线程锁ReentrantLock和Condition的使用

我们通过一个实际的例子来解释Condition的用法：

我们要打印1到9这9个数字，由A线程先打印1，2，3，然后由B线程打印4,5,6，然后再由A线程打印7，8，9. 这道题有很多种解法，现在我们使用Condition来做这道题（使用Object的wait，notify方法的解法在这里）。

package cn.outofmemory.locks;import java.util.concurrent.locks.Condition;import java.util.concurrent.locks.Lock;import java.util.concurrent.locks.ReentrantLock;public class App {static class NumberWrapper {public int value = 1;}public static void main(String[] args) {//初始化可重入锁final Lock lock = new ReentrantLock();//第一个条件当屏幕上输出到3final Condition reachThreeCondition = lock.newCondition();//第二个条件当屏幕上输出到6final Condition reachSixCondition = lock.newCondition();//NumberWrapper只是为了封装一个数字，一边可以将数字对象共享，并可以设置为final//注意这里不要用Integer, Integer 是不可变对象final NumberWrapper num = new NumberWrapper();//初始化A线程Thread threadA = new Thread(new Runnable() {@Overridepublic void run() {//需要先获得锁lock.lock();try {System.out.println("threadA start write");//A线程先输出前3个数while (num.value <= 3) {System.out.println(num.value);num.value++;}//输出到3时要signal，告诉B线程可以开始了reachThreeCondition.signal();} finally {lock.unlock();}lock.lock();try {//等待输出6的条件reachSixCondition.await();System.out.println("threadA start write");//输出剩余数字while (num.value <= 9) {System.out.println(num.value);num.value++;}} catch (InterruptedException e) {e.printStackTrace();} finally {lock.unlock();}}});Thread threadB = new Thread(new Runnable() {@Overridepublic void run() {try {lock.lock();while (num.value <= 3) {//等待3输出完毕的信号reachThreeCondition.await();}} catch (InterruptedException e) {e.printStackTrace();} finally {lock.unlock();}try {lock.lock();//已经收到信号，开始输出4，5，6System.out.println("threadB start write");while (num.value <= 6) {System.out.println(num.value);num.value++;}//4，5，6输出完毕，告诉A线程6输出完了reachSixCondition.signal();} finally {lock.unlock();}}});//启动两个线程threadB.start();threadA.start();}}

 上述代码中有完整的注释，请参考注释，理解Condition的用法。

基本思路就是首先要A线程先写1，2，3，这时候B线程应该等待reachThredCondition信号，而当A线程写完3之后就通过signal告诉B线程“我写到3了，该你了”，这时候A线程要等嗲reachSixCondition信号，同时B线程得到通知，开始写4，5，6，写完4，5，6之后B线程通知A线程reachSixCondition条件成立了，这时候A线程就开始写剩下的7，8，9了。

为了更好的理解Condition的用法，我们再看下java官方提供的例子：

package locks;import java.util.Random;import java.util.concurrent.locks.Condition;import java.util.concurrent.locks.Lock;import java.util.concurrent.locks.ReentrantLock;public class AppOfficial {/** * BoundedBuffer 是一个定长100的集合，当集合中没有元素时，take方法需要等待，直到有元素时才返回元素 * 当其中的元素数达到最大值时，要等待直到元素被take之后才执行put的操作 * @author yukaizhao * */static class BoundedBuffer {final Lock lock = new ReentrantLock();final Condition notFull = lock.newCondition();final Condition notEmpty = lock.newCondition();final Object[] items = new Object[100];int putptr, takeptr, count;public void put(Object x) throws InterruptedException {System .out.println("put wait lock");lock.lock();System.out.println("put get lock");try {while (count == items.length) {System.out.println("buffer full, please wait");notFull.await();}items[putptr] = x;if (++putptr == items.length)putptr = 0;++count;notEmpty.signal();} finally {lock.unlock();}}public Object take() throws InterruptedException {System.out.println("take wait lock");lock.lock();System.out.println("take get lock");try {while (count == 0) {System.out.println("no elements, please wait");notEmpty.await();}Object x = items[takeptr];if (++takeptr == items.length)takeptr = 0;--count;notFull.signal();return x;} finally {lock.unlock();}}}public static void main(String[] args) {final BoundedBuffer boundedBuffer = new BoundedBuffer();Thread t1 = new Thread(new Runnable() {@Overridepublic void run() {System.out.println("t1 run");for (int i=0;i<1000;i++) {try {System.out.println("putting..");boundedBuffer.put(Integer.valueOf(i));} catch (InterruptedException e) {e.printStackTrace();}}}}) ;Thread t2 = new Thread(new Runnable() {@Overridepublic void run() {for (int i=0;i<1000;i++) {try {Object val = boundedBuffer.take();System.out.println(val);} catch (InterruptedException e) {e.printStackTrace();}}}}) ;t1.start();t2.start();}}

这个示例中BoundedBuffer是一个固定长度的集合，这个在其put操作时，如果发现长度已经达到最大长度，那么会等待notFull信号，如果得到notFull信号会像集合中添加元素，并发出notEmpty的信号，而在其take方法中如果发现集合长度为空，那么会等待notEmpty的信号，同时如果拿到一个元素，那么会发出notFull的信号。