When to use AtomicReference in Java

Question

When do we use AtomicReference   Is it needed to create objects in all multithreaded programs   Provide a simple example where AtomicReference should be used

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Here s a very simple use case and has nothing to do with thread safety    To share an object between lambda invocations  the AtomicReference is an option   public void doSomethingUsingLambdas          AtomicReference lt YourObject gt  yourObjectRef   new AtomicReference lt  gt          soSomethingThatTakesALambda    - gt            yourObjectRef set youObject                soSomethingElseThatTakesALambda    - gt            YourObject yourObject   yourObjectRef get                I m not saying this is good design or anything  it s just a trivial example   but if you have have the case where you need to share an object between lambda invocations  the AtomicReference is an option   In fact you can use any object that holds a reference  even a Collection that has only one item  However  the AtomicReference is a perfect fit

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You can use AtomicReference when applying optimistic locks  You have a shared object and you want to change it from more than 1 thread    You can create a copy of the shared object Modify the shared object You need to check that the shared object is still the same as before - if yes  then update with the reference of the modified copy     As other thread might have modified it and can modify between these 2 steps  You need to do it in an atomic operation  this is where AtomicReference can help

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Another simple example is to do a safe-thread modification in a session object    public PlayerScore getHighScore         ServletContext ctx   getServletConfig   getServletContext        AtomicReference lt PlayerScore gt  holder             AtomicReference lt PlayerScore gt   ctx getAttribute  highScore        return holder get       public void updateHighScore PlayerScore newScore        ServletContext ctx   getServletConfig   getServletContext        AtomicReference lt PlayerScore gt  holder             AtomicReference lt PlayerScore gt   ctx getAttribute  highScore        while  true            HighScore old   holder get            if  old score  gt   newScore score              break          else if  holder compareAndSet old  newScore               break             Source  http   www ibm com developerworks library j-jtp09238 index html

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An atomic reference is ideal to use when you need to share and change the state of an immutable object between multiple threads  That is a super dense statement so I will break it down a bit   First  an immutable object is an object that is effectively not changed after construction   Frequently an immutable object s methods return new instances of that same class   Some examples include the wrapper classes of Long and Double  as well as String  just to name a few    According to Programming Concurrency on the JVM immutable objects are a critical part of modern concurrency      Next  why AtomicReference is better than a volatile object for sharing that shared value   A simple code example will show the difference   volatile String sharedValue  static final Object lock new Object    void modifyString      synchronized lock       sharedValue sharedValue  something to add           Every time you want to modify the string referenced by that volatile field based on its current value  you first need to obtain a lock on that object   This prevents some other thread from coming in during the meantime and changing the value in the middle of the new string concatenation   Then when your thread resumes  you clobber the work of the other thread  But honestly that code will work  it looks clean  and it would make most people happy     Slight problem   It is slow   Especially if there is a lot of contention of that lock Object   Thats because most locks require an OS system call  and your thread will block and be context switched out of the CPU to make way for other processes   The other option is to use an AtomicRefrence   public static AtomicReference lt String gt  shared   new AtomicReference lt  gt     String init  Inital Value   shared set init     now we will modify that value boolean success false  while  success     String prevValue shared get         do all the work you need to   String newValue shared get    lets add something        Compare and set   success shared compareAndSet prevValue newValue       Now why is this better   Honestly that code is a little less clean than before   But there is something really important that happens under the hood in AtomicRefrence  and that is compare and swap  It is a single CPU instruction  not an OS call  that makes the switch happen  That is a single instruction on the CPU   And because there are no locks  there is no context switch in the case where the lock gets exercised which saves even more time     The catch is  for AtomicReferences  this does not use a  equals   call  but instead an    comparison for the expected value   So make sure the expected is the actual object returned from get in the loop

