Category: Java

Java (JVM) Memory Model


What is Java Memory Model and Why it is Important?

The Java memory model specifies how the Java virtual machine works with the computer’s memory (RAM).

It is very important to understand the Java memory model if you want to design correctly behaving concurrent programs. The Java memory model specifies how and when different threads can see values written to shared variables by other threads, and how to synchronize access to shared variables when necessary.


Java Memory Model : Heap and Stack

Internal memory model in java, divides the JVM memory into two areas.

  • Heap
  • Stack (known as Thread Stack)

The heap is where all the objects live and the stacks are where the threads do their work. Please refer the below diagram.


There is a single heap per JVM and conceptually, there can be multiple thread stacks depending on the number of threads, but it can be limited by the OS based on the available resources (such as memory). Thread stack is local for each thread and heap is shared among all threads.



Heap is the area in the memory where the objects related to (created in) the program resides and lives. all the objects lives in the heap regardless of its nature and creator (creating and owning thread). It is no matter who(which thread) created the object and whether it is static, non static or method local (any scope). All the objects are stored in the heap until they are collected/claimed by the GC (Garbage Collector).

If you new to java, you might be questioning yourself about what will be treated as object?  Every variable reference and assignment that does not belong to any primitive type available (There are eight primitive types)  in java can be considered as objects. Therefore any object instantiation including Wrapper classes (Object version of primitive types e.g:- Byte, Short, LongFloatCharacter, Integer, Double, Long) will also fall into this category and lives in the heap.Therefore the heap may contain any java object that is created because of the execution of the java application.

In addition the heap stores the static variables (static primitive variables) with the class definition.



Each thread running in the JVM (Java virtual machine) has its own thread stack. This stack is private/local to each thread. The thread stack contains information about which method is currently executed and up-to which line it has been executed to reach the current point of execution. That means that each thread has a pointer into the code which points to the current position of code they’re currently running. This is known as the “call stack“. As the thread executes the code, the call stack may change.

The thread stack also contains all local variables for each method being executed (all methods on the call stack). A thread can only access it’s own thread stack. Local variables created by a thread are only visible to that thread and  No other threads can access them.

local variables :- primitive and object reference (not actual objects) variables that are local for the thread including method local variable, instance variable of the class and object variable references.

Even if two threads are executing the exact same code, the two threads will still create the local variables of that code in each their own thread stack. Thus, each thread has its own version of each local variable. . When a thread starts running a new method it saves the arguments and local variables in that method on its own stack. Some of these values might be pointers to objects on the heap. If two threads are running the same method at the same time they will both have their code pointers pointing at that method and have their own copies of arguments and local variables on their stacks. They will only interfere with each other if the things on their stacks point to the same objects on the heap.

All local variables of primitive types (booleanbyteshortcharintlongfloatdouble) are fully stored on the thread stack and therefore they are not visible to other threads. One thread may pass a copy of a primitive variable to another thread, but it cannot share the primitive local variable itself.

When the method execution is completed, all the local variables and call stacks will be removed from the memory (stack).



Where the “static” variables are stored ?

Prior to Java 8 (Java 7 and Older versions) static variables are stored in the PermGen (Permanent Generation) space. PermGem space is a special memory area available in the java heap. PermGen space was removed in Java 8 and introduced Metaspace to overcome some limitation of the PermGen space. In Java 8, static variables are stored in the Metaspace.



What is PermGen (Permanent Generation) and Metaspace?

PermGen space is a special memory area available in the JVM heap with Java 7 and older versions. PermGem was used to store meta information about the classes that has been loaded into JVM.  In addition, PermGem space is used for storing the static variables of the application. The problem with PermGem space is that it has a fixed max size. Since the PermGen space is a part of Heap, it cannot exceed the allocated memory.  Therefore it throws the following error when the it reaches the max PermGem space.

java.lang.OutOfMemoryError: PermGen space

PermGen space was completely removed in Java 8 and Metaspace was introduced as a replacement for the PermGen. Metaspace stores the Class definitions of your Objects, and some other metadata (like static variables).

The main advantage of Metaspace is that it resides in the main memory area of the system (not a part of the Heap) and it does not have a fixed max size. It can increases its size on runtime depending on the available memory of the system. Therefore OutOfMemoryError will not be occurred with Metaspace.



Consider the following code segment. 

