What is the volatile keyword useful for

Question

At work today  I came across the volatile keyword in Java  Not being very familiar with it  I found this explanation  Given the detail in which that article explains the keyword in question  do you ever use it or could you ever see a case in which you could use this keyword in the correct manner

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the volatile modifier guarantees that any thread that reads a field will see the most recently written value     - Josh Bloch If you are thinking about using volatile  read up on the package java util concurrent which deals with atomic behaviour  The Wikipedia post on a Singleton Pattern shows volatile in use

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While I see many good Theoretical explanations in the answers mentioned here  I am adding a practical example with an explanation here  1  CODE RUN WITHOUT VOLATILE USE public class VisibilityDemonstration    private static int sCount   0   public static void main String   args        new Consumer   start        try           Thread sleep 100         catch  InterruptedException e            return            new Producer   start       static class Consumer extends Thread        Override     public void run             int localValue   -1          while  true                if  localValue    sCount                    System out println  quot Consumer  detected count change  quot    sCount                   localValue   sCount                            if  sCount  gt   5                    break                                  System out println  quot Consumer  terminating quot             static class Producer extends Thread        Override     public void run             while  sCount  lt  5                int localValue   sCount              localValue                System out println  quot Producer  incrementing count to  quot    localValue               sCount   localValue              try                   Thread sleep 1000                 catch  InterruptedException e                    return                                  System out println  quot Producer  terminating quot               In the above code  there are two threads - Producer and Consumer  The producer thread iterates over the loop 5 times  with a sleep of 1000 milliSecond or 1 Sec  in between  In every iteration  the producer thread increases the value of sCount variable by 1  So  the producer changes the value of sCount from 0 to 5 in all iterations The consumer thread is in a constant loop and print whenever the value of sCount changes until the value reaches 5 where it ends  Both the loops are started at the same time  So both the producer and consumer should print the value of sCount 5 times  OUTPUT Consumer  detected count change 0 Producer  incrementing count to 1 Producer  incrementing count to 2 Producer  incrementing count to 3 Producer  incrementing count to 4 Producer  incrementing count to 5 Producer  terminating  ANALYSIS In the above program  when the producer thread updates the value of sCount  it does update the value of the variable in the main memory memory from where every thread is going to initially read the value of variable   But the consumer thread reads the value of sCount only the first time from this main memory and then caches the value of that variable inside its own memory  So  even if the value of original sCount in main memory has been updated by the producer thread  the consumer thread is reading from its cached value which is not updated  This is called VISIBILITY PROBLEM   2  CODE RUN WITH VOLATILE USE In the above code  replace the line of code where sCount is declared by the following   private volatile  static int sCount   0   OUTPUT Consumer  detected count change 0 Producer  incrementing count to 1 Consumer  detected count change 1 Producer  incrementing count to 2 Consumer  detected count change 2 Producer  incrementing count to 3 Consumer  detected count change 3 Producer  incrementing count to 4 Consumer  detected count change 4 Producer  incrementing count to 5 Consumer  detected count change 5 Consumer  terminating Producer  terminating  ANALYSIS When we declare a variable volatile  it means that all reads and all writes to this variable or from this variable will go directly into the main memory  The values of these variables will never be cached  As the value of the sCount variable is never cached by any thread  the consumer always reads the original value of sCount from the main memory where it is being updated by producer thread   So  In this case the output is correct where both the threads prints the different values of sCount 5 times  In this way  the volatile keyword solves the VISIBILITY PROBLEM

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One common example for using volatile is to use a volatile boolean variable as a flag to terminate a thread   If you ve started a thread  and you want to be able to safely interrupt it from a different thread  you can have the thread periodically check a flag   To stop it  set the flag to true   By making the flag volatile  you can ensure that the thread that is checking it will see it has been set the next time it checks it without having to even use a synchronized block

