java学习系列-高并发12:JUC中的Condition对象

内容

  1. synchronized中实现线程等待和唤醒
  2. Condition简介及常用方法介绍及相关示例
  3. 使用Condition实现生产者消费者
  4. 使用Condition实现同步阻塞队列

Object对象中的wait(),notify()方法,用于线程等待和唤醒等待中的线程,大家应该比较熟悉,想再次了解的朋友可以移步到 java学习系列-高并发05:线程的基本操作.

synchronized中等待和唤醒线程示例

package com.itsoku.chat09;

import java.util.concurrent.TimeUnit;

public class Demo1 {
     
	static Object lock = new Object();
	public static class T1 extends Thread {
     
	
		@Override
		public void run() {
     
			System.out.println(System.currentTimeMillis() + "," + this.getName() + "准备获取锁!");
			synchronized (lock) {
     
				System.out.println(System.currentTimeMillis() + "," + this.getName() + "获取锁成功!");
				try {
     
					lock.wait();
				} catch (InterruptedException e) {
     
					e.printStackTrace();
				}
			}
			System.out.println(System.currentTimeMillis() + "," + this.getName() + "释放锁成功!");
		}
	}



	public static class T2 extends Thread {
     
		@Override
		public void run() {
     
			System.out.println(System.currentTimeMillis() + "," + this.getName() + "准备获取锁!");
			synchronized (lock) {
     
				System.out.println(System.currentTimeMillis() + "," + this.getName() + "获取锁成功!");
				lock.notify();
				System.out.println(System.currentTimeMillis() + "," + this.getName() + " notify!");
				try {
     
					TimeUnit.SECONDS.sleep(5);
				} catch (InterruptedException e) {
     
					e.printStackTrace();
				}
				System.out.println(System.currentTimeMillis() + "," + this.getName() + "准备释放锁!");
			}
			System.out.println(System.currentTimeMillis() + "," + this.getName() + "释放锁成功!");
		}
	}


	public static void main(String[] args) throws InterruptedException {
     
	
		T1 t1 = new T1();
		t1.setName("t1");
		t1.start();
		TimeUnit.SECONDS.sleep(5);
		T2 t2 = new T2();
		t2.setName("t2");
		t2.start();
	}
}

输出:

11563530109234,t1准备获取锁!
21563530109234,t1获取锁成功!
31563530114236,t2准备获取锁!
41563530114236,t2获取锁成功!
51563530114236,t2 notify!
61563530119237,t2准备释放锁!
71563530119237,t2释放锁成功!
81563530119237,t1释放锁成功!

代码结合输出的结果我们分析一下:

  1. 线程t1先获取锁,然后调用了wait()方法将线程置为等待状态,然后会释放lock的锁
  2. 主线程等待5秒之后,启动线程t2,t2获取到了锁,结果中1、3行时间相差5秒左右
  3. t2调用lock.notify()方法,准备将等待在lock上的线程t1唤醒,notify()方法之后又休眠了5秒,看一下输出的5、8可知,notify()方法之后,t1并不能立即被唤醒,需要等到t2将synchronized块执行完毕,释放锁之后,t1才被唤醒
  4. wait()方法和notify()方法必须放在同步块内调用(synchronized块内),否则会报错

Condition使用简介

在了解Condition之前,需要先了解一下重入锁ReentrantLock,可以移步到: java学习系列-高并发11:JUC:ReentrantLock重入锁.

