使用 parallelStream() 导致未关闭的线程？

Question

我正在写一个 rss 媒体采集器，我的一个 parallelStream()s 导致线程未关闭。

它们都是 Daemon Thread [ForkJoinPool.commonPool-worker-xx] 个线程。

我能发现的唯一区别是这两个示例都是从不同的线程调用的。这可能是问题所在吗？即使当我尝试启动新的 Thread().start 时，parallelStream() 仍未关闭所有 ForkJoinPool 线程。而且我也尝试使用简单的 objects 像 ArrayList<int> 一样的结果。这是主线程外的通缉行为吗？

或者只是文档中描述的行为（粗体部分）：

A ForkJoinPool differs from other kinds of ExecutorService mainly by virtue of employing work-stealing: all threads in the pool attempt to find and execute tasks submitted to the pool and/or created by other active tasks (eventually blocking waiting for work if none exist).

public class MainRoutine {
    
    public static void startRoutine(SubReddit subReddit) {
        ArrayList<Entry> rssEntry = RSSgrab.pullRss(new SubReddit("pics", true, null));
        
        rssEntry.parallelStream().forEach(System.out::println); //produces unclosed threads
        System.out.println(Thread.currentThread().getName()); //prints AWT-EventQueue-0
    }

    public static void main(String[] args) {
        ArrayList<Entry> rssEntry = RSSgrab.pullRss(new SubReddit("pics", true, null));
        
        rssEntry.parallelStream().forEach(System.out::println); //this does not produce unclosed threads
        System.out.println(Thread.currentThread().getName()); //prints main
    //my bad, does also produce unclosed threads but the runtime is so short that I did not notice ofc
    }

}

条目class

public class Entry {
    String user;
    String userUri;
    String id;
    String uri;
    String date;
    String title;
    
    private ArrayList<String> media = new ArrayList<String>();
    
    public Entry(String user, String userUri, String id, String uri, String date, String title) {
        this.user = user;
        this.userUri = userUri;
        this.id = id;
        this.uri = uri;
        this.date = date;
        this.title = title;
    }

    @Override
    public String toString() {
        return "Entry [user=" + user + ", userUri=" + userUri + ", id=" + id + ", uri=" + uri + ", date=" + date
                + ", title=" + title + ", media="+getMedia().stream().map(s -> s+"; ").reduce("", String::concat)+"]";
    }

    public ArrayList<String> getMedia() {
        return media;
    }

    public void setMedia(ArrayList<String> media) {
        this.media = media;
    }
}

Answer 1

基本上...您正在尝试解决一个不是问题的问题。公共 fork-join 池是一个托管池。 JVM 会处理它。

如果这对您来说真的很重要，那么坏消息是您可能对此无能为力。公共池将忽略 shutdown() 和 shutdownNow() 调用。这是设计使然。

有一个技巧可让您在其中创建自定义 ForkJoinPool 和运行流。参见 Custom thread pool in Java 8 parallel stream。完成后，您可以关闭池以使线程消失。

但是……这可能不是个好主意。重用现有池或公共池效率更高。创建和销毁线程是昂贵的。重复执行此操作是因为您重复创建和销毁池是低效的。

常见的ForkJoinPool不应视为线程泄漏。

使用 parallelStream() 导致未关闭的线程？

Usage of parallelStream() resulting in unclosed Threads?

java

multithreading

java-stream