Java.Multithreding.What states can a thread be in?

Let’s walk through the 6 thread states in Java, according to the java.lang.Thread.State enum.

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🧵 Java Thread States

State	When It Happens
NEW	Thread object is created, but start() has not been called
RUNNABLE	Thread is ready to run or currently running
BLOCKED	Waiting to acquire a monitor lock (e.g. synchronized)
WAITING	Waiting indefinitely for another thread to signal
TIMED_WAITING	Waiting for a specified time (e.g. sleep(), join(timeout))
TERMINATED	Thread has completed execution or thrown an exception

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🔍 Detailed Breakdown

1. NEW

Thread t = new Thread(); // Not started yet

Thread object is created, but not started.

Can only move to RUNNABLE by calling start().

2. RUNNABLE

t.start();

Thread has been started and is ready to run or running

Managed by OS thread scheduler

Includes actively executing and waiting for CPU

3. BLOCKED

synchronized(obj) {
    // Thread B blocks if Thread A is holding the lock
}

Thread is waiting to acquire a monitor lock

Different from WAITING, which is voluntary

4. WAITING

obj.wait();
t.join();             // Without timeout
future.get();         // When result isn’t ready yet

Thread is waiting indefinitely

Only proceeds when another thread signals (notify(), join() completes, etc.)

5. TIMED_WAITING

Thread.sleep(1000);
t.join(5000);
obj.wait(1000);

Waiting for a specific amount of time

Will return after timeout or signal, whichever comes first

6. TERMINATED

public void run() {
    System.out.println("done"); // finishes
}

Thread has finished execution or died due to an uncaught exception

Cannot be restarted

🧠 Bonus: Transition Diagram (Text Format)

NEW --> RUNNABLE --> [BLOCKED / WAITING / TIMED_WAITING] --> RUNNABLE --> TERMINATED

BLOCKED → waiting for lock

WAITING → waiting for another thread

TIMED_WAITING → waiting for timeout or event

🧪 Example: Viewing a Thread’s State

Thread t = new Thread(() -> {
    try {
        Thread.sleep(2000);
    } catch (InterruptedException ignored) {}
});

System.out.println(t.getState()); // NEW
t.start();
System.out.println(t.getState()); // RUNNABLE or TIMED_WAITING (briefly)

🧠 Summary Table

State	Description	Common Causes
NEW	Created but not started	new Thread()
RUNNABLE	Ready or running	start() called
BLOCKED	Waiting for synchronized lock	synchronized(obj) contention
WAITING	Waiting indefinitely	wait(), join(), LockSupport.park()
TIMED_WAITING	Waiting for timeout	sleep(), join(timeout)
TERMINATED	Execution finished or failed	run() completed or exception thrown