Thread class. These methods define how threads are started, paused, coordinated, interrupted, and monitored.
In this article, we will explore the most commonly used thread methods:
start(), run(), sleep(), yield(), join(), interrupt(), and isAlive().
start() vs run()
start()
Thestart() method is used to begin the execution of a thread. When you call start(), a new thread is created and the JVM internally calls the run() method.
Thread t = new Thread(() -> {
System.out.println("Thread executed using start()");
});
t.start();
start() ensures that the task runs in a separate thread.
run()
Therun() method contains the actual logic that the thread executes. However, calling run() directly does not create a new thread—it simply runs like a normal method in the current thread.
Thread t = new Thread(() -> {
System.out.println("Inside run()");
});
t.run(); // NO new thread created
sleep() and yield()
sleep()
Thesleep() method pauses the execution of the current thread for a specified duration. During this time, the thread enters the TIMED_WAITING state.
try {
Thread.sleep(2000); // sleep for 2 seconds
} catch (InterruptedException e) {
e.printStackTrace();
}
Important Points:
- Does not release locks- Throws
InterruptedException- Used for delays, retries, or simulation
yield()
Theyield() method is a hint to the thread scheduler that the current thread is willing to pause and allow other threads of the same priority to execute.
Thread.yield();Important Points:
- It is only a hint, not guaranteed- The thread may continue execution immediately
- Rarely used in real-world applications
join() and interrupt()
join()
Thejoin() method is used when one thread needs to wait for another thread to complete before continuing execution.
Thread t = new Thread(() -> {
System.out.println("Child thread running");
});
t.start();
t.join(); // main thread waits here
System.out.println("Main thread resumes");
Output
Child thread running
Main thread resumes
Key Points:
- Causes the current thread to enter WAITING or TIMED_WAITING- Ensures sequential execution when required
interrupt()
Theinterrupt() method is used to signal a thread that it should stop what it is doing. It does not forcibly terminate the thread but sets an interrupt flag.
Thread t = new Thread(() -> {
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
System.out.println("Thread interrupted!");
}
});
t.start();
t.interrupt();
Output
Thread interrupted!t.start() is called, a new thread begins execution and enters the sleep state for 5 seconds using Thread.sleep(5000).
Immediately after starting the thread,
t.interrupt() is invoked from the main thread. This sends an interrupt signal to the sleeping thread.
Since the thread is currently in a blocking state (sleep), the interrupt causes
Thread.sleep() to throw an InterruptedException.
Key Points:
- Used for cooperative thread termination- Interrupts blocking methods like
sleep(), wait(), join()- Best practice for stopping threads safely
isAlive()
TheisAlive() method checks whether a thread is still running or not. It returns true if the thread has been started and has not yet terminated.
Thread t = new Thread(() -> {
System.out.println("Running...");
});
System.out.println(t.isAlive()); // false
t.start();
System.out.println(t.isAlive()); // true
Use Cases:
- Monitoring thread status- Debugging concurrent applications
- Ensuring thread completion
Putting It All Together
The following example demonstrates multiple thread methods working together:public class ThreadDemo {
static void main(String[] args) throws Exception {
Thread t = new Thread(() -> {
try {
System.out.println("Thread started");
Thread.sleep(1000);
System.out.println("Thread finished");
} catch (InterruptedException e) {
System.out.println("Interrupted!");
}
});
t.start();
System.out.println("Is alive: " + t.isAlive());
t.join();
System.out.println("Main thread resumes after join");
}
}
Output
Thread started
Is alive: true
Thread finished
Main thread resumes after join
