OkHttp是一个优秀的网络请求框架，我开始接触Android开发的时候就用过这个网络请求框架，官方的参考文档在这里 https://square.github.io/okhttp/ ，本文演示了使用OkHttp框架进行简单的Get、Post（表单形式和JSON形式）、Delete、附加请求头、请求异步回调，文件的上传和下载等常用操作。最后记录了一个今天调试了很久的坑，那就是response.body().string()只能有效调用一次，注意Debug的时候对结果造成的改变！JDK9的HttpURLConnection有很多变化，下次博客中会有演示和说明。

首选引入这个依赖就可以使用OkHTTP了

<dependency>
    <groupId>com.squareup.okhttp3</groupId>
    <artifactId>okhttp</artifactId>
    <version>3.6.0</version>
</dependency>

<!-- 这个是我演示Post请求JSON格式的时候用到的 -->
<dependency>
    <groupId>com.alibaba</groupId>
    <artifactId>fastjson</artifactId>
    <version>1.2.47</version>
</dependency>

如果是 gradle 管理的项目，则只需要引入：

1	compile 'com.squareup.okhttp3:okhttp:3.6.0'

1、Get请求示例

String url = "http://zouchanglin.cn";
OkHttpClient okHttpClient = new OkHttpClient();
Request request = new Request.Builder().get().url(url).build();
try {
    Response execute = okHttpClient.newCall(request).execute();
    System.out.println(execute.body().string());
} catch (IOException e) {
    e.printStackTrace();
}

2、Post请求示例

1、JSON请求

public static void main(String[] args) {
    String url = "http://zouchanglin.cn/info/create";
    OkHttpClient okHttpClient = new OkHttpClient();
    Map<String, Object> map = new HashMap<>();
    map.put("name", "Mike");
    map.put("age", "20");
    RequestBody requestBody = RequestBody.create(
        MediaType.parse("application/json; charset=utf-8"),
        JSONObject.toJSONString(map));
    Request request = new Request.Builder().post(requestBody).url(url).build();
    try {
        Response response = okHttpClient.newCall(request).execute();
        System.out.println(response.body().string());
    } catch (IOException e) {
        e.printStackTrace();
    }
}

2、表单数据

public static void main(String[] args) {
    String url = "http://zouchanglin.cn/info/create";
    OkHttpClient okHttpClient = new OkHttpClient();
    RequestBody requestBody = new FormBody.Builder()
        .add("name", "Mike")
        .add("age", "20").build();
    Request request = new Request.Builder().post(requestBody).url(url).build();
    try {
        Response response = okHttpClient.newCall(request).execute();
        System.out.println(response.body().string());
    } catch (IOException e) {
        e.printStackTrace();
    }
}

3、Delete请求示例

public static void main(String[] args) {
    String url = "http://zouchanglin.cn/info/remove";
    OkHttpClient okHttpClient = new OkHttpClient();
    RequestBody requestBody = new FormBody.Builder()
        .add("id", "001002003004").build();
    Request request = new Request.Builder()
        .delete(requestBody).url(url).build();
    try {
        Response response = okHttpClient.newCall(request).execute();
        System.out.println(response.body().string());
    } catch (IOException e) {
        e.printStackTrace();
    }
}

4、附加请求头示例

public static void main(String[] args) {
    String url = "http://zouchanglin.cn/info/remove";
    OkHttpClient okHttpClient = new OkHttpClient();
    RequestBody requestBody = new FormBody.Builder()
        .add("id", "001002003004").build();
    Request request = new Request.Builder()
        .delete(requestBody)
        .addHeader("Accept", "application/vnd..")
        .url(url).build();
    try {
        Response response = okHttpClient.newCall(request).execute();
        System.out.println(response.body().string());
    } catch (IOException e) {
        e.printStackTrace();
    }
}

5、请求异步回调示例

public static void main(String[] args) {
    String url = "http://zouchanglin.cn/info/remove";
    OkHttpClient okHttpClient = new OkHttpClient();
    RequestBody requestBody = new FormBody.Builder()
        .add("id", "0010003004").build();
    Request request = new Request.Builder().delete(requestBody).url(url).build();
    Call call = okHttpClient.newCall(request);
    call.enqueue(new Callback() {
        @Override
        public void onFailure(Call call, IOException e) {
            //TODO...
        }

        @Override
        public void onResponse(Call call, Response response) throws IOException {
            //TODO...
        }
    });
}

6、上传文件加参数示例

@PostMapping("upload")
public String uploadToRemoteHost(String ip, String path, String fileId) {
    String url = String.format("http://%s:8080//api/host/file/create/", ip);
    //找到文件对象
    Optional<ImageFile> bigFileById = fileService.getBigFileById(fileId);
    if(bigFileById.isPresent()){
        ImageFile imageFile = bigFileById.get();
        OkHttpClient okHttpClient = new OkHttpClient();
        MultipartBody.Builder requestBody = new MultipartBody.Builder();
        requestBody.setType(MultipartBody.FORM);
        RequestBody body = RequestBody.create(
            MediaType.parse("application/octet-stream"), 
            imageFile.getContent().getData());
        
        // 参数分别为 请求key 文件名称 RequestBody
        requestBody.addFormDataPart("file", imageFile.getName(), body);
        //要上传的文字参数
        Map<String, String> map = new HashMap<>();
        map.put("name", imageFile.getName());
        map.put("path", path);
        for (String key : map.keySet()) {
            requestBody.addFormDataPart(key, map.get(key));
        }
        MultipartBody build = requestBody.build();
        try {
            Request request = new Request.Builder().post(build).url(url).build();
            Response execute = okHttpClient.newCall(request).execute();
            if(execute.isSuccessful()){
                return execute.body().string();
            }
            return JSONObject.toJSONString(ResultVOUtil.error(1, "网络错误"));
        } catch (IOException e) {
            e.printStackTrace();
            return JSONObject.toJSONString(ResultVOUtil.error(2, "网络错误2"));
        }
    }
    return JSONObject.toJSONString(ResultVOUtil.error(3, "文件不存在"));
}

7、下载文件示例

public void downloadImg(View view){
    OkHttpClient client = new OkHttpClient();
    final Request request = new Request.Builder().get()
            .url("http://wwwx.yyy/a.png")
            .build();
    Call call = client.newCall(request);
    call.enqueue(new Callback() {
        @Override
        public void onFailure(Call call, IOException e) {
            Log.e("moer", "onFailure: ");;
        }

        @Override
        public void onResponse(Call call, Response response) throws IOException {
            //拿到字节流
            InputStream is = response.body().byteStream();

            int len = 0;
            File file  = new File(Environment.getExternalStorageDirectory(), "n.png");
            FileOutputStream fos = new FileOutputStream(file);
            byte[] buf = new byte[128];

            while ((len = is.read(buf)) != -1){
                fos.write(buf, 0, len);
            }

            fos.flush();
            //关闭流
            fos.close();
            is.close();
        }
    });
}

8、OkHttp的坑

OkHttp请求回调中response.body().string()只能有效调用一次，调用response.body().string()的时候数据流已经关闭了，再次调用就是提示已经closed，抛出java.lang.IllegalStateException: closed异常，所以这个坑还是有点大，我在debug的时候由于已经监视了一次response.body().string()的返回值，在代码中跑完就是IllegalStateException。调试代码调试时，表达式的监视有时候会影响代码的运行，比如就像OkHttp这种情况。

Tim's Blog

OkHttp的使用与跳坑示例