深入理解OkHttp3：（四）常用事例Demo

这里汇总编写了一些演示如何使用OkHttp解决常见问题的方法。通读它们，了解它们是如何协同工作的。可以在需要的时候，随意剪切粘贴这些示例;

1.同步GET（Synchronous GET）

下载一个文件，打印它的标题，并打印它的响应主体作为字符串。

响应体（Body）上的string()方法对于小文档来说既方便又高效。但是，如果响应主体很大(大于1 MiB)，请避免string()，因为它将把整个文档加载到内存中。在这种情况下，更喜欢把身体当作一个流来处理。

import java.io.IOException;
import okhttp3.Headers;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

public final class SynchronousGet {
  private final OkHttpClient client = new OkHttpClient();

  public void run() throws Exception {
    Request request = new Request.Builder()
        .url("https://publicobject.com/helloworld.txt")
        .build();

    try (Response response = client.newCall(request).execute()) {
      if (!response.isSuccessful()) throw new IOException("Unexpected code " + response);

      Headers responseHeaders = response.headers();
      for (int i = 0; i < responseHeaders.size(); i++) {
        System.out.println(responseHeaders.name(i) + ": " + responseHeaders.value(i));
      }

      System.out.println(response.body().string());
    }
  }

  public static void main(String... args) throws Exception {
    new SynchronousGet().run();
  }
}

2.异步GET（Asynchronous GET）

在工作线程上下载文件，并在响应可读时被调用。回调是在响应Header就绪后进行的。读取响应体仍然可能阻塞。OkHttp目前不提供异步api分段来接收响应体。

import java.io.IOException;
import okhttp3.Call;
import okhttp3.Callback;
import okhttp3.Headers;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;
import okhttp3.ResponseBody;

public final class AsynchronousGet {
  private final OkHttpClient client = new OkHttpClient();

  public void run() throws Exception {
    Request request = new Request.Builder()
        .url("http://publicobject.com/helloworld.txt")
        .build();

    client.newCall(request).enqueue(new Callback() {
      @Override public void onFailure(Call call, IOException e) {
        e.printStackTrace();
      }

      @Override public void onResponse(Call call, Response response) throws IOException {
        try (ResponseBody responseBody = response.body()) {
          if (!response.isSuccessful()) throw new IOException("Unexpected code " + response);

          Headers responseHeaders = response.headers();
          for (int i = 0, size = responseHeaders.size(); i < size; i++) {
            System.out.println(responseHeaders.name(i) + ": " + responseHeaders.value(i));
          }

          System.out.println(responseBody.string());
        }
      }
    });
  }

  public static void main(String... args) throws Exception {
    new AsynchronousGet().run();
  }
}

3.访问Headers(Accessing Headers)

通常HTTP报头的工作方式类似于Map<String, String>:每个字段都有一个值或没有。但有些标头允许多个值，比如Guava的Multimap。例如，HTTP响应提供多个不同的Headers是合法和常见的。OkHttp的api试图让所有情况都变得简单易处理。

在编写请求Headers时，使用Headers(名称、值)方法将唯一出现的名称设置为值。如果存在现有值，则在添加新值之前将删除它们。使用addHeader(名称、值)方法添加标题，而不删除已经出现的标题。

在读取响应Headers时，使用header(name)方法返回最后一次出现的value。通常这也是唯一可能发生的情况！如果不存在值，header(name)方法将返回null。要将字段的所有值作为列表读取，请使用headers(name)方法。

要访问所有Header，请使用支持按索引访问的Headers类。

import java.io.IOException;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

public final class AccessHeaders {
  private final OkHttpClient client = new OkHttpClient();

  public void run() throws Exception {
    Request request = new Request.Builder()
        .url("https://api.github.com/repos/square/okhttp/issues")
        .header("User-Agent", "OkHttp Headers.java")
        .addHeader("Accept", "application/json; q=0.5")
        .addHeader("Accept", "application/vnd.github.v3+json")
        .build();

    try (Response response = client.newCall(request).execute()) {
      if (!response.isSuccessful()) throw new IOException("Unexpected code " + response);

      System.out.println("Server: " + response.header("Server"));
      System.out.println("Date: " + response.header("Date"));
      System.out.println("Vary: " + response.headers("Vary"));
    }
  }

  public static void main(String... args) throws Exception {
    new AccessHeaders().run();
  }
}

