ThreadLifeCycle

• Thread Lifecycle

  • State conversion

  • Create thread - implementing Runnable vs extending Thread

  • Start a thread

  • Stop a thread

  • Object methods

    • Wait, notify and notifyAll

  • Thread methods

    • Join

    • Sleep

Thread Lifecycle

State conversion

• When will be a thread blocked?

  • Blocked:

  • Waiting:

  • Timed waiting:

• New -> Runnable -> terminated is not reversible

• Timed waiting / Waiting / Blocked can only transfer to each other by going through Runnable first.

Create thread - implementing Runnable vs extending Thread

• Internal mechanism

  • There is only one way to create thread - create a Thread instance. And there are two ways to implement the run() method - Override the run() method inside Thread instance vs pass an implementation of Runnable interface into Thread constructor.

  • Thread and Runnable are complement to each other for multithreading not competitor or replacement. Because we need both of them for multi-threading.

    • For Multi-threading we need two things:

      • Something that can run inside a Thread (Runnable).

      • Something That can start a new Thread (Thread).

    • So technically and theoretically both of them is necessary to start a thread, one will run and one will make it run (Like Wheel and Engine of motor vehicle).

@Override
public void run() 
{
    if (target != null) 
    {
        target.run();
    }
}

• Best practices - Implement Runnable()

  • Code cleaniness perspective:

    • Decoupling: Implementing Runnable could separate thread creation from running.

    • Extensibility: If adopting the approach of extending Thread, then it could not extend another class because Java does not support multiple inheritance.

  • Cost of operation perspective: Thread approach will require creating and destroying a thread object each time; When combined with threadpool, Runnable approach could avoid creating a new thread object and deleting it.

// Approach 1: Runnable
public class DemoRunnable implements Runnable 
{
    public void run() 
    {
        //Code
    }
}

// Approach 2: Thread
public class DemoThread extends Thread 
{
    public DemoThread() 
    {
        super("DemoThread");
    }

    public DemoThread(Runnable ) 
    {
        super("DemoThread");
    }

    public void run() 
    {
        //Code
    }
}

Start a thread

• Best practices - Use Start()

  • Start() method responsiblity:

    • Start a new thread

    • Check the thread status

    • Add the thread to thread group

    • Kick off the Run()

  • Run() method responsibility:

    • Kick off the code inside Run()

  • Key difference is that Start() method (approach 1 below) will create a new thread to run. Run() (approach 2 below) will run everything inside main() method.

public static void main(string[] args)
{
    // Approach 1: Create a runnable instance and run it
    // Output: main
    Runnable runnable = () -> 
    {
        System.out.println(Thread.currentThread().GetName());
    };
    runnable.run();

    // Approach 2: Start a new thread
    // Output: thread0
    new Thread(runnable).start();
}

Stop a thread

• Java does not provide a way for one thread to force stop of another thread because if it does so, then the other thread might be in a state of inconsistency. Java provides a collaboration mechanism for one thread to notify another thread that it would better stop.

• Best practices: Please see this folder for sample code: https://github.com/DreamOfTheRedChamber/system-design-interviews/tree/master/code/multithreads/StopThreads

Object methods

• Please see https://github.com/DreamOfTheRedChamber/system-design-interviews/tree/master/code/multithreads/ObjectMethods for best practices

Wait, notify and notifyAll

• Wait and notify are all based on object's monitor mechanism. Therefore, they are declared as methods on top of Object.

• They are considered the native way of doing multi-threading. Java JDK has shipped packages such as Condition variable which is easier to use.

Thread methods

• Please see https://github.com/DreamOfTheRedChamber/system-design-interviews/tree/master/code/multithreads/ThreadMethods

Join

• Join thread will be in the waiting status

• Join is the native way of doing waiting. Java JDK has shipped packages such as CountDownLatch or CyclicBarrier.

• Best pratices:

Sleep

• Wait vs Sleep

  • Similarities:

    • Both wait and sleep method could make the thread come into blocked state. Wait will result in Waiting and sleep will result in Time_Waiting.

    • Both wait and sleep method could respond to interrupt.

  • Differences:

    • Wait could only be used in synchronized blocks, while sleep could be used in other scenarios.

    • Wait is a method on Object, and sleep is a method on Thread.

    • Wait will release monitor lock, and sleep will not.

    • Wait could only exit blocked state reactively, and sleep could proactive exit after specific time.

• Yield vs Sleep: Similar. However yield is non-blocking but sleep is blocking