he Thread Lifecycle in Java is a core concept in multithreading that defines how a thread progresses through different stages during its execution. Each state, New, Runnable, Running, Waiting, Timed Waiting, and Terminated, represents a specific phase in a thread’s existence, helping developers manage concurrency effectively. Understanding these stages is essential for writing efficient, responsive, and stable Java applications.
In this blog, we will explore the complete Thread Lifecycle in Java with clear explanations, diagrams, and practical examples. You will learn how threads transition between states, how lifecycle methods like start(), run(), and sleep() impact execution, and how to implement these concepts in real-world programs. By the end, you’ll have a strong foundation in Java multithreading.
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Life Cycle of a Thread
Threads, the smallest unit of a process, follow a structured progression called the lifecycle and states of a thread in Java. This lifecycle includes six main states that a thread can occupy at any point in time:
1. New
A thread is in this state when you've created an instance of the Thread class but haven't invoked the start() method yet. It remains in this state until the program starts the thread.
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2. Active
This state consists of two sub-states, Runnable and Running. Runnable implies that the thread is ready for execution and is waiting for resource allocation by the thread scheduler. Running means the thread scheduler has selected the thread and is currently executing its run() method.
3. Blocked / Waiting
A thread enters this state when it is temporarily inactive and waiting for a signal to proceed due to reasons like waiting for a resource to become available (Blocked) or waiting for another thread to perform a specific action (Waiting).
4. Timed Waiting
In this state, a thread is waiting for a specified period. A thread might enter this state through methods like Thread.sleep(long millis) or Object.wait(long timeout) where it waits for a particular duration before resuming its activities.
5. Terminated
This is the final state in the thread life cycle. The thread arrives here when it has completed its execution, i.e., its run() method has been completed, or it has been abruptly terminated due to an unhandled exception. Once in this state, the thread cannot be resumed.
As Java developers, having a profound understanding of these states and the transitioning nuances between them provides us with the power to harness threads effectively, optimizing the execution of our concurrent programs.
Here, We Explain the Life Cycle of Thread In Java with Diagram
Life Cycle Of Thread In Java With Example
Let's go through a simple example to better understand the lifecycle of a thread in Java. In this example, let’s create a simple thread and explain the states it goes through in its lifecycle.
public class ThreadExample extends Thread {
@Override
public void run() {
for (int i = 0; i < 3; i++) {
try {
Thread.sleep(1000);
System.out.println("Thread running " + i);
} catch (InterruptedException e) {
System.out.println("Thread interrupted");
}
}
System.out.println("Thread execution completed");
}
public static void main(String[] args) {
ThreadExample te = new ThreadExample();
te.start();
}
}The code creates a subclass of Thread called ThreadExample and overrides its run() method. Inside the run() method, there is a loop that runs three times, with a sleep interval of 1 second (1000 milliseconds) between each run. If the thread is interrupted during its sleep, it catches the InterruptedException and prints "Thread interrupted". After the loop completes its three runs, it prints "Thread execution completed."
In the main() method, an instance of ThreadExample is created and started, causing its run() method to execute in a separate thread.
The output of this program would look something like this:
Thread running 0
Thread running 1
Thread running 2
Thread execution completed
Let's walk through the stages this thread will go through:
- New: As soon as we create an instance of our ‘ThreadExample’ class, the new thread is in the New state. It remains in this state until the program calls the thread's ‘start()’ method. This happens in the line ‘ThreadExample te = new ThreadExample();’
- Active (Runnable and Running): After calling the ‘start()’ method (i.e., ‘te.start();’), the thread moves from the New state to the Runnable state. It's now ready to run and waiting to be selected for execution by the thread scheduler. Once the thread scheduler allocates CPU to the thread, it transitions to the Running state, and its ‘run()’ method starts to execute.
- Blocked / Waiting / Timed Waiting: In our ‘run()’ method, we have used the ‘Thread.sleep(1000);’ method, which makes the thread sleep for 1 second. This places our thread in a Timed Waiting state. If running state of thread in Java needs to wait for another thread to release a resource, it will move to the Blocked/Waiting state.
- Terminated: Once the ‘run()’ method has completed its execution, the thread moves to the Terminated state. In our example, this occurs after the "Thread execution completed" line is printed.