Student std = new Student();

after executing the above line, JVM memory will be like this.

  • Heap:  stores “new Student()” (object that has been created)
  • Stack: stores information about “std” (reference variable)
  • PermGen Space / Metaspace: stores the meta information about Student class



Here is the diagram that illustrates how the objects are stored in the heap, local variables are stored in the stack and method invocations are stored in the call stack.


If the local variable is a primitive type, then it is stored in the Thread Stack,

If the local variable is a reference variable to an object, then the reference variable is stored in the Thread Stack and Object is stored in the Heap.

An object may contain methods and these methods may contain local variables. These local variables are also stored on the thread stack, even if the object belongs to the method, it is stored on the heap. (any object is stored in the heap regardless of its scope)

An object’s member variables (member variables of  instance) are stored on the heap along with the object itself. That is true in cases when the member variable is  a primitive type or  if it is a reference to an object.

static variables (class members) are also stored on the heap (PermGen space or Metaspace) along with the class definition.

Objects on the heap can be accessed by all threads that have a reference to the object. When a thread has access to an object, it can also get access to that object’s member variables. If two threads call a method on the same object at the same time, they will both have access to the object’s member variables, but each thread will have its own copy of the local variables.



Consider the below code examples.

After executing the above example, the heap and stack of the JVM will looks like below (Before running the Garbage Collector) .


There are two threads that execute the MyRunnable instances.  run() method will invoke the methodOne().  Then the methodOne() will invoke the methodTwo(). Therefore the call stack for both method should reside in the each Thread stack.

methodOne() declares a primitive local variable (localVariable1) that store a primitive value and object reference variable (localVariable2) that refers the static object (object3) available in the heap.  The local primitive variable and object reference will be stored in the local thread stack.

If we look at the MySharedObject class, you can notice that it creates a static object (object3) and  two Integer wrapper class objects ( known as object2 and object4). Since all the objects live in the heap, the object3 , object2 and object4 should reside in the heap. In addition, the object3 will have the references for the object2 and object4.

Since the object3 is static, both threads will have the reference for the same object. In other words, both threads will share the object3.

methodOne() will invoke the methodTwo(). if you look at the methodTwo() implementation, you may notice that it creates an Integer wrapper class object which is local to the thread and method. Even if it is local to the thread itself, it is a object creation. Therefore the object should resides in the heap. Since there are two threads running, there should be two another objects available in the heap and each thread stack should have the reference for related object (please see object1 and object5).

References :-

CountDownLatch in Java Concurrency

What is CountDownLatch?

In CountDownLatch, it enables one thread (calling thread) to wait until other threads completes their tasks.

e.g. Assume that a situation where the application’s main thread want to wait until other service threads have completed their works. (media player will wait until the file is loaded, initialized and making ready for the playing)

CountDownLatch works by having a counter initialized with number of threads, which is decremented each time a thread complete its execution (by calling countDown() method available in the CountDownLatch class). When count reaches to zero, it means all threads have completed their execution, and the thread waiting on latch (caller thread)  resume its execution.


How CountDownLatch works? class defines one constructor inside:

public CountDownLatch(int count) {...}

This will create a CountDownLatch that will wait until the given number of thread completes their executions.


CountDownLatch latch1 = new CountDownLatch(4);

The latch1.await()  hold the currently running thread until 4 threads have completed their execution with latch1. each completion is acknowledged to latch1 with countDown() method.

 This count is essentially the number of threads, for which latch should wait. This value can be set only once, and CountDownLatch provides no other mechanism to reset this count. (You cannot reset or change this value later)

The first interaction with CountDownLatch is made by the main thread (caller thread) which is going to wait for other threads.

This main thread (caller thread) must call, CountDownLatch.await() method immediately after starting other threads. Then await() method will hold the execution of the caller thread until all the given threads are executed. The execution of the await() method will be expired once the count of the CountDownLatch is zero and the caller method will be resumed.

(as i have mentioned earlier, each thread should call the countDown() method on the latch upon completion of the thread. each countDown() call will decrement the count value by one)



Lets look at the diagram in detailed.

ThreadA (the main thread) has created a CountDownLatch that should wait for  3 number of threads.

ThreadA has created another three threads and started them. As soon as they are started,  ThreadA call the await() method of the CountDownLatch.

The await() method holds the execution of the caller method (that is ThreadA) until the specified number of threads (In this case three threads) are completed (in the CountDownLatch).

