java - 如何在运行时提供 MapStruct Mapping 注解映射元数据

Question

我们使用 MapStruct在商业模型和我们的用户界面模型之间。
当 UI 客户端想要获取排序数据时，它可以从 ui-model 中指定一个字段。
我们的 MapStructParser可以获取相应的业务模型字段名称并创建所需的 Criteria 对其进行排序。
示例:

public interface ModelMapping extends BridgeMapping<BusinessModel, UiModel> {
   @Mapping(source = "zip", target = "plz")
   UiModel modelToUiModel(BusinessModel model, @MappingTarget UiModel uiModel);
}

问题:
如何阅读 @Mapping(source = "zip", target = "plz")注释并获得 source和 target值(value)？ Mapping -注解有 RetentionPolicy.CLASS ，因此无法通过反射访问它。

Answer 1

我们通过使用 ASM 解决了这个问题(字节码操作和分析框架)
阅读 Mapping -注释并在元模型中提供它:
示例 (还有 available on Github )

public class AnnotationParser {
   public void parse(Class<?> mapper) {
      ClassInfoCollector classPrinter = new ClassInfoCollector(annotationInfos);
      ClassReader cr = new ClassReader(mapper.getCanonicalName());
      cr.accept(classPrinter, 0);
   }
}

public class ClassInfoCollector extends ClassVisitor {

   private final List<MethodAnnotationInfo> mapStructParser;

   public ClassInfoCollector(List<MethodAnnotationInfo> mapStructParser) {
      super(ASMversion);
      this.mapStructParser = mapStructParser;
   }

   @Override
   public void visit(int version, int access, String name, String signature, String superName, String[] interfaces) {
      super.visit(version, access, name, signature, superName, interfaces);
   }

   @Override
   public MethodVisitor visitMethod(int access, String methodName, String descriptor, String signature, String[] exceptions) {
      return new MethodInfoCollector(methodName,  mapStructParser);
   }

}

public class ClassInfoCollector extends ClassVisitor {

   private final List<MethodAnnotationInfo> mapStructParser;

   public ClassInfoCollector(List<MethodAnnotationInfo> mapStructParser) {
      super(ASMversion);
      this.mapStructParser = mapStructParser;
   }

   @Override
   public void visit(int version, int access, String name, String signature, String superName, String[] interfaces) {
      super.visit(version, access, name, signature, superName, interfaces);
   }

   @Override
   public MethodVisitor visitMethod(int access, String methodName, String descriptor, String signature, String[] exceptions) {
      return new MethodInfoCollector(methodName,  mapStructParser);
   }

}

class MethodInfoCollector extends MethodVisitor {

    private final String methodName;
    private final List<MethodAnnotationInfo> mapStructParser;

   public MethodInfoCollector(String method, List<MethodAnnotationInfo> mapStructParser) {
      super(ASMversion);
      this.methodName = method;
      this.mapStructParser = mapStructParser;
   }

   @Override
   public AnnotationVisitor visitAnnotation(String descriptor, boolean visible) { 
      return new MethodAnnotationInfoCollector(methodName, descriptor,  mapStructParser);
   }
}

class MethodAnnotationInfoCollector extends AnnotationVisitor {

   private final String method;
   private final String annotationType;
   private final List<MethodAnnotationInfo> mapStructParser;

   public MethodAnnotationInfoCollector(String method, String annotationType, List<MethodAnnotationInfo> mapStructParser) {
      super(ASMversion);
      this.method = method;
      this.annotationType = annotationType;
      this.mapStructParser = mapStructParser;
   }

   @Override
   public void visit(String name, Object value) {
      MethodAnnotationInfo annotationInfo = new MethodAnnotationInfo(method,  annotationType, name, value.toString());
      mapStructParser.add(annotationInfo);
      super.visit(name, value);
   }

}

机智的 AnnotationParser可以通过这种方式获取映射信息:

class BusinessModel{
   String zip; 
}

class UiModel{
   String plz; 
}

public interface ModelMapping extends BridgeMapping<BusinessModel, UiModel> {
   @Mapping(source = "zip", target = "plz")
   UiModel modelToUiModel(BusinessModel model, @MappingTarget UiModel uiModel);
}

@Test
public testMappingInfo(){
   MapStructParser mappingInfo = new MapStructParser();
   mappingInfo.parseMappingInterface(ModelMapping.class);
   assertEquals("zip", mappingInfo.mapToTargetField("plz"));
}

mappingInfo.mapToTargetField("plz")返回 if BusinessModel 的映射字段( zip )。 AnnotationParser是一个通用的注释解析器，它提供了 MethodAnnotationInfo 的列表. MapStructParser使用 AnnotationParser模型搭建 MapStructMappingInfo通过收集 Mapping - 注释。
完整的可运行和经过测试的示例可在此处获得:
https://github.com/TobseF/mapstruct-metadata-example

Theoretically it also possible to hook into the MapStruct annotation processing process and generate MetaModel Java classes (ModelElementProcessor). But I wasn't able to get it working. Adding additional Annotation processors is't straight forward and debugging Annotation processing during class compilation is more than cumbersome. Four our purpose the ASM way and a simple mapping is sufficient.