1、Activity

import android.Manifest;
import android.content.pm.PackageManager;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.v4.app.ActivityCompat;
import android.support.v4.content.ContextCompat;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.Toast;

import com.bumptech.glide.Glide;
import com.google.gson.Gson;

import java.io.File;
import java.io.IOException;

import io.reactivex.Observer;
import io.reactivex.android.schedulers.AndroidSchedulers;
import io.reactivex.disposables.Disposable;
import io.reactivex.schedulers.Schedulers;
import okhttp3.Call;
import okhttp3.Callback;
import okhttp3.MediaType;
import okhttp3.MultipartBody;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.RequestBody;
import okhttp3.Response;
import retrofit2.Retrofit;
import retrofit2.adapter.rxjava2.RxJava2CallAdapterFactory;
import retrofit2.converter.gson.GsonConverterFactory;

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    private static final String TAG = "MainActivity";

    private Button btn;
    private ImageView img;
    File file = new File("/storage/sdcard0/bb.jpg");

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initView();
    }

    private void initView() {
        btn = (Button) findViewById(R.id.btn);
        img = (ImageView) findViewById(R.id.img);

        btn.setOnClickListener(this);
    }

    @Override
    public void onClick(View v) {
        switch (v.getId()) {
            case R.id.btn:
                initData();
                break;
        }
    }

    private void initData() {
        if (ContextCompat.checkSelfPermission(MainActivity.this, Manifest.permission.READ_EXTERNAL_STORAGE)== PackageManager.PERMISSION_GRANTED){
//            uploadFile();
            uploadFileByRetrofit();
        }else {
            ActivityCompat.requestPermissions(MainActivity.this,new String[]{Manifest.permission.READ_EXTERNAL_STORAGE},1);
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        if (requestCode==1&&grantResults[0]==PackageManager.PERMISSION_GRANTED){
//            uploadFile();
            uploadFileByRetrofit();
        }else {
            Toast.makeText(this, "授权失败", Toast.LENGTH_SHORT).show();
        }
    }

    private void uploadFile() {

        OkHttpClient okHttpClient = new OkHttpClient.Builder()
                .build();

        RequestBody requestBody = RequestBody.create(MediaType.parse("image/jpg"), file);

        MultipartBody multipartBody = new MultipartBody.Builder()
                .setType(MultipartBody.FORM)
                .addFormDataPart("key", "H1808C")
                .addFormDataPart("file", file.getName(),requestBody)
                .build();

        Request request = new Request.Builder()
                .url("http://yun918.cn/study/public/file_upload.php")
                .post(multipartBody)
                .addHeader("multipart","form-data")
                .build();

        Call call = okHttpClient.newCall(request);

        call.enqueue(new Callback() {
            @Override
            public void onFailure(Call call, IOException e) {
                Log.e(TAG, "onFailure: " + e.getMessage());
            }

            @Override
            public void onResponse(Call call, Response response) throws IOException {
                String string = response.body().string();
                Gson gson = new Gson();
                UploadBean uploadBean = gson.fromJson(string, UploadBean.class);
                final UploadBean.DataBean data = uploadBean.getData();
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        Glide.with(MainActivity.this).load(data.getUrl()).into(img);
                    }
                });
                Log.e(TAG, "onResponse: " + string);
            }
        });
    }

    private void uploadFileByRetrofit() {
        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(HttpService.url)
                .addConverterFactory(GsonConverterFactory.create())
                .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
                .build();

        HttpService httpService = retrofit.create(HttpService.class);

        RequestBody body = RequestBody.create(MediaType.parse("application/octet-stream"), "H1808C");

        RequestBody requestBody = RequestBody.create(MediaType.parse("image/jpg"), file);

        MultipartBody.Part file = MultipartBody.Part.createFormData("file", this.file.getName(), requestBody);

        httpService.getData(body,file)
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(new Observer<UploadBean>() {
                    @Override
                    public void onSubscribe(Disposable d) {

                    }

                    @Override
                    public void onNext(UploadBean uploadBean) {
                        String url = uploadBean.getData().getUrl();
                        Log.e(TAG, "onNext: " + url);
                        Glide.with(MainActivity.this).load(url).into(img);
                    }

                    @Override
                    public void onError(Throwable e) {
                        Log.e(TAG, "onError: " + e.getMessage());
                    }

                    @Override
                    public void onComplete() {

                    }
                });
    }
}

2、HttpService接口

import io.reactivex.Observable;
import okhttp3.MultipartBody;
import okhttp3.RequestBody;
import retrofit2.http.Multipart;
import retrofit2.http.POST;
import retrofit2.http.Part;

public interface HttpService {
    String url="http://yun918.cn/study/public/";
    @POST("file_upload.php")
    @Multipart
    Observable<UploadBean> getData(@Part("key")RequestBody requestBody,@Part MultipartBody.Part file);
}