多线程中等待/通知模式最经典的案例就是“生产者/消费者”模式。但此模式在使用上有几种“变形”，还有一些小的注意事项，但原理是基于wait/notify的。该模式可以通过操作值来实现。
一生产与一消费
代码清单1
//生产者
public class P {
private String lock;
public P(String lock) {
super();
this.lock = lock;
}
public void setValue() {
try {
synchronized (lock) {
if (!ValueObject.value.equals("")) {
lock.wait();
}
String value = System.currentTimeMillis() + "_" + System.nanoTime();
System.out.println("set的值是" + value);
ValueObject.value = value;
lock.notify();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
//消费者
public class C {
private String lock;
public C(String lock) {
super();
this.lock = lock;
}
public void getValue() {
try {
synchronized (lock) {
if (ValueObject.value.equals("")) {
lock.wait();
}
System.out.println("get的值是" + ValueObject.value);
ValueObject.value = "";
lock.notify();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
//存储值的对象
public class ValueObject {
public static String value = "";
}
//生产者线程
public class ThreadP extends Thread {
private P p;
public ThreadP(P p) {
super();
this.p = p;
}
@Override
public void run() {
while (true) {
p.setValue();
}
}
}
//消费者线程
public class ThreadC extends Thread {
private C r;
public ThreadC(C r) {
super();
this.r = r;
}
@Override
public void run() {
while (true) {
r.getValue();
}
}
}
//运行类
public class Run {
public static void main(String[] args) {
String lock = new String("");
P p = new P(lock);
C r = new C(lock);
ThreadP pThread = new ThreadP(p);
ThreadC rThread = new ThreadC(r);
pThread.start();
rThread.start();
}
}
运行结果
set的值是1514222349229_7168218698116
get的值是1514222349229_7168218698116
set的值是1514222349231_7168221053134
get的值是1514222349231_7168221053134
...
本示例是一个生产者和一个消费者进行数据的交互，在控制台中打印的日志set与get是交替运行的。
多生产与多消费
代码清单2
//生产者
public class P {
private String lock;
public P(String lock) {
super();
this.lock = lock;
}
public void setValue() {
try {
synchronized (lock) {
while (!ValueObject.value.equals("")) {
System.out.println("生产者 "
+ Thread.currentThread().getName() + " WAITING了★");
lock.wait();
}
System.out.println("生产者 " + Thread.currentThread().getName()
+ " RUNNABLE了");
String value = System.currentTimeMillis() + "_"
+ System.nanoTime();
ValueObject.value = value;
lock.notifyAll();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
//消费者
public class C {
private String lock;
public C(String lock) {
super();
this.lock = lock;
}
public void getValue() {
try {
synchronized (lock) {
while (ValueObject.value.equals("")) {
System.out.println("消费者"
+ Thread.currentThread().getName() + " WAITING了☆");
lock.wait();
}
System.out.println("消费者" + Thread.currentThread().getName()
+ " RUNNABLE了");
ValueObject.value = "";
lock.notifyAll();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
//生产者线程
public class ThreadP extends Thread {
private P p;
public ThreadP(P p) {
super();
this.p = p;
}
@Override
public void run() {
while (true) {
p.setValue();
}
}
}
//消费者x
public class ThreadC extends Thread {
private C r;
public ThreadC(C r) {
super();
this.r = r;
}
@Override
public void run() {
while (true) {
r.getValue();
}
}
}
public class ValueObject {
public static String value = "";
}
public class Run {
public static void main(String[] args) throws InterruptedException {
String lock = new String("");
P p = new P(lock);
C r = new C(lock);
ThreadP[] pThread = new ThreadP[2];
ThreadC[] rThread = new ThreadC[2];
for (int i = 0; i < 2; i++) {
pThread[i] = new ThreadP(p);
pThread[i].setName("生产者" + (i + 1));
rThread[i] = new ThreadC(r);
rThread[i].setName("消费者" + (i + 1));
pThread[i].start();
rThread[i].start();
}
Thread.sleep(5000);
Thread[] threadArray = new Thread[Thread.currentThread()
.getThreadGroup().activeCount()];
Thread.currentThread().getThreadGroup().enumerate(threadArray);
for (int i = 0; i < threadArray.length; i++) {
System.out.println(threadArray[i].getName() + " "
+ threadArray[i].getState());
}
}
}
运行结果
生产者 生产者1 RUNNABLE了
生产者 生产者1 WAITING了★
消费者消费者1 RUNNABLE了
消费者消费者1 WAITING了☆
消费者消费者2 WAITING了☆
生产者 生产者2 RUNNABLE了
生产者 生产者2 WAITING了★
消费者消费者2 RUNNABLE了
消费者消费者2 WAITING了☆
消费者消费者1 WAITING了☆
生产者 生产者1 RUNNABLE了
生产者 生产者1 WAITING了★
...
注意事项
比较上面两种模式，主要改动了两处。
- if()判断改为while()循环，重新判断条件，避免逻辑混乱;
- notify()改为notifyAll()避免进入“假死”状态。
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