Future interface was added in Java 5 but it had a major downside: you had to wait for it to return the result if you wanted to do something with it. Spring framework added
SettableListenableFuture which allowed to register callbacks, but now Java 8 added
CompletableFuture class and in release 4.2 Spring added support for it.
CompletableFuture allows to chain several asynchronous calls which takes asynchronous to another level. But the class is rather complex, it has more than 40 methods. How do we take advantage of it together with asynchronous support offered by Spring?
Lets say we are implementing a method which returns a
String result:
public String getResult() {
return "result";
}
Now we want to make this method asynchronous and return
CompletableFuture. Let's assume we already have all the necessary configurations for executing methods with
@Async (i.e. we have
@EnableAsync annotation in the configuration and defined
TaskExecutor bean).
First, we'll add
@Async annotation to the method and change the return type.
@Async
public CompletableFuture<String> getResult() {
return "result";
}
This will not even compile. But how do we convert string to
CompletableFuture? We can use the static method
completedFuture:
@Async
public CompletableFuture<String> getResult() {
return CompletableFuture.completedFuture("result");
}
We also need to process the result in the calling method. If we want to process both successes and failures we can execute
whenComplete method:
getResult().whenComplete(...);
Now you've taken advantage of
CompletableFuture.
