Retrofit注解的理解

Retrofit使用注解，使网络请求的代码特别简洁，但是内部是怎样通过注解转换成一个正常格式的网络请求链接的呢？
阅读源码理解如下：

POST注解

  /**
     * 登录
     *
     * @return
     */
    @POST("service-auth/login")
    Observable<Result> login(@Query("userPhone") String userPhone, @Query("userPwd") String userPwd);

点击POST看看他是什么

/** Make a POST request. */
@Documented
@Target(METHOD)
@Retention(RUNTIME)
public @interface POST {
  /**
   * A relative or absolute path, or full URL of the endpoint. This value is optional if the first
   * parameter of the method is annotated with {@link Url @Url}.
   * <p>
   * See {@linkplain retrofit2.Retrofit.Builder#baseUrl(HttpUrl) base URL} for details of how
   * this is resolved against a base URL to create the full endpoint URL.
   */
  String value() default "";
}

@Target(METHOD)说明这是一个方法注解
@Retention(RUNTIME) 表示需要在什么级别保存该注解信息

然后我们找找POST注解在哪使用了：
ServiceMethod

 private void parseMethodAnnotation(Annotation annotation) {
      if (annotation instanceof DELETE) {
        parseHttpMethodAndPath("DELETE", ((DELETE) annotation).value(), false);
      } else if (annotation instanceof GET) {
        parseHttpMethodAndPath("GET", ((GET) annotation).value(), false);
      } else if (annotation instanceof HEAD) {
        parseHttpMethodAndPath("HEAD", ((HEAD) annotation).value(), false);
        if (!Void.class.equals(responseType)) {
          throw methodError("HEAD method must use Void as response type.");
        }
      } else if (annotation instanceof PATCH) {
        parseHttpMethodAndPath("PATCH", ((PATCH) annotation).value(), true);
      } else if (annotation instanceof POST) {
        parseHttpMethodAndPath("POST", ((POST) annotation).value(), true);
      } else if (annotation instanceof PUT) {
        parseHttpMethodAndPath("PUT", ((PUT) annotation).value(), true);
      } else if (annotation instanceof OPTIONS) {
        parseHttpMethodAndPath("OPTIONS", ((OPTIONS) annotation).value(), false);
      } else if (annotation instanceof HTTP) {
        HTTP http = (HTTP) annotation;
        parseHttpMethodAndPath(http.method(), http.path(), http.hasBody());
      } else if (annotation instanceof retrofit2.http.Headers) {
        String[] headersToParse = ((retrofit2.http.Headers) annotation).value();
        if (headersToParse.length == 0) {
          throw methodError("@Headers annotation is empty.");
        }
        headers = parseHeaders(headersToParse);
      } else if (annotation instanceof Multipart) {
        if (isFormEncoded) {
          throw methodError("Only one encoding annotation is allowed.");
        }
        isMultipart = true;
      } else if (annotation instanceof FormUrlEncoded) {
        if (isMultipart) {
          throw methodError("Only one encoding annotation is allowed.");
        }
        isFormEncoded = true;
      }
    }

发现在ServiceMethod中被使用了。

image.png

。

ServiceMethod

public ServiceMethod build() {
...
  for (Annotation annotation : methodAnnotations) {
        parseMethodAnnotation(annotation);
      }
...
    }

ServiceMethod

Builder(Retrofit retrofit, Method method) {
      this.retrofit = retrofit;
      this.method = method;
      this.methodAnnotations = method.getAnnotations();
      this.parameterTypes = method.getGenericParameterTypes();
      this.parameterAnnotationsArray = method.getParameterAnnotations();
    }

Retrofit

  ServiceMethod<?, ?> loadServiceMethod(Method method) {
   ...
        result = new ServiceMethod.Builder<>(this, method).build();
       ...
  }

Retrofit

  private void eagerlyValidateMethods(Class<?> service) {
    Platform platform = Platform.get();
    for (Method method : service.getDeclaredMethods()) {
      if (!platform.isDefaultMethod(method)) {
        loadServiceMethod(method);
      }
    }
  }

整个过程从上到下是，1.传给eagerlyValidateMethods一个Class对象，
2.通过反射 service.getDeclaredMethods()从Class的得到Method，
3.this.methodAnnotations = method.getAnnotations();从Method获得方法的注解，4.在 parseMethodAnnotation中根据获取的注解做分支操作。

使用时，在

ServiceFactory.getInstance()
                .createService(AccountService.class)
                .login(userCode, pwd);

被执行时,注解修饰的interface login()方法

  /**
     * 登录
     *
     * @return
     */
    @POST("service-auth/login")
    Observable<Result> login(@Query("userPhone") String userPhone, @Query("userPwd") String userPwd);

就被转化为post 请求形式。

curl -X POST "http://xxx.service-auth/login?userPhone=1&userPwd=1" -H "accept: */*" -H "token: 1"

参考： 对注解的使用，和自定义注解有一定的了解后，会更容易理解理解Retrofit的注解。