Java线程池如何优雅地等待所有任务执行完？
https://blog.csdn.net/flycp/article/details/106337294
https://blog.csdn.net/qq_25806863/article/details/71214033
随着项目的体量越来越大，对代码的执行效率要求越来越高，在实际应用过程中我们会经常使用线程池。
那么如果线程池嵌入在业务代码中，如何正确的等待线程池执行完，在执行后续操作呢？或者想要获取执行结果有应该怎么处理呢？
下面走一下场景：
package com.example.demo1.entity;
/**
* create by c-pown on 2019-12-06
*/
public class Student {
private String name;
private Integer age;
private Integer heigh;
private String hoby;
public Student(String name, Integer age, Integer heigh, String hoby) {
this.name = name;
this.age = age;
this.heigh = heigh;
this.hoby = hoby;
}
static String getAllname(){
return "张三";
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Integer getAge() {
return age;
}
public void setAge(Integer age) {
this.age = age;
}
public Integer getHeigh() {
return heigh;
}
public void setHeigh(Integer heigh) {
this.heigh = heigh;
}
public String getHoby() {
return hoby;
}
public void setHoby(String hoby) {
this.hoby = hoby;
}
@Override
public String toString() {
return "Student{" +
"name='" + name + '\'' +
", age=" + age +
", heigh=" + heigh +
", hoby='" + hoby + '\'' +
'}';
}
}
package com.example.demo1.service.TestThreadPool;
import com.example.demo1.entity.Student;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.*;
/**
* create by c-pown on 2020-05-25
*/
public class TestThreadPool {
/**
* 手动创建线程池
*/
private static ThreadPoolExecutor executor = new ThreadPoolExecutor(20,25,100L,
TimeUnit.SECONDS,new LinkedBlockingQueue<>(),new ThreadPoolExecutor.CallerRunsPolicy());
public static void main(String[] args) {
Student student = null;
List<Student> students = new ArrayList<>();
//添加五十万个学生元素
for (int i = 0; i < 500000; i++) {
student = new Student("name"+i,20,183,"玩");
students.add(student);
}
for (Student student1 : students) {
/**
* 给元素添加后缀
*/
executor.submit(()-> student1.setName(student1.getName()+"这是后缀"));
}
//查看添加情况
System.out.println("添加数量："+students.stream().filter(x->x.getName().contains("这是后缀")).count());
}
}
我们给List里面添加500000个学生元素，然后使用线程池给name属性添加后缀，看一下执行结果：
添加数量：475371
我们发现添加成功的数量是少了两万多，这是由于线程池中的子线程任务没有执行完，而主线程已经开始执行业务代码，导致成功数量变少。
下面我们修改一下代码：
一、使用CountDownLatch
package com.example.demo1.service.TestThreadPool;
import com.example.demo1.entity.Student;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.*;
/**
* create by c-pown on 2020-05-25
*/
public class TestThreadPool {
/**
* 手动创建线程池
*/
private static ThreadPoolExecutor executor = new ThreadPoolExecutor(20,25,100L,
TimeUnit.SECONDS,new LinkedBlockingQueue<>(),new ThreadPoolExecutor.CallerRunsPolicy());
public static void main(String[] args) {
Student student = null;
List<Student> students = new ArrayList<>();
//添加五十万个学生元素
for (int i = 0; i < 500000; i++) {
student = new Student("name"+i,20,183,"玩");
students.add(student);
}
CountDownLatch countDownLatch = new CountDownLatch(students.size());
for (Student student1 : students) {
/**
* 给元素添加后缀
*/
executor.submit(()-> {
try {
student1.setName(student1.getName()+"这是后缀");
} catch (Exception e) {
e.printStackTrace();
}finally {
countDownLatch.countDown();
}
});
}
try {
countDownLatch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
//查看添加情况
System.out.println("添加数量："+students.stream().filter(x->x.getName().contains("这是后缀")).count());
}
}
结果：
添加数量：500000
1
这是一个计数器操作，在线程池执行之前，给计数器指定数值（与要执行代码的次数一致）也就是students.size()，在线程池执行代码体里面要加上countDownLatch.countDown();代表每执行一次数值减少一，最后在循环体外边写上countDownLatch.await();代表等待计数器归零。
可以查看下源码介绍：
/**
* Decrements the count of the latch, releasing all waiting threads if
* the count reaches zero.
