Lock锁住的是什么

通常lock对象都是final的，它锁住的是mLock.lock()和mLock.unlock()之间的代码块。也就是说所有使用了同一mLock对象的锁块之间是互斥的。

import java.util.concurrent.locks.ReentrantLock;
public class LockTest{
public static void main(String[] args){
final ReentrantLock mLock = new ReentrantLock();
Thread t = new Thread(){
public void run(){

for(int i = 0; i < 3; i++){
try{
mLock.lock();
System.out.println("thread 2 running i = " + i + " start");
sleep(3000);
System.out.println("thread 2 running i = " + i + " end");
}catch(Exception e){

}finally{
mLock.unlock();
}
}

}
};
t.start();

for(int i = 0; i < 3; i++){
try{
mLock.lock();
System.out.println("thread main running i = " + i + " start");
Thread.sleep(2000);
System.out.println("thread main running i = " + i + " end");
}catch(Exception e){

}finally{
mLock.unlock();
}
}
}
}

如上代码段，注释掉两个mLock.lock()，执行结果如下：

thread main running i = 0 start
thread 2 running i = 0 start
thread main running i = 0 end
thread main running i = 1 start
thread 2 running i = 0 end
thread 2 running i = 1 start
thread main running i = 1 end
thread main running i = 2 start
thread 2 running i = 1 end
thread 2 running i = 2 start
thread main running i = 2 end
thread 2 running i = 2 end

　线程是交替执行的，如果代码区内有共享内容，将会出现线程安全问题。

有锁的情况下，执行结果如下：

thread main running i = 0 start
thread main running i = 0 end
thread main running i = 1 start
thread main running i = 1 end
thread main running i = 2 start
thread main running i = 2 end
thread 2 running i = 0 start
thread 2 running i = 0 end
thread 2 running i = 1 start
thread 2 running i = 1 end
thread 2 running i = 2 start
thread 2 running i = 2 end

　不会同时访问锁块之间的内容。

当使用公平锁——mLock =new ReentrantLock(true)，执行结果如下：

thread main running i = 0 start
thread main running i = 0 end
thread 2 running i = 0 start
thread 2 running i = 0 end
thread main running i = 1 start
thread main running i = 1 end
thread 2 running i = 1 start
thread 2 running i = 1 end
thread main running i = 2 start
thread main running i = 2 end
thread 2 running i = 2 start
thread 2 running i = 2 end

　ReentrantLock(重入锁)以及公平性

《Java并发编程从入门到精通》显示锁Lock和ReentrantLock