An HTTP & HTTP/2 client for Android and Java applications
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，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 分两种类型：、，如何理解这两种拦截器，我们先看一张图片：
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
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 nullUser-Agent: OkHttp ExampleINFO: Received response for https://publicobject.com/helloworld.txt in 1179.7msServer: nginx/1.4.6 (Ubuntu)Content-Type: text/plainContent-Length: 1759Connection: 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
2. Network Interceptors
将日志拦截器添加为Network Interceptors，并访问
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 ExampleHost: www.publicobject.comConnection: Keep-AliveAccept-Encoding: gzipINFO: Received response for http://www.publicobject.com/helloworld.txt in 115.6msServer: nginx/1.4.6 (Ubuntu)Content-Type: text/htmlContent-Length: 193Connection: keep-aliveLocation: https://publicobject.com/helloworld.txtINFO: 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 ExampleHost: publicobject.comConnection: Keep-AliveAccept-Encoding: gzipINFO: Received response for https://publicobject.com/helloworld.txt in 80.9msServer: nginx/1.4.6 (Ubuntu)Content-Type: text/plainContent-Length: 1759Connection: 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
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。