*
* <p>If the current count is greater than zero then it is decremented.
* If the new count is zero then all waiting threads are re-enabled for
* thread scheduling purposes.
*
* <p>If the current count equals zero then nothing happens.
*/
public void countDown() {
sync.releaseShared(1);
}
/**
* Causes the current thread to wait until the latch has counted down to
* zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
*
* <p>If the current count is zero then this method returns immediately.
*
* <p>If the current count is greater than zero then the current
* thread becomes disabled for thread scheduling purposes and lies
* dormant until one of two things happen:
* <ul>
* <li>The count reaches zero due to invocations of the
* {@link #countDown} method; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread.
* </ul>
*
* <p>If the current thread:
* <ul>
* <li>has its interrupted status set on entry to this method; or
* <li>is {@linkplain Thread#interrupt interrupted} while waiting,
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* @throws InterruptedException if the current thread is interrupted
* while waiting
*/
public void await() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
介绍中写到等待计数器数量减少直至为0为止。也可以给await()设置超时时间
countDownLatch.await(300,TimeUnit.SECONDS);
1
如果超过300s（也可以是时，分）则不再等待，直接执行下面代码。
二、使用Future.get()
package com.example.demo1.service.TestThreadPool;
import com.example.demo1.entity.Student;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.*;
/**
* create by c-pown on 2020-05-25
*/
public class TestThreadPool {
/**
* 手动创建线程池
*/
private static ThreadPoolExecutor executor = new ThreadPoolExecutor(20,25,100L,
TimeUnit.SECONDS,new LinkedBlockingQueue<>(),new ThreadPoolExecutor.CallerRunsPolicy());
public static void main(String[] args) {
Student student = null;
List<Student> students = new ArrayList<>();
//添加五十万个学生元素
for (int i = 0; i < 500000; i++) {
student = new Student("name"+i,20,183,"玩");
students.add(student);
}
List<Future> futures = new ArrayList<>();
for (Student student1 : students) {
/**
* 给元素添加后缀
*/
Future future = executor.submit(()-> {
try {
student1.setName(student1.getName()+"这是后缀");
} catch (Exception e) {
e.printStackTrace();
}
});
futures.add(future);
}
for (Future future : futures) {
try {
future.get();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
}
//查看添加情况
System.out.println("添加数量："+students.stream().filter(x->x.getName().contains("这是后缀")).count());
}
}
结果：
添加数量：500000
1
Future.get()可以同步等待线程执行完成，并且可以监听执行结果
/**
* Waits if necessary for the computation to complete, and then
* retrieves its result.
*
* @return the computed result
* @throws CancellationException if the computation was cancelled
* @throws ExecutionException if the computation threw an
* exception
* @throws InterruptedException if the current thread was interrupted
* while waiting
*/
V get() throws InterruptedException, ExecutionException;
源码中可以看出方法是有返回值得，可以监听线程池子线程执行状态及执行结果。
直接return 结果 Future<?>添加泛型即可。
同样的 Future.get()也是可以指定超时时间的，超过等待时间可以直接执行后续代码。
最后 如果线程池是方法内部创建的，可以直接使用shutdown（）也会等待线程池的执行结果。同时会关闭线程池资源。
executor.shutdown();
try {
executor.awaitTermination(300,TimeUnit.SECONDS);
} catch (InterruptedException e) {
e.printStackTrace();
}
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原文链接：https://blog.csdn.net/flycp/article/details/106337294
