关键字synchronized与wait和notify/notifyAll方法相结合可以实现等待/通知模式，类ReentrantLock也可以实现同样的功能，但需要借助于Condition对象。Condition类是JDK5中出现的技术，使用它有更好的灵活性，比如可以实现多路通知功能，也就是在一个Lock对象里面可以创建多个Condition（即对象监视器）实例，线程对象可以注册在指定的Condition中，从而可以有选择性地进行线程通知，在调度线程上更加灵活。
在使用notify/notifyAll方法进行通知时，被通知的线程却是由JVM随机选择的。但使用ReentrantLock结合Condition类是可以实现前面介绍过的“选择性通知”，这个功能是非常重要的，而且在Condition类中是默认提供的。
而synchronized就相当于整个Lock对象中只有一个单一的Condition对象，所有的线程都注册到它一个对象的身上。线程开始notifyAll时，需要通知所有的WAITING线程，没有选择权，会出现相当大的效率问题。
package org.github.lujiango;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class Test06 {
static class Service {
private Lock lock = new ReentrantLock();
public Condition condition = lock.newCondition();
public void await() {
try {
lock.lock();
System.out.println("await time: " + System.currentTimeMillis());
condition.await();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public void signal() {
try {
lock.lock();
System.out.println("signal time: " + System.currentTimeMillis());
condition.signal();
} finally {
lock.unlock();
}
}
}
static class ThreadA extends Thread {
private Service service;
public ThreadA(Service service) {
this.service = service;
}
@Override
public void run() {
service.await();
}
}
public static void main(String[] args) throws InterruptedException {
Service service = new Service();
ThreadA a = new ThreadA(service);
a.start();
Thread.sleep(3000);
service.signal();
}
}
如果想单独唤醒部分线程该怎么处理呢？这时就有必要使用多个Condition对象了，也就是Condition对象可以唤醒部分指定线程，有助于提升程序运行的效率。可以先对线程进行分组，然后在唤醒指定组中的线程。
package org.github.lujiango;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class Test07 {
static class MyService {
private Lock lock = new ReentrantLock();
public Condition condition1 = lock.newCondition();
public Condition condition2 = lock.newCondition();
public void await1() {
try {
lock.lock();
System.out.println(Thread.currentThread().getName() + " begin await1");
condition1.await();
System.out.println(Thread.currentThread().getName() + " end await1");
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public void await2() {
try {
lock.lock();
System.out.println(Thread.currentThread().getName() + " begin await2");
condition2.await();
System.out.println(Thread.currentThread().getName() + " end await2");
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public void signalAll1() {
try {
lock.lock();
System.out.println(Thread.currentThread().getName() + " signalAll1");
condition1.signalAll();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public void signalAll2() {
try {
lock.lock();
System.out.println(Thread.currentThread().getName() + " signalAll2");
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
}
static class ThreadA extends Thread {
private MyService service;
public ThreadA(MyService service) {
this.service = service;
}
@Override
public void run() {
service.await1();
}
}
static class ThreadB extends Thread {
private MyService service;
public ThreadB(MyService service) {
this.service = service;
}
@Override
public void run() {
service.await2();
}
}
public static void main(String[] args) throws InterruptedException {
MyService service = new MyService();
ThreadA a = new ThreadA(service);
a.setName("A");
a.start();
ThreadB b = new ThreadB(service);
b.setName("B");
b.start();
Thread.sleep(3000);
service.signalAll2();
}
}
实现生产者/消费者模式：一对一交替打印
package org.github.lujiango;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class Test08 {
static class Service {
private Lock lock = new ReentrantLock();
private Condition condition = lock.newCondition();
private Boolean hasValue = Boolean.FALSE;
public void producer() {
try {
lock.lock();
while (hasValue) {
condition.await();
}
System.out.println("producer");
hasValue = Boolean.TRUE;
condition.signalAll();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public void consumer() {
try {
lock.lock();
while (!hasValue) {
condition.await();
}
System.out.println("consumer");
hasValue = Boolean.FALSE;
condition.signalAll();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
}
static class ThreadA extends Thread {
private Service service;
public ThreadA(Service service) {
this.service = service;
}
@Override
public void run() {
for (int i = 0; i < Integer.MAX_VALUE; i++) {
service.producer();
}
}
}
static class ThreadB extends Thread {
private Service service;
public ThreadB(Service service) {
this.service = service;
}
@Override
public void run() {
for (int i = 0; i < Integer.MAX_VALUE; i++) {
service.consumer();
}
}
}
public static void main(String[] args) {
Service service = new Service();
ThreadA a = new ThreadA(service);
a.start();
ThreadB b = new ThreadB(service);
b.start();
}
}