[置顶] Retrofit2 multpart多文件上传详解

Retrofit2是目前很流行的Android网络框架，运用注解和动态代理，极大的简化了网络请求的繁琐步骤，非常适合处理restfull网络请求。在项目中，经常需要上传文件到服务器，有时候是需要上传多个文件。网上文章基本都是单文件上传教程，这篇文章主要讲retrofit的多文件上传实现。 
个人觉得有必要深入理解http协议，这样无论使用哪个网络框架，碰到类似这样上传的问题，一眼就能知道问题出在哪里。因此就有必要了解http协议的上传机制。

了解 multipart/form-data

在最初的http协议中，没有定义上传文件的Method，为了实现这个功能，http协议组改造了post请求，添加了一种post规范，设定这种规范的Content-Type为multipart/form-data;boundary= bound,其中 {bound}是定义的分隔符，用于分割各项内容(文件,key-value对)，不然服务器无法正确识别各项内容。post body里需要用到,尽量保证随机唯一。

post格式如下:

–${bound} 
Content-Disposition: form-data; name=”Filename”

HTTP.pdf 
–${bound} 
Content-Disposition: form-data; name=”file000”; filename=”HTTP协议详解.pdf” 
Content-Type: application/octet-stream

%PDF-1.5 
file content 
%%EOF

–${bound} 
Content-Disposition: form-data; name=”Upload”

Submit Query 
–${bound}–

${bound}是Content-Type里boundary的值

Retrofit2 对multipart/form-data的封装

Retrofit其实是个网络代理框架，负责封装请求，然后把请求分发给http协议具体实现者-httpclient。retrofit默认的httpclient是okhttp。

既然Retrofit不实现http，为啥还用它呢。因为他方便！！ 
Retrofit会根据注解封装网络请求，待httpclient请求完成后，把原始response内容通过转化器(converter)转化成我们需要的对象(object)。

具体怎么使用 retrofit2,请参考: Retrofit2官网

那么Retrofit和okhttp怎么封装这些multipart/form-data上传数据呢

• 在retrofit中: 
  @retrofit2.http.Multipart: 标记一个请求是multipart/form-data类型,需要和@retrofit2.http.POST一同使用，并且方法参数必须是@retrofit2.http.Part注解。 
  @retrofit2.http.Part: 代表Multipart里的一项数据,即用${bound}分隔的内容块。
• 在okhttp3中: 
  okhttp3.MultipartBody: multipart/form-data的抽象封装,继承okhttp3.RequestBody 
  okhttp3.MultipartBody.Part: multipart/form-data里的一项数据。

Service接口定义

假设服务器上传接口返回数据类型为application/json,字段如下

<code class="hljs css has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-rules" style="box-sizing: border-box;">{
<span class="hljs-rule" style="box-sizing: border-box;"><span class="hljs-attribute" style="box-sizing: border-box;">data</span>:<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 102, 102);"> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"上传了3个文件"</span>,
msg: <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"访问成功"</span>,
code: <span class="hljs-number" style="box-sizing: border-box;">200</span>
</span></span></span>}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

因此需要对返回数据封装成一个对象，考虑到复用性,封装成泛型最佳:

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> BaseResponse<T> {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> code;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> String msg;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> T data;
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

接着定义一个上传的网络请求Service:

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">interface</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">FileuploadService</span> {</span>
    <span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**
     * 通过 List<MultipartBody.Part> 传入多个part实现多文件上传
     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> parts 每个part代表一个
     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @return</span> 状态信息
     */</span>
    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Multipart</span>
    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@POST</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"users/image"</span>)
    Call<BaseResponse<String>> uploadFilesWithParts(<span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Part</span>() List<MultipartBody.Part> parts);


    <span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**
     * 通过 MultipartBody和@body作为参数来上传
     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> multipartBody MultipartBody包含多个Part
     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @return</span> 状态信息
     */</span>
    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@POST</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"users/image"</span>)
    Call<BaseResponse<String>> uploadFileWithRequestBody(<span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Body</span> MultipartBody multipartBody);
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li></ul>

由上可知，有两种方式实现上传

• 使用@Multipart注解方法，并用@Part注解方法参数，类型是List< okhttp3.MultipartBody.Part>
• 不使用@Multipart注解方法，直接使用@Body注解方法参数，类型是okhttp3.MultipartBody

可以看到,无论方法参数类型是MultipartBody.Part还是MultipartBody，这些类都不是Retrofit的类，而是okhttp实现上传的源生类。

