创建接口ServerApi

public interface MyServerApi {

    @POST("/upload")

    public upload(@Query("usrid") String usrid, @Body RequestBody pcmBody);

}

1. 使用retrofit创建一个MyServerApi对象

        Retrofit retrofit = new Retrofit.Builder().addConverterFactory(GsonConverterFactory.create()).baseUrl("http://192.168.1.20:8080").build();
        MyServerApi myServerApi = retrofit.create(MyServerApi.class);

2. 自定义一个RequestBody的子类FileRequestBody

/**
 * author : Richile
 * date   : 2017/3/29
 * email  : ruichao1001@foxmail.com
 */

public class FileRequestBody extends RequestBody{

    private final static String TAG = FileRequestBody.class.getSimpleName();
    /* 上传文件的类型 */
    private MediaType mediaType;
    /* 需要上传的文件 */
    private File file;
    /* 已经发送的字节数 */
    private long writen = 0L;

    /**
     * 构造FileRequestBody
     * @param mediaType     上传文件的类型
     * @param file          上传的文件
     */
    public FileRequestBody(MediaType mediaType, File file) {
        this.mediaType = mediaType;
        this.file = file;
    }

    @Override
    public long contentLength() throws IOException {
        return file.length();
    }

    @Override
    public MediaType contentType() {
        return mediaType;
    }

    @Override
    public void writeTo(BufferedSink sink) throws IOException {
        BufferedSource bufferedSource = Okio.buffer(Okio.source(file));

        byte[] buffer = new byte[8192];
        long len = 0;

        /**
         * 从文件一次读取8k字节，发送给服务器
         */
        while((len = bufferedSource.read(buffer)) > 0){
            sink.write(buffer, 0, (int) len);
            writen += len;
            /* 这里就可以写一个回调接口，用户监听进度 */
            Log.d(TAG, "writeTo: progress=" + (int)((writen/(float)contentLength()) * 100) + "%");
        }
    }


}

然后就可以使用retrofit上传文件了

        /* 创建一个用于传输文件的RequestBody，使用POST方式发送出去 */
        FileRequestBody fileRequestBody = new FileRequestBody(MediaType.parse("audio/pcm;rate=16000"), pcmFile);

        retrofit2.Call<UploadResult> call = smartServerApi.upload(usrid, fileRequestBody);
        call.enqueue(new retrofit2.Callback<UploadResult>() {
            @Override
            public void onResponse(retrofit2.Call<UploadResult> call, retrofit2.Response<UploadResult> response) {
                if(response.isSuccessful()){
                    UploadResult result = response.body();
                }else{
                    Log.d(TAG, "Upload onResponse: " + response.code() + ":" + response.message());
                }
            }

            @Override
            public void onFailure(retrofit2.Call<UploadResult> call, Throwable t) {
                Log.d(TAG, "Upload onFailure: " + t.getMessage());
            }
        });