Modern software systems must execute multiple operations concurrently, such as serving user requests, processing data, invoking APIs, and performing database updates. Threads in Java enable this concurrency model by allowing multiple execution paths within a single process, preventing the performance bottlenecks of purely sequential execution.
This is where threads in Java become essential.
A thread is the smallest unit of execution within a program. Java’s multithreading capability allows developers to build fast, responsive, and high-performance applications.
Table of Contents
This article explains threads in Java from fundamentals to internal working and production-level practices.
What is a Thread?
A thread is an independent path of execution within a program.
Every Java application starts with one thread called:
Main Thread
When you create additional threads, the program can perform multiple tasks concurrently.
Example:
- One thread handles UI
- One thread processes data
- One thread performs network calls
All running at the same time.
Why Multithreading is Important
Multithreading helps in:
- Better CPU utilization
- Faster performance
- Improved responsiveness
- Handling concurrent users
- Background processing
Without threads, tasks execute one after another, wasting CPU resources.
Process vs Thread
| Process | Thread |
|---|---|
| Independent program | Part of a process |
| Has its own memory | Shares process memory |
| Heavyweight | Lightweight |
| Slower context switch | Faster context switch |
A Java program is a process; threads run inside it.
How Threads Work Internally
When a thread is created:
- JVM asks OS for a thread
- OS schedules it on CPU
- Thread gets its own stack memory
- Threads share heap memory
- CPU switches between threads (context switching)
The scheduler decides which thread runs and for how long.
Thread Lifecycle in Java
A thread goes through these states:
1) New
Thread is created but not started.
Thread t = new Thread();
2) Runnable
Thread is ready and waiting for CPU.
t.start();
3) Running
Thread is executing instructions.
4) Blocked / Waiting
Thread waits for resources or another thread.
Examples:
- sleep()
- wait()
- join()
5) Terminated
Thread finishes execution.
Creating Threads in Java
Method 1: Extending Thread Class
class MyThread extends Thread {
public void run() {
System.out.println("Thread running");
}
}
new MyThread().start();
Method 2: Implementing Runnable (Preferred)
class MyTask implements Runnable {
public void run() {
System.out.println("Task running");
}
}
Thread t = new Thread(new MyTask());
t.start();
Why preferred?
- Allows inheritance from other classes
- Separates task from thread logic
Method 3: Lambda Expression
Thread t = new Thread(() ->
System.out.println("Running"));
t.start();
Thread Methods You Must Know
sleep()
Pauses execution.
Thread.sleep(1000);
join()
Waits for another thread.
t.join();
yield()
Suggests scheduler to switch threads.
interrupt()
Requests thread to stop.
Synchronization in Threads
When multiple threads access shared data, problems occur:
- Race conditions
- Data inconsistency
- Unexpected behavior
Example problem:
Two threads updating same bank balance.
synchronized Keyword
synchronized void update() {
balance++;
}
Only one thread enters at a time.
Locks and Concurrency Utilities
Java provides advanced tools:
- ReentrantLock
- ExecutorService
- CountDownLatch
- ConcurrentHashMap
These give better control than basic synchronization.
ExecutorService (Production Standard)
Instead of manually creating threads, use thread pools.
ExecutorService executor =
Executors.newFixedThreadPool(3);
executor.submit(() ->
System.out.println("Task"));
executor.shutdown();
Benefits:
- Reuses threads
- Controls concurrency
- Improves performance
- Prevents resource exhaustion
Common Multithreading Problems
Deadlock
Two threads waiting on each other forever.
Starvation
A thread never gets CPU time.
Livelock
Threads keep reacting to each other without progress.
Best Practices in Production
✔ Prefer ExecutorService over manual threads
✔ Avoid shared mutable state
✔ Use immutability where possible
✔ Always handle InterruptedException
✔ Limit thread creation
✔ Monitor thread usage
Interview Insight
If asked:
“What is multithreading in Java?”
You can say:
Multithreading in Java allows concurrent execution of multiple threads within a single process, improving performance and responsiveness. Threads share heap memory but maintain separate stacks, and JVM relies on OS scheduling for execution.
Simple Mental Model
Think of threads like workers in a kitchen:
- One cuts vegetables
- One cooks
- One plates food
Work finishes faster when tasks run in parallel.
Conclusion
Threads in Java are powerful but must be used carefully. They enable scalable, responsive applications but introduce complexity like synchronization and concurrency issues.
Mastering threads means understanding:
- Lifecycle
- Synchronization
- Thread pools
- Concurrency utilities
Once you grasp these, you can build high-performance Java systems.