By observing this simple example, we can see the thread moving through each state in its lifecycle: New, Active (Runnable and Running), Timed Waiting, and finally, Terminated.
Applet Life Cycle In Java
Applets in Java also have a distinct lifecycle, much like threads, governed by a set of specific methods. The lifecycle consists of the following stages:
- Initialization
- Starting
- Stopping
- Destruction
Let's delve into each of these stages:
1. Initialization
This is the first phase of the lifecycle. When an applet is loaded into the browser, the ‘init()’ method is called. This method is executed only once during the lifecycle of the applet. This method is used to perform one-time operations such as loading images or sounds, initializing variables, etc.
public void init() {
}2. Starting
After initialization, the applet enters the Starting stage. The ‘start()’ method is called, signaling the applet to start its execution. This method is called every time an applet's webpage is opened or revisited, or when the applet is deiconified.
public void start() {
}3. Stopping
When a user navigates away from the webpage containing the applet, or the webpage is minimized, the ‘stop()’ method is called. This method allows you to suspend tasks that don't need to run when the applet is not visible.
public void stop() {
}4. Destruction
This is the final stage of the applet's lifecycle. The ‘destroy()’ method is called when the browser closes, or the applet's webpage is no longer in use. This method gives the applet a chance to free up system resources and perform cleanup tasks.
Remember
Each stage can be overridden as per the need of the applet to execute specific tasks at each stage of the lifecycle. By understanding these stages, developers can control an applet's behavior throughout its existence in a web browser.
Also Read: How to Code, Compile, and Run Java Projects: A Beginner’s Guide
Implementation of Thread States
In Java, understanding the thread states is key for effective multithreading. Let's explore each of these thread states in Java with some examples:
1. New: A thread is in the new state when an instance of the Thread class gets created but the ‘start()’ method is not invoked yet.
Thread thread = new Thread();
2. Runnable: The thread transitions to the Runnable state when the ‘start()’ method is invoked. The thread might not be executed immediately but is ready for execution.
thread.start();
3. Running: The thread scheduler picks a thread from the pool of Runnable threads and moves it to the Running state where the thread's ‘run()’ method executes.
public void run() {
}4. Blocked/Waiting: A thread enters this state when it is waiting for a resource, like I/O completion, and cannot proceed until it gets that resource or if it's waiting for another thread to perform a specific action.
synchronized(object) {
}5. Timed Waiting: A thread enters this state when it calls methods like ‘Thread.sleep(long millis)’ or ‘Object.wait(long timeout)’, i.e., it's in a sleeping or waiting state for a specified period.
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}6. Terminated: The thread enters the Terminated state when it has completed its execution, i.e., its ‘run()’ method has been completed, or it has been abruptly terminated due to an unhandled exception.
public void run() {
}In these examples, we have implemented and observed the thread states in Java. Remember, managing these states effectively is pivotal for writing efficient multithreaded programs in Java.
Java Program for Demonstrating Thread States
Here is a Java program demonstrating thread states using thread methods in Java, with priority setting and creating threads in Java:
class ThreadDemo extends Thread {
@Override
public void run() {
for(int i = 0; i < 5; i++) {
try {
Thread.sleep(1000);
System.out.println(Thread.currentThread().getId() + " Value " + i);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
public class Main {
public static void main(String args[]) {
ThreadDemo t1 = new ThreadDemo();
ThreadDemo t2 = new ThreadDemo();
ThreadDemo t3 = new ThreadDemo();
t1.start();
t2.setPriority(Thread.MAX_PRIORITY);
t2.start();
System.out.println("Is thread t1 alive? " + t1.isAlive());
try {
t1.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
t3.start();
t3.setPriority(Thread.MIN_PRIORITY);
System.out.println("Is thread t1 alive? " + t1.isAlive());
}
}In this program, we've created three threads, t1, t2, and t3, demonstrating the creation of threads in Java. The thread methods in Java are showcased via the ‘start()’, ‘setPriority()’, ‘isAlive()’, and ‘join()’ methods.
The thread t2's priority is set to MAX_PRIORITY, and t3's to MIN_PRIORITY, demonstrating thread priority in Java.
Code Explanation:
The code defines a class ThreadDemo that extends the Thread class in Java. Inside this class, the run() method is overridden. The run() method is where the execution of a new thread starts after the start() method is called.