4.用POST发送一个字符串（Posting a String）

使用HTTP POST向服务端发送请求体。这个示例将标记文档发送给使用HTML呈现的web服务端。同时因为整个请求主体在内存中，所以避免使用此API发送较大(大于1 MiB)的文档。

import java.io.IOException;
import okhttp3.MediaType;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.RequestBody;
import okhttp3.Response;

public final class PostString {
  public static final MediaType MEDIA_TYPE_MARKDOWN
      = MediaType.get("text/x-markdown; charset=utf-8");

  private final OkHttpClient client = new OkHttpClient();

  public void run() throws Exception {
    String postBody = ""
        + "Releases\n"
        + "--------\n"
        + "\n"
        + " * _1.0_ May 6, 2013\n"
        + " * _1.1_ June 15, 2013\n"
        + " * _1.2_ August 11, 2013\n";

    Request request = new Request.Builder()
        .url("https://api.github.com/markdown/raw")
        .post(RequestBody.create(MEDIA_TYPE_MARKDOWN, postBody))
        .build();

    try (Response response = client.newCall(request).execute()) {
      if (!response.isSuccessful()) throw new IOException("Unexpected code " + response);

      System.out.println(response.body().string());
    }
  }

  public static void main(String... args) throws Exception {
    new PostString().run();
  }
}

5.发送流（Post Streaming）

在这里，我们将请求主体作为流发送给服务端。这个请求主体的内容在编写时生成。这个示例直接流到Okio缓冲接收器中。您的程序可能更喜欢OutputStream，您可以从BufferedSink.outputStream()中获得它。

import java.io.IOException;
import okhttp3.MediaType;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.RequestBody;
import okhttp3.Response;
import okio.BufferedSink;

public final class PostStreaming {
  public static final MediaType MEDIA_TYPE_MARKDOWN
      = MediaType.get("text/x-markdown; charset=utf-8");

  private final OkHttpClient client = new OkHttpClient();

  public void run() throws Exception {
    RequestBody requestBody = new RequestBody() {
      @Override public MediaType contentType() {
        return MEDIA_TYPE_MARKDOWN;
      }

      @Override public void writeTo(BufferedSink sink) throws IOException {
        sink.writeUtf8("Numbers\n");
        sink.writeUtf8("-------\n");
        for (int i = 2; i <= 997; i++) {
          sink.writeUtf8(String.format(" * %s = %s\n", i, factor(i)));
        }
      }

      private String factor(int n) {
        for (int i = 2; i < n; i++) {
          int x = n / i;
          if (x * i == n) return factor(x) + " × " + i;
        }
        return Integer.toString(n);
      }
    };

    Request request = new Request.Builder()
        .url("https://api.github.com/markdown/raw")
        .post(requestBody)
        .build();

    try (Response response = client.newCall(request).execute()) {
      if (!response.isSuccessful()) throw new IOException("Unexpected code " + response);

      System.out.println(response.body().string());
    }
  }

  public static void main(String... args) throws Exception {
    new PostStreaming().run();
  }
}

6.用Post方法发送一个文件（Posting a file）

用一个文件来作为请求body，是一件很简单的事情。

import java.io.File;
import java.io.IOException;
import okhttp3.MediaType;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.RequestBody;
import okhttp3.Response;

public final class PostFile {
  public static final MediaType MEDIA_TYPE_MARKDOWN
      = MediaType.get("text/x-markdown; charset=utf-8");

  private final OkHttpClient client = new OkHttpClient();

  public void run() throws Exception {
    File file = new File("README.md");

    Request request = new Request.Builder()
        .url("https://api.github.com/markdown/raw")
        .post(RequestBody.create(MEDIA_TYPE_MARKDOWN, file))
        .build();

    try (Response response = client.newCall(request).execute()) {
      if (!response.isSuccessful()) throw new IOException("Unexpected code " + response);

      System.out.println(response.body().string());
    }
  }

  public static void main(String... args) throws Exception {
    new PostFile().run();
  }
}

7.用post发送表单数据到服务端口（Posting form parameters）

使用FormBody.Builder构建器来构建一个类似于HTML <form>标记的请求体。名称和值将使用html兼容的表单URL编码进行编码。

import java.io.IOException;
import okhttp3.FormBody;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.RequestBody;
import okhttp3.Response;

public final class PostForm {
  private final OkHttpClient client = new OkHttpClient();

  public void run() throws Exception {
    RequestBody formBody = new FormBody.Builder()
        .add("search", "Jurassic Park")
        .build();
    Request request = new Request.Builder()
        .url("https://en.wikipedia.org/w/index.php")
        .post(formBody)
        .build();

    try (Response response = client.newCall(request).execute()) {
      if (!response.isSuccessful()) throw new IOException("Unexpected code " + response);

      System.out.println(response.body().string());
    }
  }

  public static void main(String... args) throws Exception {
    new PostForm().run();
  }
}

