Threads are a fundamental concept in Java for performing concurrent and parallel processing. However, working with threads effectively requires understanding their lifecycle. This article will provide a comprehensive overview of the various states a thread transitions through during execution. We’ll illustrate each stage with code examples and also cover best practices around managing threads. By the end, you’ll have an in-depth understanding of how to control thread lifecycles for building robust multi-threaded applications.
What is a Thread?
Before jumping into lifecycles, we need to understand what a thread fundamentally represents.
A thread is an execution flow within a program that runs independently. For example, one thread may handle user input, while another performs calculations.
Each thread has its own:
- Program counter to keep track of execution state
- Stack space to store local data and method calls
- Access to shared data/resources for communication
This enables concurrent processing since multiple logical flows execute simultaneously.
Multithreading also enables utilizing multi-core CPUs efficiently. For computation tasks, different threads can run parallel on separate cores.
Some common examples of using threads:
- GUI application – separate thread for handling user interactions
- Web server – individual thread per client request
- Scientific computing – parallel threads for simulations
So why worry about lifecycles? Well-managed threads ensure robust, smooth execution flow. Now let’s understand what these lifecycle stages entail.
Overview of Thread Lifecycle in Java
A thread in Java transitions through various stages, starting from its creation to termination when its task is complete. The diagram summarizes the main thread states:
Let‘s cover what each represents:
New
This is the initial state when a Thread instance is created using the constructor. For example:
Thread t = new Thread(() -> {
// task
}); The thread now exists, but its start() method has not been called yet. So it is inactive and not eligible for scheduling execution yet.
Runnable
When start() is invoked on the thread, its state changes to runnable.
t.start();This adds the thread to the pool of executable threads managed by the thread scheduler. Note multiple threads may be in runnable state and await scheduling.
Running
The thread scheduler selects a runnable thread and dispatches it to run on a CPU core. This is the actually running state where the thread executes the task‘s instructions, iterating through its control flow.
It will occupy the CPU, caches, pipelines, busses to ultimately perform useful computational work.
Waiting/Blocked
The thread may temporarily transition into a waiting or blocked state due to several reasons:
- Waiting – It needs to wait for certain conditions to proceed further (synchronization)
- Blocked – Waiting to acquire a lock or monitor required for entering next section
- Sleeping – Explicitly put to sleep for defined time (
Thread.sleep)
Once the underlying trigger condition changes, the thread will shift back to runnable pool.
Terminated
A thread‘s lifecycle ends when the run() method completes execution. Some ways a thread may terminate:
- Natural completion of the
run()method - Unhandled exception thrown from
run() - Forcibly terminated using
stop()(deprecated) orinterrupt()
Once terminated, a thread cannot restart again.
To illustrate thread transitions through these states, consider this simple example:
class LifecycleThread extends Thread {
public void run() {
// Running
System.out.println("Thread executing");
try {
// Blocked - accessing synchronized method
processData();
} catch (InterruptedException e) {
e.printStackTrace();
}
// Completed - run() exits
System.out.println("Ending thread");
}
private synchronized void processData() throws InterruptedException {
Thread.sleep(2000);
// ... process something
}
}
public class App {
public static void main(String[] args) {
// New thread
LifecycleThread t = new LifecycleThread();
// Runnable - start() called
t.start();
// Do something else while thread runs
// ...
// Thread terminates eventually after run() exits
}
}This covers the major states. But additional states exist for specific mechanisms like timed waiting.
Special Thread States
Beyond core states of new, runnable, running and terminated, some other thread states exist:
Timed Waiting – Thread waits for a specified time period to elapse by using methods like sleep(), wait() etc.
Parked – Thread has been suspended or parked using methods like LockSupport.park(). This is a more advanced use case.
Terminated (not Garbage Collected) – The thread has completed execution (run() returned). But it has not been garbage collected yet and lingers until GC cycle.
So based on the use case, a thread may transition to these states as part of its lifecycle flow.
Next, let‘s go through some guidelines around managing threads effectively.
Best Practices for Thread Management
When architecting a robust, concurrent system using threads, consider these tips:
1. Control Termination
Carefully control termination conditions for threads since restart is not possible. Premature termination may cause:
- Loss of pending work/tasks
- Incomplete workflow execution
- Inconsistent application state
Allow threads to complete either by joining or checking an isAlive flag.
2. Follow Early Initialization
Initialize key components like pools, executors, queues etc on startup rather than lazily. This prevents erratic delays.
3. Use Thread Pools
Maintain a pool of reusable threads rather than spawning short-lived ones for each task. This reduces resource overhead.
4. Handle Shared Resources
Use proper synchronization via locks, semaphores when multiple threads access shared data.
5. Implement Error Handling
Log errors, collect diagnostics and handle unexpected exceptions within worker threads to prevent abrupt failures.
6. Shutdown Gracefully
On application exit, clean up threads by allowing completion or interruption. Abrupt halting may corrupt state.
Adhering to these principles results in robust, production-grade systems. Let‘s look at some common pitfalls next.
Common Concurrency Bugs
While extremely useful, threads also introduce whole classes of complex bugs if not written properly:
-
Race conditions – threads access inconsistent shared data during reads and writes when order of execution varies across runs, without proper locking.
-
Deadlocks – two or more threads cyclically wait on resources held by the other, preventing progress.
-
Starvation – lack of fair resource allocation deprives a thread from executing
-
Livelocks – threads get stuck retrying actions in response to changes by others repeatedly, forgetting to make progress
-
Indefinite blocking – failure to unblock under error scenarios leads to hangs
The non-deterministic nature of threads makes these extremely hard to tackle. Some mitigation options are:
- Carefully model interactions early in design
- Adhere to standard concurrency patterns
- Use high-level constructs like atomics/monitors over locks directly
- Rigorously test for race windows using thread sanitizers
- Set timeouts for operations to prevent permanent blocking
- Handle deadlock detection/recovery automatically
Get it right and threads become a really useful technique!
Hopefully this summary gives you a complete understanding into the lifecycle now. Let‘s conclude with some key takeaways.
Conclusion
The key points about thread lifecycles in Java are:
- Threads transition through different stages like new, runnable, and terminated
- Understanding this lifecycle flow allows controlling execution appropriately
- Properly initializing, synchronizing and shutting down threads is vital
- Special states like timed waiting may be entered conditionally
- Terminated threads cannot restart, making control essential
- Following standard patterns prevents concurrency errors
Modeling thread state transitions clearly early on and managing them effectively leads to scalable concurrent systems without crashes or data corruption.
Understanding these fundamentals will help immensely while architecting robust, multi-threaded applications. Let me know if you have any other queries!
