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Java多线程 生产者消费者模型实例详解

2020-03-19 12:07 726 查看

生产者消费者模型

生产者:生产任务的个体;

消费者:消费任务的个体;

缓冲区:是生产者和消费者之间的媒介,对生产者和消费者解耦。


缓冲区元素为满,生产者无法生产,消费者继续消费;

缓冲区元素为空,消费者无法消费,生产者继续生产;

wait()/notify()生产者消费者模型

制作一个简单的缓冲区ValueObject,value为空表示缓冲区为空,value不为空表示缓冲区满

public class ValueObject {
public static String value = "";
}

生产者,缓冲区满则wait(),不再生产,等待消费者notify(),缓冲区为空则开始生产

public class Producer {
private Object lock;

public Producer(Object 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,缓冲区为空则wait(),等待生产者notify(),缓冲区为满,消费者开始消费

public class Customer {
private Object lock;

public Customer(Object 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();
}
}
}

main方法,启动一个生产者和一个消费者

public class Main {
public static void main(String[] args)
{
Object lock = new Object();
final Producer producer = new Producer(lock);
final Customer customer = new Customer(lock);
Runnable producerRunnable = new Runnable()
{
public void run()
{
while (true)
{
producer.setValue();
}
}
};
Runnable customerRunnable = new Runnable()
{
public void run()
{
while (true)
{
customer.getValue();
}
}
};
Thread producerThread = new Thread(producerRunnable);
Thread CustomerThread = new Thread(customerRunnable);
producerThread.start();
CustomerThread.start();
}
}

运行结果如下

Set的值是:1564733938518_27520480474279
Get的值是:1564733938518_27520480474279
Set的值是:1564733938518_27520480498378
Get的值是:1564733938518_27520480498378
Set的值是:1564733938518_27520480540254
Get的值是:1564733938518_27520480540254
・・・・・・

生产者和消费者交替运行,生产者生产一个字符串,缓冲区为满,消费者消费一个字符串,缓冲区为空,循环往复,满足生产者/消费者模型。

await()/signal()生产者/消费者模型

缓冲区

public class ValueObject {
public static String value = "";
}

ThreadDomain48继承ReentrantLock,set方法生产,get方法消费

public class ThreadDomain48 extends ReentrantLock
{
private Condition condition = newCondition();

public void set()
{
try
{
lock();
while (!"".equals(ValueObject.value))
condition.await();
ValueObject.value = "123";
System.out.println(Thread.currentThread().getName() + "生产了value, value的当前值是" + ValueObject.value);
condition.signal();
}
catch (InterruptedException e)
{
e.printStackTrace();
}
finally
{
unlock();
}
}

public void get()
{
try
{
lock();
while ("".equals(ValueObject.value))
condition.await();
ValueObject.value = "";
System.out.println(Thread.currentThread().getName() + "消费了value, value的当前值是" + ValueObject.value);
condition.signal();
}
catch (InterruptedException e)
{
e.printStackTrace();
}
finally
{
unlock();
}
}
}

MyThread41启动两个生产线程和一个消费线程

public class MyThread41 {
public static void main(String[] args)
{
final ThreadDomain48 td = new ThreadDomain48();
Runnable producerRunnable = new Runnable()
{
public void run()
{
for (int i = 0; i < Integer.MAX_VALUE; i++)
td.set();
}
};
Runnable customerRunnable = new Runnable()
{
public void run()
{
for (int i = 0; i < Integer.MAX_VALUE; i++)
td.get();
}
};
Thread ProducerThread1 = new Thread(producerRunnable);
ProducerThread1.setName("Producer1");
Thread ProducerThread2 = new Thread(producerRunnable);
ProducerThread2.setName("Producer2");
Thread ConsumerThread = new Thread(customerRunnable);
ConsumerThread.setName("Consumer");
ProducerThread1.start();
ProducerThread2.start();
ConsumerThread.start();
}
}

输出结果如下

Producer1生产了value, value的当前值是123
Consumer消费了value, value的当前值是
Producer1生产了value, value的当前值是123

为什么Producer2无法生产,消费者无法消费呢?是因为此时缓冲区为满,Producer1的notify()应该唤醒Consumer却唤醒了Producer2,导致Producer2因为缓冲区为满和Consumer没有被唤醒而处于waiting状态,此时三个线程均在等待,出现了假死。

解决方案有两种:

1.让生产者唤醒所有线程,在set方法中使用condition.signalAll();

2.使用两个Condition,生产者Condition和消费者Condition,唤醒指定的线程;

正常输入如下:

・・・・・・
Producer2生产了value, value的当前值是123
Consumer消费了value, value的当前值是
Producer2生产了value, value的当前值是123
Consumer消费了value, value的当前值是
Producer2生产了value, value的当前值是123
Consumer消费了value, value的当前值是Producer1生产了value, value的当前值是123
Consumer消费了value, value的当前值是
Producer1生产了value, value的当前值是123
Consumer消费了value, value的当前值是Producer1生产了value, value的当前值是123
Consumer消费了value, value的当前值是
Producer1生产了value, value的当前值是123
Consumer消费了value, value的当前值是
Producer1生产了value, value的当前值是123
Consumer消费了value, value的当前值是
・・・・・・

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