How to make non daemon thread as daemon in Java?

In Java, threads can be categorised as either daemon or non-daemon (user) threads. Non-daemon threads are typical threads that the Java Virtual Machine (JVM) waits to complete before shutting down, while daemon threads are background threads that do not prevent the JVM from exiting when all user threads finish execution. Sometimes, there might be a need to convert a non-daemon thread into a daemon thread.

Non-Daemon Threads

Non-daemon threads, also known as user threads, are standard threads that keep the JVM running until they complete their execution. These threads are used for tasks that the application relies on to function correctly, such as processing user requests, performing computations, or managing application states.

Daemon Threads

Daemon threads are generally used for background-supporting tasks like garbage collection, monitoring, or housekeeping. The JVM exits when all user threads (non-daemon threads) finish execution, even if daemon threads are still running. Daemon threads are background service threads that do not prevent the JVM from exiting when all user threads have finished their execution. These threads are typically used for background tasks like garbage collection, periodic monitoring, or other tasks that should not block the application from shutting down.

Creating and Converting Threads in Java

1. Creating a Non-Daemon Thread:

In Java, when we create any thread is a non-daemon thread by default. Let's see an example.

File Name: NonDaemonThreadExample.java

2. Converting Non-Daemon to Daemon

To convert a non-daemon thread to a daemon thread, we need to call the setDaemon(true) method on the Thread object before starting the thread. Once a thread is started, you cannot change its daemon status. Here is how we can modify the above example to make the thread a daemon thread:

File Name: DaemonThreadExample.java

Output:

 
Main thread ending

Explanation

In the example above, the main thread ends quickly, and since the daemonThread() is a daemon, the JVM might exit before the daemonThread() finishes its execution. The setDaemon(true) method must be called before the thread is started. If you try to set a thread as daemon after it has been started, the JVM will throw an IllegalThreadStateException.Daemon threads should not be used for tasks that require a guaranteed completion, such as writing data to a file or sending a crucial network packet.

Complete Code Example

Here's the complete code to convert a non-daemon thread to a daemon thread in a more detailed context:

File Name: DaemonThreadExample.java

Output:

 
Main thread ending
Daemon thread running
Daemon thread running
Non-daemon thread running
Daemon thread running

Conclusion

Daemon and non-daemon threads play distinct roles in Java's thread management, each serving different purposes. Non-daemon threads, being the backbone of an application's functionality, keep the JVM alive until they finish execution.

On the other hand, daemon threads provide background services that support the application without preventing the JVM from shutting down when user threads are complete. Converting a non-daemon thread to a daemon thread is straightforward: invoke setDaemon(true) before the thread starts.

This conversion is crucial for tasks that need to run in the background without blocking the application's termination, such as monitoring, logging, or periodic cleanup tasks.