AtomicLong
Jakob Jenkov |
The AtomicLong class provides you with a long variable which can be read and written
atomically, and which also contains advanced atomic operations like compareAndSet(). The AtomicLong
class is located in the java.util.concurrent.atomic package, so the full class name is
java.util.concurrent.atomic.AtomicLong . This text describes the version of AtomicLong
found in Java 8, but the first version was added in Java 5.
The reasoning behind the AtomicLong design is explained in my Java Concurrency tutorial in the text about
Compare and Swap.
Creating an AtomicLong
Creating an AtomicLong is done like this:
AtomicLong atomicLong = new AtomicLong();
This example creates an AtomicLong with the initial value 0 .
If you want to create an AtomicLong with an initial value, you can do so like this:
AtomicLong atomicLong = new AtomicLong(123);
This example passes a value of 123 as parameter to the AtomicLong contructor,
which sets the initial value of the AtomicLong instance to 123 .
Getting the AtomicLong Value
You can get the value of an AtomicLong instance via the get() method.
Here is an AtomicLong.get() example:
AtomicLong atomicLong = new AtomicLong(123); long theValue = atomicLong.get();
Setting the AtomicLong Value
You can set the value of an AtomicLong instance via the set() method.
Here is an AtomicLong.set() example:
AtomicLong atomicLong = new AtomicLong(123); atomicLong.set(234);
This example creates an AtomicLong example with an initial value of 123, and then sets its
value to 234 in the next line.
Compare and Set the AtomicLong Value
The AtomicLong class also has an atomic compareAndSet() method. This method
compares the current value of the AtomicLong instance to an expected value, and if the two
values are equal, sets a new value for the AtomicLong instance. Here is an
AtomicLong.compareAndSet() example:
AtomicLong atomicLong = new AtomicLong(123); long expectedValue = 123; long newValue = 234; atomicLong.compareAndSet(expectedValue, newValue);
This example first creates an AtomicLong instance with an initial value of 123 .
Then it compares the value of the AtomicLong to the expected value 123 and
if they are equal the new value of the AtomicLong becomes 234;
Adding to the AtomicLong Value
The AtomicLong class contains a few methods you can use to add a value to the AtomicLong
and get its value returned. These methods are:
addAndGet()getAndAdd()getAndIncrement()incrementAndGet()
The first method, addAndGet() adds a number to the AtomicLong and returns its value
after the addition. The second method, getAndAdd() also adds a number to the AtomicLong
but returns the value the AtomicLong had before the value was added. Which of these two methods
you should use depends on your use case. Here are two examples:
AtomicLong atomicLong = new AtomicLong(); System.out.println(atomicLong.getAndAdd(10)); System.out.println(atomicLong.addAndGet(10));
This example will print out the values 0 and 20. First the example gets the value
of the AtomicLong before adding 10 to. Its value before addition is 0. Then the example
adds 10 to the AtomicLong and gets the value after the addition. The value is now 20.
You can also add negative numbers to the AtomicLong via these two methods. The result is
effectively a subtraction.
The methods getAndIncrement() and incrementAndGet() works like getAndAdd()
and addAndGet() but just add 1 to the value of the AtomicLong.
Subtracting From the AtomicLong Value
The AtomicLong class also contains a few methods for subtracting values from the AtomicLong
value atomically. These methods are:
decrementAndGet()getAndDecrement()
The decrementAndGet() subtracts 1 from the AtomicLong value and returns its value
after the subtraction. The getAndDecrement() also subtracts 1 from the AtomicLong value
but returns the value the AtomicLong had before the subtraction.
Using an AtomicLong as a Counter in a Lambda Expression
A Java Lambda Expression cannot contain any member fields, and thus they cannot keep any state internally between calls to the lambda expression. However, you can bypass this limitation by creating an AtomicLong outside the Lambda Expression and use it from inside the Lambda Expression. Here is an example of that:
import java.util.concurrent.atomic.AtomicLong;
import java.util.function.Function;
public class AtomicLongExample {
public static void main(String[] args) {
AtomicLong atomicLong = new AtomicLong();
Function<Long, Long> myLambda = (input) -> {
long noOfCalls = atomicLong.incrementAndGet();
System.out.println("Lambda called " + noOfCalls + " times.");
return input * 2;
};
System.out.println(myLambda.apply(1L));
System.out.println(myLambda.apply(3L));
System.out.println(myLambda.apply(5L));
}
}
The output from running the above code look like this:
Lambda called 1 times. 2 Lambda called 2 times. 6 Lambda called 3 times. 10
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Jakob Jenkov | |
