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.
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:
1:1563530109234,t1准备获取锁!
2:1563530109234,t1获取锁成功!
3:1563530114236,t2准备获取锁!
4:1563530114236,t2获取锁成功!
5:1563530114236,t2 notify!
6:1563530119237,t2准备释放锁!
7:1563530119237,t2释放锁成功!
8:1563530119237,t1释放锁成功!
代码结合输出的结果我们分析一下:
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:
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接口提供的常用方法有:
和Object中wait类似的方法
和Object的notify/notifyAll类似的方法
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。
void signalAll():与1的区别在于能够唤醒所有等待在condition上的线程
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
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
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。
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。
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。
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。