守护线程

在后台运行的线程称为守护线程。

Daemon线程示例:

垃圾收集器。 信号分配器。

守护线程的目标:

守护进程线程的主要目标是为非守护进程线程提供支持。

有关守护线程的其他信息:

通常，守护线程在MIN_PRIORITY中运行，但是，也可以使用MAX_PRIORITY运行守护线程。 示例:当需要额外的内存时，GC通常以MIN_PRIORITY优先级运行。JVM将GC的优先级从MIN_PRIORITY增加到MAX_PRIORITY。

一旦线程已经启动，就不能将其从守护线程更改为非守护线程，这会导致IllegalThreadStateException异常。 例子: public static void main(String[] args) { Thread.currentThread () .setDaemon(真正的); ｝ 输出: java.lang.IllegalThreadStateException线程异常 在java.base / java.lang.Thread.setDaemon (Thread.java: 1403)

If we are branching off a thread, the child thread inherits the nature of the parent thread. If the parent thread is a non-daemon thread automatically the child thread will be non-daemon as well and if the parent thread is a daemon, the child thread will be a daemon as well. class Scratch { public static void main(String[] args) { CustomThread customThread = new CustomThread(); customThread.start(); } } class CustomThread extends Thread{ @Override public void run() { System.out.println(currentThread().isDaemon()); } } Output: false

When the last non-daemon thread terminates, all the daemon threads get terminated automatically. class Scratch { public static void main(String[] args) { System.out.println("Main Thread Started."); CustomThread customThread = new CustomThread(); customThread.setDaemon(true); customThread.start(); System.out.println("Main Thread Finished."); } } class CustomThread extends Thread{ @Override public void run() { System.out.println("Custom Thread Started."); try { sleep(2000); } catch (InterruptedException ignore) {} System.out.println("Custom Thread Finished."); //Won't get executed. } } Output: Main Thread Started. Main Thread Finished. Custom Thread Started.

守护进程： d（isk） a（nd） e（xecution） mon（itor） or or from de（vice） mon（itor）

Daemon(计算)的定义:

处理打印假脱机和文件传输等服务请求的后台进程，在不需要时处于休眠状态。

——来源:牛津词典英文版

Java中的守护线程是什么?

Daemon threads can shut down any time in between their flow, Non-Daemon i.e. user thread executes completely. Daemon threads are threads that run intermittently in the background as long as other non-daemon threads are running. When all of the non-daemon threads complete, daemon threads terminates automatically. Daemon threads are service providers for user threads running in the same process. The JVM does not care about daemon threads to complete when in Running state, not even finally block also let execute. JVM do give preference to non-daemon threads that is created by us. Daemon threads acts as services in Windows. The JVM stops the daemon threads when all user threads (in contrast to the daemon threads) are terminated. Hence daemon threads can be used to implement, for example, a monitoring functionality as the thread is stopped by the JVM as soon as all user threads have stopped.

Daemon thread is like daemon process which is responsible for managing resources,a daemon thread is created by the Java VM to serve the user threads. example updating system for unix,unix is daemon process. child of daemon thread is always daemon thread,so by default daemon is false.you can check thread as daemon or user by using "isDaemon()" method. so daemon thread or daemon process are basically responsible for managing resources. for example when you starting jvm there is garbage collector running that is daemon thread whose priority is 1 that is lowest,which is managing memory. jvm is alive as long as user thread is alive,u can not kill daemon thread.jvm is responsible to kill daemon threads.

再讲一点(参考:Java并发实践)

当创建一个新线程时，它将继承其守护进程状态 的父母。 当所有非守护进程线程完成时，JVM停止，并放弃所有剩余的守护进程线程: 最后，块不执行， 栈不会被解开——JVM只是退出。 由于这个原因，应该谨慎使用守护线程，将它们用于可能执行任何类型的I/O的任务是危险的。

守护线程是在后台执行一些任务的线程，比如处理请求或应用程序中可能存在的各种chronjob。

当你的程序只剩下守护线程时，它将退出。这是因为这些线程通常与普通线程一起工作，并提供事件的后台处理。

你可以使用setDaemon方法指定一个线程是守护线程，它们通常不会退出，也不会被中断。当应用程序停止时，它们就会停止。

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。