OkHttp之ApplicationInterceptors与NetworkInterceptors

An HTTP & HTTP/2 client for Android and Java applications

OKHttp对于安卓童鞋来说已经非常熟悉，几乎天天都会与之打交道。Server端虽然用的最多的还是Apache的HttpClient，但OKHttp以其简洁、方便的API也受到越来越多童鞋的关注。

言归正传，这里聊一聊okhttp的interceptors.

做java的童鞋应该对拦截器再熟悉不过，细心的童鞋可能发现，okhttp interceptors分两种类型：Application Interceptors、Network Interceptors，如何理解这两种拦截器，我们先看一张图片：

interceptor

我们将一次HTTP请求类比为一次明信片邮寄，Application Interceptors发生在将明信片投入邮筒的前后，Network Interceptors发生在邮局投送明信片的前后。

可能这样类比不是特别具体，我们以官方wiki中的示例解释。

首先，定义一个日志拦截器，记录一次请求Request及Response的请求内容

class LoggingInterceptor implements Interceptor {
  @Override public Response intercept(Interceptor.Chain chain) throws IOException {
    Request request = chain.request();

    long t1 = System.nanoTime();
    logger.info(String.format("Sending request %s on %s%n%s",
        request.url(), chain.connection(), request.headers()));

    Response response = chain.proceed(request);

    long t2 = System.nanoTime();
    logger.info(String.format("Received response for %s in %.1fms%n%s",
        response.request().url(), (t2 - t1) / 1e6d, response.headers()));

    return response;
  }
}

1. Application Interceptors

将日志拦截器添加为Application Interceptors，并访问http://www.publicobject.com/h...

OkHttpClient client = new OkHttpClient.Builder()
    .addInterceptor(new LoggingInterceptor())
    .build();

Request request = new Request.Builder()
    .url("http://www.publicobject.com/helloworld.txt")
    .header("User-Agent", "OkHttp Example")
    .build();

Response response = client.newCall(request).execute();
response.body().close();

其输出为

INFO: Sending request http://www.publicobject.com/helloworld.txt on null
User-Agent: OkHttp Example

INFO: Received response for https://publicobject.com/helloworld.txt in 1179.7ms
Server: nginx/1.4.6 (Ubuntu)
Content-Type: text/plain
Content-Length: 1759
Connection: keep-alive

从输出结果可以看出，请求(至少)被重定向过一次(请求地址与响应地址不一)，但拦截器只被执行了一次
就好比，寄送给小明的明信片，邮局在投送到小明后被打回，之后又投送给了小李，小李回信给我，但我只关心 寄出明信片、收到回信

2. Network Interceptors

将日志拦截器添加为Network Interceptors，并访问http://www.publicobject.com/h...

OkHttpClient client = new OkHttpClient.Builder()
    .addNetworkInterceptor(new LoggingInterceptor())
    .build();

Request request = new Request.Builder()
    .url("http://www.publicobject.com/helloworld.txt")
    .header("User-Agent", "OkHttp Example")
    .build();

Response response = client.newCall(request).execute();
response.body().close();

其输出为

INFO: Sending request http://www.publicobject.com/helloworld.txt on Connection{www.publicobject.com:80, proxy=DIRECT hostAddress=54.187.32.157 cipherSuite=none protocol=http/1.1}
User-Agent: OkHttp Example
Host: www.publicobject.com
Connection: Keep-Alive
Accept-Encoding: gzip

INFO: Received response for http://www.publicobject.com/helloworld.txt in 115.6ms
Server: nginx/1.4.6 (Ubuntu)
Content-Type: text/html
Content-Length: 193
Connection: keep-alive
Location: https://publicobject.com/helloworld.txt

INFO: Sending request https://publicobject.com/helloworld.txt on Connection{publicobject.com:443, proxy=DIRECT hostAddress=54.187.32.157 cipherSuite=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA protocol=http/1.1}
User-Agent: OkHttp Example
Host: publicobject.com
Connection: Keep-Alive
Accept-Encoding: gzip

INFO: Received response for https://publicobject.com/helloworld.txt in 80.9ms
Server: nginx/1.4.6 (Ubuntu)
Content-Type: text/plain
Content-Length: 1759
Connection: keep-alive

从输出结果可以看出，请求被重定向一次，拦截器被执行两次，每次请求均被记录
就好比，邮局会记录每次投送信息

选择

由此，okhttp给出了几点建议，以帮助开发者在两种拦截器之间选择

Application interceptors

Don't need to worry about intermediate responses like redirects and retries.

Are always invoked once, even if the HTTP response is served from the cache.

Observe the application's original intent. Unconcerned with OkHttp-injected headers like If-None-Match.

Permitted to short-circuit and not call Chain.proceed().

Permitted to retry and make multiple calls to Chain.proceed().

Network Interceptors

Able to operate on intermediate responses like redirects and retries.

Not invoked for cached responses that short-circuit the network.

Observe the data just as it will be transmitted over the network.

Access to the Connection that carries the request.

典型应用

记录日志

相信使用okhttp的童鞋一定都使用过HttpLoggingInterceptor来记录请求日志，同时会将此拦截器添加到Network Interceptors

OkHttpClient.Builder().addNetworkInterceptor(HttpLoggingInterceptor { 
    logger.debug(it) 
}).build()

此目的在于更精准地记录http请求过程

动态添加请求参数

在对接很多第三方应用的时候，都会要求在每次请求中根据请求参数计算签名sign，以防数据篡改。
这里便可以使用拦截器统一为每个请求计算签名sign并添加到请求参数中

OkHttpClient.Builder().addInterceptor(Interceptor {
    val original = it.request()
    val url = original.url().newBuilder().addQueryParameter("sign", sign()).build()
    val requestBuilder = original.newBuilder().url(url)

    it.proceed(requestBuilder.build())
}).build()

这里使用Application Interceptors的目的在于，签名在一次请求中只需要计算一次，同时还可以检查参数
完整性、合法性决定是否拒绝请求。

订阅号