When are daemon threads useful?

I know that Deamon threads backgro开发者_JAVA技巧und threads. We can create our own daemon thread by calling setDaemon(true).

My question is: why and when do we need to create our thread as daemon thread?

The JVM exits when all the running threads are daemon threads. So imagine you're writing a simple game where your main method loops until you decide to quit. And imagine that at the start of the game, you start a thread that will endlessly poll some website to trigger alerts. You would like the JVM to exit when you decide to end the game. You don't want the endless polling to prevent the game from ending. So you make this polling thread a daemon thread.

A Deamon thread is automatically terminated by the JVM when all "normal" threads are terminated. Normal threads are never automatically terminated.

Services that you wish to offer to your consumers without any user-interation by way of essentially user-threads form the primary use-case for setting a user thread as a daemon.

As a consequence, until user-threads exist JVM gurantees that daemon threads run continously. You can find examples like GC, UI Thread etc.. those are daemons.

Hope it helps.

As other have pointed, a daemon thread does not prevent the JVM from exiting when the program finishes when this thread is still running.

In general you'd rather not create daemon threads, unless you are absolutely certain the thread has no side effects. Since you can't tell when the thread stops, finalizer blocks are not run, nor are any stack unwound. So try avoiding using IO operations in daemon threads because it can corrupt data.

Normally program terminates when all its threads exited their run() method. Daemon threads do not prevent program to terminate even if they are still running, i.e. executing run().

So, you should use daemon thread if you wish not to prevent program termination when the thread is still running. It is typical for example for long-time periodic tasks but actually depend very much on your program, your design and your taste.

Daemon threads in Java are like a service providers for other threads or objects running in the same process as the daemon thread. Daemon threads are used for background supporting tasks and are only needed while normal threads are executing. If normal threads are not running and remaining threads are daemon threads then the interpreter exits.

When a new thread is created it inherits the daemon status of its parent. Normal thread and daemon threads differ in what happens when they exit. When the JVM halts any remaining daemon threads are abandoned: finally blocks are not executed, stacks are not unwound – JVM just exits. Due to this reason daemon threads should be used sparingly and it is dangerous to use them for tasks that might perform any sort of I/O.

I used them with Timer to delete files that cannot be deleted immediately. That is, I generate .exe files, run and then delete them. But there is 50% chance that executable.delete fails, seemingly because image is still blocked by the process in termination. You can reliably delete executable image only after process has finished completely. But, you never know how long it takes. You set .deleteOnExit therefore instead of .delete. But, you do not want to wait until java machine terminates also. It can take very long and you do not want millions of useless stupid .exe files, that you do not need anymore, hanging in the file system. You therefore schedule executable.delete in the timer to happen one-two seconds later. The timer however cannot be usual thread. If it is so, it will block your program from terminating even if there are no files to delete. I can easily make it daemon however because whether my files are deleted or not by timer is immaterial -- the files will be removed either way: either by daemon or java exit. I think it is perfect use of daemon.