任何一个java对象都天然继承于Object类,在线程间实现通信的往往会应用到Object的几个方法,比如wait()、wait(long timeout)、wait(long timeout, int nanos)与notify()、notifyAll()几个方法实现等待/通知机制,同样的, 在java Lock体系下依然会有同样的方法实现等待/通知机制。

从整体上来看Object的wait和notify/notify是与对象监视器配合完成线程间的等待/通知机制,而Condition与Lock配合完成等待通知机制,前者是java底层级别的,后者是语言级别的,具有更高的可控制性和扩展性。两者除了在使用方式上不同外,在功能特性上还是有很多的不同:

  1. Condition能够支持不响应中断,而通过使用Object方式不支持
  2. Condition能够支持多个等待队列(new 多个Condition对象),而Object方式只能支持一个
  3. Condition能够支持超时时间的设置,而Object不支持

Condition由ReentrantLock对象创建,并且可以同时创建多个,Condition接口在使用前必须先调用ReentrantLock的lock()方法获得锁,之后调用Condition接口的await()将释放锁,并且在该Condition上等待,直到有其他线程调用Condition的signal()方法唤醒线程,使用方式和wait()、notify()类似。

示例代码:

package com.itsoku.chat09;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

public class Demo2 {
     
	static ReentrantLock lock = new ReentrantLock();
	static Condition condition = lock.newCondition();
	
	public static class T1 extends Thread {
     
	
		@Override
		public void run() {
     
			System.out.println(System.currentTimeMillis() + "," + this.getName() + "准备获取锁!");
			lock.lock();
			try {
     
				System.out.println(System.currentTimeMillis() + "," + this.getName() + "获取锁成功!");
				condition.await();
			} catch (InterruptedException e) {
     
				e.printStackTrace();
			} finally {
     
				lock.unlock();
			}
			System.out.println(System.currentTimeMillis() + "," + this.getName() + "释放锁成功!");
		}
	}

public static class T2 extends Thread {
     

	@Override
	public void run() {
     
		System.out.println(System.currentTimeMillis() + "," + this.getName() + "准备获取锁!");
		lock.lock();
		try {
     
			System.out.println(System.currentTimeMillis() + "," + this.getName() + "获取锁成功!");
			condition.signal();
			System.out.println(System.currentTimeMillis() + "," + this.getName() + " signal!");
			try {
     
				TimeUnit.SECONDS.sleep(5);
			} catch (InterruptedException e) {
     
				e.printStackTrace();
			}
				System.out.println(System.currentTimeMillis() + "," + this.getName() + "准备释放锁!");
			} finally {
     
				lock.unlock();
			}
			System.out.println(System.currentTimeMillis() + "," + this.getName() + "释放锁成功!");
		}
	}

	public static void main(String[] args) throws InterruptedException {
     
	
		T1 t1 = new T1();
		t1.setName("t1");
		t1.start();
		TimeUnit.SECONDS.sleep(5);
		T2 t2 = new T2();
		t2.setName("t2");
		t2.start();
	}
}

输出:

1563532185827,t1准备获取锁!
1563532185827,t1获取锁成功!
1563532190829,t2准备获取锁!
1563532190829,t2获取锁成功!
1563532190829,t2 signal!
1563532195829,t2准备释放锁!
1563532195829,t2释放锁成功!
1563532195829,t1释放锁成功!

输出的结果和使用synchronized关键字的实例类似。

Condition.await()方法和Object.wait()方法类似,当使用Condition.await()方法时,需要先获取Condition对象关联的ReentrantLock的锁,在Condition.await()方法被调用时,当前线程会释放这个锁,并且当前线程会进行等待(处于阻塞状态)。在signal()方法被调用后,系统会从Condition对象的等待队列中唤醒一个线程,一旦线程被唤醒,被唤醒的线程会尝试重新获取锁,一旦获取成功,就可以继续执行了。因此,在signal被调用后,一般需要释放相关的锁,让给其他被唤醒的线程,让他可以继续执行。

Condition常用方法

Condition接口提供的常用方法有:

