Safer Annotations helps you define constraints on your annotations that are richer than the stock Java constraints (type, method, field…).

Usage, for library authors

Import the Safer Annotations module in your pom.xml:

<dependency>
    <groupId>io.smallrye</groupId>
    <artifactId>smallrye-safer-annotations</artifactId>
    <version>1.0.2</version>
</dependency>

And start annotating your library annotations with safer constraints:

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
// Restrict the type signature of target methods
@TargetMethod(
  /* set of allowed return types */
  returnTypes = { void.class, Response.class, UniResponse.class, OptionalResponse.class }, 
  /* set of allowed parameter types */
  parameterTypes = { ContainerRequestContext.class, ContainerResponseContext.class, ResourceInfo.class, SimpleResourceInfo.class, ThrowableSubtype.class}
)
public @interface ServerResponseFilter {
}

// Allows us to pass generic types to the @TargetMethod annotation
class UniResponse extends TargetMethod.GenericType<Uni<Response>> {
}

class OptionalResponse extends TargetMethod.GenericType<Optional<Response>> {
}

// Allows us to declare parameter types that can be any subtype of Throwable
class ThrowableSubtype extends TargetMethod.Subtype<Throwable> {
}

Usage, for your library's users

Tell your build tool to invoke our compiler plugin:

<build>
    <plugins>
        <plugin>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>${compiler-plugin.version}</version>
            <configuration>
                <annotationProcessorPaths>
                    <annotationProcessorPath>
                        <groupId>io.smallrye</groupId>
                        <artifactId>smallrye-safer-annotations</artifactId>
                        <version>1.0.2</version>
                    </annotationProcessorPath>
                </annotationProcessorPaths>
            </configuration>
        </plugin>
    </plugins>
</build>

Now your users can get compile-time errors if they misplace your annotations:

@ServerResponseFilter
// ERROR: Invalid parameter type: must be one of: […]
public void myFilter(NonSupportedType something){
}

Supported annotation constraints

• @TargetAccessor: annotated method must be a Java Bean getter or setter
• @TargetMethod: annotated method must have a compatible method signature

Declaring or overriding annotation constraints on external annotations

If you cannot annotate the contained annotation directly, because it is external to your project, you can declare a DefinitionOverride implementation and place the constaints on it instead.

First, create a META-INF/services/io.smallrye.safer.annotations.DefinitionOverride file containing the list of override class names:

com.example.foo.MyOverride

And place your constraints on your override class:

package com.example.foo;

import io.smallrye.safer.annotations.DefinitionOverride;
import io.smallrye.safer.annotations.OverrideTarget;
import io.smallrye.safer.annotations.TargetMethod;


// Designate which external annotation we are targeting
@OverrideTarget(ServerExceptionMapper.class)

// Restrict the type signature of target methods
@TargetMethod(
  /* set of allowed return types */
  returnTypes = { void.class, Response.class, UniResponse.class, OptionalResponse.class }, 
  /* set of allowed parameter types */
  parameterTypes = { ContainerRequestContext.class, ContainerResponseContext.class, ResourceInfo.class, SimpleResourceInfo.class, ThrowableSubtype.class}
)
public class MyOverride implements DefinitionOverride {
}