Services and interceptors

For common concerns like caching, auditing etc, it can be fancy to use interceptor like API. It is enabled by the framework on services.

An interceptor defines an annotation marked with @Intercepts which defines the implementation of the interceptor (an InterceptorHandler).

Here is an example:

@Intercepts(LoggingHandler.class)
@Target({ TYPE, METHOD })
@Retention(RUNTIME)
public @interface Logged {
    String value();
}

Then handler is created from its constructor and can take service injections (by type). The first parameter, however, can be a BiFunction<Method, Object[], Object> which representes the invocation chain if your interceptor can be used with others.

if you do a generic interceptor it is important to pass the invoker as first parameter. If you don’t do so you can’t combine interceptors at all.

Here is an interceptor implementation for our @Logged API:

public class LoggingHandler implements InterceptorHandler {
    // injected
    private final BiFunction<Method, Object[], Object> invoker;
    private final SomeService service;

    // internal
    private final ConcurrentMap<Method, String> loggerNames = new ConcurrentHashMap<>();

    public CacheHandler(final BiFunction<Method, Object[], Object> invoker, final SomeService service) {
        this.invoker = invoker;
        this.service = service;
    }

    @Override
    public Object invoke(final Method method, final Object[] args) {
        final String name = loggerNames.computeIfAbsent(method, m -> findAnnotation(m, Logged.class).get().value());
        service.getLogger(name).info("Invoking {}", method.getName());
        return invoker.apply(method, args);
    }
}

This implementation is compatible with interceptor chains since it takes the invoker as first constructor parameter and it also takes a service injection. Then the implementation just does what is needed - logging the invoked method here.

the findAnnotation annotation - inherited from InterceptorHandler is an utility method to find an annotation on a method or class (in this order).
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