How to use the atomic compare and swap technique in Java
Compare and swap
Compare and swap (CAS) is a concurrent programming technique. It compares an expected value to the actual value of the variable and modifies it only if it matches.
Compare and swap as an atomic operation
Modern CPUs have built-in support for atomic compare and swap operations.
Java5 onwards allows us to leverage the CPU functions via the set of classes in the java.util.concurrent.atomic package.
Some examples are:
AtomicBooleanAtomicLongAtomicInteger
This is not an exhaustive list of classes in the Java atomic package.
Let’s see an example of how to use the AtomicInteger package.
Example of AtomicInteger
import java.util.concurrent.atomic.AtomicInteger;public class MyAtomicLong {public static void main(String[] args) {// Create an atomic integerAtomicInteger atomicInteger = new AtomicInteger();// Creating an atomic integer with an initial valueAtomicInteger atomicInteger123 = new AtomicInteger(123);// Getting the atomic integer valuelong theValue = atomicInteger123.get();//Setting the valueatomicInteger123.set(234);//Compare and setint expectedValue = 123;int newValue = 234;atomicInteger123.compareAndSet(expectedValue, newValue);//Add and getSystem.out.println(atomicInteger123.getAndAdd(10));System.out.println(atomicInteger123.addAndGet(10));}}
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Line 6: We create an atomic integer without any initial value. In order to create an atomic integer with an initial value, we can use the syntax on line 9.
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Line 12: We get the value of an atomic integer.
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Line 15: We set a new value of the atomic integer.
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Line 20: We see the usage of the
compareAndSetfunction. It first checks the value of the variable to see if it matches the expected value. It then goes ahead and sets the value to the new value. The function returnstrueif the value was set to the new value. If not, the function returnsfalse. -
Line 24 and 25: We see how the
getAndAddandaddAndGetmethods are used. As the name suggests, thegetAndAddreturns the value and then adds to it. Meanwhile, theaddAndGetfunction first adds and then returns the resulting value.
Conclusion
Because the set of atomic classes uses the CPU’s support for comparison and swap, these are faster than traditional locks.
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Anjana Shankar