最近开始学习基于Java的并发,我在windows(jdk 11)上运行以下代码
import java.util.*;
import java.util.concurrent.*;
class TaskWithResult implements Callable<String>{
private int id;
public TaskWithResult(int id){
this.id = id;
}
public String call(){
return "Result of TaskWithResult "+id;
}
}
public class TestCallable{
public static void main(String[] args){
ExecutorService exec = Executors.newCachedThreadPool();
ArrayList<Future<String>> results =
new ArrayList<Future<String>>();
for(int i = 0;i<10;i++){
results.add(exec.submit(new TaskWithResult(i)));
for(Future<String> fs:results){
try{
System.out.println(fs.get());
}catch(InterruptedException e){
System.out.println(e);
return;
}catch(ExecutionException e){
System.out.println(e);
}finally{
exec.shutdown();
}
}
}
}
}
每次运行都会出现sanme异常:
\\output:
Result of TaskWithResult 0
Exception in thread "main" java.util.concurrent.RejectedExecutionException: Task java.util.concurrent.FutureTask@380fb434[Not completed, task = me.rexjz.a.TaskWithResult@21bcffb5] rejected from java.util.concurrent.ThreadPoolExecutor@3cda1055[Terminated, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 1]
at java.base/java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2055)
at java.base/java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:825)
at java.base/java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1355)
at java.base/java.util.concurrent.AbstractExecutorService.submit(AbstractExecutorService.java:140)
at javaBin/me.rexjz.a.TestCallable.main(TestCallable.java:22)
代码摘自Thinging in Java(4th),我初步猜测是在所有任务提交之前驱动main()执行的隐式线程
shutdown()
,因为第一个任务执行成功并且异常信息表明 pool size= 1
,但这是不可能的,因为 main() 中的所有内容都是顺序执行的。所有 Callable 对象都应该在关闭之前提交。
然后我把ThreadPool的类型改为
Executors.newFixedThreadPool(10)
,还是出现了Exception,池大小还是1。
这是怎么发生的?
如果你仔细观察你的
for
循环,你就会发现问题(特别是当代码按照惯例缩进时):
for (int i = 0; i < 10; i++) {
results.add(exec.submit(new TaskWithResult(i)));
for (Future<String> fs : results) {
try {
System.out.println(fs.get());
} catch (InterruptedException e) {
System.out.println(e);
return;
} catch (ExecutionException e) {
System.out.println(e);
} finally {
exec.shutdown();
}
}
}
请注意,查询每个
for
的 Future
循环是 nested 于提交任务的 for
循环内。这意味着您提交一个任务,等待结果,关闭执行器,然后尝试提交另一项任务。以下应该可以解决您的问题:
for (int i = 0; i < 10; i++) {
results.add(exec.submit(new TaskWithResult(i)));
}
executor.shutdown(); // shutdown() allows already-submitted tasks to execute
for (Future<String> fs : results) {
try {
System.out.println(fs.get());
} catch (InterruptedException e) {
e.printStackTrace();
return;
} catch (ExecutionException e) {
e.printStackTrace();
}
}
我移动了
executor.shutdown()
调用,因为这只需要在您提交最后一个任务后发生一次。当然,如果您要继续重用执行器,那么您就不想关闭它。
我还将
System.out.println(e)
更改为e.printStackTrace()
。通常最好打印堆栈跟踪,而不仅仅是打印异常类型和消息(默认情况下,这是 Throwable#toString()
返回的内容)。在像您的示例这样的短程序中,它可能并不明显,但堆栈跟踪在更复杂的应用程序中非常有价值,因为它直接指向抛出异常的位置。请参阅什么是堆栈跟踪,以及如何使用它来调试应用程序错误?了解更多信息。
package com.springboot.testapplication;
import java.util.*;
import java.util.concurrent.*;
class TaskWithResult implements Callable<String> {
private int id;
public TaskWithResult(int id) {
this.id = id;
}
public String call() {
return "Result of TaskWithResult " + id;
}
}
public class TestCallable {
public static void main(String[] args) {
ExecutorService exec = Executors.newCachedThreadPool();
ArrayList<Future<String>> results = new ArrayList<Future<String>>();
for (int i = 0; i < 10; i++) {
results.add(exec.submit(new TaskWithResult(i)));
}
for (Future<String> fs : results) {
try {
System.out.println(fs.get());
} catch (InterruptedException e) {
System.out.println(e);
return;
} catch (ExecutionException e) {
System.out.println(e);
} finally {
exec.shutdown();
}
}
}
}
在shutDown()或awaitTermination()之前编写invokeAny()或invokeAll()方法。
Integer result = executorService.invokeAny(futureList);
System.out.println(result);
executorService.shutdown();