Thread Fundamentals

A thread is the smallest unit of execution in a program. In Java, every program runs on at least one thread, known as the main thread.

1. What is a Thread?

• A thread is a lightweight process that runs independently.

• Java supports multithreading, meaning multiple threads can run simultaneously.

• Each thread has its own execution path but shares resources with other threads in the same process.

Analogy

Imagine a thread as a train running on a track.

• A single-threaded program is like one train running on a single track, completing one task at a time.

• A multi-threaded program is like multiple trains running on parallel tracks, performing tasks simultaneously.

Here’s a visual representation of threads:

Main Thread  -----> Task 1 -----> Task 2 -----> Task 3  (Sequential Execution)

Multi-Threading:
Thread 1  -----> Task A -----> Task B  
Thread 2  -----> Task X -----> Task Y  
(Thread 1 and Thread 2 run in parallel)

Java Thread Package

Threads in Java are part of the java.lang package, primarily using the following classes and interfaces:

• java.lang.Thread → The main class for creating and managing threads.

• java.lang.Runnable → Functional interface for defining thread tasks.

• java.util.concurrent package → Provides higher-level concurrency utilities.

2. Why Use Threads?

• Improves application performance by utilizing multiple CPU cores.

• Allows multiple tasks to execute simultaneously (e.g., UI responsiveness, background tasks).

• Enables parallel execution for tasks like data processing, computations, and network requests.

3. Thread Lifecycle

A thread goes through multiple states:

1. New → Thread is created but not started.

2. Runnable → Ready to run but waiting for CPU.

3. Running → Actively executing.

4. Blocked/Waiting → Paused, waiting for a resource.

5. Terminated → Execution is completed or stopped.

4. Thread Priorities

Each thread in Java has a priority (range 1 to 10). Default priority is 5 (NORM_PRIORITY).

• Thread.MIN_PRIORITY → 1

• Thread.NORM_PRIORITY → 5

• Thread.MAX_PRIORITY → 10

5. Daemon vs User Threads

• User Threads → Standard application threads.

• Daemon Threads → Background threads that terminate when no user thread is running.