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Java:使用synchronized和Lock对象获取对象锁
www.jz123.cn 2009-09-08 来源: IT专家网 责任编辑(袁袁) 我要投递新闻
1.对象的锁
所有对象都自动含有单一的锁。
JVM负责跟踪对象被加锁的次数。如果一个对象被解锁,其计数变为0。在任务(线程)第一次给对象加锁的时候,计数变为1。每当这个相同的任务(线程)在此对象上获得锁时,计数会递增。
只有首先获得锁的任务(线程)才能继续获取该对象上的多个锁。
每当任务离开一个synchronized方法,计数递减,当计数为0的时候,锁被完全释放,此时别的任务就可以使用此资源。
2.synchronized同步块
2.1同步到单一对象锁
当使用同步块时,如果方法下的同步块都同步到一个对象上的锁,则所有的任务(线程)只能互斥的进入这些同步块。
Resource1.java演示了三个线程(包括main线程)试图进入某个类的三个不同的方法的同步块中,虽然这些同步块处在不同的方法中,但由于是同步到同一个对象(当前对象 synchronized (this)),所以对它们的方法依然是互斥的。
Resource1.java package com.zj.lock; import java.util.concurrent.TimeUnit; public class Resource1 { public void f() { // other operations should not be locked... System.out.println(Thread.currentThread().getName() + ":not synchronized in f()"); synchronized (this) { for (int i = 0; i < 5; i++) { System.out.println(Thread.currentThread().getName() + ":synchronized in f()"); try { TimeUnit.SECONDS.sleep(3); } catch (InterruptedException e) { e.printStackTrace(); } } } } public void g() { // other operations should not be locked... System.out.println(Thread.currentThread().getName() + ":not synchronized in g()"); synchronized (this) { for (int i = 0; i < 5; i++) { System.out.println(Thread.currentThread().getName() + ":synchronized in g()"); try { TimeUnit.SECONDS.sleep(3); } catch (InterruptedException e) { e.printStackTrace(); } } } } public void h() { // other operations should not be locked... System.out.println(Thread.currentThread().getName() + ":not synchronized in h()"); synchronized (this) { for (int i = 0; i < 5; i++) { System.out.println(Thread.currentThread().getName() + ":synchronized in h()"); try { TimeUnit.SECONDS.sleep(3); } catch (InterruptedException e) { e.printStackTrace(); } } } } public static void main(String[] args) { final Resource1 rs = new Resource1(); new Thread() { public void run() { rs.f(); } }.start(); new Thread() { public void run() { rs.g(); } }.start(); rs.h(); } } |
结果:
Thread-0:not synchronized in f()
Thread-0:synchronized in f()
main:not synchronized in h()
Thread-1:not synchronized in g()
Thread-0:synchronized in f()
Thread-0:synchronized in f()
Thread-0:synchronized in f()
Thread-0:synchronized in f()
Thread-1:synchronized in g()
Thread-1:synchronized in g()
Thread-1:synchronized in g()
Thread-1:synchronized in g()
Thread-1:synchronized in g()
main:synchronized in h()
main:synchronized in h()
main:synchronized in h()
main:synchronized in h()
main:synchronized in h()
2.2 同步到多个对象锁
Resource1.java演示了三个线程(包括main线程)试图进入某个类的三个不同的方法的同步块中,这些同步块处在不同的方法中,并且是同步到三个不同的对象(synchronized (this),synchronized (syncObject1),synchronized (syncObject2)),所以对它们的方法中的临界资源访问是独立的。
Resource2.java package com.zj.lock; import java.util.concurrent.TimeUnit; public class Resource2 { private Object syncObject1 = new Object(); private Object syncObject2 = new Object(); public void f() { // other operations should not be locked... System.out.println(Thread.currentThread().getName() + ":not synchronized in f()"); synchronized (this) { for (int i = 0; i < 5; i++) { System.out.println(Thread.currentThread().getName() + ":synchronized in f()"); try { TimeUnit.SECONDS.sleep(3); } catch (InterruptedException e) { e.printStackTrace(); } } } } public void g() { // other operations should not be locked... System.out.println(Thread.currentThread().getName() + ":not synchronized in g()"); synchronized (syncObject1) { for (int i = 0; i < 5; i++) { System.out.println(Thread.currentThread().getName() + ":synchronized in g()"); try { TimeUnit.SECONDS.sleep(3); } catch (InterruptedException e) { e.printStackTrace(); } } } } public void h() { // other operations should not be locked... System.out.println(Thread.currentThread().getName() + ":not synchronized in h()"); synchronized (syncObject2) { for (int i = 0; i < 5; i++) { System.out.println(Thread.currentThread().getName() + ":synchronized in h()"); try { TimeUnit.SECONDS.sleep(3); } catch (InterruptedException e) { e.printStackTrace(); } } } } public static void main(String[] args) { final Resource2 rs = new Resource2(); new Thread() { public void run() { rs.f(); } }.start(); new Thread() { public void run() { rs.g(); } }.start(); rs.h(); } } |
结果:
Thread-0:not synchronized in f()
Thread-0:synchronized in f()
main:not synchronized in h()
main:synchronized in h()
Thread-1:not synchronized in g()
Thread-1:synchronized in g()
Thread-0:synchronized in f()
main:synchronized in h()
Thread-1:synchronized in g()
Thread-0:synchronized in f()
main:synchronized in h()
Thread-1:synchronized in g()
Thread-0:synchronized in f()
main:synchronized in h()
Thread-1:synchronized in g()
Thread-0:synchronized in f()
main:synchronized in h()
Thread-1:synchronized in g()
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