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Here is a use case for AtomicReference   Consider this class that acts as a number range  and uses individual AtmomicInteger variables to maintain lower and upper number bounds   public class NumberRange          INVARIANT  lower  lt   upper     private final AtomicInteger lower   new AtomicInteger 0       private final AtomicInteger upper   new AtomicInteger 0        public void setLower int i               Warning -- unsafe check-then-act         if  i  gt  upper get                throw new IllegalArgumentException                       can t set lower to     i      gt  upper            lower set i              public void setUpper int i               Warning -- unsafe check-then-act         if  i  lt  lower get                throw new IllegalArgumentException                       can t set upper to     i      lt  lower            upper set i              public boolean isInRange int i            return  i  gt   lower get    amp  amp  i  lt   upper get               Both setLower and setUpper are check-then-act sequences  but they do not use sufficient locking to make them atomic  If the number range holds  0  10   and one thread calls setLower 5  while another thread calls setUpper 4   with some unlucky timing both will pass the checks in the setters and both modifications will be applied  The result is that the range now holds  5  4 an invalid state  So while the underlying AtomicIntegers are thread-safe  the composite class is not  This can be fixed by using a AtomicReference instead of using individual AtomicIntegers for upper and lower bounds   public class CasNumberRange          Immutable     private static class IntPair           final int lower      Invariant  lower  lt   upper         final int upper           private IntPair int lower  int upper                this lower   lower              this upper   upper                       private final AtomicReference lt IntPair gt  values                new AtomicReference lt IntPair gt  new IntPair 0  0         public int getLower             return values get   lower             public void setLower int lower            while  true                IntPair oldv   values get                if  lower  gt  oldv upper                  throw new IllegalArgumentException                       Can t set lower to     lower      gt  upper                IntPair newv   new IntPair lower  oldv upper               if  values compareAndSet oldv  newv                   return                       public int getUpper             return values get   upper             public void setUpper int upper            while  true                IntPair oldv   values get                if  upper  lt  oldv lower                  throw new IllegalArgumentException                       Can t set upper to     upper      lt  lower                IntPair newv   new IntPair oldv lower  upper               if  values compareAndSet oldv  newv                   return

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Atomic reference should be used in a setting where you need to do simple atomic  i e  thread-safe  non-trivial  operations on a reference  for which monitor-based synchronization is not appropriate  Suppose you want to check to see if a specific field only if the state of the object remains as you last checked   AtomicReference lt Object gt  cache   new AtomicReference lt Object gt      Object cachedValue   new Object    cache set cachedValue          time passes     Object cachedValueToUpdate   cache get          do some work to transform cachedValueToUpdate into a new version Object newValue   someFunctionOfOld cachedValueToUpdate   boolean success   cache compareAndSet cachedValue cachedValueToUpdate     Because of the atomic reference semantics  you can do this even if the cache object is shared amongst threads  without using synchronized  In general  you re better off using synchronizers or the java util concurrent framework rather than bare Atomic  unless you know what you re doing    Two excellent dead-tree references which will introduce you to this topic      Herlihy s excellent Art of Multiprocessor Programming Java Concurrency in Practice   Note that  I don t know if this has always been true  reference assignment  i e     is itself atomic  updating primitive 64-bit types like long or double may not be atomic  but updating a reference is always atomic  even if it s 64 bit  without explicitly using an Atomic   See the Java Language Specification 3ed  Section 17 7