In the run() method, there's a for-loop that runs five times. Inside the loop, Thread.sleep(1000) makes the current thread sleep for 1 second (or 1000 milliseconds). The program then prints the ID of the current thread and the value of i. If the thread is interrupted while sleeping, an InterruptedException is caught and its stack trace is printed.
The main() method in the Main class is where the program starts execution. Three ThreadDemo objects, t1, t2, and t3, are created, illustrating creating threads in Java.
t1 is started with t1.start(), which causes the run() method in t1 to begin execution in a new thread.
t2 is given maximum priority using setPriority(Thread.MAX_PRIORITY), after which it is started. Thread priorities in Java range from 1 (minimum priority) to 10 (maximum priority), and they influence the order in which threads are scheduled to run.
The program then checks if t1 is still running (alive) with t1.isAlive() and prints the result.
Next, t1.join() is called, which causes the current thread (main thread) to pause execution until t1 has finished executing (is no longer alive).
T3 is then started and given minimum priority.
Finally, the program checks again whether t1 is still running and prints the result.
Example Output:
Thread-0 Value 0
Thread-1 Value 0
Is thread t1 alive? true
Thread-0 Value 1
Thread-1 Value 1
Thread-0 Value 2
Thread-1 Value 2
Thread-0 Value 3
Thread-1 Value 3
Thread-0 Value 4
Thread-1 Value 4
Is thread t1 alive? false
Thread-2 Value 0
Thread-2 Value 1
Thread-2 Value 2
Thread-2 Value 3
Thread-2 Value 4
This program presents a solid example of the lifecycle and states of a thread in Java, demonstrating thread methods, thread creation, and thread priority in action. Each thread transitions between the various states (New, Runnable, Timed Waiting, and Terminated) during the course of execution.
Conclusion
Understanding the Thread Lifecycle in Java is essential for creating reliable and efficient multithreaded applications. The lifecycle and states of a thread in Java, New, Runnable, Running, Blocked, Waiting, Timed Waiting, and Terminated, represent distinct phases that govern how a thread operates within a program. By applying methods like start(), run(), sleep(), and join(), developers can manage these transitions with precision.
Proper handling of threads enhances application responsiveness, ensures better resource utilization, and supports concurrent execution. Additionally, considering synchronization and thread safety prevents issues such as race conditions or deadlocks. A solid grasp of the thread lifecycle empowers Java developers to design high-performing applications that run smoothly in real scenarios.
FAQs
1. What impact does the Thread Lifecycle in Java have on the performance of a Java application?
The Thread Lifecycle in Java directly affects performance by managing how threads start, run, wait, and terminate. Proper lifecycle management ensures optimal CPU utilization, reduces idle time, and minimizes resource conflicts. By understanding these states, developers can create responsive and efficient Java applications that scale better under heavy workloads.
2. How does understanding the Thread Lifecycle in Java aid in debugging multithreaded applications?
A clear understanding of the Thread Lifecycle in Java helps detect issues like race conditions and deadlocks. By knowing which state a thread is in, developers can analyze thread behavior and troubleshoot errors effectively. This knowledge improves debugging efficiency and ensures smoother execution of multithreaded programs.
3. How does thread synchronization relate to the Thread Lifecycle in Java?
Thread synchronization is essential in the lifecycle and states of a thread in Java. It prevents multiple threads from accessing shared resources simultaneously. Synchronization directly impacts the Blocked and Waiting states, ensuring thread safety, data consistency, and preventing deadlocks when threads compete for the same resource.
4. How does the Thread Lifecycle in Java differ from other programming languages?
The Thread Lifecycle in Java defines six distinct states: New, Runnable, Running, Waiting, Timed Waiting, and Terminated. While other languages may follow similar principles, the terminology and state transitions differ. Java’s structured lifecycle gives developers precise control over concurrency and thread management compared to other programming languages.
5. What are the different states in the lifecycle of a thread in Java?
The lifecycle and states of a thread in Java include New, Runnable, Running, Waiting, Timed Waiting, and Terminated. Each state represents a specific execution stage. Understanding these states allows developers to track thread behavior, manage concurrency, and ensure stable and efficient application performance.