为什么可以这样写：

• Retrofit会判断@Body的参数类型，如果参数类型为okhttp3.RequestBody,则Retrofit不做包装处理，直接丢给okhttp3处理。而MultipartBody是继承RequestBody，因此Retrofit不会自动包装这个对象。
• 同理，Retrofit会判断@Part的参数类型，如果参数类型为okhttp3.MultipartBody.Part,则Retrofit会把RequestBody封装成MultipartBody，再把Part添加到MultipartBody。

上传多个文件

写好service接口后，来看看怎么构造MultipartBody 
可以写一个方法，用于把File对象转化成MultipartBody:

<code class="hljs avrasm has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">public static MultipartBody filesToMultipartBody(List<File> files) {
        MultipartBody<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.Builder</span> builder = new MultipartBody<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.Builder</span>()<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>

        for (File file : files) {
            // TODO: <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">16</span>-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>  这里为了简单起见，没有判断file的类型 
            RequestBody requestBody = RequestBody<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.create</span>(MediaType<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.parse</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"image/png"</span>), file)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
            builder<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.addFormDataPart</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"file"</span>, file<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.getName</span>(), requestBody)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
        }

        builder<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.setType</span>(MultipartBody<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.FORM</span>)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
        MultipartBody multipartBody = builder<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.build</span>()<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
        return multipartBody<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li></ul>

或者把File转化成MultipartBody.Part:

<code class="hljs avrasm has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">    public static List<MultipartBody<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.Part</span>> filesToMultipartBodyParts(List<File> files) {
        List<MultipartBody<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.Part</span>> parts = new ArrayList<>(files<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.size</span>())<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
        for (File file : files) {
            // TODO: <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">16</span>-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>  这里为了简单起见，没有判断file的类型
            RequestBody requestBody = RequestBody<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.create</span>(MediaType<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.parse</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"image/png"</span>), file)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
            MultipartBody<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.Part</span> part = MultipartBody<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.Part</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.createFormData</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"file"</span>, file<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.getName</span>(), requestBody)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
            parts<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.add</span>(part)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
        }
        return parts<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul>

最后就剩下调用了

为了复用，因此把构建Retrofit简单封装成一个builder类:

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> RetrofitBuilder {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> Retrofit retrofit;

    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> synchronized <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> Retrofit <span class="hljs-title" style="box-sizing: border-box;">buildRetrofit</span>() {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (retrofit == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {
            HttpLoggingInterceptor logging = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> HttpLoggingInterceptor();
            Gson gson = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> GsonBuilder().setDateFormat(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"yyyy-MM-dd HH:mm:ss"</span>).create();
            GsonConverterFactory gsonConverterFactory = GsonConverterFactory.create(gson);
            logging.setLevel(HttpLoggingInterceptor.Level.BODY);
            OkHttpClient client = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> OkHttpClient.Builder()
                    .addInterceptor(logging).retryOnConnectionFailure(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>)
                    .build();
            retrofit = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Retrofit.Builder().client(client)
                    .baseUrl(Config.NetURL.BASE_URL)
                    .addConverterFactory(gsonConverterFactory)
                    .addCallAdapterFactory(RxJavaCallAdapterFactory.create())
                    .build();
        }
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> retrofit;
    }
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li></ul>

接着可以在activity里调用FileUploadService的接口了:

<code class="hljs avrasm has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">    private void uploadFile() {
        MultipartBody body = MultipartBuilder<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.filesToMultipartBody</span>(mFileList)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>

        RetrofitBuilder<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.buildRetrofit</span>()<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.create</span>(FileUploadService<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.class</span>)<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.uploadFileWithRequestBody</span>(body)
        <span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.enqueue</span>(new Callback<BaseResponse<String>>() {
            @Override
            public void onResponse(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">Call</span><BaseResponse<String>> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">call</span>, Response<BaseResponse<String>> response) {

                if (response<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.isSuccessful</span>()) {
                    BaseResponse<String> body = response<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.body</span>()<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
                    Toast<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.makeText</span>(LoginActivity<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.this</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"上传成功:"</span>+response<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.body</span>()<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.getMsg</span>(), Toast<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.LENGTH</span>_SHORT)<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.show</span>()<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
                } else {
                        Log<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.d</span>(TAG,<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"上传失败"</span>)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
                        Toast<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.makeText</span>(RegisterActivity<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.this</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"上传失败"</span>, Toast<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.LENGTH</span>_SHORT)<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.show</span>()<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
                }
            }

            @Override
            public void onFailure(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">Call</span><BaseResponse<String>> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">call</span>, Throwable t) {
                Toast<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.makeText</span>(RegisterActivity<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.this</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"网络连接失败"</span>, Toast<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.LENGTH</span>_SHORT)<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.show</span>()<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
            }
        })<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li></ul>

转自：http://www.chenkaihua.com/2016/04/02/retrofit2-upload-multipart-files.html