参考：https://www.jb51.net/article/138804.htm

首先看一下Interceptor源码：

/**
* Observes, modifies, and potentially short-circuits requests going out and the corresponding
* responses coming back in. Typically interceptors add, remove, or transform headers on the request
* or response.
*/
 
public interface Interceptor {
  Response intercept(Chain chain) throws IOException;
 
  interface Chain {
    Request request();
 
    Response proceed(Request request) throws IOException;
 
    /**
     * Returns the connection the request will be executed on. This is only available in the chains
     * of network interceptors; for application interceptors this is always null.
     */
    @Nullable Connection connection();
  }
}

先看一下api描述，翻译过来其实就是可以通过拦截器拦截即将发出的请求及对响应结果做相应处理，典型的处理方式是修改header。其实我们可能不仅要处理header，有时也需要添加统一参数，都可以在拦截器内部完成。

看一下Interceptor接口，只有intercept(Chain chain)方法，其返回值是Response，顾名思义，是响应数据，我们要做的也就是重写该方法以达到我们的目的。intercept(Chain chain)方法中有个Chain参数，Chain是Interceptor接口内部中定义的另一个接口，我们暂且不管Retrofit内部是如何实现该接口的(这部分内容将会在新的文章中统一讲解)，现在只需要知道调用其request()方法可以拿到Request，调用其proceed(Request request)方法可以得到相应数据即可。

到此为止，Interceptor基本用法已经知晓，下面上示例代码：

public class CommonInterceptor implements Interceptor {
 
  private static Map<String, String> commonParams;
 
  public synchronized static void setCommonParam(Map<String, String> commonParams) {
    if (commonParams != null) {
      if (CommonInterceptor.commonParams != null) {
        CommonInterceptor.commonParams.clear();
      } else {
        CommonInterceptor.commonParams = new HashMap<>();
      }
      for (String paramKey : commonParams.keySet()) {
        CommonInterceptor.commonParams.put(paramKey, commonParams.get(paramKey));
      }
    }
  }
 
  public synchronized static void updateOrInsertCommonParam(@NonNull String paramKey, @NonNull String paramValue) {
    if (commonParams == null) {
      commonParams = new HashMap<>();
    }
    commonParams.put(paramKey, paramValue);
  }
 
  @Override
  public synchronized Response intercept(Chain chain) throws IOException {
    Request request = rebuildRequest(chain.request());
    Response response = chain.proceed(request);
    // 输出返回结果
    try {
      Charset charset;
      charset = Charset.forName("UTF-8");
      ResponseBody responseBody = response.peekBody(Long.MAX_VALUE);
      Reader jsonReader = new InputStreamReader(responseBody.byteStream(), charset);
      BufferedReader reader = new BufferedReader(jsonReader);
      StringBuilder sbJson = new StringBuilder();
      String line = reader.readLine();
      do {
        sbJson.append(line);
        line = reader.readLine();
      } while (line != null);
      LogUtil.e("response: " + sbJson.toString());
    } catch (Exception e) {
      e.printStackTrace();
      LogUtil.e(e.getMessage(), e);
    }
//    saveCookies(response, request.url().toString());
    return response;
  }
 
 
  public static byte[] toByteArray(RequestBody body) throws IOException {
    Buffer buffer = new Buffer();
    body.writeTo(buffer);
    InputStream inputStream = buffer.inputStream();
    ByteArrayOutputStream output = new ByteArrayOutputStream();
    byte[] bufferWrite = new byte[4096];
    int n;
    while (-1 != (n = inputStream.read(bufferWrite))) {
      output.write(bufferWrite, 0, n);
    }
    return output.toByteArray();
  }
 
  private Request rebuildRequest(Request request) throws IOException {
    Request newRequest;
    if ("POST".equals(request.method())) {
      newRequest = rebuildPostRequest(request);
    } else if ("GET".equals(request.method())) {
      newRequest = rebuildGetRequest(request);
    } else {
      newRequest = request;
    }
    LogUtil.e("requestUrl: " + newRequest.url().toString());
    return newRequest;
  }
 
