Asynchronous Execution of Methods

Methods annotated with @Async will return immediately to its caller while its operation executes asynchronously:

public class Foo {
  @Async
  public void save() {
    // perform some operation here
  }
}

The save() method will immediately return control to its caller when invoked, without having to wait for the operation to complete. Note that the save() operation will execute in a separate thread, so you must ensure thread safety for any shared data being manipulated within save().

An asynchronous method can also pass a result to its caller by returning a Future through the returned value of Future.get().

public class Foo {
  @Async
  public Future<Integer> calculate() {
    // perform some operation here
    // return an instance of Future interface
  }
}

Methods annotated with @Async must strictly have a void or Future return type. Otherwise, an exception will be thrown at runtime when the method is invoked. If you wish to check for non-compliant methods at compile time, you may incorporate the annotation processor AsyncReturnTypeProcessor during your build. Example usage with maven-processor-plugin:

<plugin>
  <groupId>org.bsc.maven</groupId>
  <artifactId>maven-processor-plugin</artifactId>
  <executions>
    <execution>
      <id>process</id>
      <goals>
        <goal>process</goal>
      </goals>
      <phase>generate-sources</phase>
      <configuration>
        <processors>
          <processor>com.jcabi.aspects.apt.AsyncReturnTypeProcessor</processor>
        </processors>
      </configuration>
    </execution>
  </executions>
</plugin>

The mechanism is implemented with AOP/AspectJ. Read how to integrate it into your pom.xml.