unsafe类：java不能直接访问操作系统底层，而是通过本地方法来访问。Unsafe类提供了硬件级别的原子操作

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public class Counter{
private Lock lock = new Lock();
private int count = 0;
public int inc(){
lock.lock();
this.count++;
lock.unlock();
return count;
}
}
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不可重入锁

先来设计一种锁

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public class Lock{
private boolean isLocked = false;
public synchronized void lock() throws InterruptedException{
while(isLocked){
wait();
}
isLocked = true;
}
public synchronized void unlock(){
isLocked = false;
notify();
}
}
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这其实是个不可重入锁，举个例子

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public class Count{
Lock lock = new Lock();
public void print(){
lock.lock();
doAdd();
lock.unlock();
}
public void doAdd(){
lock.lock();
//do something
lock.unlock();
}
}
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当调用print()方法时，获得了锁，这时就无法再调用doAdd()方法，这时必须先释放锁才能调用，所以称这种锁为不可重入锁，也叫自旋锁。

可重入锁

设计如下:

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public class Lock{
boolean isLocked = false;
Thread lockedBy = null;
int lockedCount = 0;
public synchronized void lock()
throws InterruptedException{
Thread thread = Thread.currentThread();
while(isLocked && lockedBy != thread){
wait();
}
isLocked = true;
lockedCount++;
lockedBy = thread;
}
public synchronized void unlock(){
if(Thread.currentThread() == this.lockedBy){
lockedCount--;
if(lockedCount == 0){
isLocked = false;
notify();
}
}
}
}
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