Question

Explain the life cycle of the Thread.

Short answer

A thread starts in 'New', goes through 'Runnable', 'Running', 'Blocked/Waiting', 'Timed Waiting', and ends in 'Terminated'.

Step-by-step solution

New State

The life cycle of a thread begins with the 'New' state. In this state, a Thread object is created but it is not yet eligible for running. At this point, the thread is merely an instance of the Thread class, and does not yet encapsulate any active behavior.

Runnable State

Once the thread's 'start()' method is invoked, the thread moves to the 'Runnable' state. In this state, the thread is ready to run and awaits the Operating System's thread scheduler to allocate CPU time for execution. The thread transitions to this state even if it is not immediately running.

Running State

The thread enters the 'Running' state when the thread scheduler selects it from the 'Runnable' pool and allocates CPU time for execution. In this state, the 'run()' method of the thread is executed.

Blocked/Waiting State

A thread transitions to a 'Blocked' or 'Waiting' state when it is inactive for some reason. This can occur if the thread is waiting for an I/O operation to complete or when it is waiting to acquire a lock on a resource.

Timed Waiting State

A thread is in the 'Timed Waiting' state when it calls methods like 'sleep()', 'wait(time)', or 'join(time)', making it wait for a specified amount of time before transitioning back to the 'Runnable' state once the time expires or the event it's waiting for occurs.

Terminated State

Finally, a thread enters the 'Terminated' state when its 'run()' method has finished executing or when it is manually exited using the 'stop()' method. At this point, the thread's life cycle is complete, and the resources used by it are released.

Key concepts

New State

In the world of programming, the journey of a thread begins with the 'New State'. This is a phase where the thread is born as an object of the Thread class in Java. However, it's just a prospective thread. At this point, the thread hasn't been scheduled for execution; it merely exists in the program, like a plane waiting on the tarmac. Think of it as being in the starting blocks, ready but not yet set off by the starting gun.

Here are some characteristics of the 'New State':

• The thread has been instantiated, using the 'new' keyword.
• No code from the thread's 'run()' method is being executed.
• The thread does not yet hold resources or processor time.

This state is purely preparatory, setting the stage for the actions to come once it joins the race of execution.

Runnable State

After a thread has been created in the 'New State', it transitions to the 'Runnable State' when the 'start()' method is invoked on it. This is an interesting phase because while the thread is ready to run, it isn't necessarily executing just yet. It's awaiting its turn, poised for action, similar to cars on a busy highway waiting for the light to turn green.

Key points about the 'Runnable State' include:

• The thread is eligible to run and is waiting for CPU allocation.
• The Operating System's thread scheduler (a whimsical sort of traffic cop) is tasked with deciding which thread in the pool gets CPU time.
• It is important to note that 'Runnable' does not mean that the thread is running; it just signifies readiness.

This state underscores how a thread is dependent on the system scheduler for execution time.

Running State

The magical moment for any thread is when it enters the 'Running State'. This occurs when the thread scheduler picks the thread from the 'Runnable' pool and provides it CPU time. Now, the thread is actively executing, its 'run()' method in full stride like an athlete on the track.

In the 'Running State', the thread exhibits these features:

• The thread is in execution, conducting whatever task it was given.
• The critical 'run()' method is actively executing code.
• A thread can yield control back to the scheduler voluntarily by calling 'yield()', especially if it's a good citizen in a multi-threading environment.

This state is transient, as threads often have to return to 'Runnable' or move on to different states as part of their lifecycle.

Blocked/Waiting State

Not every journey is smooth, and threads face their share of roadblocks and waiting periods — that's when they enter the 'Blocked/Waiting State'. This state is all about interruptions:

There are several reasons a thread might transition into this state:

• A thread is waiting for a resource or lock to become available.
• It might be waiting for input/output operations that are pending.
• In Java, it might occur when calling methods like 'wait()' or when synchronized context tries to access locks.

When the roadblock clears, the thread is ushered back to the 'Runnable State', eager for another opportunity at the CPU.

Timed Waiting State

In some scenarios, threads might be in a 'Timed Waiting State', reflecting a calculated patience rather than indefinite holding. This state occurs when a thread voluntarily waits for a specified amount of time until it returns back to the 'Runnable State'.

Here's when the 'Timed Waiting State' is typically seen:

• Using 'sleep()' to pause execution for a certain period.
• With 'wait(time)' to relinquish the lock for a specific time.
• Utilizing 'join(time)', allowing thread joining with a timeout.

The timed nature gives programmers more control, ensuring threads don't wait endlessly unless explicitly required.

Terminated State

Every journey comes to an end, and so does a thread's lifecycle with the 'Terminated State'. A thread reaches this state when its 'run()' method completes, or if it ceases execution through an explicit 'stop()' instruction.

In the 'Terminated State', the thread's characteristics are:

• The thread's task has completed its execution.
• It is no longer eligible for CPU time, as its journey has ended.
• Resources tied to the thread are released back to the system.

Understanding this final state is important for resource management and ensuring efficient system performance in a multi-threaded environment.