As per my knowledge main() in Java is a non daemon thread by default, so is it possible to convert it into a daemon thread?
If there are only daemon threads running then the JVM will shutdown. If the main thread was a daemon thread then the program couldn't run without shutting down right away. Also you're not allowed to set the daemon property on a thread after it's started, you can't change a non-daemon thread to a daemon thread while it's running:
public final void setDaemon(boolean on)
Marks this thread as either a daemon thread or a user thread. The Java
Virtual Machine exits when the only threads running are all daemon
threads.
This method must be invoked before the thread is started.
Related
This question already has answers here:
How to create a daemon thread? and what for? [duplicate]
(4 answers)
Closed 2 years ago.
why to use daemon threads if we can use normal threads instead. what is the thing that a daemon thread can do but normal thread can’t. because up to I know, there is not much difference in those two
The Java VM will exit when the last non-daemon thread has finished. That is, a non-daemon thread will prevent the VM from exiting as long as it is running, whereas a daemon thread will not.
You would use a daemon thread for operations which make sense only as long as any non-daemon thread is running. One such example (used internally by the VM) is garbage collection.
A caveat with daemon threads is that the VM kills them off the hard way and does not even guarantee execution of finally blocks. That makes daemon threads dangerous to use for any operations that have effects outside the process itself.
I'm wondering what would happen in the following scenario:
Let's say I have the need to monitor some log file and roll it occasionaly. For that purpose I make a daemon thread that runs in while(true) loop, occasionaly spawning the user threads that close the file and open a new one if needed.
But what happens when, at some point all user threads terminate and JVM decides it is time to shut down and initiates the shutdown sequence, but before it actually shuts down, my daemon spawns another user thread? Can this even happen? If it can, will the user thread just die like a daemon or does jvm have some mechanisms to prevent this?
I'm currently working with daemon threads in Java. Is a thread started by a daemon thread a daemon thread itself too? I personally think it is, but I'm not sure.
Yes, by default, any thread created from a daemon thread is also a daemon thread.
In any case, you can always test this, very easily - with isDaemon().
yes we can make the hierarchy of daemon thread one belongs to other..........
If the thread created by the daemon thread by default he is daemon thread.
On the contrary if the thread created by the user thread , he is user thread.
More on my video about Daemon threads.
I know that Deamon threads background 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.
Is garbage collector a daemon (background) thread?
Thanks.
I will assume yes, Garbage collector thread is a daemon thread. Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection operation or other requests for the java runtime system.
It's not a thread from a java.lang.Thread perspective at least.
Yes: http://www.javaperspective.com/daemon-threads.html :
(Daemon threads are considered as threads that run in the background and they are generally used as service providers for user threads. For example, the Java garbage collector is a daemon thread)
on jdk 1.8 following threads are listed with
ThreadMXBean mxbean = ManagementFactory.getThreadMXBean();
for(long id:mxbean.getAllThreadIds())
System.out.println(mxbean.getThreadInfo(id));
Output -
"Attach Listener" Id=5 RUNNABLE
"Signal Dispatcher" Id=4 RUNNABLE
"Finalizer" Id=3 WAITING on java.lang.ref.ReferenceQueue$Lock#63947c6b
"Reference Handler" Id=2 WAITING on java.lang.ref.Reference$Lock#2b193f2d
"main" Id=1 RUNNABLE
There is no GC thread. It can safely be said garbage collection process is native.
A daemon thread is also a thread that continues to run even after the JVM exits. From Oracle documentation
When a Java Virtual Machine starts up, there is usually a single non-daemon thread (which typically calls the method named main of some designated class). The Java Virtual Machine continues to execute threads until either of the following occurs:
•The exit method of class Runtime has been called and the security manager has permitted the exit operation to take place.
•All threads that are not daemon threads have died, either by returning from the call to the run method or by throwing an exception that propagates beyond the run method.
So if GC is a daemon thread, it should be a native thread spawned by the java run time, but can continue to run after he JVM exits