Each thread should call the countDown() method of the CountDownLatch upon completion of its execution.  countDown() method will decrement the count value of the CountDownLatch by one.

As soon as the count is zero, CountdownLatch get to know that all the threads has completed their tasks. Therefore the caller thread (ThreadA) that is waiting on the CountDownLatch will be resumed.


Now lets look at the code implementation for the above scenario.


WorkerThread has been implemented in a way to sleep for 5 seconds. After that it will print the name property and call the countDown() method of the CountDownLatch.



After executing the above Application, you will get the following output.

 main thread has started 
 all the worker threads have been started. next calling await() method 
 thread [thread2] completed 
 thread [thread4] completed 
 thread [thread3] completed 
 thread [thread1] completed 
 Resume the main thread after executing of all worker threads





Java – Thread : run() or start() method?

If you just invoke run() directly, it’s executed on the calling thread, just like any other method call.

Thread.start() will create a new thread so that the runnable’s run method is executed in parallel.

when program calls start() method a new Thread is created and code inside run() method is executed in new Thread while if you call run() method directly no new Thread is created and code inside run() will execute on currently running (existing) Thread.

Another difference between start() vs run()  in Java thread is that you cannot call start() method more than one time on thread object. once started, second call of start() will throw IllegalStateException in Java while you can call run() method any number of times.

Starting and Running Thread in java

How to create Thread in Java?

There are two ways to create a Thread in java.

  • Create a subclass of the Thread class and override the run() method.
  • Create a class that implements the Runnable interface and implements the run()  method.  After that an instance of Thread class is created by passing the instance of implemented Runnable interface.

Lets look at those two methods in detailed.


Creating a sub class of Thread class

Create a sub class of a Thread class and override the run() method. Then create an object of  the class and call the start() method. Then it will be executed in a separate thread.

If you check the console log, you will notice that MyThread is running in a different Thread from the main thread.

12:57:40.208 [main] INFO - Application Running on : [main]
12:57:40.223 [Thread-0] INFO - MyThread Running on : [Thread-0]


Implementing the Runnable interface

In this way, create a class that implements the Runnable interface and implement the run() method. Then create an instance of Thread class by passing the just created runnable instance.  Then call the start() method. We will modify the above example again to demonstrate how the Thread works with Runnable interface.


If you check the console log, you will notice that MyWork is running in a different Thread from the main thread.

12:57:40.208 [main] INFO - Application Running on : [main]
12:57:40.223 [Thread-0] INFO - MyWork Running on : [Thread-0]


What is the best way to create and run threads?  Sub class of Thread or implementation of Runnable interface?

Any of above approaches can be used to create and run Thread in java. The choice between one of them can be made depends on the personal preferences and special requires.  for instance, if you need to override the some of the default behavior of the Thread class, then you can go with extending Thread class. otherwise you can go with implementing Runnable interface.

Personally i prefer, implementing the Runnable interface and delegating the runnable instance to the Thread object.


Worker and Work (Work to be Done)

The relationship between Thread and Runnable can be describes as the relationship between worker and work to be done.

Runnable – Work to be done

Thread  – Worker who is handling the work (Runnable)


Executing the run() method directly without executing the start() method

If you just invoke run() directly, it’s executed on the calling thread, just like any other method call.

Thread.start() will create a new thread so that the runnable’s run method is executed in parallel.

when program calls start() method a new Thread is created and code inside run() method is executed in new Thread while if you call run() method directly no new Thread is created and code inside run() will execute on currently running (existing) Thread.

Another difference between start() vs run()  in Java thread is that you cannot call start() method more than one time on thread object. once started, second call of start() will throw IllegalStateException in Java while you can call run() method any number of times.

Please refer the below example.



If you check the console log, you will notice that MyThread is running in the same thread (main thread).

04:31:55.412 [main] INFO - Application Running on : [main]
04:31:55.419 [main] INFO - MyThread Running on : [main]


Creating a Thread in Java 8 with Lambda

Runnable interface is marked as a functional interface. Therefore we can use a lambda expression to create a thread as follows.


Execution of above program will give you the following console output.

04:37:24.334 [main] INFO - Application Running on : [main]
04:37:24.412 [Thread-0] INFO - Runnable instance running on : [Thread-0]



Thread.currentThread() will return the currently running thread object.

