，两把锁，故互不影响，各自执行各自的。
再来看看下面这个例子
import java.util.Random;
public class MyData{
public synchronized void increment() {
for (int i = 0; i < 10; i++) {
try {
Thread.sleep(new Random().nextInt(200));
} catch (Exception e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + ":" +i);
}
}
public static synchronized void decrement() {
for (int i = 0; i < 10; i++) {
try {
Thread.sleep(new Random().nextInt(200));
} catch (Exception e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + ":" +i);
}
}
public static void main(String[] args) {
final MyData myData1 = new MyData();
// final MyData myData2 = new MyData();
new Thread(new Runnable() {
@Override
public void run() {
myData1.increment();
}
}).start();
new Thread(new Runnable() {
@Override
public void run() {
MyData.decrement();
}
}).start();
}
}
结果也是无序的，原因和上面一样，static方法是属于Class对象的，故decrement方法锁的MyData.Class对象，而myData1.increment();锁的是myData1对象，互不干扰。
只需记住synchronized锁的是对象，每个对象有一把对象锁，拿到锁之后才能执行synchronized的方法
原文：http://www.cnblogs.com/duan-to-success/p/6888520.html