What does atomic mean in programming

Question

In the Effective Java book  it states       The language specification guarantees that reading or writing a   variable is atomic unless the variable is of type long or double  JLS    17 4 7     What does  atomic  mean in the context of Java programming  or programming in general

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Just found a post Atomic vs  Non-Atomic Operations to be very helpful to me       An operation acting on shared memory is atomic if it completes in a single step relative to other threads        When an atomic store is performed on a shared memory  no other thread can observe the modification half-complete        When an atomic load is performed on a shared variable  it reads the entire value as it appeared at a single moment in time

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Here s an example  because an example is often clearer than a long explanation  Suppose foo is a variable of type long  The following operation is not an atomic operation   foo   65465498L    Indeed  the variable is written using two separate operations  one that writes the first 32 bits  and a second one which writes the last 32 bits  That means that another thread might read the value of foo  and see the intermediate state    Making the operation atomic consists in using synchronization mechanisms in order to make sure that the operation is seen  from any other thread  as a single  atomic  i e  not splittable in parts   operation  That means that any other thread  once the operation is made atomic  will either see the value of foo before the assignment  or after the assignment  But never the intermediate value   A simple way of doing this is to make the variable volatile   private volatile long foo    Or to synchronize every access to the variable   public synchronized void setFoo long value        this foo   value     public synchronized long getFoo         return this foo       no other use of foo outside of these two methods  unless also synchronized   Or to replace it with an AtomicLong   private AtomicLong foo

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Atomic operation  means an operation that appears to be instantaneous from the perspective of all other threads  You don t need to worry about a partly complete operation when the guarantee applies

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It s something that  appears to the rest of the system to occur instantaneously   and falls under categorisation of Linearizability in computing processes  To quote that linked article further      Atomicity is a guarantee of isolation from concurrent processes    Additionally  atomic operations commonly have a succeed-or-fail   definition     they either successfully change the state of the system    or have no apparent effect    So  for instance  in the context of a database system  one can have  atomic commits   meaning that you can push a changeset of updates to a relational database and those changes will either all be submitted  or none of them at all in the event of failure  in this way data does not become corrupt  and consequential of locks and or queues  the next operation will be a different write or a read  but only after the fact  In the context of variables and threading this is much the same  applied to memory   Your quote highlights that this need not be expected behaviour in all instances

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In Java reading and writing fields of all types except long and double occurs atomically  and if the field is declared with the volatile modifier  even long and double are atomically read and written  That is  we get 100  either what was there  or what happened there  nor can there be any intermediate result in the variables

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If you have several threads executing the methods m1 and m2 in the code below   class SomeClass       private int i   0       public void m1     i   5        public int m2     return i        you have the guarantee that any thread calling m2 will either read 0 or 5   On the other hand  with this code  where i is a long    class SomeClass       private long i   0       public void m1     i   1234567890L        public long m2     return i        a thread calling m2 could read 0  1234567890L  or some other random value because the statement i   1234567890L is not guaranteed to be atomic for a long  a JVM could write the first 32 bits and the last 32 bits in two operations and a thread might observe i in between

[java] What does "atomic" mean in programming?

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Examples related to atomic