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。

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。

当最后一个非守护进程线程执行完成时，JVM将完成这项工作。默认情况下，JVM将创建一个线程作为非守护进程，但我们可以通过setDaemon(true)方法将线程创建为守护进程。Daemon线程的一个很好的例子是GC线程，它将在所有非Daemon线程完成后立即完成它的工作。

Java中的守护线程是指运行在后台的线程，主要由JVM创建，用于执行后台任务，如垃圾收集和其他家政任务。

注意事项:

Any thread created by main thread, which runs main method in Java is by default non daemon because Thread inherits its daemon nature from the Thread which creates it i.e. parent Thread and since main thread is a non daemon thread, any other thread created from it will remain non-daemon until explicitly made daemon by calling setDaemon(true). Thread.setDaemon(true) makes a Thread daemon but it can only be called before starting Thread in Java. It will throw IllegalThreadStateException if corresponding Thread is already started and running.

Java中守护线程与非守护线程的区别:

1) JVM在存在之前不会等待任何守护线程完成。

2)当JVM终止时，守护线程与用户线程被区别对待，最终块不被调用，堆栈不受伤，JVM只是退出。

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.