What s the difference between Thread start and Runnable run

Question

Say we have these two Runnables   class R1 implements Runnable       public void run                      class R2 implements Runnable       public void run                       Then what s the difference between this   public static void main         R1 r1   new R1        R2 r2   new R2         r1 run        r2 run        And this   public static void main         R1 r1   new R1        R2 r2   new R2        Thread t1   new Thread r1       Thread t2   new Thread r2        t1 start        t2 start

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Actually Thread start   creates a new thread and have its own execution scenario   Thread start   calls the run   method asynchronously which changes the state of new Thread to Runnable   But Thread run   does not create any new thread  Instead it execute the run method in the current running thread synchronously   If you are using Thread run   then you are not using the features of multi threading at all

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If you directly call run   method  you are not using multi-threading feature since run   method is executed as part of caller thread    If you call start   method on Thread  the Java Virtual Machine will call run   method and two threads will run concurrently - Current Thread  main   in your example  and Other Thread  Runnable r1 in your example    Have a look at source code of start   method in Thread class              Causes this thread to begin execution  the Java Virtual Machine        calls the  lt code gt run lt  code gt  method of this thread          lt p gt         The result is that two threads are running concurrently  the        current thread  which returns from the call to the         lt code gt start lt  code gt  method  and the other thread  which executes its         lt code gt run lt  code gt  method           lt p gt         It is never legal to start a thread more than once         In particular  a thread may not be restarted once it has completed        execution                 exception  IllegalThreadStateException  if the thread was already                      started          see         run           see         stop               public synchronized void start                            This method is not invoked for the main method thread or  system             group threads created set up by the VM  Any new functionality added            to this method in the future may have to also be added to the VM                        A zero status value corresponds to state  NEW                       if  threadStatus    0              throw new IllegalThreadStateException            group add this           start0            if  stopBeforeStart                stop0 throwableFromStop                        private native void start0      In above code  you can t see invocation to run   method    private native void start0   is responsible for calling run   method  JVM executes this native method

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The separate start   and run   methods in the Thread class provide two ways to create threaded programs  The start   method starts the execution of the new thread and calls the run   method  The start   method returns immediately and the new thread normally continues until the run   method returns   The Thread class  run   method does nothing  so sub-classes should override the method with code to execute in the second thread  If a Thread is instantiated with a Runnable argument  the thread s run   method executes the run   method of the Runnable object in the new thread instead   Depending on the nature of your threaded program  calling the Thread run   method directly can give the same output as calling via the start   method  but in the latter case the code is actually executed in a new thread

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In the first case you are just invoking the run   method of the r1 and r2 objects   In the second case you re actually creating 2 new Threads   start   will call run   at some point

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Start   method call run override method of Thread extended class and Runnable implements interface   But by calling run   it search for run method but if class implementing Runnable interface then it call run   override method of Runnable   ex         public class Main1   A a new A    B b new B    a run     This call run   of Thread because run   of Thread only call when class            implements with Runnable not when class extends Thread  b run     This not run anything because no run method found in class B but it            didn t show any error   a start     this call run   of Thread b start     this call run   of Thread    class A implements Runnable   Override     public void run                 System out println  A              class B extends Thread         Override     public void run                 System out println  B

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If you just invoke run   directly  it s executed on the calling thread  just like any other method call  Thread start   is required to actually create a new thread so that the runnable s run method is executed in parallel

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run method   -  is an abstract method originally created in the Runnable interface and overridden in the Thread class as well as Thread subclasses  As Thread implements Runnable in its source code  and any other implementing classes of Runnable interface  - It is used to load the thread  runnable object  with the task it is intended to do thus you override it to write that task   start method   - is defined in Thread class  When start method is called on a Thread object 1- it calls internally native  nonjava  method called start0    method   start0    method  is responsible for low processing  stack creation for a thread and allocating thread in processor queue  at this point we have a thread in Ready Runnable state   2- At a time when Thread scheduler decides that a thread enters a processor core acorrding to  thread priority as well as OS scheduling algorithm  run method is invoked on the Runnable object  whether it is the current Runnable thread object or the Runnable object passed to the thread constructor  here a thread enters a Running state and starts executing its task  run method

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If you do run   in main method  the thread of main method will invoke the run method instead of the thread you require to run   The start   method creates new thread and for which the run   method has to be done

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Thread start   code registers the Thread with scheduler and the scheduler calls the run   method  Also  Thread is class while Runnable is an interface

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The difference is that when program calls start   method  a new thread is created and code inside run   is executed in the new thread  while if you call run   method directly  no new thread will be created and code inside run   will execute in the current thread directly  Another difference between start   and run   in Java thread is that you cannot call start   twice  Once started  second start   call will throw IllegalStateException in Java while you can call run   method several times since it s just an ordinary method

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t start   is the method that the library provides for your code to call when you want a new thread   r run   is the method that you provide for the library to call in the new thread     Most of these answers miss the big picture  which is that  as far as the Java language is concerned  there is no more difference between t start   and r run   than there is between any other two methods   They re both just methods   They both run in the thread that called them   They both do whatever they were coded to do  and then they both return  still in the same thread  to their callers   The biggest difference is that most of the code for t start   is native code while  in most cases  the code for r run   is going to be pure Java   But that s not much of a difference   Code is code   Native code is harder to find  and harder to understand when you find it  but it s still just code that tells the computer what to do   So  what does t start   do   It creates a new native thread  it arranges for that thread to call t run    and then it tells the OS to let the new thread run   Then it returns   And what does r run   do   The funny thing is  the person asking this question is the person who wrote it   r run   does whatever you  i e   the developer who wrote it  designed it to do

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The points  that the members made are all right so I just want to add something  The thing is that JAVA supports no Multi-inheritance  But What is if you want to derive a class B from another class A  but you can only derive from one Class  The problem now is how to  derive  from both classes  A and Thread  Therefore you can use the Runnable Interface   public class ThreadTest     public void method          Thread myThread   new Thread new B           myThread start          public class B extends A implements Runnable

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The difference is that Thread start   starts a thread that calls the run   method  while Runnable run   just calls the run   method on the current thread

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invoke run   is  executing on the calling thread  like any other method call  whereas  Thread start   creates a new thread  invoking run   is a programmatic bug

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First example  No multiple threads  Both execute in single  existing  thread  No thread creation   R1 r1   new R1    R2 r2   new R2      r1 and r2 are just two different objects of classes that implement the Runnable interface and thus implement the run   method   When you call r1 run   you are executing it in the current thread   Second example  Two separate threads   Thread t1   new Thread r1   Thread t2   new Thread r2     t1 and t2 are objects of the class Thread   When you call t1 start    it starts a new thread and calls the run   method of r1 internally to execute it within that new thread

[java] What's the difference between Thread start() and Runnable run()

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