问题描述
Java 8环境.
同时使用CompletableFuture.allOf()运行任务,并然后从每个线程获取每个结果,然后将所有结果合并到一个CombinedResult中并返回.
run tasks using CompletableFuture.allOf() concurrently andthen get each result from each thread and then combine all results into one combinedResult and return it.
在下面的代码中,获取结果(= List<Student>
)不必是I.和II之间的代码.他们说我需要使用join()但没有用
In the below code, to get the result ( = List<Student>
) it doesn't have to be the code between I. and II.They say I need to use join() but didn't work
我也从
带有Collection或Java 8的Java 8 CompletableFuture.allOf(...)列表
和其他链接,但对我没有任何帮助.我想我错过了一些非常简单的部分.有人知道如何使它工作吗?
Java 8 CompletableFuture.allOf(...) with Collection or List
and other links, but nothing works for me.I think I miss some very easy part. Does someone know how to make it work?
public class Test1 {
public static void main(String[] args) {
Test1 t = new Test1();
Map<Major, List<Student>> allMajorStudentListMap = new HashMap<>();
// fill out some data toallMajorStudentListMap
t.getData(allMajorStudentListMap);
}
List<Student> getData(Map<Major, List<Student>> allMajorStudentListMap) {
List<CompletableFuture<List<Student>>> completableFutures = new ArrayList<>();
// suppose the size of completableFutures is 10
for(Map.Entry<Major, List<Student>> entry: allMajorStudentListMap.entrySet()) {
CompletableFuture<List<Student>> future = CompletableFuture.supplyAsync(() -> getDetailedStudents(entry));
completableFutures.add(future);
}
// want to run 10 jobs concurrently --> get the 10 result and then combine these 10 results into one
// finally want to sent the combined 10 results at one in this method
// I. ======================= I got this code from somewhere ==========================
CompletableFuture.allOf(completableFutures.toArray(new CompletableFuture[0]))
.exceptionally(ex -> null)
.join();
Map<Boolean, List<CompletableFuture<List<Student>>>> result =
completableFutures.stream()
.collect(Collectors.partitioningBy(CompletableFuture::isCompletedExceptionally));
result.forEach((k, clist) -> {
System.out.printf("k = " + k);
for(CompletableFuture<List<Student>> student: clist) {
// 3) don't know how to get only List<Student> and then print here
// tried this and that but didn't work
// student.get() has compile error
}
});
// II. =============================================================================================
// want to return combined List<Student>
return ???;
}
List<Student> getDetailedStudents(Map.Entry<Major, List<Student>> entry)
{
List<Student> studentList = new ArrayList<>();
Major major = entry.getKey();
String majorCode = major.getMajorCode();
String majorName = major.getMajorName();
List<Student> studentListList = entry.getValue();
studentList.addAll(getDataFromRemote(majorCode, majorName, studentList)));
return studentList;
}
List<Student> getDataFromRemote(String majorCode, String majorName, List<studentList> studentList) {
// do something and then return list of Student
return detailedStudentList;
}
}
推荐答案
在这里,我创建了一个稍作改动的(直接使用List<Student>
代替Map<K,V>
)版本的工作示例.您可以将您的解决方案与此解决方案进行比较.
Here I have created a slightly altered (directly used List<Student>
instead of Map<K,V>
) version of working sample. You can compare your solution with this one.
总共查询五次学生名单,并且每次并发执行一次,这在人为延迟3秒后返回了其中有一个学生对象的完整学生名单.因此,从理论上讲,如果每个对象同时运行,则在延迟3秒后,应该显示所有5个学生对象.
Totally five times the students list is queried and each gets executed concurrently which returns a completable list of students with ONE student object in it after an artificial delay of 3 seconds. So in theory if each runs concurrently, after 3 seconds of delay all of the 5 student object should get displayed.
如果您注意到主方法的开始和结束之间的时间间隔,则大约为3秒.
if you notice the time gap between start and end of the main method, it will be around 3 seconds.
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CompletableFuture;
import java.util.*;
import java.util.stream.Collectors;
public class CompletableFutureTest {
private static int counter = 0;
public static void main(String[] args) {
System.out.println("Program started at " + new Date());
List<Student> allStudents = new ArrayList<>();
new CompletableFutureTest().getData(allStudents);
for(Student st : allStudents){
System.out.println(st.getName());
}
System.out.println("Program ended at " + new Date());
}
private void getData(List<Student> resultToFillIn){
List<CompletableFuture<List<Student>>> completableFutures = new ArrayList<>();
//for simulation purpose just running regular for loop 5 times instead of yours
final Integer integer = new Integer(0);
for(int i=0; i < 5; i++){
completableFutures.add(CompletableFuture.supplyAsync(() -> getStudentsBatch()));
}
CompletableFuture<List<Student>>[] cfArray = new CompletableFuture[completableFutures.size()];
cfArray = completableFutures.toArray(cfArray);
CompletableFuture.allOf(cfArray)
.exceptionally(ex ->
{
ex.printStackTrace();
return null;
}).join();
List<CompletableFuture<List<Student>>> completedFutures = completableFutures.stream().filter(cf -> !cf.isCompletedExceptionally()).collect(Collectors.toList());
for(CompletableFuture<List<Student>> cf : completedFutures){
resultToFillIn.addAll(cf.join());
}
}
private List<Student> getStudentsBatch() {
//adding some delay
try {
Thread.sleep( 3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
List<Student> students = new ArrayList<>();
Student student = new Student();
student.setName("Student " + ++counter);
students.add(student);
return students;
}
public static class Student{
private String name;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
public static class Major{
private String name;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
}
由于此操作如上所述,所以CompletableFuture.allOf(...)
正常工作.
Since this works as explained above, CompletableFuture.allOf(...)
is working.
但是,请尽量避免使用j oin()
,因为它会暂停当前正在执行的线程.如果选择真正的异步编程,则可以使用thenAccept(x -> {})
,thenApply(x -> {})
回调方法来代替join()
.
But, try avoid using join()
whenever you can, as it will stall the currently executing thread. If you opt for truly asynchronous programming, then instead of join()
you can use thenAccept(x -> {})
, thenApply(x -> {})
callback methods.
希望这对您有所帮助.
Hope this helps you.
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