Java多线程生产者消费者模式实现过程解析

Java多线程生产者消费者模式实现过程解析

单生产者与单消费者

示例：

public class ProduceConsume {

public static void main(String[] args) {

String lock = new String("");

Produce produce = new Produce(lock);

Consume consume = new Consume(lock);

new Thread(() -> {

while (true) {

produce.setValue();

}

}, "ProductThread").start();

new Thread(() -> {

while (true) {

consume.getValue();

}

}, "ConsumeThread").start();

}

/**

* 生产者

*/

static class Produce {

private String lock;

public Produce(String lock) {

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

*/

static class Consume {

private String lock;

public Consume(String lock) {

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();

}

}

}

static class ValueObject {

public static String value = "";

}

}

执行结果如下：

多生产者与多消费者

这种模式下，容易出现“假死”，也就是全部线程都进入了 WAITNG 状态，程序不在执行任何业务功能了，整个项目呈停止状态。

示例：

public class MultiProduceConsume {

public static void main(String[] args) throws InterruptedException {

String lock = new String("");

Produce produce = new Produce(lock);

Consume consume = new Consume(lock);

Thread[] pThread = new Thread[2];

Thread[] cThread = new Thread[2];

for (int i = 0; i < 2; i++) {

pThread[i] = new Thread(() ->PWotKt {

while (true) {

produce.setValue();

}

}, "生产者" + (i + 1));

cThread[i] = new Thread(() -> {

while (true) {

consume.getValue();

}

}, "消费者" + (i + 1));

pThread[i].start();

cThread[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());

}

}

static class Produce {

PWotKtprivate String lock;

public Produce(String lock) {

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.notify();

}

} catch (InterruptedException e) {

e.printStackTrace();

}

}

}

static class Consume {

private String lock;

public Consume(String lock) {

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.notify();

}

} catch (InterruptedException e) {

e.printStackTrace();PWotKt

}

}

}

static class ValueObject {

public static String value = "";

}

}

运行结果如图：

分析：

虽然代码中通过 wait/notify 进行通信了，但是不能保证 notify 唤醒的一定是异类，也可能是同类，比如“生产者”唤醒了“生产者”这样的情况。

解决方案：

假死出现的主要原因是有可能连续唤醒了同类。所以解决方案很简单，就是把 notify() 改为 notifyAll() 即可。

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