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volatile   gt  synchronized About  volatile says for a programmer that the value always will be up to date  The problem is that the value can be saved on different types of hardware memory  For example it can be CPU registers  CPU cache  RAM     PU registers and CPU cache belong to CPU and can not share a data unlike of RAM which is on the rescue in multithreading envirompment  volatile keyword says that a variable will be read and written from to RAM memory directly  It has some computation footprint Java 5 extended volatile by supporting happens-before About   A write to a volatile field happens-before every subsequent read of that field   volatile keyword does not cure a race condition situation when several threads can write some values simultaneously  The answer is synchronized keyword About  As a result it safety only when one thread writes and others just read the volatile value

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No one has mentioned the treatment of read and write operation for long and double variable type  Reads and writes are atomic operations for reference variables and for most primitive variables  except for long and double variable types  which must use the volatile keyword to be atomic operations   link

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Assume that a thread modifies the value of a shared variable  if you didn t use volatile modifier for that variable  When other threads want to read this variable s value  they don t see the updated value because they read the variable s value from the CPU s cache instead of RAM memory  This problem also known as Visibility Problem  By declaring the shared variable volatile  all writes to the counter variable will be written back to main memory immediately  Also  all reads of the counter variable will be read directly from main memory  public class SharedObject       public volatile int sharedVariable   0     With non-volatile variables there are no guarantees about when the Java Virtual Machine  JVM  reads data from main memory into CPU caches  or writes data from CPU caches to main memory  This can cause several problems which I will explain in the following sections   Example  Imagine a situation in which two or more threads have access to a shared object which contains a counter variable declared like this  public class SharedObject       public int counter   0     Imagine too  that only Thread 1 increments the counter variable  but both Thread 1 and Thread 2 may read the counter variable from time to time  If the counter variable is not declared volatile there is no guarantee about when the value of the counter variable is written from the CPU cache back to main memory  This means  that the counter variable value in the CPU cache may not be the same as in main memory  This situation is illustrated here   The problem with threads not seeing the latest value of a variable because it has not yet been written back to main memory by another thread  is called a  quot visibility quot  problem  The updates of one thread are not visible to other threads

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The     volatile key when used with a variable  will make sure that threads reading this variable will see the same value   Now if you have multiple threads reading and writing to a variable  making the variable volatile will not be enough and data will be corrupted   Image threads have read the same value but each one has done some chages  say incremented a counter    when writing back to the memory  data integrity is violated   That is why it is necessary to make the varible synchronized  diffrent ways are possible   If the changes are done by 1 thread and the others need just to read this value  the volatile will be suitable

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There are two different uses of volatile keyword    Prevents JVM from reading values from register  assume as cache   and forces its value to be read from memory  Reduces the risk of memory in-consistency errors       Prevents JVM from reading values in register  and forces its   value to be read from memory    A busy flag is used to prevent a thread from continuing while the device is busy and the flag is not protected by a lock   while  busy           do something else        The testing thread will continue when another thread turns off the busy flag   busy   0    However  since busy is accessed frequently in the testing thread  the JVM may optimize the test by placing the value of busy in a register  then test the contents of the register without reading the value of busy in memory before every test  The testing thread would never see busy change and the other thread would only change the value of busy in memory  resulting in deadlock  Declaring the busy flag as volatile forces its value to be read before each test      Reduces the risk of memory consistency errors    Using volatile variables reduces the risk of memory consistency errors  because any write to a volatile variable establishes a  happens-before  relationship with subsequent reads of that same variable  This means that changes to a volatile variable are always visible to other threads   The technique of reading  writing without memory consistency errors is called atomic action   An atomic action is one that effectively happens all at once  An atomic action cannot stop in the middle  it either happens completely  or it doesn t happen at all  No side effects of an atomic action are visible until the action is complete   Below are actions you can specify that are atomic    Reads and writes are atomic for reference variables and for most primitive variables  all types except long and double   Reads and writes are atomic for all variables declared volatile  including long and double variables     Cheers

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volatile is very useful to stop threads   Not that you should be writing your own threads  Java 1 6 has a lot of nice thread pools   But if you are sure you need a thread  you ll need to know how to stop it   The pattern I use for threads is   public class Foo extends Thread      private volatile boolean close   false     public void run         while  close             do work             public void close         close   true         interrupt here if needed         In the above code segment  the thread reading close in the while loop is different from the one that calls close    Without volatile  the thread running the loop may never see the change to close   Notice how there s no need for synchronization

