Helper library and annotation processor for building incremental annotation processors

Gradle 4.7 comes with some level incremental annotation processing support. Gradle 4.8 goes farther by making it possibly dynamic.

This library and annotation processor helps you generate the META-INF descriptor, and return the appropriate value from your processor's getSupportedOptions() if it's dynamic.

1. Add the incap library to your compile-time dependencies, and the incap-processor to your annotation processor path:

   with Gradle

   dependencies {
       compileOnly("net.ltgt.gradle.incap:incap:${incap.version}")
       annotationProcessor("net.ltgt.gradle.incap:incap-processor:${incap.version}")
   }

   with Maven

   <dependencies>
       <dependency>
           <groupId>net.ltgt.gradle.incap</groupId>
           <artifactId>incap</artifactId>
           <version>${incap.version}</version>
           <scope>provided</scope>
           <optional>true</optional>
       </dependency>
   </dependencies>
   <build>
       <plugins>
           <plugin>
               <groupId>org.apache.maven.plugins</groupId>
               <artifactId>maven-compiler-plugin</artifactId>
               <configuration>
                   <annotationProcessorPaths>
                       <path>
                           <groupId>net.ltgt.gradle.incap</groupId>
                           <artifactId>incap-processor</artifactId>
                           <version>${incap.version}</version>
                       </path>
                   </annotationProcessorPaths>
               </configuration>
           </plugin>
       </plugins>
   </build>

   Note: it's OK to use compileOnly in Gradle, or the provided scope in Maven, despite the annotation having class retention, because annotation processors aren't libraries that others compile against.

2. Annotate your annotation processor with @IncrementalAnnotationProcessor

   @IncrementalAnnotationProcessor(ISOLATING)
   public class MyProcessor extends AbstractProcessor {

3. If the choice of incrementality support is dynamic (i.e. you used the DYNAMIC value above), use the IncrementalAnnotationProcessorType enumeration's getProcessorOption() from your getSupportedOptions() to get the appropriate constant.

   @Override
   public Set<String> getSupportedOptions() {
       if (someCondition) {
           return Collections.singleton(ISOLATING.getProcessorOption());
       } else {
           return Collections.emptySet();
       }
   }

This processor works great with @AutoService, which also inspired some of the code here.

Shout-out Gradle, Inc. (and Groupon) who built this feature.