和Object中wait类似的方法
  1. void await() throws InterruptedException:当前线程进入等待状态,如果其他线程调用condition的signal或者signalAll方法并且当前线程获取Lock从await方法返回,如果在等待状态中被中断会抛出被中断异常;
  2. long awaitNanos(long nanosTimeout):当前线程进入等待状态直到被通知,中断或者超时;
  3. boolean await(long time, TimeUnit unit) throws InterruptedException:同第二种,支持自定义时间单位,false:表示方法超时之后自动返回的,true:表示等待还未超时时,await方法就返回了(超时之前,被其他线程唤醒了)
  4. boolean awaitUntil(Date deadline) throws InterruptedException:当前线程进入等待状态直到被通知,中断或者到了某个时间
  5. void awaitUninterruptibly();:当前线程进入等待状态,不会响应线程中断操作,只能通过唤醒的方式让线程继续
和Object的notify/notifyAll类似的方法
  1. void signal():唤醒一个等待在condition上的线程,将该线程从等待队列中转移到同步队列中,如果在同步队列中能够竞争到Lock则可以从等待方法中返回。

  2. void signalAll():与1的区别在于能够唤醒所有等待在condition上的线程

Condition.await()过程中被打断

package com.itsoku.chat09;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;


public class Demo4 {
     

	static ReentrantLock lock = new ReentrantLock();
	static Condition condition = lock.newCondition();
	
	public static class T1 extends Thread {
     
		@Override
		public void run() {
     
			lock.lock();
			try {
     
				condition.await();
			} catch (InterruptedException e) {
     
				System.out.println("中断标志:" + this.isInterrupted());
				e.printStackTrace();
			} finally {
     
				lock.unlock();
			}
		}
	}

	public static void main(String[] args) throws InterruptedException {
     
	
		T1 t1 = new T1();
		t1.setName("t1");
		t1.start();
		TimeUnit.SECONDS.sleep(2);
		//给t1线程发送中断信号
		System.out.println("1、t1中断标志:" + t1.isInterrupted());
		t1.interrupt();
		System.out.println("2、t1中断标志:" + t1.isInterrupted());
	}
}

输出:

1、t1中断标志:false
2、t1中断标志:true
中断标志:false
java.lang.InterruptedException
	at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.reportInterruptAfterWait(AbstractQueuedSynchronizer.java:2014)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2048)
	at com.itsoku.chat09.Demo4$T1.run(Demo4.java:19)

调用condition.await()之后,线程进入阻塞中,调用t1.interrupt(),给t1线程发送中断信号,await()方法内部会检测到线程中断信号,然后触发 InterruptedException异常,线程中断标志被清除。从输出结果中可以看出,线程t1中断标志的变换过程:false->true->false

await(long time, TimeUnit unit)超时之后自动返回

package com.itsoku.chat09;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;


public class Demo5 {
     

	static ReentrantLock lock = new ReentrantLock();
	static Condition condition = lock.newCondition();
	
	public static class T1 extends Thread {
     
		@Override
		public void run() {
     
			lock.lock();
			try {
     
				System.out.println(System.currentTimeMillis() + "," + this.getName() + ",start");
				boolean r = condition.await(2, TimeUnit.SECONDS);
				System.out.println(r);
				System.out.println(System.currentTimeMillis() + "," + this.getName() + ",end");
			} catch (InterruptedException e) {
     
				e.printStackTrace();
			} finally {
     
				lock.unlock();
			}
		}
	}


	public static void main(String[] args) throws InterruptedException {
     
	
		T1 t1 = new T1();
		t1.setName("t1");
		t1.start();
	}
}

输出:

1563541624082,t1,start
false
1563541626085,t1,end

t1线程等待2秒之后,自动返回继续执行,最后await方法返回false,await返回false表示超时之后自动返回

await(long time, TimeUnit unit)超时之前被唤醒

package com.itsoku.chat09;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

public class Demo6 {
     

	static ReentrantLock lock = new ReentrantLock();
	static Condition condition = lock.newCondition();
	
	public static class T1 extends Thread {
     
	
		@Override
		public void run() {
     
			lock.lock();
			try {
     
				System.out.println(System.currentTimeMillis() + "," + this.getName() + ",start");
				boolean r = condition.await(5, TimeUnit.SECONDS);
				System.out.println(r);
				System.out.println(System.currentTimeMillis() + "," + this.getName() + ",end");
			} catch (InterruptedException e) {
     