  /**
   * 对post请求添加统一参数
   */
  private Request rebuildPostRequest(Request request) {
//    if (commonParams == null || commonParams.size() == 0) {
//      return request;
//    }
    Map<String, String> signParams = new HashMap<>(); // 假设你的项目需要对参数进行签名
    RequestBody originalRequestBody = request.body();
    assert originalRequestBody != null;
    RequestBody newRequestBody;
    if (originalRequestBody instanceof FormBody) { // 传统表单
      FormBody.Builder builder = new FormBody.Builder();
      FormBody requestBody = (FormBody) request.body();
      int fieldSize = requestBody == null ? 0 : requestBody.size();
      for (int i = 0; i < fieldSize; i++) {
        builder.add(requestBody.name(i), requestBody.value(i));
        signParams.put(requestBody.name(i), requestBody.value(i));
      }
      if (commonParams != null && commonParams.size() > 0) {
        signParams.putAll(commonParams);
        for (String paramKey : commonParams.keySet()) {
          builder.add(paramKey, commonParams.get(paramKey));
        }
      }
      // ToDo 此处可对参数做签名处理 signParams
      /**
       * String sign = SignUtil.sign(signParams);
       * builder.add("sign", sign);
       */
      newRequestBody = builder.build();
    } else if (originalRequestBody instanceof MultipartBody) { // 文件
      MultipartBody requestBody = (MultipartBody) request.body();
      MultipartBody.Builder multipartBodybuilder = new MultipartBody.Builder();
      if (requestBody != null) {
        for (int i = 0; i < requestBody.size(); i++) {
          MultipartBody.Part part = requestBody.part(i);
          multipartBodybuilder.addPart(part);
 
          /*
           上传文件时，请求方法接收的参数类型为RequestBody或MultipartBody.Part参见ApiService文件中uploadFile方法
           RequestBody作为普通参数载体，封装了普通参数的value; MultipartBody.Part即可作为普通参数载体也可作为文件参数载体
           当RequestBody作为参数传入时，框架内部仍然会做相关处理，进一步封装成MultipartBody.Part，因此在拦截器内部，
           拦截的参数都是MultipartBody.Part类型
           */
 
          /*
           1.若MultipartBody.Part作为文件参数载体传入，则构造MultipartBody.Part实例时，
           需使用MultipartBody.Part.createFormData(String name, @Nullable String filename, RequestBody body)方法，
           其中name参数可作为key使用(因为你可能一次上传多个文件，服务端可以此作为区分)且不能为null，
           body参数封装了包括MimeType在内的文件信息，其实例创建方法为RequestBody.create(final @Nullable MediaType contentType, final File file)
           MediaType获取方式如下：
           String fileType = FileUtil.getMimeType(file.getAbsolutePath());
           MediaType mediaType = MediaType.parse(fileType);
 
           2.若MultipartBody.Part作为普通参数载体，建议使用MultipartBody.Part.createFormData(String name, String value)方法创建Part实例
            name可作为key使用，name不能为null,通过这种方式创建的实例，其RequestBody属性的MediaType为null；当然也可以使用其他方法创建
           */
 
          /*
           提取非文件参数时,以RequestBody的MediaType为判断依据.
           此处提取方式简单暴力。默认part实例的RequestBody成员变量的MediaType为null时，part为非文件参数
           前提是:
           a.构造RequestBody实例参数时，将MediaType设置为null
           b.构造MultipartBody.Part实例参数时,推荐使用MultipartBody.Part.createFormData(String name, String value)方法，或使用以下方法
            b1.MultipartBody.Part.create(RequestBody body)
            b2.MultipartBody.Part.create(@Nullable Headers headers, RequestBody body)
            若使用方法b1或b2，则要求
 
           备注：
           您也可根据需求修改RequestBody的MediaType，但尽量保持外部传入参数的MediaType与拦截器内部添加参数的MediaType一致，方便统一处理
           */
 