Thread thread = Thread.currentThread();





Java Concurrency: ScheduledExecutorService

The ScheduledExecutorService is an extended interface of ExecutorService. It is used to executes after given initial delay or execute tasks repeatedly with given intervals or both (both initial delay and repeatedly execution).  It will repeatedly execute the task until it is explicitly stopped.


Different thread pool implementations

  • Single Thread for handling task. (with all repeated executions)

As the name implies this will create only a single worker thread and it will be reused for all repeated execution.

Please refer the below example.

The initial delay is 5 seconds and the interval between each execution is 2 seconds.


  • Thread pool for handling task (with repeated executions)

This will create a thread pool with five threads and those threads will be reused for all repeated execution. Each repeated execution will be handled by a randomly selected thread from the available thread pool.

Please refer the below example.



Java Concurrency: ExecutorService (A better way of asynchronous tasks handling)

The Traditional/Conventional way of handling asynchronous tasks with Thread class

When you develop a multi-threaded java application, you create a Runnable instance and corresponding Thread instance for executing them.  This is the most common and preferred way of doing concurrent programming among most of the developers. Dealing with Thread class directly may lead to few issues and those will be described below.

The issues with dealing with Thread class directly. 

The creation of new thread for each task is really expensive. This is because it involves few steps with managing the lifecycle of the Thread.  It needs to be created, started and  the entire lifecycle of the thread instance should be managed. This consumes more time and system resources. When the number of tasks are increasing/becoming high, the number of threads that need to be created will also be high.

In addition, working directly with Thread class can be error-prone. For instance, if some any error/exception occurs, it is hard to handle them with Thread class. This is because the run() method does not throw any exception. On the other hand, there is no way to check whether the thread has executed successfully because, run() method does not return anything (return type of run() method is void).

When the thread object is started, there is no way to control its behavior. Simply, there is no way to suspend its execution later. (If there is a situation to suspend the execution of some started threads, we cannot do it if we manually created and started threads). even if there are suspend() and stop() methods available in Thread class, those have been deprecated now.

The Solution

Maintaining and managing a pool of  threads that can be used to handle/execute the submitted tasks. The thread pool can be reused to handle the upcoming tasks without creating new threads for each request. This will help to reduce the number of threads created.

Find a way to throw exception and  return the output after the execution. This is possible with Callable and Future. Callable supports throwing exception from run() method and returning output with Future interface.

A Better solution with ExecutorService

ExecutorService is a higher level replacement for working with threads directly.  This is capable of executing tasks asynchronously (in background) with managed thread pool. The main advantage of ExecutorService over traditional Thread class is that,it maintains a pool of worker threads and re-use them for executing tasks (in the background). Once the execution of the task is completed, the worker thread will not go to dead state and it will come back to the pool for serving next available task. Therefore it eliminates the unnecessary cost and overhead associated with creating separate new thread for each task in traditional Thread class approach.

The ExecutorService has some useful methods which accept either Runnable or Callable instances as method parameters. The most of the methods support of returning the output that is wrapped by Future interface. Therefore the calling process(caller) can know the ultimate execution status/result of the background task.

The  ExecutorService has some methods like shutDown() and shutDownNow(). Those methods will help to control/manage the ExecutorService with already submitted and running tasks.

What are the Executor, ExecutorService and Executors?

Executor is the top level interface and ExecutorService is an extended interface of it. Executors is an utility class which has few factory methods for creating different executor services with different types of thread pools.

Here is just a class and interface level representation.

Screen Shot 2017-12-27 at 10.32.51 PM.png

Click here to view the ScheduledExecutorService article.

ThreadPoolExecutor is almost similar to the ExecutorService. 

What are the different types of executor services available?

  • ExecutorService
  • ScheduledExecutorService
  • ThreadPoolExecutor

ExecutorService with different Thread Pool implementations.


  • Single Thread for handling all the tasks.


Please refer the below code example for Executors.newSingleThreadExecutor();

If you run the above program, you will notice that it will create just a single thread and reuse it for executing all the submitted callable objects.


  • Fixed number of threads for handling all tasks
   Executors.newFixedThreadPool(5); // This will create a thread pool with 5 threads. 


Please refer the below code example for Executors.newFixedThreadPool(5);

If you run the above program, you will notice that there are only five threads in the thread pool. those threads will be reused to execute all the tasks. No additional threads will be created and the available five threads will be reused.


