有人能告诉我Java中的守护线程是什么吗?


当前回答

守护线程类似于助手线程。非daemon线程就像前面的表演者。助手帮助表演者完成一项工作。工作完成后,执行者不再需要帮助来执行。由于不需要帮助,助手们离开了这个地方。因此,当非守护线程的任务结束时,守护线程就会离开。

其他回答

java线程中的用户线程与守护线程

守护进程的线程

这些线程在Java中是低优先级的线程,在后台运行以执行诸如垃圾收集之类的任务。Java中的守护线程也是一个服务提供者线程,它向用户线程提供服务。

用户线程

这些线程是高优先级线程。JVM将等待任何用户线程在终止它之前完成它的任务

“请记住用户和守护线程都包装在操作系统线程上”

最近OpenJdk在Loom项目中提出了虚拟线程(它们也是基于用户的),你可以在这里找到更多关于Java虚拟机线程的纤维和延续。

守护进程线程通常被称为“服务提供者”线程。这些线程不应该用来执行程序代码,而应该用来执行系统代码。这些线程与您的代码并行运行,但JVM可以随时终止它们。当JVM没有发现用户线程时,它将停止用户线程,所有守护进程线程立即终止。我们可以使用以下方法将非守护线程设置为守护线程:

setDaemon(true)

守护线程就像一个服务提供者,为运行在与守护线程相同进程中的其他线程或对象提供服务。守护线程用于后台支持任务,只有在正常线程执行时才需要守护线程。如果正常线程不运行,其余线程是守护线程,则解释器退出。

例如,HotJava浏览器使用最多4个名为“Image Fetcher”的守护线程从文件系统或网络中为任何需要的线程获取图像。

守护线程通常用于为应用程序/applet执行服务(例如加载“fiddley bits”)。用户线程和守护线程之间的核心区别在于,JVM只会在所有用户线程都终止时关闭程序。当不再有任何用户线程在运行(包括执行主线程)时,守护线程将由JVM终止。

setDaemon(真/假)?此方法用于指定一个线程是守护线程。

公共boolean isDaemon() ?此方法用于确定线程是否是守护线程。

Eg:

public class DaemonThread extends Thread {
    public void run() {
        System.out.println("Entering run method");

        try {
            System.out.println("In run Method: currentThread() is" + Thread.currentThread());

            while (true) {
                try {
                    Thread.sleep(500);
                } catch (InterruptedException x) {}

                System.out.println("In run method: woke up again");
            }
        } finally {
            System.out.println("Leaving run Method");
        }
    }
    public static void main(String[] args) {
        System.out.println("Entering main Method");

        DaemonThread t = new DaemonThread();
        t.setDaemon(true);
        t.start();

        try {
            Thread.sleep(3000);
        } catch (InterruptedException x) {}

        System.out.println("Leaving main method");
    }

}

输出:

C:\java\thread>javac DaemonThread.java

C:\java\thread>java DaemonThread
Entering main Method
Entering run method
In run Method: currentThread() isThread[Thread-0,5,main]
In run method: woke up again
In run method: woke up again
In run method: woke up again
In run method: woke up again
In run method: woke up again
In run method: woke up again
Leaving main method

C:\j2se6\thread>

下面是一个示例,用于测试在jvm由于不存在用户线程而退出时守护线程的行为。

请注意下面输出的倒数第二行,当主线程退出时,守护线程也死了,并且没有在finally块中打印finally executed9语句。这意味着如果由于不存在用户线程而导致JVM退出,守护线程finally块内关闭的任何i/o资源都不会被关闭。

public class DeamonTreadExample {

public static void main(String[] args) throws InterruptedException {

    Thread t = new Thread(() -> {
        int count = 0;
        while (true) {
            count++;
            try {
                System.out.println("inside try"+ count);
                Thread.currentThread().sleep(1000);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } finally {
                System.out.println("finally executed"+ count);
            }
        }
    });
    t.setDaemon(true);
    t.start();

    Thread.currentThread().sleep(10000);
    System.out.println("main thread exited");
  }
}

输出

inside try1
finally executed1
inside try2
finally executed2
inside try3
finally executed3
inside try4
finally executed4
inside try5
finally executed5
inside try6
finally executed6
inside try7
finally executed7
inside try8
finally executed8
inside try9
finally executed9
inside try10
main thread exited

答案已经有很多了;然而,也许我可以更清楚地解释这一点,因为当我阅读Daemon Threads时,最初,我有一种感觉,我很好地理解了它;然而,在玩了它并调试了一下之后,我发现了一个奇怪的行为。

我被教导:

如果我想让线程在主线程有序完成执行后立即死亡,我应该将其设置为Diamond。

我尝试了什么:

我从主线程中创建了两个线程,我只将其中一个设置为菱形; 在主线程有序完成执行后,那些新创建的线程都没有退出,但我预计,守护线程应该已经退出; 我浏览了许多博客和文章,到目前为止,我找到的最好、最清晰的定义来自《Java并发实践》一书,它非常清楚地指出:

7.4.2守护线程

Sometimes you want to create a thread that performs some helper function but you don’t want the existence of this thread to prevent the JVM from shutting down. This is what daemon threads are for. Threads are divided into two types: normal threads and daemon threads. When the JVM starts up, all the threads it creates (such as garbage collector and other housekeeping threads) are daemon threads, except the main thread. When a new thread is created, it inherits the daemon status of the thread that created it, so by default any threads created by the main thread are also normal threads. Normal threads and daemon threads differ only in what happens when they exit. When a thread exits, the JVM performs an inventory of running threads, and if the only threads that are left are daemon threads, it initiates an orderly shutdown. When the JVM halts, any remaining daemon threads are abandoned— finally blocks are not executed, stacks are not unwound—the JVM just exits. Daemon threads should be used sparingly—few processing activities can be safely abandoned at any time with no cleanup. In particular, it is dangerous to use daemon threads for tasks that might perform any sort of I/O. Daemon threads are best saved for “housekeeping” tasks, such as a background thread that periodically removes expired entries from an in-memory cache.