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Volatile Variables are light-weight synchronization  When visibility of latest data among all threads is requirement and atomicity can be compromised   in such situations Volatile Variables must be preferred  Read on volatile variables always return most recent write done by any thread since they are neither cached in registers nor in caches where other processors can not see  Volatile is Lock-Free  I use volatile  when scenario meets criteria as mentioned above

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volatile only guarantees that all threads  even themselves  are incrementing  For example  a counter sees the same face of the variable at the same time  It is not used instead of synchronized or atomic or other stuff  it completely makes the reads synchronized  Please do not compare it with other java keywords  As the example shows below volatile variable operations are also atomic they fail or succeed at once   package io netty example telnet   import java util ArrayList  import java util List   public class Main        public static volatile  int a   0      public static void main String args    throws InterruptedException           List lt Thread gt  list   new  ArrayList lt Thread gt             for int i   0   i lt 11  i                 list add new Pojo                        for  Thread thread   list                thread start                       Thread sleep 20000           System out println a           class Pojo extends Thread      int a   10001      public void run             while a-- gt 0               try                   Thread sleep 1                 catch  InterruptedException e                    e printStackTrace                              Main a                System out println  a     Main a                       Even you put volatile or not results will always differ  But if you use AtomicInteger as below results will be always same  This is same with synchronized also       package io netty example telnet       import java util ArrayList      import java util List      import java util concurrent atomic AtomicInteger       public class Main            public static volatile  AtomicInteger a   new AtomicInteger 0           public static void main String args    throws InterruptedException               List lt Thread gt  list   new  ArrayList lt Thread gt                 for int i   0   i lt 11  i                     list add new Pojo                                for  Thread thread   list                    thread start                               Thread sleep 20000               System out println a get                          class Pojo extends Thread          int a   10001          public void run                 while a-- gt 0                   try                       Thread sleep 1                     catch  InterruptedException e                        e printStackTrace                                      Main a incrementAndGet                    System out println  a     Main a

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From oracle documentation page  the need for volatile variable arises to fix memory consistency issues      Using volatile variables reduces the risk of memory consistency errors  because any write to a volatile variable establishes a happens-before relationship with subsequent reads of that same variable     This means that changes to a volatile variable are always visible to other threads  It also means that when a thread reads a volatile variable  it sees not just the latest change to the volatile  but also the side effects of the code that led up the change   As explained in Peter Parker answer  in absence of volatile modifier  each thread s stack may have their own copy of variable  By making the variable as volatile  memory consistency issues have been fixed    Have a look at jenkov tutorial page for better understanding    Have a look at related SE question for some more details on volatile  amp  use cases to use volatile   Difference between volatile and synchronized in Java  One practical use case   You have many threads  which need to print current time in a particular format for example   java text SimpleDateFormat  HH-mm-ss    Yon can have one class  which converts current time into SimpleDateFormat and updated the variable for every one second  All other threads can simply use this volatile variable to print current time in log files

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A variable declared with volatile keyword  has two main qualities which make it special    If we have a volatile variable  it cannot be cached into the computer s microprocessor  cache memory by any thread  Access always happened from main memory  If there is a write operation going on a volatile variable  and suddenly a read operation is requested  it is guaranteed that the write operation will be finished prior to the read operation    Two above qualities deduce that   All the threads reading a volatile variable will definitely read the latest value  Because no cached value can pollute it  And also the read request will be granted only after the completion of the current write operation    And on the other hand    If we further investigate the  2 that I have mentioned  we can see that volatile keyword is an ideal way to maintain a shared variable which has  n  number of reader threads and only one writer thread to access it  Once we add the volatile keyword  it is done  No any other overhead about thread safety    Conversly    We can t make use of volatile keyword solely  to satisfy a shared variable which has more than one writer thread accessing it