				e.printStackTrace();
			} finally {
     
				lock.unlock();
			}
		}
	}


	public static void main(String[] args) throws InterruptedException {
     
	
		T1 t1 = new T1();
		t1.setName("t1");
		t1.start();
		//休眠1秒之后,唤醒t1线程
		TimeUnit.SECONDS.sleep(1);
		lock.lock();
		try {
     
			condition.signal();
		} finally {
     
			lock.unlock();
		}
	}
}

输出:

1563542046046,t1,start
true
1563542047048,t1,end

t1线程中调用 condition.await(5,TimeUnit.SECONDS);方法会释放锁,等待5秒,主线程休眠1秒,然后获取锁,之后调用signal()方法唤醒t1,输出结果中发现await后过了1秒(1、3行输出结果的时间差),await方法就返回了,并且返回值是true。true表示await方法超时之前被其他线程唤醒了。

long awaitNanos(long nanosTimeout)超时返回

package com.itsoku.chat09;


import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;


public class Demo7 {
     

static ReentrantLock lock = new ReentrantLock();
static Condition condition = lock.newCondition();

	public static class T1 extends Thread {
     
	
		@Override
		public void run() {
     
			lock.lock();
			try {
     
				System.out.println(System.currentTimeMillis() + "," + this.getName() + ",start");
				long r = condition.awaitNanos(TimeUnit.SECONDS.toNanos(5));
				System.out.println(r);
				System.out.println(System.currentTimeMillis() + "," + this.getName() + ",end");
			} catch (InterruptedException e) {
     
				e.printStackTrace();
			} finally {
     
				lock.unlock();
			}
		}
	}

	public static void main(String[] args) throws InterruptedException {
     
		T1 t1 = new T1();
		t1.setName("t1");
		t1.start();
	}

}

输出:

1563542547302,t1,start
-258200
1563542552304,t1,end

awaitNanos参数为纳秒,可以调用TimeUnit中的一些方法将时间转换为纳秒。

t1调用await方法等待5秒超时返回,返回结果为负数,表示超时之后返回的。

waitNanos(long nanosTimeout)超时之前被唤醒

package com.itsoku.chat09;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

public class Demo8 {
     

	static ReentrantLock lock = new ReentrantLock();
	static Condition condition = lock.newCondition();
	
	public static class T1 extends Thread {
     
	
		@Override
		public void run() {
     
			lock.lock();
			try {
     
				System.out.println(System.currentTimeMillis() + "," + this.getName() + ",start");
				long r = condition.awaitNanos(TimeUnit.SECONDS.toNanos(5));
				System.out.println(r);
				System.out.println(System.currentTimeMillis() + "," + this.getName() + ",end");
			} catch (InterruptedException e) {
     
				e.printStackTrace();
			} finally {
     
				lock.unlock();
			}
		}
	}



	public static void main(String[] args) throws InterruptedException {
     
	
		T1 t1 = new T1();
		t1.setName("t1");
		t1.start();
		//休眠1秒之后,唤醒t1线程
		TimeUnit.SECONDS.sleep(1);
		lock.lock();
		try {
     
			condition.signal();
		} finally {
     
			lock.unlock();
		}
	}
}

输出:

1563542915991,t1,start
3999988500
1563542916992,t1,end

t1中调用await休眠5秒,主线程休眠1秒之后,调用signal()唤醒线程t1,await方法返回正数,表示返回时距离超时时间还有多久,将近4秒,返回正数表示,线程在超时之前被唤醒了。

其他几个有参的await方法和无参的await方法一样,线程调用interrupt()方法时,这些方法都会触发InterruptedException异常,并且线程的中断标志会被清除。

