技术分享 | APT结合JavaPoet生成模板化Java源代码文件

public interface AttributeConverter<X,Y> {
    /**     * Converts the value stored in the entity attribute into the      * data representation to be stored in the database.     *     * @param attribute  the entity attribute value to be converted     * @return  the converted data to be stored in the database      *          column     */    public Y convertToDatabaseColumn (X attribute);
    /**     * Converts the data stored in the database column into the      * value to be stored in the entity attribute.     * Note that it is the responsibility of the converter writer to     * specify the correct <code>dbData</code> type for the corresponding      * column for use by the JDBC driver: i.e., persistence providers are      * not expected to do such type conversion.     *     * @param dbData  the data from the database column to be      *                converted     * @return  the converted value to be stored in the entity      *          attribute     */    public X convertToEntityAttribute (Y dbData);}

public abstract class AbstractJsonConverter<T> implements AttributeConverter<T, String> {
  protected static ObjectMapper objectMapper;
  @Autowired  public void setObjectMapper(ObjectMapper objectMapper) {    AbstractJsonConverter.objectMapper = objectMapper;  }
  @Override  public String convertToDatabaseColumn(T attribute) {    objectMapper.setConfig(objectMapper.getSerializationConfig().without(SerializationFeature.FAIL_ON_EMPTY_BEANS));    ObjectWriter writer = objectMapper.writerFor(getJsonType());    try {      return writer.writeValueAsString(attribute);    } catch (JsonProcessingException e) {      e.printStackTrace();      return null;    }  }
  @Override  public T convertToEntityAttribute(String dbData) {    ObjectReader reader = objectMapper.readerFor(getJsonType());    try {      return dbData == null ? null : reader.readValue(dbData);    } catch (IOException e) {      e.printStackTrace();      return null;    }  }
  /**   * 获取子类中 Json 转换的 TypeReference   *   * @return TypeReference<T>   */  protected TypeReference<T> getJsonType() {    Type[] actualTypeArguments = ((ParameterizedType) (getClass().getGenericSuperclass())).getActualTypeArguments();    if (actualTypeArguments != null && actualTypeArguments.length > 0) {      return new TypeReference<T>() {        @Override        public Type getType() {          return actualTypeArguments[0];        }
      };    }
    return null;  }}

@Documented@Target(ElementType.TYPE)@Retention(RetentionPolicy.SOURCE)public @interface JsonAutoConverter {
  /**   * 是否开启自动转换   *   * @return   */  boolean autoApply() default true;
}

@AutoService(Processor.class)@SupportedSourceVersion(SourceVersion.RELEASE_8)@SupportedAnnotationTypes("com.zhucan.jpa.conversion.annotation.JsonAutoConverter")public class JsonConvertProcessor extends AbstractProcessor {
    private Filer filer;    private Messager messager;    private Elements elementUtils;    public static final String doc = "\n This codes are generated automatically. Do not modify! \n -.- \n created by zhuCan \n";
    @Override    public synchronized void init(ProcessingEnvironment processingEnvironment) {        super.init(processingEnvironment);        filer = processingEnv.getFiler();        messager = processingEnvironment.getMessager();        elementUtils = processingEnvironment.getElementUtils();    }
    @Override    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
        messager.printMessage(Diagnostic.Kind.NOTE, "Processor : " + getClass().getSimpleName());
        Set<? extends Element> elements = roundEnv.getElementsAnnotatedWith(JsonAutoConverter.class);
        elements.forEach(x -> {            // 被扫描的类的包路径            PackageElement packageElement = elementUtils.getPackageOf(x);            String packageName = packageElement.getQualifiedName().toString();
            // 获取类上面的注解            JsonAutoConverter annotation = x.getAnnotation(JsonAutoConverter.class);
            // 构建类            TypeSpec clazz = TypeSpec.classBuilder(x.getSimpleName() + "Converter")                    .addModifiers(Modifier.PUBLIC, Modifier.FINAL)                    .addAnnotation(AnnotationSpec.builder(Converter.class).addMember("autoApply", CodeBlock.builder().add("$L", annotation.autoApply()).build()).build())                    .addJavadoc(" generator for Json converter " + doc)                    .superclass(ParameterizedTypeName.get(ClassName.get(AbstractJsonConverter.class),                            ClassName.get((TypeElement) x)))                    .build();            try {                // 创建java文件                JavaFile javaFile = JavaFile.builder(packageName, clazz)                        .build();                // 写入                javaFile.writeTo(filer);            } catch (IOException e) {                e.printStackTrace();            }
            // 构建集合转换类            TypeSpec listClazz = TypeSpec.classBuilder(x.getSimpleName() + "ListConverter")                    .addModifiers(Modifier.PUBLIC, Modifier.FINAL)                    .addAnnotation(AnnotationSpec.builder(Converter.class).addMember("autoApply", CodeBlock.builder().add("$L", annotation.autoApply()).build()).build())                    .addJavadoc(" generator for Json converter " + doc)                    .superclass(ParameterizedTypeName.get(ClassName.get(AbstractJsonConverter.class),                            ParameterizedTypeName.get(ClassName.get(List.class), ClassName.get((TypeElement) x))))                    .build();            try {                // 创建java文件                JavaFile javaFile = JavaFile.builder(packageName, listClazz)                        .build();                // 写入                javaFile.writeTo(filer);            } catch (IOException e) {                e.printStackTrace();            }
        });        return false;    }}