深入研究ReentrantLock（重入锁）之引出话题篇
synchronized原语和ReentrantLock在一般情况下没有什么区别，但是在非常复杂的同步应用中，请考虑使用ReentrantLock，特别是遇到下面2种需求的时候。
1.某个线程在等待一个锁的控制权的这段时间需要中断
2.需要分开处理一些wait-notify，ReentrantLock里面的Condition应用，能够控制notify哪个线程
3.具有公平锁功能，每个到来的线程都将排队等候
下面细细道来……
先说第一种情况，ReentrantLock的lock机制有2种，忽略中断锁和响应中断锁，这给我们带来了很大的灵活性。比如：如果A、B2个线程去竞争锁，A线程得到了锁，B线程等待，但是A线程这个时候实在有太多事情要处理，就是一直不返回，B线程可能就会等不及了，想中断自己，不再等待这个锁了，转而处理其他事情。这个时候ReentrantLock就提供了2种机制，第一，B线程中断自己（或者别的线程中断它），但是ReentrantLock不去响应，继续让B线程等待，你再怎么中断，我全当耳边风（synchronized原语就是如此）；第二，B线程中断自己（或者别的线程中断它），ReentrantLock处理了这个中断，并且不再等待这个锁的到来，完全放弃。（如果你没有了解java的中断机制，请参考下相关资料，再回头看这篇文章，80%的人根本没有真正理解什么是java的中断，呵呵）
这里来做个试验，首先搞一个Buffer类，它有读操作和写操作，为了不读到脏数据，写和读都需要加锁，我们先用synchronized原语来加锁，如下：
Java代码
public class Buffer {???
????
??? private Object lock;???
????
??? public Buffer() {???
??????? lock = this;???
??? }???
????
??? public void write() {???
??????? synchronized (lock) {???
??????????? long startTime = System.currentTimeMillis();???
??????????? System.out.println("开始往这个buff写入数据…");???
??????????? for (;;)// 模拟要处理很长时间???
??????????? {???
??????????????? if (System.currentTimeMillis()???
??????????????????????? - startTime > Integer.MAX_VALUE)???
??????????????????? break;???
??????????? }???
??????????? System.out.println("终于写完了");???
??????? }???
??? }???
????
??? public void read() {???
??????? synchronized (lock) {???
??????????? System.out.println("从这个buff读数据");???
??????? }???
??? }???
}??
接着，我们来定义2个线程，一个线程去写，一个线程去读。
Java代码
public class Writer extends Thread {???
????
??? private Buffer buff;???
????
??? public Writer(Buffer buff) {???
??????? this.buff = buff;???
??? }???
????
??? @Override???
??? public void run() {???
??????? buff.write();???
??? }???
????
}???
????
public class Reader extends Thread {???
????
??? private Buffer buff;???
????
??? public Reader(Buffer buff) {???
??????? this.buff = buff;???
??? }???
????
??? @Override???
??? public void run() {???
????
??????? buff.read();//这里估计会一直阻塞???
????
??????? System.out.println("读结束");???
????
??? }???
????
}??
好了，写一个Main来试验下，我们有意先去“写”，然后让“读”等待，“写”的时间是无穷的，就看“读”能不能放弃了。
Java代码
public class Test {???
??? public static void main(String[] args) {???
??????? Buffer buff = new Buffer();???
????
??????? final Writer writer = new Writer(buff);???
??????? final Reader reader = new Reader(buff);???
????
??????? writer.start();???
??????? reader.start();???
????
??????? new Thread(new Runnable() {???
????
??????????? @Override???
??????????? public void run() {???
??????????????? long start = System.currentTimeMillis();???
??????????????? for (;;) {???
??????????????????? //等5秒钟去中断读???
??????????????????? if (System.currentTimeMillis()???
??????????????????????????? - start > 5000) {???
??????????????????????? System.out.println("不等了，尝试中断");???
??????????????????????? reader.interrupt();???
??????????????????????? break;???
??????????????????? }???
????
??????????????? }???
????
??????????? }???
??????? }).start();???
????
??? }???
}??
我们期待“读”这个线程能退出等待锁，可是事与愿违，一旦读这个线程发现自己得不到锁，就一直开始等待了，就算它等死，也得不到锁，因为写线程要21亿秒才能完成 T_T ，即使我们中断它，它都不来响应下，看来真的要等死了。这个时候，ReentrantLock给了一种机制让我们来响应中断，让“读”能伸能屈，勇敢放弃对这个锁的等待。我们来改写Buffer这个类，就叫BufferInterruptibly吧，可中断缓存。
Java代码
import java.util.concurrent.locks.ReentrantLock;???
????
public class BufferInterruptibly {???
????
??? private ReentrantLock lock = new ReentrantLock();???
????
??? public void write() {???
??????? lock.lock();???
??????? try {???
??????????? long startTime = System.currentTimeMillis();???
??????????? System.out.println("开始往这个buff写入数据…");???
??????????? for (;;)// 模拟要处理很长时间???
??????????? {???
??????????????? if (System.currentTimeMillis()???
??????????????????????? - startTime > Integer.MAX_VALUE)???
??????????????????? break;???
??????????? }???
??????????? System.out.println("终于写完了");???
??????? } finally {???
??????????? lock.unlock();???
??????? }???
??? }???
????
??? public void read() throws InterruptedException {???
??????? lock.lockInterruptibly();// 注意这里，可以响应中断???
??????? try {???
??????????? System.out.println("从这个buff读数据");???
??????? } finally {???
??????????? lock.unlock();???
??????? }???
??? }???
????
}??
当然，要对reader和writer做响应的修改
Java代码
public class Reader extends Thread {???
????
??? private BufferInterruptibly buff;???
????
??? public Reader(BufferInterruptibly buff) {???
??????? this.buff = buff;???
??? }???
????
??? @Override???
??? public void run() {???
????
??????? try {???
??????????? buff.read();//可以收到中断的异常，从而有效退出???
??????? } catch (InterruptedException e) {???
??????????? System.out.println("我不读了");???
??????? }???
??????????
??????? System.out.println("读结束");???
????
??? }???
????
}???
????
/**??
* Writer倒不用怎么改动??
*/???
public class Writer extends Thread {???
????
??? private BufferInterruptibly buff;???
????
??? public Writer(BufferInterruptibly buff) {???
??????? this.buff = buff;???
??? }???
????
??? @Override???
??? public void run() {???
??????? buff.write();???
??? }???
????
}???
????
public class Test {???
??? public static void main(String[] args) {???
??????? BufferInterruptibly buff = new BufferInterruptibly();???
????
??????? final Writer writer = new Writer(buff);???
??????? final Reader reader = new Reader(buff);???
????
??????? writer.start();???
??????? reader.start();???
????
??????? new Thread(new Runnable() {???
????
??????????? @Override???
??????????? public void run() {???
??????????????? long start = System.currentTimeMillis();???
??????????????? for (;;) {???
??????????????????? if (System.currentTimeMillis()???
??????????????????????????? - start > 5000) {???
??????????????????????? System.out.println("不等了，尝试中断");???
??????????????????????? reader.interrupt();???
??????????????????????? break;???
??????????????????? }???
????
??????????????? }???
????
??????????? }???
??????? }).start();???
????
??? }???
}??
这次“读”线程接收到了lock.lockInterruptibly()中断，并且有效处理了这个“异常”。
