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类锁和对象锁是否会冲突?对象锁和私有锁是否会冲突?通过实例来进行说明。
一、相关约定
为了明确后文的描述,先对本文涉及到的锁的相关定义作如下约定:
1. 类锁:在代码中的方法上加了static和synchronized的锁,或者synchronized(xxx.class)的代码段,如下文中的increament();
2.对象锁:在代码中的方法上加了synchronized的锁,或者synchronized(this)的代码段,如下文中的synOnMethod()和synInMethod();
3.私有锁:在类内部声明一个私有属性如private Object lock,在需要加锁的代码段synchronized(lock),如下文中的synMethodWithObj()。
二、测试代码
1.编写一个启动类ObjectLock
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public class ObjectLock { |
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public static void main(String[] args) { |
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System.out.println( "start time = " + System.currentTimeMillis()+ "ms" ); |
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LockTestClass test = new LockTestClass(); |
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for ( int i = 0 ; i < 3 ; i++) { |
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Thread thread = new ObjThread(test, i); |
2.编写一个线程类ObjThread,用于启动同步方法(注意它的run方法可能会调整以进行不同的测试)
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public class ObjThread extends Thread { |
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public ObjThread(LockTestClass lock, int i) { |
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LockTestClass.increment(); |
3.再编写一个锁的测试类LockTestClass,包括各种加锁方法
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public class LockTestClass { |
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private static int i = 0 ; |
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private Object object = new Object(); |
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public void noSynMethod( long threadID, ObjThread thread) { |
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System.out.println( "nosyn: class obj is " + thread + ", threadId is" |
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public synchronized void synOnMethod() { |
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System.out.println( "synOnMethod begins" + ", time = " |
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+ System.currentTimeMillis() + "ms" ); |
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} catch (InterruptedException e) { |
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System.out.println( "synOnMethod ends" ); |
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* 对象锁方法2,采用synchronized (this)来加锁 |
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public void synInMethod() { |
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System.out.println( "synInMethod begins" + ", time = " |
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+ System.currentTimeMillis() + "ms" ); |
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} catch (InterruptedException e) { |
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System.out.println( "synInMethod ends" ); |
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public void synMethodWithObj() { |
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synchronized (object) { |
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System.out.println( "synMethodWithObj begins" + ", time = " |
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+ System.currentTimeMillis() + "ms" ); |
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} catch (InterruptedException e) { |
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System.out.println( "synMethodWithObj ends" ); |
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public static synchronized void increament() { |
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System.out.println( "class synchronized. i = " + i + ", time = " |
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+ System.currentTimeMillis() + "ms" ); |
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} catch (InterruptedException e) { |
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System.out.println( "class synchronized ends." ); |
三、测试结果
1.测试类锁和对象锁,ObjectThread的run方法修改如下:
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LockTestClass.increament(); |
终端输出:
start time = 1413101360231ms
synInMethod begins, time = 1413101360233ms
synInMethod ends
class synchronized. i = 0, time = 1413101362233ms
synInMethod begins, time = 1413101362233ms
class synchronized ends.
synInMethod ends
class synchronized. i = 1, time = 1413101364233ms
synInMethod begins, time = 1413101364233ms
class synchronized ends.
synInMethod ends
class synchronized. i = 2, time = 1413101366234ms
class synchronized ends.
可以看到对象锁方法(synInMothod)第一次启动时比类锁方法(increament)快2秒,这是因为在synInMehtod执行时sleep了2秒再执行的increament,而这两个方法共用一个线程,所以会慢2秒,如果increament在run中放到synInMethod前面,那么第一次启动时就是increament快2秒。
而当类锁方法启动时,另一个线程时的对象锁方法也几乎同时启动,说明二者使用的并非同一个锁,不会产生竞争。
结论:类锁和对象锁不会产生竞争,二者的加锁方法不会相互影响。
2.私有锁和对象锁,ObjectThread的run方法修改如下:
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lock.synMethodWithObj(); |
终端输出:
start time = 1413121912406ms
synInMethod begins, time = 1413121912407ms.
synInMethod ends.
synMethodWithObj begins, time = 1413121914407ms
synInMethod begins, time = 1413121914407ms.
synInMethod ends.
synMethodWithObj ends
synInMethod begins, time = 1413121916407ms.
synMethodWithObj begins, time = 1413121916407ms
synInMethod ends.
synMethodWithObj ends
synMethodWithObj begins, time = 1413121918407ms
synMethodWithObj ends
和类锁和对象锁非常类似。
结论:私有锁和对象锁也不会产生竞争,二者的加锁方法不会相互影响。
3.synchronized直接加在方法上和synchronized(this),ObjectThread的run方法修改如下:
public void run() {
//无锁方法
// lock.noSynMethod(this.getId(),this);
//对象锁方法1,采用synchronized synInMethod的方式
lock.synInMethod();
//对象锁方法2,采用synchronized(this)的方式
lock.synOnMethod();
//私有锁方法,采用synchronized(object)的方式
// lock.synMethodWithObj();
//类锁方法,采用static synchronized increment的方式
// LockTestClass.increament();
}
终端输出:
start time = 1413102913278ms
synInMethod begins, time = 1413102913279ms
synInMethod ends
synInMethod begins, time = 1413102915279ms
synInMethod ends
synOnMethod begins, time = 1413102917279ms
synOnMethod ends
synInMethod begins, time = 1413102919279ms
synInMethod ends
synOnMethod begins, time = 1413102921279ms
synOnMethod ends
synOnMethod begins, time = 1413102923279ms
synOnMethod ends
可以看到,二者严格地串行输出(当然再次执行时先运行synInMethod还是先运行synOnMethod并不是确定的,取决于谁获得了锁)。
结论:synchronized直接加在方法上和synchronized(this)都是对当前对象加锁,二者的加锁方法够成了竞争关系,同一时刻只能有一个方法能执行。
四、参考资料: