Daemon thread

• Daemon thread in java is a service provider thread that provides services to the user thread. 

• Its life depend on the mercy of user threads i.e. when all the user threads dies, JVM terminates this thread automatically.

• There are many java daemon threads running automatically e.g. gc, finalizer etc

• It provides services to user threads for background supporting tasks. It has no role in life than to serve user threads.

• Its life depends on user threads.

• It is a low priority thread.

• The user thread can be made ad daemon thread before calling start() function else it will  throw illegalThreadStateException.

• A daemon thread runs in background and doesn’t prevent JVM from terminating. 

• When there are no user threads running, JVM shutdown the program and quits. 

• A child thread created from daemon thread is also a daemon thread.

Methods:

• public void setDaemon(boolean status)

• publicbooleanisDaemon()

Why JVM terminates the daemon thread if there is no user thread?

• The sole purpose of the daemon thread is that it provides services to user thread for background supporting task. 

• If there is no user thread, why should JVM keep running this thread? That is why JVM terminates the daemon thread if there is no user thread.