Daemon Threads

In the world of programming, daemon threads are a special type of thread that run in the background to provide support services to the main application threads, often referred to as user or non-daemon threads. The key characteristic of daemon threads is that they do not prevent the application from exiting when all user threads have completed their execution.

A daemon thread is often called a service thread or a nonessential thread.

The other kind of thread is often called a user thread.

The JVM will exit when all user threads have returned from their run methods. Once all of the user threads have returned, any active daemon threads will be forced to stop running.

Daemon threads are a valuable tool in concurrent programming for managing background tasks that support the primary functions of an application without preventing its timely termination.

What are they?

1. Daemon threads will not prevent the JVM from exiting when the main program finishes - even if these daemon threads are still running in the background.
2. Daemon threads will be terminated by the JVM when none of the other threads are running. It includs main thread of execution as well. When the main thread terminates, all the daemon threads will also be terminated - irrespective of whether the work that they are doing is completed or not.
3. Daemon threads acts like service providers for other threads running in the same process.

Simple example

1. main method spawns one or more daemon threads
2. the daemon thread/threads keep running in the background
3. the main method terminates
4. even if some of the work in the daemon thread/threads is still pending, the daemon thread/threads will also terminate

Examples

1. An example for a daemon thread is the garbage collection.

Another abstract example

Pretend that we have a client-server GUI-based chat client. We will typically have at least one thread sitting around and listening for input from the other program (the server will listen for input from the client and the client will listen for input from the server). These threads can be set as daemon threads, because we don’t want the JVM to keep executing if those are the only threads left.

Now let’s look at the GUI for our programs. These will be run in a user thread, since we want to make sure that the JVM does not exit until the GUI thread finishes running (we will generally stop these threads from running when the user clicks to close the window).

Sample implementation

https://github.com/explorer436/programming-playground/tree/main/java-playground/my-implementations/java-multithreading-and-concurrency-demo

The Core Purpose and Behavior of Daemon Threads

The primary role of a daemon thread is to perform tasks that are helpful but not critical to the application’s core functionality. Think of them as “service providers” for the foreground threads.

A crucial distinction lies in how the program’s lifecycle is handled. The Java Virtual Machine (JVM) or the Python interpreter will wait for all non-daemon threads to finish their tasks before terminating the program. However, if only daemon threads are left running, the program will exit, and the daemon threads will be abruptly stopped.

Key Characteristics of Daemon Threads

1. Background Execution: They operate in the background to support the main threads.
2. Low Priority: Daemon threads are typically low-priority threads.
3. Automatic Termination: The program’s exit is not dependent on daemon threads. They are automatically terminated when all user threads have finished.
4. Non-Essential Tasks: They are intended for tasks that can be safely abandoned, as they might not complete their execution.

Common Use Cases for Daemon Threads

Daemon threads are well-suited for a variety of background tasks, including:

1. Garbage Collection: One of the most classic examples is garbage collection in Java, which runs in the background to free up memory used by objects that are no longer needed.
2. Monitoring and Logging: Daemon threads can be used for application monitoring, tracking performance, or logging information without interfering with the main application flow.
3. Background Data Synchronization: They can periodically sync data with a server or database in the background.
4. Heartbeat Signals: Sending periodic “heartbeat” signals to indicate that a system is still active.
5. Asynchronous I/O Tasks: Handling asynchronous input/output requests.
6. Cache Management: Performing background tasks like cleaning up an in-memory cache.

How to Create a Daemon Thread

How do we make a thread into a daemon thread?

1. We can change any java thread into daemon thread.
2. Before a thread starts, if we use the setDaemon(boolean) method to true, we are converting a regular thread into a daemon thread.
3. To specify that a thread is a daemon thread, call the setDaemon method with the argument true.
4. To determine if a thread is a daemon thread, use the accessor method isDaemon.

In both Java and Python, a standard thread can be designated as a daemon thread before it is started.

In Java: you can use the setDaemon(true) method of the Thread class.

Thread daemonThread = new Thread(() -> {
    // Background task logic
});
daemonThread.setDaemon(true);
daemonThread.start();

In Python: you can set the daemon property of a threading.Thread object to True.

import threading
import time

def background_task():
    while True:
        print("Daemon is running...")
        time.sleep(1)

daemon_thread = threading.Thread(target=background_task)
daemon_thread.daemon = True
daemon_thread.start()

print("Main thread is finishing.")