  • Creating new threads as needed (based on the demand) and re-using the previously constructed threads when they are available.


Please refer the below code example for Executors.newCachedThreadPool();

Controlling the ExecutorService

shutdown() :- The ExecutorService will not accept any new tasks once the shutdown method is called. All the previous submitted tasks will be executed before shut downing the executorService.

shutdownNow() :- The ExecutorService will be shutdown immediately by terminating already running tasks. The tasks that are pending on execution will no be executed and will be returned back as a list. So it is guaranteed that calling this method will shut down the ExecutorService immediately.


Either Runnable or Callable instances (Single Object or List of Objects)  will be accepted as method arguments by the execution related methods in the ExecutorService.  If you want to access the currently running thread, you can get it by Thread.currentThread() and it will return the current worker thread.


Java Thread: Lifecycle of Thread

The lifecycle of the Thread can be described as follows.

Screen Shot 2017-12-24 at 4.41.24 AM.png

According to the above diagram, a Thread object can be in one of the following life cycle statuses.



When the thread object is created it is in the new state.

Thread threadA = new Thread();

Now the threadA is in the new state.



This is commonly known as “ready to run state“. Here the thread object is started and ready to run. There will be multiple thread objects in this state and it it the responsibility of the Thread scheduler to select a thread object for running. The thread object in new state can be moved to Runnable state by calling the start() method on it.



Thread object can be started only once. if you try to start it more than once,  it will throw IllegalThreadStateException.



If the Thread scheduler has selected a thread from runnable state and call the run() method, then it is in the Running state. Here the run() method is still executing and not competed yet. It is important to note that the run() method must be called by the Thread scheduler.


Not Runnable

This is also known as blocking, sleeping or waiting state. The currently running thread can be moved to “Not Runnable State” due to several reasons.  for instance

  • calling the sleep() method
  • calling the wait() method on the shared resource
  • calling the join() method etc….

In this case, the execution of the running thread might be hold and moved to not runnable state from running state.


Back to Runnable from Not Runnable state

The threads that are in the not runnable state can be moved to runnable state again on following situations.

  • sleeping/waiting time has expired
  • call the interrupt() method
  • call the notify()/notifyAll() on waited resource/object

After  it is moved back to the runnable state, it is the responsibility of the thread scheduler to select them and resume their execution.



Once the run() method is completed, the execution of the thread object will be over and thread will be moved to the dead state.




Java Thread: yield() method example


This is a static method available in thread class and give the hint for the thread scheduler that the currently running thread is willing to yield the execution time for some other thread.

Thread.yield() will just give a recommendation/hint for the thread scheduler and it is up to thread scheduler to accept or ignore. Therefore we cannot guarantee that calling the Thread.yield() will immediately move the currently running thread not “Not Running  state” and allow some other thread to utilize the CPU time. It will be decided by the thread scheduler.

Please refer the below example about Thread.yield()



It is very difficult to give the exact output upon running the above code sample as the Thread.yield() method is not guaranteed to run.  Therefore if you run above program few times, you might get different results.




Java Thread : Difference between wait(), join() and sleep()


The call of wait() method on the object will hold the execution of the currently running thread until some other thread calls the notify() or notifyAll() method on the same object. The currently running thread will release the object monitor of the object on which the wait() method is invoked/called. Then some other thread can acquire the object monitor and continue its work with that object. The thread (that owns the object monitor) will call the notify() or notifyAll() method to notify the threads that are waiting on the on that object to inform that the object monitor will be released soon and get ready to acquire it.

wait() method should be called in a synchronized context. that means either in synchronized method or synchronized block.

since the wait() method is inherited from the Object class, it can be invoked on any java object(that is directly or indirectly inherited from Object class)

Click here to see my blog article about wait() method with code examples.



The join() method belongs to the Thread class. therefore join() method can be invoked only on the Thread objects. The join() method is used to postpone the execution of the currently running thread after the execution of some other thread.

ThreadA threadA =  new ThreadA();

Here the currently running thread (lets say it as the main thread)  will hold the execution until the threadA completes. Once the threadA is completed, the waiting/sleeping thread (the main thread) will be resumed its execution.  Simply the main thread will be resumed once the threadA  is dead(after completing the run method)

Click here to see my blog article about join() method with code examples.



