synchronized关键字的使用
- 在日常开发中我们经常看到synchronized关键字,常见的几种应用是synchronized(this)、synchronized(class)与synchronized(Object)，和synchronize方法，静态同步synchronized方法，这几种方式究竟有什么区别呢，当初学习java同步锁那块的时候好像就没怎么注意过，现在来通过实例比较下：
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对于synchronized(this):
//核心业务代码 public class HotProductService { public void serviceMethodA(){//业务A try { synchronized (this) { Log.d("HotProductService","A begin time="+System.currentTimeMillis()); Thread.sleep(2000); Log.d("HotProductService","A end time="+System.currentTimeMillis()); } } catch (InterruptedException e) { e.printStackTrace(); } } public void serviceMethodB(){ synchronized (this) { Log.d("HotProductService","B begin time="+System.currentTimeMillis()); Log.d("HotProductService","B end time="+System.currentTimeMillis()); } } } //线程1 public class ThreadA extends Thread{ private HotProductService objectService; public ThreadA(HotProductService objectService){ super(); this.objectService=objectService; } @Override public void run() { super.run();//调用业务a objectService.serviceMethodA(); } } //线程2 public class ThreadB extends Thread { private HotProductService objectService; public ThreadB(HotProductService objectService){ super(); this.objectService=objectService; } @Override public void run() { super.run()；//调用业务b objectService.serviceMethodB(); } } //测试代码 private void test1() { HotProductService service=new HotProductService(); ThreadA a=new ThreadA(service); a.start(); ThreadB b=new ThreadB(service); b.start(); }
运行结果:
02-24 10:05:12.135 6434-7477/com.example.refreshdemo D/HotProductService: A begin time=1519437912141
02-24 10:05:14.135 6434-7477/com.example.refreshdemo D/HotProductService: A end time=1519437914142
02-24 10:05:14.135 6434-7478/com.example.refreshdemo D/HotProductService: B begin time=1519437914145
02-24 10:05:14.135 6434-7478/com.example.refreshdemo D/HotProductService: B end time=1519437914146
上面的结果我们能看到即使A业务耗时较长，B业务也并不会获得执行的机会，如果我们将核心业务代码中B业务或者A业务的同步锁去掉，其他照旧会是怎么样
...
比如去掉B业务的同步锁
public void serviceMethodB(){
//synchronized (this)
{
Log.d("HotProductService","B begin time="+System.currentTimeMillis());
Log.d("HotProductService","B end time="+System.currentTimeMillis());
}
}
...
运行结果:
02-24 02:27:19.248 3562-3629/com.example.refreshdemo D/HotProductService: A begin time=1519439239248
02-24 02:27:19.248 3562-3630/com.example.refreshdemo D/HotProductService: B begin time=1519439239248
02-24 02:27:19.248 3562-3630/com.example.refreshdemo D/HotProductService: B end time=1519439239248
02-24 02:27:21.248 3562-3629/com.example.refreshdemo D/HotProductService: A end time=1519439241248
结论：synchronized (this)使用的对象监视器是一个,即是该对象自身，当一个线程访问HotProductService的一个synchronized (this)同步代码块时，其它线程对同一个HotProductService中其它的synchronized (this)同步代码块的访问将是堵塞，实现了代码顺序的同步执行。
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对于synchronized修饰的方法:
上面的结论对于synchronized修饰的方法是否同样有效呢，通过实例看下://核心业务代码 public class HotProductService { //业务A public synchronized void objectMethodA(){ Log.d("HotProductService","run----objectMethodA2"); } //业务B public void objectMethodB(){ synchronized (this) { try { for (int i = 1; i <= 10; i++) { Log.d("HotProductService","synchronized thread name:"+Thread.currentThread().getName()+"-->i="+i); Thread.sleep(1000); } } catch (InterruptedException e) { e.printStackTrace(); } } } } //A线程 public class ThreadA2 extends Thread{ private HotProductService objectService; public ThreadA2(HotProductService objectService){ super(); this.objectService=objectService; } @Override public void run() { super.run(); objectService.objectMethodA(); } } //B线程 public class ThreadB2 extends Thread { private HotProductService objectService; public ThreadB2(HotProductService objectService){ super(); this.objectService=objectService; } @Override public void run() { super.run(); objectService.objectMethodB(); } } private void test2() { HotProductService service=new HotProductService(); ThreadA2 a=new ThreadA2(service); a.start(); ThreadB2 b=new ThreadB2(service); b.start(); }
运行结果：
02-24 02:48:32.212 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=1
02-24 02:48:33.213 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=2
02-24 02:48:34.214 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=3
02-24 02:48:35.215 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=4
02-24 02:48:36.216 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=5
02-24 02:48:37.217 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=6
02-24 02:48:38.218 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=7
02-24 02:48:39.219 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=8
02-24 02:48:40.220 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=9
02-24 02:48:41.221 3562-3765/com.example.refreshdemo D/HotProductService: synchronized thread name:Thread-166-->i=10
02-24 02:48:42.223 3562-3764/com.example.refreshdemo D/HotProductService: run----objectMethodA2
同样验证了只能同时被一个线程所访问synchronized代码块。其他线程遇到synchronized代码块将会堵塞。
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那么synchronized(Object)呢
