CORE JAVA: Multithreading


  • Multithreading in java is a process of executing multiple threads simultaneously.
  • A thread is basically a lightweight sub-process, the smallest unit of processing. Multiprocessing and multithreading, both are used to achieve multitasking.
  • But we use multithreading than multiprocessing because threads share a common memory area. They don’t allocate separate memory area so saves memory, and context-switching between the threads takes less time than process.
  • Java Multithreading is mostly used in games, animation etc.

Advantages of Java Multithreading

1) It doesn’t block the user because threads are independent and you can perform multiple operations at the same time.

2) You can perform many operations together so it saves time.

3) Threads are independent so it doesn’t affect other threads if exception occurs in a single thread.

Thread Creations

What is Thread in java?

  • A thread is a lightweight sub process, the smallest unit of processing. It is a separate path of execution.
  • Threads are independent, if there occurs exception in one thread, it doesn’t affect other threads. It shares a common memory area.


  • As shown in the above figure, a thread is executed inside the process. There is context-switching between the threads. There can be multiple processes inside the OS and one process can have multiple threads.

How to create a thread ?

There are two ways to create a thread:

  1. By extending Thread class
  2. By implementing Runnable interface.
  • Thread class:

Thread class provide constructors and methods to create and perform operations on a thread.Thread class extends Object class and implements Runnable interface.

Commonly used Constructors of Thread class:

  • Thread()
  • Thread(String name)
  • Thread(Runnable r)
  • Thread(Runnable r, String name)


 Java Thread Example by extending Thread class:


  • Runnable interface:

The Runnable interface should be implemented by any class whose instances are intended to be executed by a thread. Runnable interface has only one method named run().
  1. public void run(): is used to perform an action for a thread.

Starting a thread:

start() method of Thread class is used to start a newly created thread. It performs following tasks:

  • A new thread start(with new callstack).
  • The thread moves from New state to the Runnable state.
  • When the thread gets a chance to execute, its target run() method will run.

Java Thread Example by implementing a Runnable interface:


  • If you are not extending the Thread class, your class object would not be treated as a thread object.So you need to explicitly create Thread class object.We are passing the object of your class that implements Runnable so that your class run() method may execute.

Thread Life Cycle

  • A thread goes through various stages in its life cycle. For example, a thread is born, started, runs, and then dies. The following diagram shows the complete life cycle of a thread.


  • The life cycle of the thread in java is controlled by JVM. The Java thread states are as follows:
  1. New − A new thread begins its life cycle in the new state. It remains in this state until the program starts the thread. It is also referred to as a born thread.
  2. Runnable − After a newly born thread is started, the thread becomes runnable. A thread in this state is considered to be executing its task.
  3. Waiting  − Sometimes, a thread transitions to the waiting state while the thread waits for another thread to perform a task. A thread transitions back to the runnable state only when another thread signals the waiting thread to continue executing.
  4. Timed Waiting − A runnable thread can enter the timed waiting state for a specified interval of time. A thread in this state transitions back to the runnable state when that time interval expires or when the event it is waiting for occurs.
  5. Terminated (Dead) − A runnable thread enters the terminated state when it completes its task or otherwise terminates.

Thread Priority

Each thread has a priority. Priorities are represented by a number between 1 and 10. In most cases, thread scheduler schedules the threads according to their priority (known as preemptive scheduling). But it is not guaranteed because it depends on JVM specification that which scheduling it chooses.

3 constants defined in Thread class:

  1. public static int MIN_PRIORITY
  2. public static int NORM_PRIORITY
  3. public static int MAX_PRIORITY
The default priority of a thread is 5 (NORM_PRIORITY). The value of MIN_PRIORITY is 1 and the value of MAX_PRIORITY is 10.

Life Cycle Methods

  1. public void run(): is used to perform an action for a thread.
  2. public void start(): starts the execution of the thread.JVM calls the run() method on the thread.
  3. public void sleep(long milliseconds): Causes the currently executing thread to sleep (temporarily cease execution) for the specified number of milliseconds.
  4. public void join(): waits for a thread to die.
  5. public void join(long milliseconds): waits for a thread to die for the specified milliseconds.
  6. public int getPriority(): returns the priority of the thread.
  7. public int setPriority(int priority): changes the priority of the thread.
  8. public String getName(): returns the name of the thread.
  9. public void setName(String name): changes the name of the thread.
  10. public Thread currentThread(): returns the reference of currently executing thread.
  11. public int getId(): returns the id of the thread.
  12. public Thread.State getState(): returns the state of the thread.
  13. public boolean isAlive(): tests if the thread is alive.
  14. public void yield(): causes the currently executing thread object to temporarily pause and allow other threads to execute.
  15. public void suspend(): is used to suspend the thread(deprecated).
  16. public void resume(): is used to resume the suspended thread(deprecated).
  17. public void stop(): is used to stop the thread(deprecated).
  18. public boolean isDaemon(): tests if the thread is a daemon thread.
  19. public void setDaemon(boolean b): marks the thread as daemon or user thread.
  20. public void interrupt(): interrupts the thread.
  21. public boolean isInterrupted(): tests if the thread has been interrupted.
  22. public static boolean interrupted(): tests if the current thread has been interrupted.




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