拦截器(Interceptors)¶

拦截器是一种强大的机制，可以监视、重写和重试调用。这是一个简单的拦截器，它记录传出请求和传入响应。

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;
  }
}

对chain.proceed(request)的调用是每个拦截器实现的关键部分。这种看起来简单的方法是所有HTTP工作发生的地方，产生满足请求的响应。如果chain.proceed(request)被多次调用，则必须关闭之前的响应Body。

拦截器可以是链式的。假设你同时拥有一个压缩拦截器和一个校验和拦截器：你需要确定数据是先压缩后校验，还是先校验后压缩。 OkHttp使用列表来跟踪拦截器，拦截器是按顺序调用的。

Interceptors Diagram

应用拦截器¶

拦截器注册为_application_或_network_拦截器。我们将使用上面定义的LoggingInterceptor 来显示差异。

在OkHttpClient.Builder上调用addInterceptor()注册an_app_interceptor：

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();

The URL http://www.publicobject.com/helloworld.txt redirects to https://publicobject.com/helloworld.txt, and OkHttp follows this redirect automatically. Our application interceptor is called once and the response returned from chain.proceed() has the redirected response:

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

我们可以看到我们被重定向了，因为 response.request().url() 与 request.url() 不同。这两条LOG语句记录两个不同的URL。

网络拦截器¶

注册网络拦截器非常类似。调用 addNetworkInterceptor() 而不是 addInterceptor() :

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();

当我们运行这段代码时，拦截器运行两次。一次是初始请求到 http://www.publicobject.com/helloworld.txt，另一次是重定向到 https://publicobject.com/helloworld.txt。

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

网络请求也包含更多的数据，比如OkHttp添加了Accept-Ending: gzip头部来宣传支持响应压缩。网络拦截器的 Chain 具有非空的 Connection，可用于询问用于连接到web服务器的IP地址和TLS配置。

在应用程序和网络拦截器之间选择¶

每个拦截器链都有相对的优点。

应用拦截器

无需担心诸如重定向和重试之类的中间响应。
始终调用一次，即使从缓存提供HTTP响应也是如此。
遵守应用程序的原始意图。与OkHttp注入的标头无关，如 “If-None-match”。
允许短路行为，不调用Chain.Progress()。
允许重试并多次调用 Chain.proceed()。
可以使用withConnectTimeout、withReadTimeout、withWriteTimeout调整调用超时。

**网络拦截器 **

能够对重定向和重试等中间响应进行操作。
不会对网络短路的缓存响应调用。
观察数据时，就像数据将通过网络传输一样。
访问承载请求的Connection。

重写请求¶

拦截器可以添加、删除或替换请求头。它们还可以转换具有此类请求的正文。例如，如果你正在连接到已知支持它的web服务器，则可以使用应用程序拦截器添加请求主体压缩。

/** This interceptor compresses the HTTP request body. Many webservers can't handle this! */
final class GzipRequestInterceptor implements Interceptor {
  @Override public Response intercept(Interceptor.Chain chain) throws IOException {
    Request originalRequest = chain.request();
    if (originalRequest.body() == null || originalRequest.header("Content-Encoding") != null) {
      return chain.proceed(originalRequest);
    }

    Request compressedRequest = originalRequest.newBuilder()
        .header("Content-Encoding", "gzip")
        .method(originalRequest.method(), gzip(originalRequest.body()))
        .build();
    return chain.proceed(compressedRequest);
  }

  private RequestBody gzip(final RequestBody body) {
    return new RequestBody() {
      @Override public MediaType contentType() {
        return body.contentType();
      }

      @Override public long contentLength() {
        return -1; // We don't know the compressed length in advance!
      }

      @Override public void writeTo(BufferedSink sink) throws IOException {
        BufferedSink gzipSink = Okio.buffer(new GzipSink(sink));
        body.writeTo(gzipSink);
        gzipSink.close();
      }
    };
  }
}

重写响应¶

对称地，拦截器可以重写响应头以及转换响应体。这通常比重写请求头更危险，因为它可能会违反Web服务器的期望！

如果你处于必要的情况并准备应对后果，那么重写响应头是解决问题的强大方法。例如，你可以修复服务器错误配置的Cache-Control响应头，以实现更好的响应缓存：

/** Dangerous interceptor that rewrites the server's cache-control header. */
private static final Interceptor REWRITE_CACHE_CONTROL_INTERCEPTOR = new Interceptor() {
  @Override public Response intercept(Interceptor.Chain chain) throws IOException {
    Response originalResponse = chain.proceed(chain.request());
    return originalResponse.newBuilder()
        .header("Cache-Control", "max-age=60")
        .build();
  }
};

通常，这种方法在补充网络服务器上的相应修复时效果最好!