//核心业务代码 public class HotProductService { private String uname; private String pwd; String lock=new String(); public void setUserNamePassWord(String userName,String passWord){ try { synchronized (lock) { Log.d("HotProductService","thread name=" + Thread.currentThread().getName() + " 进入代码快:" + System.currentTimeMillis()); uname=userName; Thread.sleep(3000); pwd=passWord; Log.d("HotProductService","thread name="+Thread.currentThread().getName() +" 进入代码快:"+System.currentTimeMillis()+"入参uname:"+uname+"入参pwd:"+pwd); } } catch (InterruptedException e) { e.printStackTrace(); } } } //A线程 public class ThreadA3 extends Thread{ private HotProductService objectService; public ThreadA3(HotProductService objectService){ super(); this.objectService=objectService; } @Override public void run() { super.run(); objectService.setUserNamePassWord("a","aa"); } } //B线程 public class ThreadB3 extends Thread { private HotProductService objectService; public ThreadB3(HotProductService objectService){ super(); this.objectService=objectService; } @Override public void run() { super.run(); objectService.setUserNamePassWord("b","bb"); } } HotProductService service=new HotProductService(); ThreadA2 a=new ThreadA2(service); a.start(); ThreadB2 b=new ThreadB2(service); b.start();
运行结果：
02-24 02:57:41.380 3975-4016/? D/HotProductService: thread name=A 进入代码快:1519441061380
02-24 02:57:44.382 3975-4016/com.example.refreshdemo D/HotProductService: thread name=A 进入代码快:1519441064382入参uname:a入参pwd:aa
02-24 02:57:44.382 3975-4017/com.example.refreshdemo D/HotProductService: thread name=B 进入代码快:1519441064382
02-24 02:57:47.383 3975-4017/com.example.refreshdemo D/HotProductService: thread name=B 进入代码快:1519441067383入参uname:b入参pwd:bb
如果我们将
//核心业务代码
public class HotProductService {
private String uname;
private String pwd;
//String lock=new String();移入方法会得到什么样的结果呢
public void setUserNamePassWord(String userName,String passWord){
try {
String lock=new String();
synchronized (lock) {
Log.d("HotProductService","thread name=" + Thread.currentThread().getName()
+ " 进入代码快:" + System.currentTimeMillis());
uname=userName;
Thread.sleep(3000);
pwd=passWord;
Log.d("HotProductService","thread name="+Thread.currentThread().getName()
+" 进入代码快:"+System.currentTimeMillis()+"入参uname:"+uname+"入参pwd:"+pwd);
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
其他代码照旧得到
运行结果：
02-24 03:04:16.757 4148-4187/? D/HotProductService: thread name=A 进入代码快:1519441456757
02-24 03:04:16.758 4148-4188/? D/HotProductService: thread name=B 进入代码快:1519441456758
02-24 03:04:19.758 4148-4187/com.example.refreshdemo D/HotProductService: thread name=A 进入代码快:1519441459758入参uname:b入参pwd:aa
02-24 03:04:19.759 4148-4188/com.example.refreshdemo D/HotProductService: thread name=B 进入代码快:1519441459759入参uname:b入参pwd:bb
是不是很奇怪出错了？为什么呢 移入方法内部就不对了，比较前后改变，发现修改后每次调用方法时（哪个线程调用方法时）都会重新new一个对象作为对象锁，但是修改前不管哪次进入方法时（哪个线程调用方法时）用的都是同一个对象，所以我们得到结论:
使用自定义任意对象进行同步锁 不同线程必须为同一对象，否则仍旧是异步运行的
- synchronized(.class)与静态同步synchronized方法
可以直接给出的结论是这两种方式的同步是一样的，使用的同步锁都是对应的类作为对象锁，在jvm中类是唯一的，那也就是说对任何对象都是同步的，因为具有唯一的对象锁。下面来验证下：
//核心业务代码
public class HotProductService {
//业务A
public void methodA(){
try {
synchronized (HotProductService.class){
Log.d("HotProductService","static methodA begin Name:" + Thread.currentThread().getName() + " times:" + System.currentTimeMillis());
Thread.sleep(2000);
Log.d("HotProductService","static methodA end Name:"+Thread.currentThread().getName()+" times:"+System.currentTimeMillis());
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
//业务B
public void methodB(){
synchronized (HotProductService.class){
Log.d("HotProductService","static methodB begin Name:"+Thread.currentThread().getName()+" times:"+System.currentTimeMillis());
Log.d("HotProductService","static methodB end Name:"+Thread.currentThread().getName()+" times:"+System.currentTimeMillis());
}
}
}
//A线程
public class ThreadA5 extends Thread{
private HotProductService mHotProductService;
public ThreadA5(HotProductService hotProductService){
mHotProductService = hotProductService;
}
@Override
public void run() {
super.run();
mHotProductService.methodA();
}
}
//B线程
public class ThreadB5 extends Thread {
private HotProductService mHotProductService;
public ThreadB5(HotProductService hotProductService){
mHotProductService = hotProductService;
}
@Override
public void run() {
super.run();
mHotProductService.methodB();
}
}
private void test5() {
HotProductService service= new HotProductService();
ThreadA5 a=new ThreadA5(service);
a.start();
ThreadB5 b=new ThreadB5(service);
b.start();
}
运行结果：
02-24 03:24:11.250 4148-4282/com.example.refreshdemo D/HotProductService: static methodA begin Name:Thread-174 times:1519442651250
02-24 03:24:13.252 4148-4282/com.example.refreshdemo D/HotProductService: static methodA end Name:Thread-174 times:1519442653252
02-24 03:24:13.252 4148-4283/com.example.refreshdemo D/HotProductService: static methodB begin Name:Thread-175 times:1519442653252
02-24 03:24:13.252 4148-4283/com.example.refreshdemo D/HotProductService: static methodB end Name:Thread-175 times:1519442653252
使用.class作为对象锁保证了不能线程的同步。
引用网上常见的说法就是:
synchronize修饰的方法和 synchronize(this) 都是锁住自己本身的对象 而synchronize(class) synchronize(object) 都是锁别的对象