          MediaType mediaType = part.body().contentType();
          if (mediaType == null) {
            String normalParamKey;
            String normalParamValue;
            try {
              normalParamValue = getParamContent(requestBody.part(i).body());
              Headers headers = part.headers();
              if (!TextUtils.isEmpty(normalParamValue) && headers != null) {
                for (String name : headers.names()) {
                  String headerContent = headers.get(name);
                  if (!TextUtils.isEmpty(headerContent)) {
                    String[] normalParamKeyContainer = headerContent.split("name=\"");
                    if (normalParamKeyContainer.length == 2) {
                      normalParamKey = normalParamKeyContainer[1].split("\"")[0];
                      signParams.put(normalParamKey, normalParamValue);
                      break;
                    }
                  }
                }
              }
            } catch (Exception e) {
              e.printStackTrace();
            }
          }
        }
      }
      if (commonParams != null && commonParams.size() > 0) {
        signParams.putAll(commonParams);
        for (String paramKey : commonParams.keySet()) {
          // 两种方式添加公共参数
          // method 1
          multipartBodybuilder.addFormDataPart(paramKey, commonParams.get(paramKey));
          // method 2
//          MultipartBody.Part part = MultipartBody.Part.createFormData(paramKey, commonParams.get(paramKey));
//          multipartBodybuilder.addPart(part);
        }
      }
      // ToDo 此处可对参数做签名处理 signParams
      /**
       * String sign = SignUtil.sign(signParams);
       * multipartBodybuilder.addFormDataPart("sign", sign);
       */
      newRequestBody = multipartBodybuilder.build();
    } else {
      try {
        JSONObject jsonObject;
        if (originalRequestBody.contentLength() == 0) {
          jsonObject = new JSONObject();
        } else {
          jsonObject = new JSONObject(getParamContent(originalRequestBody));
        }
        if (commonParams != null && commonParams.size() > 0) {
          for (String commonParamKey : commonParams.keySet()) {
            jsonObject.put(commonParamKey, commonParams.get(commonParamKey));
          }
        }
        // ToDo 此处可对参数做签名处理
        /**
         * String sign = SignUtil.sign(signParams);
         * jsonObject.put("sign", sign);
         */
        newRequestBody = RequestBody.create(originalRequestBody.contentType(), jsonObject.toString());
        LogUtil.e(getParamContent(newRequestBody));
 
      } catch (Exception e) {
        newRequestBody = originalRequestBody;
        e.printStackTrace();
      }
    }
//    可根据需求添加或修改header,此处制作示意
//    return request.newBuilder()
//        .addHeader("header1", "header1")
//        .addHeader("header2", "header2")
//        .method(request.method(), newRequestBody)
//        .build();
    return request.newBuilder().method(request.method(), newRequestBody).build();
  }
 
  /**
   * 获取常规post请求参数
   */
  private String getParamContent(RequestBody body) throws IOException {
    Buffer buffer = new Buffer();
    body.writeTo(buffer);
    return buffer.readUtf8();
  }
 
  /**
   * 对get请求做统一参数处理
   */
  private Request rebuildGetRequest(Request request) {
    if (commonParams == null || commonParams.size() == 0) {
      return request;
    }
    String url = request.url().toString();
    int separatorIndex = url.lastIndexOf("?");
    StringBuilder sb = new StringBuilder(url);
    if (separatorIndex == -1) {
      sb.append("?");
    }
    for (String commonParamKey : commonParams.keySet()) {
      sb.append("&").append(commonParamKey).append("=").append(commonParams.get(commonParamKey));
    }
    Request.Builder requestBuilder = request.newBuilder();
    return requestBuilder.url(sb.toString()).build();
  }
}

该拦截器示例代码提供了插入公共参数及对添加header功能(该功能在代码中被注释掉，如需要，放开即可)。对Request的拦截处理在rebuildRequest(Request request) 方法中完成，该方法只处理了GET与POST请求，内部有较为详尽的注释，较为复杂的是文件传输，有些需要注意的事项也做了尽可能完善的说明；对响应数据的处理，代码示例中只做了结果输出处理，仅仅做个示范。

完整示例： https://github.com/670832188/TestApp

补充：

RequestBody有两个子类：FormBody和MultipartBody

RequestBody--json数据提交

public static final MediaType JSON = MediaType.parse("application/json; charset=utf-8");
OkHttpClient client = new OkHttpClient();
String post(String url, String json) throws IOException {
     RequestBody body = RequestBody.create(JSON, json);
      Request request = new Request.Builder()
      .url(url)
      .post(body)
      .build();
//同步
      Response response = client.newCall(request).execute();
    f (response.isSuccessful()) {
        return response.body().string();
    } else {
        throw new IOException("Unexpected code " + response);
    }
}

FromBody---表单提交这种能满足大部分的需求

FromBody用于提交表单键值对,key-value,其作用类似于HTML中的<form>标记。比如username="LHX",age="21"等类似的键值对

MultipartBody---文件上传

MultipartBody可以构建与HTML文件上传格式兼容的复杂请求体。

图片下载，文件下载

图片下载是通过回调的Response拿到byte[]然后decode成图片；
文件下载，就是拿到inputStream做写文件操作；

详解Retrofit Interceptor(拦截器) 拦截请求并做相关处理

RequestBody--json数据提交

FromBody---表单提交这种能满足大部分的需求

MultipartBody---文件上传

图片下载，文件下载

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MultipartBody---文件上传

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