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I won t talk much  Already my respected fellow friends have given their valuable input  The full fledged running code at the last of this blog should remove any confusion  It s about a movie seat booking small program in multi-threaded scenario   Some important elementary facts are as follows  1  Different threads can only contend for instance and static member variables in the heap space  2  Volatile read or write are completely atomic and serialized happens before and only done from memory  By saying this I mean that any read will follow the previous write in memory  And any write will follow the previous read from memory  So any thread working with a volatile will always see the most up-to-date value  AtomicReference uses this property of volatile   Following are some of the source code of AtomicReference  AtomicReference refers to an object reference  This reference is a volatile member variable in the AtomicReference instance as below   private volatile V value    get   simply returns the latest value of the variable  as volatiles do in a  happens before  manner    public final V get     Following is the most important method of AtomicReference   public final boolean  compareAndSet V expect  V update            return unsafe compareAndSwapObject this  valueOffset  expect  update       The compareAndSet expect update  method calls the compareAndSwapObject   method of the unsafe class of Java  This method call of unsafe invokes the native call  which invokes a single instruction to the processor   expect  and  update  each reference an object   If and only if the AtomicReference instance member variable  value  refers to the same object is referred to by  expect    update  is assigned to this instance variable now  and  true  is returned  Or else  false is returned  The whole thing is done atomically  No other thread can intercept in between  As this is a single processor operation  magic of modern computer architecture   it s often faster than using a synchronized block  But remember that when multiple variables need to be updated atomically  AtomicReference won t help   I would like to add a full fledged running code  which can be run in eclipse  It would clear many confusion  Here 22 users  MyTh threads  are trying to book 20 seats  Following is the code snippet followed by the full code   Code snippet where 22 users are trying to book 20 seats   for  int i   0  i  lt  20  i        20 seats             seats add new AtomicReference lt Integer gt                        Thread   ths   new Thread 22     22 users         for  int i   0  i  lt  ths length  i                  ths i    new MyTh seats  i               ths i  start                Following is the full running code   import java util ArrayList  import java util List  import java util concurrent ThreadLocalRandom  import java util concurrent atomic AtomicInteger  import java util concurrent atomic AtomicReference   public class Solution        static List lt AtomicReference lt Integer gt  gt  seats    Movie seats numbered as per                                                    list index      public static void main String   args  throws InterruptedException              TODO Auto-generated method stub         seats   new ArrayList lt  gt             for  int i   0  i  lt  20  i        20 seats             seats add new AtomicReference lt Integer gt                        Thread   ths   new Thread 22     22 users         for  int i   0  i  lt  ths length  i                  ths i    new MyTh seats  i               ths i  start                      for  Thread t   ths                t join                      for  AtomicReference lt Integer gt  seat   seats                System out print       seat get                                     id is the id of the user                 author sankbane                    static class MyTh extends Thread     each thread is a user         static AtomicInteger full   new AtomicInteger 0           List lt AtomicReference lt Integer gt  gt  l   seats         int id   id of the users         int seats           public MyTh List lt AtomicReference lt Integer gt  gt  list  int userId                l   list              this id   userId              seats   list size                        Override         public void run                 boolean reserved   false              try                   while   reserved  amp  amp  full get    lt  seats                        Thread sleep 50                       int r   ThreadLocalRandom current   nextInt 0  seats     excludes                                                                                seats                                                                                                    AtomicReference lt Integer gt  el   l get r                       reserved   el compareAndSet null  id     null means no user                                                                has reserved this                                                                seat                     if  reserved                          full getAndIncrement                                      if   reserved  amp  amp  full get      seats                      System out println  user     id     did not get a seat                  catch  InterruptedException ie                       log it

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When do we use AtomicReference    AtomicReference is flexible way to update the variable value atomically without use of synchronization    AtomicReference support lock-free thread-safe programming on single variables   There are multiple ways of achieving Thread safety with high level concurrent API   Atomic variables is one of the multiple options   Lock objects support locking idioms that simplify many concurrent applications   Executors define a high-level API for launching and managing threads  Executor implementations provided by java util concurrent provide thread pool management suitable for large-scale applications   Concurrent collections make it easier to manage large collections of data  and can greatly reduce the need for synchronization   Atomic variables have features that minimize synchronization and help avoid memory consistency errors      Provide a simple example where AtomicReference should be used    Sample code with AtomicReference   String initialReference    value 1    AtomicReference lt String gt  someRef       new AtomicReference lt String gt  initialReference    String newReference    value 2   boolean exchanged   someRef compareAndSet initialReference  newReference   System out println  exchanged      exchanged        Is it needed to create objects in all multithreaded programs    You don t have to use AtomicReference in all multi threaded programs   If you want to guard a single variable  use AtomicReference  If you want to guard a code block  use other constructs like Lock  synchronized etc

[java] When to use AtomicReference in Java?

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