6. What is the significance of the New state in the Thread Lifecycle in Java?
In the New state, a thread object is created but has not started execution. This stage allows developers to initialize resources, assign priorities, or configure properties before the thread begins. Once the start() method is invoked, the thread transitions to the Runnable state, awaiting CPU allocation.
7. What happens in the Runnable state of the Thread Lifecycle in Java?
In the Runnable state, the thread is eligible for execution but awaits CPU scheduling. Multiple threads may exist in this state simultaneously. The thread scheduler decides which thread moves to the Running state. This phase is crucial for concurrency and resource sharing in multithreaded applications.
8. What is the difference between the Waiting and Timed Waiting states in the lifecycle of a thread in Java?
In the Waiting state, a thread pauses indefinitely until another thread signals it to resume. In contrast, the Timed Waiting state allows a thread to pause for a defined time using methods like sleep() or join(timeout). Both states help manage thread coordination and resource usage.
9. What occurs when a thread reaches the Terminated state in Java?
A thread enters the Terminated state once its execution completes. At this stage, the thread cannot be restarted, and attempts to do so result in IllegalThreadStateException. This state signifies the end of the thread’s lifecycle and ensures resources are released for system efficiency.
10. Can a thread move back from the Terminated state in the Thread Lifecycle in Java?
No, once a thread reaches the Terminated state, it cannot return to any previous state. To execute the same task again, developers must create a new thread instance. This restriction maintains lifecycle integrity and prevents unexpected program behavior in Java multithreading.
11. Why is it important to understand the lifecycle and states of a thread in Java for multithreading?
Understanding the lifecycle and states of a thread in Java ensures efficient concurrency management. Developers can optimize CPU usage, prevent deadlocks, and improve response time. This knowledge helps build stable and scalable multithreaded applications that function effectively under varying loads.
12. Which methods in Java are commonly used to manage thread states?
Common methods include start(), run(), sleep(), wait(), notify(), join(), and yield(). These methods control transitions between states in the Thread Lifecycle in Java. Using them effectively allows developers to manage concurrency, scheduling, and synchronization across multiple threads.
13. How does the thread scheduler affect the Thread Lifecycle in Java?
The thread scheduler determines which thread in the Runnable pool enters the Running state. Its decision depends on the JVM implementation and the operating system. Since scheduling is non-deterministic, developers cannot control the exact execution order, making lifecycle understanding essential.
14. What is the role of the start() method in the lifecycle of a thread in Java?
The start() method transitions a thread from the New state to the Runnable state. This method signals the JVM to schedule the thread for execution. Without invoking start(), the thread remains inactive and never participates in the Thread Lifecycle in Java.
15. What happens if run() is called directly instead of start() in Java?
If run() is called directly, it behaves like a normal method and executes within the current thread. No new thread is created, and the Thread Lifecycle in Java is bypassed. To create a new concurrent thread, the start() method must be used.
16. How does thread priority influence the lifecycle and states of a thread in Java?
Thread priority provides hints to the scheduler about which thread should run first. Higher-priority threads are generally executed sooner. However, exact execution order depends on the JVM and OS. Priorities impact the Runnable state but do not guarantee strict control.
17. What exceptions are commonly associated with the Thread Lifecycle in Java?
The most common exceptions are InterruptedException, which occurs when a waiting or sleeping thread is interrupted, and IllegalThreadStateException, raised when attempting to restart a terminated thread. These exceptions highlight the importance of understanding lifecycle rules in Java multithreading.
18. How can understanding the Thread Lifecycle in Java prevent deadlocks?
By knowing the states in which threads may block or wait, developers can design synchronization strategies that avoid circular waits and resource conflicts. Understanding the lifecycle helps prevent deadlocks by ensuring proper thread coordination and resource allocation.
19. What tools are available to monitor the Thread Lifecycle in Java?
Developers can use Java VisualVM, JConsole, or thread dumps to monitor the lifecycle and states of a thread in Java. These tools provide real-time insights into thread states, helping identify performance issues, deadlocks, and inefficient resource usage.
20. How does mastering the lifecycle and states of a thread in Java improve application design?
Mastering the lifecycle and states of a thread in Java allows developers to design efficient, concurrent applications. It improves resource management, prevents synchronization issues, and ensures responsive performance. This knowledge is crucial for building scalable, high-performance Java systems.
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