OnJava8-Examples/annotations/ifx/IfaceExtractorProcessor.java

// annotations/ifx/IfaceExtractorProcessor.java
// (c)2021 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
// javac-based annotation processing
package annotations.ifx;
import javax.annotation.processing.*;
import javax.lang.model.SourceVersion;
import javax.lang.model.element.*;
import javax.lang.model.util.*;
import java.util.*;
import java.util.stream.*;
import java.io.*;

@SupportedAnnotationTypes(
  "annotations.ifx.ExtractInterface")
@SupportedSourceVersion(SourceVersion.RELEASE_8)
public class IfaceExtractorProcessor
extends AbstractProcessor {
  private ArrayList<Element>
    interfaceMethods = new ArrayList<>();
  Elements elementUtils;
  private ProcessingEnvironment processingEnv;
  @Override public void init(
    ProcessingEnvironment processingEnv) {
    this.processingEnv = processingEnv;
    elementUtils = processingEnv.getElementUtils();
  }
  @Override public boolean process(
    Set<? extends TypeElement> annotations,
    RoundEnvironment env) {
    for(Element elem:env.getElementsAnnotatedWith(
        ExtractInterface.class)) {
      String interfaceName = elem.getAnnotation(
        ExtractInterface.class).interfaceName();
      for(Element enclosed :
          elem.getEnclosedElements()) {
        if(enclosed.getKind()
           .equals(ElementKind.METHOD) &&
           enclosed.getModifiers()
           .contains(Modifier.PUBLIC) &&
           !enclosed.getModifiers()
           .contains(Modifier.STATIC)) {
          interfaceMethods.add(enclosed);
        }
      }
      if(interfaceMethods.size() > 0)
        writeInterfaceFile(interfaceName);
    }
    return false;
  }
  private void
  writeInterfaceFile(String interfaceName) {
    try(
      Writer writer = processingEnv.getFiler()
        .createSourceFile(interfaceName)
        .openWriter()
    ) {
      String packageName = elementUtils
        .getPackageOf(interfaceMethods
                      .get(0)).toString();
      writer.write(
        "package " + packageName + ";\n");
      writer.write("public interface " +
        interfaceName + " {\n");
      for(Element elem : interfaceMethods) {
        ExecutableElement method =
          (ExecutableElement)elem;
        String signature = "  public ";
        signature += method.getReturnType() + " ";
        signature += method.getSimpleName();
        signature += createArgList(
          method.getParameters());
        System.out.println(signature);
        writer.write(signature + ";\n");
      }
      writer.write("}");
    } catch(Exception e) {
      throw new RuntimeException(e);
    }
  }
  private String createArgList(
    List<? extends VariableElement> parameters) {
    String args = parameters.stream()
      .map(p -> p.asType() + " " + p.getSimpleName())
      .collect(Collectors.joining(", "));
    return "(" + args + ")";
  }
}