What is the difference between atomic volatile synchronized

Question

How do atomic   volatile   synchronized work internally   What is the difference between the following code blocks   Code 1  private int counter   public int getNextUniqueIndex         return counter         Code 2  private AtomicInteger counter   public int getNextUniqueIndex         return counter getAndIncrement        Code 3  private volatile int counter   public int getNextUniqueIndex         return counter           Does volatile work in the following way  Is  volatile int i   0  void incIBy5         i    5      equivalent to   Integer i   5  void incIBy5         int temp      synchronized i    temp   i       synchronized i    i   temp   5       I think that two threads cannot enter a synchronized block at the same time    am I right  If this is true then how does atomic incrementAndGet   work without synchronized  And is it thread-safe   And what is the difference between internal reading and writing to volatile variables   atomic variables  I read in some article that the thread has a local copy of the variables - what is that

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The Java volatile modifier is an example of a special mechanism to guarantee that communication happens between threads  When one thread writes to a volatile variable  and another thread sees that write  the first thread is telling the second about all of the contents of memory up until it performed the write to that volatile variable   Atomic operations are performed in a single unit of task without interference from other operations  Atomic operations are necessity in multi-threaded environment to avoid data inconsistency

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A volatile   synchronization is a fool proof solution for an operation statement  to be fully atomic which includes multiple instructions to the CPU   Say for eg volatile int i   2   i    which is nothing but i   i   1  which makes i as the value 3 in the memory after the execution of this statement  This includes reading the existing value from memory for i which is 2   load into the CPU accumulator register and do with the calculation by increment the existing value with one 2   1   3 in accumulator   and then write back that incremented value back to the memory   These operations are not atomic enough though the value is of i is volatile   i being volatile guarantees only that a SINGLE read write from memory is atomic and not with MULTIPLE   Hence  we need to have synchronized also around i    to keep it to be fool proof atomic statement   Remember the fact that a statement includes multiple statements   Hope the explanation is clear enough

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volatile   volatile is a keyword  volatile forces all threads to get latest value of the variable from main memory instead of cache  No locking is required to access volatile variables  All threads can access volatile variable value at same time   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  What s more  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   When to use  One thread modifies the data and other threads have to read latest value of data  Other threads will take some action but they won t update data   AtomicXXX   AtomicXXX classes support lock-free thread-safe programming on single variables  These AtomicXXX classes  like AtomicInteger  resolves memory inconsistency errors   side effects of modification of volatile variables  which have been accessed in multiple threads     When to use  Multiple threads can read and modify data    synchronized   synchronized is keyword used to guard a method or code block   By making method as synchronized has two effects    First  it is not possible for two invocations of synchronized methods on the same object to interleave  When one thread is executing a synchronized method for an object  all other threads that invoke synchronized methods for the same object block  suspend execution  until the first thread is done with the object  Second  when a synchronized method exits  it automatically establishes a happens-before relationship with any subsequent invocation of a synchronized method for the same object  This guarantees that changes to the state of the object are visible to all threads    When to use  Multiple threads can read and modify data  Your business logic not only update the data but also executes atomic operations   AtomicXXX is equivalent of volatile   synchronized even though the implementation is different  AmtomicXXX extends volatile variables   compareAndSet methods but does not use synchronization    Related SE questions   Difference between volatile and synchronized in Java  Volatile boolean vs AtomicBoolean  Good articles to read    Above content is taken from these documentation pages   https   docs oracle com javase tutorial essential concurrency sync html  https   docs oracle com javase tutorial essential concurrency atomic html  https   docs oracle com javase 8 docs api java util concurrent atomic package-summary html

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I know that two threads can not enter in Synchronize block at the same time   Two thread cannot enter a synchronized block on the same object twice  This means that two threads can enter the same block on different objects   This confusion can lead to code like this   private Integer i   0   synchronized i       i        This will not behave as expected as it could be locking on a different object each time      if this is true than How this atomic incrementAndGet   works without Synchronize    and is thread safe      yes  It doesn t use locking to achieve thread safety   If you want to know how they work in more detail  you can read the code for them      And what is difference between internal reading and writing to Volatile Variable   Atomic Variable      Atomic class uses volatile fields  There is no difference in the field   The difference is the operations performed   The Atomic classes use CompareAndSwap or CAS operations      i read in some article that thread has local copy of variables what is that      I can only assume that it referring to the fact that each CPU has its own cached view of memory which can be different from every other CPU   To ensure that your CPU has a consistent view of data  you need to use thread safety techniques    This is only an issue when memory is shared at least one thread updates it