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Volatile does following   1  Read and write of volatile variables by different threads are always from memory  not from thread s own cache or cpu register  So each thread always deals with the latest value  2  When 2 different threads work with same instance or static variables in heap  one may see other s actions as out of order  See jeremy manson s blog on this  But volatile helps here   Following fully running code shows how a number of threads can execute in predefined order and print outputs without using synchronized keyword   thread 0 prints 0 thread 1 prints 1 thread 2 prints 2 thread 3 prints 3 thread 0 prints 0 thread 1 prints 1 thread 2 prints 2 thread 3 prints 3 thread 0 prints 0 thread 1 prints 1 thread 2 prints 2 thread 3 prints 3   To achieve this we may use the following full fledged running code   public class Solution       static volatile int counter   0      static int print   0      public static void main String   args               TODO Auto-generated method stub         Thread   ths   new Thread 4           for  int i   0  i  lt  ths length  i                  ths i    new Thread new MyRunnable i  ths length                ths i  start                        static class MyRunnable implements Runnable           final int thID          final int total          public MyRunnable int id  int total                thID   id              this total   total                     Override         public void run                    TODO Auto-generated method stub             while  true                    if  thID    counter                        System out println  thread     thID     prints     print                       print                        if  print    total                          print   0                      counter                        if  counter    total                          counter   0                    else                       try                           Thread sleep 30                         catch  InterruptedException e                               log it                                                                           The following github link has a readme  which gives proper explanation  https   github com sankar4git volatile thread ordering

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Yes  volatile must be used whenever you want a mutable variable to be accessed by multiple threads  It is not very common usecase because typically you need to perform more than a single atomic operation  e g  check the variable state before modifying it   in which case you would use a synchronized block instead

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volatile variable is basically used for instant update  flush  in main shared cache line once it updated  so that changes reflected to all worker threads immediately

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Below is a very simple code to demonstrate the requirement of volatile for variable which is used to control the Thread execution from other thread  this is one scenario where volatile is required       Code to prove importance of  volatile  when state of one thread is being mutated from another thread     Try running this class with and without  volatile  for  state  property of Task class  public class VolatileTest       public static void main String   a  throws Exception           Task task   new Task            new Thread task  start             Thread sleep 500           long stoppedOn   System nanoTime             task stop       ----- gt  do this to stop the thread          System out println  Stopping on      stoppedOn            class Task implements Runnable          Try running with and without  volatile  here     private volatile boolean state   true      private int i   0       public void stop             state   false               Override     public void run             while state                i                      System out println i     gt  Stopped on      System nanoTime                When volatile is not used  you ll never see  Stopped on  xxx  message even after  Stopping on  xxx   and the program continues to run   Stopping on  1895303906650500   When volatile used  you ll see the  Stopped on  xxx  immediately   Stopping on  1895285647980000 324565439 gt  Stopped on  1895285648087300   Demo  https   repl it repls SilverAgonizingObjectcode

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Yes  I use it quite a lot - it can be very useful for multi-threaded code   The article you pointed to is a good one   Though there are two important things to bear in mind    You should only use volatile if you completely understand what it does and how it differs to synchronized   In many situations volatile appears  on the surface  to be a simpler more performant alternative to synchronized  when often a better understanding of volatile would make clear that synchronized is the only option that would work  volatile doesn t actually work in a lot of older JVMs  although synchronized does   I remember seeing a document that referenced the various levels of support in different JVMs but unfortunately I can t find it now   Definitely look into it if you re using Java pre 1 5 or if you don t have control over the JVMs that your program will be running on

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Absolutely  yes   And not just in Java  but also in C    There are times when you need to get or set a value that is guaranteed to be an atomic operation on your given platform  an int or boolean  for example  but do not require the overhead of thread locking  The volatile keyword allows you to ensure that when you read the value that you get the current value and not a cached value that was just made obsolete by a write on another thread

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volatile has semantics for memory visibility  Basically  the value of a volatile field becomes visible to all readers  other threads in particular  after a write operation completes on it  Without volatile  readers could see some non-updated value   To answer your question  Yes  I use a volatile variable to control whether some code continues a loop  The loop tests the volatile value and continues if it is true  The condition can be set to false by calling a  stop  method  The loop sees false and terminates when it tests the value after the stop method completes execution   The book  Java Concurrency in Practice   which I highly recommend  gives a good explanation of volatile  This book is written by the same person who wrote the IBM article that is referenced in the question  in fact  he cites his book at the bottom of that article   My use of volatile is what his article calls the  pattern 1 status flag    If you want to learn more about how volatile works under the hood  read up on the Java memory model  If you want to go beyond that level  check out a good computer architecture book like Hennessy  amp  Patterson and read about cache coherence and cache consistency