Thread.sleep causes the current thread to suspend execution for a specified time period. This is an efficient means of making processor time available to the other threads of an application or other applications that might be running on a computer system.

Click here to see my blog article about sleep() method with code examples


The more comparison of wait(), sleep() and join method can be found on my next blog article.


Java Thread: Comparison of wait(), sleep() and join() methods


sleep() is a method which is used to hold the process for few seconds or the time you wanted but in case of wait() method thread goes in waiting state and it won’t come back automatically until we call the notify() or notifyAll().

The major difference is that wait() releases the lock or monitor while sleep() and join() doesn’t releases any lock or monitor while waiting. wait() is used for synchronizing the access for the shared object in multi-threaded environment and will be used in inter-threaded communication. sleep is used to pause the execution on currently thread for a defined time period.

Thread.sleep() sends the current thread into the “Not Runnable” state for some amount of time. The thread keeps the monitors it has acquired.

i.e. if the thread is currently in a synchronized block or method no other thread can enter this block or method. If another thread calls t.interrupt() it will wake up the sleeping thread. Note that sleep is a static method, which means that it always affects the current thread (the one that is executing the sleep method). A common mistake is to call t.sleep() where t is a different thread; even then, it is the current thread that will sleep, not the t thread.

object.wait() sends the current thread into the “Not Runnable” state, like sleep(), but with wait() is called on an object, not a thread; we call this object the “lock object.”

Before lock.wait() is called, the current running thread must synchronize on the lock object; wait() then releases this lock, and adds the thread to the “wait list” associated with the lock. Later, another thread can synchronize on the same lock object and call lock.notify(). This wakes up the original, waiting thread. Basically, wait()/notify() is like sleep()/interrupt(), only the active thread does not need a direct pointer to the sleeping thread, but only to the shared lock object.

thread.join() sends the current running thread into the “Not Runnable” state, until joined thread is completing its execution. once the joined thread completes its execution, the waiting thread will comes back to the runnable state. Then the waiting thread will be resumed again. join() method will not release any monitor or lock object acquired by the running thread and it will be waiting with those monitors(if there is any)

The wait() method must be called in a synchronized context, but sleep() and join() methods does not require to be.Thread.join() waits for the thread to completely finish, whereas a synchronized block can be used to prevent two threads from executing the same piece of code at the same time.

It is recommended, not use waitnotify or notifyAll on Thread instances. They should be used on objects for synchronizing the access among the multiple threads.


Lets compare each of these methods in detailed.

Derived from or Belongs to:

  • wait(): Object class
  • sleep(): Thread class
  • join():  Thread class

Call/Invoked on:

  • wait(): Call on an object; current thread must synchronize on the lock object.
  • sleep(): Call on a Thread; always currently executing thread will be sleep.
  • join():  Call on a Thread. currently executing thread will be joined at the end of the execution of the calling thread.

Synchronized Context:

  • wait(): must be called in a synchronized context. (either in synchronized block or method)
  • sleep(): synchronization context not required
  • join(): synchronization context not required

Hold lock/monitor:

  • wait(): release the object monitor for giving the chance for other threads to execute.
  • sleep(): keep object monitor for at least the timeout specified or somebody interrupt.
  • join(): keep the monitor and goes to sleep until the joined() method completes its execution.


  • wait():  Call notify(), notifyAll() on the object.
  • sleep():  After expiring the sleeping time or call interrupt().
  • join():  completing the run method of the joined thread (dead of the joined thread)


  • sleep():  for holding the execution of currently running thread for a specific time duration
  • wait(): for multi-thread-synchronization on an shared object.
  • join(): for joining the execution of the currently running thread at the end of the another thread.

What is really happening after waiting or sleeping?

sleep(), wait() and join() method will cause to move the currently running thread from running state to not runnable state (waiting/blocking/sleeping state) . However, the waiting/blocking thread will become runnable after expiring the time duration or calling the relevant method like notify(). notifyAll(), interrupt() etc.  what will be really happening after that?  Will the waiting thread get resumed/run as soon after it wakes up?

The simple answer is NO. The waiting thread will be moved from “Not runnable state” to “runnable state“. After that it is the responsibility of the Thread scheduler to move it to “running state“.  Since the Thread scheduler is working and shifting the execution of thread with milliseconds and nano seconds, we feel like that it just resumed as soon as  it wakes up.