What are the thread states?

In Java, a thread can exist in one of several states during its lifecycle. Understanding these states is crucial for effective multithreaded programming. Knowing the states helps in debugging, optimizing, and managing the execution of threads, ensuring that your concurrent applications run smoothly and efficiently.

Thread States Overview

A Java thread can be in one of the following states:

1. NEW: A thread that has not yet started execution. This is the state when a Thread object is created but start() method has not been called.
2. RUNNABLE: A thread that is eligible to run. This doesn't mean the thread is currently running, but that it's ready and waiting for its turn to be executed by the JVM. It can be running or ready to run.
3. BLOCKED: A thread that is waiting to acquire a lock to enter a synchronized block or method. It pauses and waits for other threads to release a lock.
4. WAITING: A thread that is waiting indefinitely for another thread to perform a particular action. It pauses indefinitely waiting for another thread to wake it up (using notify() or notifyAll() methods).
5. TIMED_WAITING: A thread that is waiting for another thread to perform a particular action for up to a specified waiting time. Similar to WAITING but with a timeout (using methods like sleep(long millis) or wait(long timeout)).
6. TERMINATED: A thread that has completed its execution. Once a thread enters this state, it cannot be restarted.

Code Example: Demonstrating Thread States

This code demonstrates the different states a thread can be in. Here's a breakdown:

1. A new thread myThread is created, and its state is printed (NEW).
2. The thread is started using myThread.start(), and its state is printed (RUNNABLE).
3. A short delay is introduced using Thread.sleep(100), giving the thread a chance to run and possibly enter the TIMED_WAITING state if it calls Thread.sleep internally during the execution of the lambda, or remains RUNNABLE.
4. The main thread waits for myThread to complete using myThread.join().
5. After the thread completes its execution, its state is printed (TERMINATED).

public class ThreadStateExample {

    public static void main(String[] args) throws InterruptedException {
        Thread myThread = new Thread(() -> {
            try {
                Thread.sleep(5000); // Simulate some work
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
        });

        System.out.println("Thread State after creation: " + myThread.getState()); // NEW

        myThread.start();
        System.out.println("Thread State after start(): " + myThread.getState()); // RUNNABLE

        Thread.sleep(100); // Give the thread some time to run

        System.out.println("Thread State after short running time: " + myThread.getState()); // TIMED_WAITING or RUNNABLE

        myThread.join(); // Wait for the thread to complete

        System.out.println("Thread State after completion: " + myThread.getState()); // TERMINATED
    }
}

Concepts Behind the Snippet

This snippet showcases the progression of a thread through its lifecycle: from its initial state (NEW), to being eligible for execution (RUNNABLE), potentially entering a waiting state (TIMED_WAITING in this instance due to the internal Thread.sleep), and finally reaching its termination state (TERMINATED) after completing its task.

The key concepts here are:

• Thread Creation: Creating a Thread object puts it in the NEW state.
• Thread Scheduling: The JVM is responsible for scheduling RUNNABLE threads for execution.
• Waiting States: Threads can enter waiting states for various reasons, such as waiting for time to pass or waiting for a lock.
• Thread Termination: Once a thread's run() method completes, the thread transitions to the TERMINATED state.

Real-Life Use Case

Consider a web server that handles incoming requests concurrently. Each request might be handled by a separate thread. Understanding thread states is critical for:

• Load Balancing: Monitoring the number of RUNNABLE threads to ensure the server isn't overloaded.
• Deadlock Detection: Identifying threads in the BLOCKED state for extended periods, which could indicate a deadlock.
• Resource Management: Optimizing resource allocation based on the number of threads in different states. For example, if a large number of threads are in WAITING states, it might indicate that the server is waiting for external resources (like database connections) and needs to be optimized.

Best Practices

• Avoid Long-Running Synchronized Blocks: Long synchronized blocks can cause excessive blocking, leading to performance bottlenecks.
• Use Thread Pools: Thread pools reduce the overhead of creating and destroying threads, improving performance and resource utilization.
• Be Mindful of Waiting States: Minimize the amount of time threads spend in WAITING or BLOCKED states to improve overall throughput.
• Use Non-Blocking Algorithms: Consider using non-blocking algorithms and data structures (e.g., those in java.util.concurrent) to avoid blocking and improve concurrency.

Interview Tip

When discussing thread states in an interview, emphasize your understanding of the state transitions and the reasons behind them. Be prepared to discuss scenarios where threads might be in different states and how you would diagnose and address potential issues like deadlocks or excessive waiting.

When to use them

Understanding thread states is fundamental when working with concurrent applications. You should use your knowledge of thread states during:

• Debugging: Diagnosing performance issues, deadlocks, and race conditions.
• Performance Tuning: Identifying bottlenecks and optimizing thread execution.
• System Design: Making informed decisions about thread management and resource allocation.
• Monitoring: Gaining real-time insights into your application's threading behavior

Memory footprint

Each thread has its own stack space to store local variables and execution context. Thus, creating too many threads can put a strain on memory resources. Thread pools and careful thread management are essential to controlling memory footprint in concurrent applications.

Alternatives

Alternatives to traditional threads include:

• Executors and Thread Pools: Manage and reuse threads efficiently.
• Fork/Join Framework: Suited for problems that can be divided into smaller subproblems.
• CompletableFuture: Enables asynchronous computations and simplifies complex workflows.
• Reactive Programming (RxJava, Project Reactor): Handle asynchronous data streams in a declarative way.
• Virtual Threads (Project Loom): Lightweight threads that minimize resource consumption.

Pros

Advantages of understanding and managing thread states effectively include:

• Improved Performance: Optimal thread scheduling and resource utilization.
• Enhanced Responsiveness: Avoid blocking and maintain application responsiveness.
• Scalability: Handle increased workload with multiple threads.
• Resource Efficiency: Minimize resource usage with proper thread management

Cons

Potential drawbacks and challenges:

• Complexity: Concurrent programming can be complex and error-prone.
• Synchronization Overhead: Managing access to shared resources introduces overhead.
• Deadlocks and Race Conditions: Improper synchronization can lead to deadlocks and race conditions.
• Debugging: Debugging multithreaded applications can be difficult.