同一个锁支持创建多个Condition

使用两个Condition来实现一个阻塞队列的例子:

package com.itsoku.chat09;


import java.util.LinkedList;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;


public class BlockingQueueDemo<E> {
     

	int size;//阻塞队列最大容量
	ReentrantLock lock = new ReentrantLock();
	LinkedList<E> list = new LinkedList<>();//队列底层实现
	Condition notFull = lock.newCondition();//队列满时的等待条件
	Condition notEmpty = lock.newCondition();//队列空时的等待条件

	public BlockingQueueDemo(int size) {
     
		this.size = size;
	}

	public void enqueue(E e) throws InterruptedException {
     
		lock.lock();
		try {
     
			while (list.size() == size)//队列已满,在notFull条件上等待
			notFull.await();
			list.add(e);//入队:加入链表末尾
			System.out.println("入队:" + e);
			notEmpty.signal(); //通知在notEmpty条件上等待的线程
		} finally {
     
			lock.unlock();
		}
	}

	public E dequeue() throws InterruptedException {
     
		E e;
		lock.lock();
		try {
     
			while (list.size() == 0)//队列为空,在notEmpty条件上等待
			notEmpty.await();
			e = list.removeFirst();//出队:移除链表首元素
			System.out.println("出队:" + e);
			notFull.signal();//通知在notFull条件上等待的线程
			return e;
		} finally {
     
			lock.unlock();
		}
	}



	public static void main(String[] args) {
     
	
		BlockingQueueDemo<Integer> queue = new BlockingQueueDemo<>(2);
		for (int i = 0; i < 10; i++) {
     
			int data = i;
			new Thread(new Runnable() {
     
				@Override
				public void run() {
     
					try {
     
						queue.enqueue(data);
					} catch (InterruptedException e) {
     
					}
				}
			}).start();
		}
	
		for (int i = 0; i < 10; i++) {
     
			new Thread(new Runnable() {
     
				@Override
				public void run() {
     
					try {
     
						Integer data = queue.dequeue();
					} catch (InterruptedException e) {
     
						e.printStackTrace();
					}
				}
			}).start();
		}
	}
}

代码非常容易理解,创建了一个阻塞队列,大小为3,队列满的时候,会被阻塞,等待其他线程去消费,队列中的元素被消费之后,会唤醒生产者,生产数据进入队列。上面代码将队列大小置为1,可以实现同步阻塞队列,生产1个元素之后,生产者会被阻塞,待消费者消费队列中的元素之后,生产者才能继续工作。

Object的监视器方法与Condition接口的对比java学习系列-高并发12:JUC中的Condition对象_第1张图片

总结

  1. 使用condition的步骤:创建condition对象,获取锁,然后调用condition的方法
  2. 一个ReentrantLock支持床多个condition对象
  3. voidawait()throwsInterruptedException;方法会释放锁,让当前线程等待,支持唤醒,支持线程中断
  4. voidawaitUninterruptibly();方法会释放锁,让当前线程等待,支持唤醒,不支持线程中断
  5. longawaitNanos(longnanosTimeout)throwsInterruptedException;参数为纳秒,此方法会释放锁,让当前线程等待,支持唤醒,支持中断。超时之后返回的,结果为负数;超时之前被唤醒返回的,结果为正数(表示返回时距离超时时间相差的纳秒数)
  6. booleanawait(longtime,TimeUnitunit)throwsInterruptedException;方法会释放锁,让当前线程等待,支持唤醒,支持中断。超时之后返回的,结果为false;超时之前被唤醒返回的,结果为true
  7. booleanawaitUntil(Datedeadline)throwsInterruptedException;参数表示超时的截止时间点,方法会释放锁,让当前线程等待,支持唤醒,支持中断。超时之后返回的,结果为false;超时之前被唤醒返回的,结果为true
  8. voidsignal();会唤醒一个等待中的线程,然后被唤醒的线程会被加入同步队列,去尝试获取锁
  9. voidsignalAll();会唤醒所有等待中的线程,将所有等待中的线程加入同步队列,然后去尝试获取锁

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