有人能告诉我Java中的守护线程是什么吗?
当前回答
守护线程是在进程的其他非守护线程仍在运行时在后台运行的线程。因此,当所有非守护进程线程完成时,守护进程线程将终止。非守护进程线程的一个例子是运行Main的线程。 通过在线程启动之前调用setDaemon()方法,将线程设置为守护进程
更多参考:Java中的守护进程线程
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。
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。
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
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.
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