AtomicOperations

Atomic operations are singular ( an operation that cannot be stopped in between). Just like a transaction, it happens completely or does not happen at all. The operations that are atomic :

1. Read and write for reference variables
2. Read and write for byte, short, int, char, float, and Boolean
3. Read and write for all variables declared with the volatile qualifier ( this includes all primitive types)

Operations like increment and decrement look like atomic operations, but there is no guarantee that they are atomic. As any increment operation x++ can be a group of three operations

a)      Temporary copy of x will be created

b)      Temporary copy will be incremented

c)       Newly incremented value will be written back to variable x

Variables of datatype double and long are 64-bit and can be accessed by 2 32-bit operations. During concurrent programming, this can compromise the integrity of data. Java.util.concurrent provides atomic APIs to solve this problem. Let us take a look at an example:

package atomicops;

import java.util.concurrent.atomic.AtomicLong;

/**
 *
 * @author Nancy
 */
public class TestAtomicLong {
    public static void main(String[] args) {
        AtomicLong myLong= new AtomicLong(25);
     
        long x = myLong.getAndAdd(20);
        System.out.println("x ="+x);
        x=myLong.get();
        System.out.println("x ="+x);
     
        x = myLong.addAndGet(30);
        System.out.println("x ="+x);
     
        boolean b = myLong.compareAndSet(75,100);
        System.out.println("b ="+b);
        x=myLong.get();
        System.out.println("x ="+x);
     
        x= myLong.incrementAndGet();
        System.out.println("x="+x);
     
         x= myLong.decrementAndGet();
        System.out.println("x="+x);
    }  
}

Output :

x =25

x =45

x =75

b =true

x =100

x=101

x=100

Other Atomic operation classes are : 

• AtomicBoolean
• AtomicInteger
• AtomicIntegerArray
• AtomicIntegerFieldUpdater
• AtomicLong
• AtomicLongArray
• AtomicLongFieldUpdater
• AtomicMarkableReference
• AtomicReference
• AtomicReferenceArray
• AtomicReferenceFieldUpdater
• AtomicStampedReference

©Nancy

11th April, 2015

2:34 am