在Java中，我们可以使用注解处理器（Annotation Processor）来在编译时自动生成Dto类。下面是一个示例实现：

1. 定义注解

@Retention(RetentionPolicy.SOURCE)
@Target(ElementType.TYPE)
public @interface Dto {
    String value();
}

2. 创建注解处理器

@AutoService(Processor.class)
public class DtoProcessor extends AbstractProcessor {
     
    @Override
    public Set getSupportedAnnotationTypes() {
        Set annotations = new LinkedHashSet<>();
        annotations.add(Dto.class.getCanonicalName());
        return annotations;
    }
 
    @Override
    public SourceVersion getSupportedSourceVersion() {
        return SourceVersion.latestSupported();
    }
 
    @Override
    public boolean process(Set annotations, RoundEnvironment roundEnv) {
          
        for (Element element : roundEnv.getElementsAnnotatedWith(Dto.class)) {
            TypeElement typeElement = (TypeElement) element;
 
            String packageName = getPackageName(typeElement);
            String className = getClassName(typeElement);
            String dtoName = typeElement.getAnnotation(Dto.class).value();
 
            String source = getSource(packageName, className, dtoName);
 
            try {
                JavaFileObject file = processingEnv.getFiler().createSourceFile(packageName + "." + dtoName);
                Writer writer = file.openWriter();
                writer.write(source);
                writer.close();
            } catch (IOException e) {
                error(e.getMessage(), typeElement);
            }
        }
 
        return true;
    }
 
    private String getPackageName(TypeElement type) {
        return processingEnv.getElementUtils().getPackageOf(type).getQualifiedName().toString();
    }
 
    private String getClassName(TypeElement type) {
        String packageName = getPackageName(type);
        int packageLength = packageName.length() + 1;
        return type.getQualifiedName().toString().substring(packageLength);
    }
 
    private String getSource(String packageName, String className, String dtoName) {
        StringBuilder builder = new StringBuilder();
        builder.append("package ").append(packageName).append(";\n\n");
        builder.append("public class ").append(dtoName).append(" {\n\n");
        builder.append("    private ").append(className).append(" entity;\n\n");
 
        // 生成getter、setter方法
        // ...
 
        builder.append("}");
 
        return builder.toString();
    }
}

3. 在Dto类中使用注解

@Dto("UserDto")
public class User {
    // 省略属性和方法
}

当编译时，注解处理器会扫描所有使用@Dto注解的Dto类，并自动生成相应