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Every thread accessing a volatile field will read its current value before continuing  instead of  potentially  using a cached value   Only member variable can be volatile or transient

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Important point about volatile     Synchronization in Java is possible by using Java keywords synchronized and volatile and locks  In Java  we can not have synchronized variable  Using synchronized keyword with a variable is illegal and will result in compilation error  Instead of using the synchronized variable in Java  you can use the java volatile variable  which will instruct JVM threads to read the value of volatile variable from main memory and don   t cache it locally  If a variable is not shared between multiple threads then there is no need to use the volatile keyword    source  Example usage of volatile   public class Singleton       private static volatile Singleton  instance     volatile variable     public static Singleton getInstance             if   instance    null                synchronized  Singleton class                    if   instance    null                       instance   new Singleton                                    return  instance            We are creating instance lazily at the time the first request comes   If we do not make the  instance variable volatile then the Thread which is creating the instance of Singleton is not able to communicate to the other thread  So if Thread A is creating Singleton instance and just after creation  the CPU corrupts etc  all other threads will not be able to see the value of  instance as not null and they will believe it is still assigned null     Why does this happen  Because reader threads are not doing any locking and until the writer thread comes out of a synchronized block  the memory will not be synchronized and value of  instance will not be updated in main memory  With the Volatile keyword in Java  this is handled by Java itself and such updates will be visible by all reader threads        Conclusion  volatile keyword is also used to communicate the content of memory between threads      Example usage of without volatile   public class Singleton          private static Singleton  instance      without volatile variable     public static Singleton getInstance                 if  instance    null                   synchronized Singleton class                    if  instance    null   instance   new Singleton                            return  instance            The code above is not thread-safe  Although it checks the value of instance once again within the synchronized block  for performance reasons   the JIT compiler can rearrange the bytecode in a way that the reference to the instance is set before the constructor has finished its execution  This means the method getInstance   returns an object that may not have been initialized completely  To make the code thread-safe  the keyword volatile can be used since Java 5 for the instance variable  Variables that are marked as volatile get only visible to other threads once the constructor of the object has finished its execution completely  Source    volatile usage in Java   The fail-fast iterators are typically implemented using a volatile counter on the list object    When the list is updated  the counter is incremented    When an Iterator is created  the current value of the counter is embedded in the Iterator object  When an Iterator operation is performed  the method compares the two counter values and throws a ConcurrentModificationException if they are different    The implementation of fail-safe iterators is typically light-weight   They typically rely on properties of the specific list implementation s data structures   There is no general pattern

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You ll need to use  volatile  keyword  or  synchronized  and any other concurrency control tools and techniques you might have at your disposal if you are developing a multithreaded application  Example of such application is desktop apps   If you are developing an application that would be deployed to application server  Tomcat  JBoss AS  Glassfish  etc  you don t have to handle concurrency control yourself as it already addressed by the application server  In fact  if I remembered correctly the Java EE standard prohibit any concurrency control in servlets and EJBs  since it is part of the  infrastructure  layer which you supposed to be freed from handling it  You only do concurrency control in such app if you re implementing singleton objects  This even already addressed if you knit your components using frameworkd like Spring   So  in most cases of Java development where the application is a web application and using IoC framework like Spring or EJB  you wouldn t need to use  volatile

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In my opinion  two important scenarios other than stopping thread in which volatile keyword is used are    Double-checked locking mechanism  Used often in Singleton design pattern  In this the singleton object needs to be declared volatile  Spurious Wakeups  Thread may sometimes wake up from wait call even if no notify call has been issued  This behavior is called spurious wakeup  This can be countered by using a conditional variable  boolean flag   Put the wait   call in a while loop as long as the flag is true  So if thread wakes up from wait call due to any reasons other than Notify NotifyAll then it encounters flag is still true and hence calls wait again  Prior to calling notify set this flag to true  In this case the boolean flag is declared as volatile

[java] What is the volatile keyword useful for?

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