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You are specifically asking about how they internally work  so here you are   No synchronization  private int counter   public int getNextUniqueIndex       return counter         It basically reads value from memory  increments it and puts back to memory  This works in single thread but nowadays  in the era of multi-core  multi-CPU  multi-level caches it won t work correctly  First of all it introduces race condition  several threads can read the value at the same time   but also visibility problems  The value might only be stored in  local  CPU memory  some cache  and not be visible for other CPUs cores  and thus - threads   This is why many refer to local copy of a variable in a thread  It is very unsafe  Consider this popular but broken thread-stopping code   private boolean stopped   public void run         while  stopped              do some work          public void pleaseStop         stopped   true      Add volatile to stopped variable and it works fine - if any other thread modifies stopped variable via pleaseStop   method  you are guaranteed to see that change immediately in working thread s while  stopped  loop  BTW this is not a good way to interrupt a thread either  see  How to stop a thread that is running forever without any use and Stopping a specific java thread   AtomicInteger  private AtomicInteger counter   new AtomicInteger     public int getNextUniqueIndex       return counter getAndIncrement        The AtomicInteger class uses CAS  compare-and-swap  low-level CPU operations  no synchronization needed   They allow you to modify a particular variable only if the present value is equal to something else  and is returned successfully   So when you execute getAndIncrement   it actually runs in a loop  simplified real implementation    int current  do     current   get      while  compareAndSet current  current   1      So basically  read  try to store incremented value  if not successful  the value is no longer equal to current   read and try again  The compareAndSet   is implemented in native code  assembly    volatile without synchronization  private volatile int counter   public int getNextUniqueIndex       return counter         This code is not correct  It fixes the visibility issue  volatile makes sure other threads can see change made to counter  but still has a race condition  This has been explained multiple times  pre post-incrementation is not atomic   The only side effect of volatile is  flushing  caches so that all other parties see the freshest version of the data  This is too strict in most situations  that is why volatile is not default   volatile without synchronization  2   volatile int i   0  void incIBy5       i    5      The same problem as above  but even worse because i is not private  The race condition is still present  Why is it a problem  If  say  two threads run this code simultaneously  the output might be   5 or   10  However  you are guaranteed to see the change   Multiple independent synchronized  void incIBy5       int temp    synchronized i    temp   i     synchronized i    i   temp   5       Surprise  this code is incorrect as well  In fact  it is completely wrong  First of all you are synchronizing on i  which is about to be changed  moreover  i is a primitive  so I guess you are synchronizing on a temporary Integer created via autoboxing     Completely flawed  You could also write   synchronized new Object          thread-safe  SRSLy      No two threads can enter the same synchronized block with the same lock  In this case  and similarly in your code  the lock object changes upon every execution  so synchronized effectively has no effect   Even if you have used a final variable  or this  for synchronization  the code is still incorrect  Two threads can first read i to temp synchronously  having the same value locally in temp   then the first assigns a new value to i  say  from 1 to 6  and the other one does the same thing  from 1 to 6    The synchronization must span from reading to assigning a value  Your first synchronization has no effect  reading an int is atomic  and the second as well  In my opinion  these are the correct forms   void synchronized incIBy5       i    5     void incIBy5       synchronized this        i    5         void incIBy5       synchronized this        int temp   i      i   temp   5

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Synchronized Vs Atomic Vs Volatile    Volatile and Atomic is apply only on variable   While Synchronized apply on method  Volatile ensure about visibility not atomicity consistency of object   While other both   ensure about visibility and atomicity  Volatile variable store in RAM and it   s faster in access but we can   t achive Thread safety   or synchronization whitout synchronized keyword  Synchronized implemented as synchronized block or synchronized method while both not  We can thread safe multiple line of code with help of synchronized keyword while with both we can   t achieve the same  Synchronized can lock the same class object or different class object while both can   t    Please correct me if anything i missed

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Declaring a variable as volatile means that modifying its value immediately affects the actual memory storage for the variable  The compiler cannot optimize away any references made to the variable  This guarantees that when one thread modifies the variable  all other threads see the new value immediately   This is not guaranteed for non-volatile variables    Declaring an atomic variable guarantees that operations made on the variable occur in an atomic fashion  i e   that all of the substeps of the operation are completed within the thread they are executed and are not interrupted by other threads  For example  an increment-and-test operation requires the variable to be incremented and then compared to another value  an atomic operation guarantees that both of these steps will be completed as if they were a single indivisible uninterruptible operation   Synchronizing all accesses to a variable allows only a single thread at a time to access the variable  and forces all other threads to wait for that accessing thread to release its access to the variable    Synchronized access is similar to atomic access  but the atomic operations are generally implemented at a lower level of programming  Also  it is entirely possible to synchronize only some accesses to a variable and allow other accesses to be unsynchronized  e g   synchronize all writes to a variable but none of the reads from it    Atomicity  synchronization  and volatility are independent attributes  but are typically used in combination to enforce proper thread cooperation for accessing variables   Addendum  April 2016   Synchronized access to a variable is usually implemented using a monitor or semaphore  These are low-level mutex  mutual exclusion  mechanisms that allow a thread to acquire control of a variable or block of code exclusively  forcing all other threads to wait if they also attempt to acquire the same mutex  Once the owning thread releases the mutex  another thread can acquire the mutex in turn   Addendum  July 2016   Synchronization occurs on an object  This means that calling a synchronized method of a class will lock the this object of the call  Static synchronized methods will lock the Class object itself   Likewise  entering a synchronized block requires locking the this object of the method   This means that a synchronized method  or block  can be executing in multiple threads at the same time if they are locking on different objects  but only one thread can execute a synchronized method  or block  at a time for any given single object

[java] What is the difference between atomic / volatile / synchronized?

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