8.发布MultipartBody请求（Posting a multipartBody request）

MultipartBody.Builder构造器可以构建与HTML文件上传表单兼容的复杂请求体。多部分请求体的每个部分本身就是请求体，可以定义自己的头。如果存在，这些header应该描述对应部分主体，例如其内容配置。如果内容长度和内容类型header可用，则会自动添加它们。

import java.io.File;
import java.io.IOException;
import okhttp3.MediaType;
import okhttp3.MultipartBody;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.RequestBody;
import okhttp3.Response;

public final class PostMultipart {
  /**
   * The imgur client ID for OkHttp recipes. If you're using imgur for anything other than running
   * these examples, please request your own client ID! https://api.imgur.com/oauth2
   */
  private static final String IMGUR_CLIENT_ID = "9199fdef135c122";
  private static final MediaType MEDIA_TYPE_PNG = MediaType.get("image/png");

  private final OkHttpClient client = new OkHttpClient();

  public void run() throws Exception {
    // Use the imgur image upload API as documented at https://api.imgur.com/endpoints/image
    RequestBody requestBody = new MultipartBody.Builder()
        .setType(MultipartBody.FORM)
        .addFormDataPart("title", "Square Logo")
        .addFormDataPart("image", "logo-square.png",
            RequestBody.create(MEDIA_TYPE_PNG, new File("website/static/logo-square.png")))
        .build();

    Request request = new Request.Builder()
        .header("Authorization", "Client-ID " + IMGUR_CLIENT_ID)
        .url("https://api.imgur.com/3/image")
        .post(requestBody)
        .build();

    try (Response response = client.newCall(request).execute()) {
      if (!response.isSuccessful()) throw new IOException("Unexpected code " + response);

      System.out.println(response.body().string());
    }
  }

  public static void main(String... args) throws Exception {
    new PostMultipart().run();
  }
}

9.用Moshi解析JSON响应

Moshi是一个方便的API，用于在JSON和Java对象之间进行转换。这里，我们使用它从GitHub API解码JSON响应。

注意，ResponseBody.charStream()使用Content-Type响应头来确定解码响应主体时使用的字符集。如果没有指定字符集，则默认为UTF-8。

import com.squareup.moshi.JsonAdapter;
import com.squareup.moshi.Moshi;
import java.io.IOException;
import java.util.Map;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

public final class ParseResponseWithMoshi {
  private final OkHttpClient client = new OkHttpClient();
  private final Moshi moshi = new Moshi.Builder().build();
  private final JsonAdapter<Gist> gistJsonAdapter = moshi.adapter(Gist.class);

  public void run() throws Exception {
    Request request = new Request.Builder()
        .url("https://api.github.com/gists/c2a7c39532239ff261be")
        .build();
    try (Response response = client.newCall(request).execute()) {
      if (!response.isSuccessful()) throw new IOException("Unexpected code " + response);

      Gist gist = gistJsonAdapter.fromJson(response.body().source());

      for (Map.Entry<String, GistFile> entry : gist.files.entrySet()) {
        System.out.println(entry.getKey());
        System.out.println(entry.getValue().content);
      }
    }
  }

  static class Gist {
    Map<String, GistFile> files;
  }

  static class GistFile {
    String content;
  }

  public static void main(String... args) throws Exception {
    new ParseResponseWithMoshi().run();
  }
}

10.Responses缓存

要缓存响应，您需要一个可以读写的缓存目录，以及对缓存大小的限制。缓存目录应该是私有的，不受信任的应用程序不应该能够读取它的内容!

让多个缓存同时访问同一个缓存目录是错误的。大多数应用程序应该只调用一次新的OkHttpClient()，使用它们的缓存配置它，并在任何地方使用相同的实例。否则，这两个缓存实例将相互参杂在一起，破坏响应缓存，并可能导致程序崩溃。

响应缓存对所有配置使用HTTP Header。你可以添加像cache-control: max-stale=3600这样的请求header，OkHttp的缓存将会支持它们。您的服务端通过它自己的响应header来配置缓存响应的时间，比如Cache-Control: max-age=9600。缓存header用于强制缓存响应、强制网络响应或强制使用条件GET验证网络响应。

import java.io.File;
import java.io.IOException;
import okhttp3.Cache;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

public final class CacheResponse {
  private final OkHttpClient client;

  public CacheResponse(File cacheDirectory) throws Exception {
    int cacheSize = 10 * 1024 * 1024; // 10 MiB
    Cache cache = new Cache(cacheDirectory, cacheSize);

    client = new OkHttpClient.Builder()
        .cache(cache)
        .build();
  }

  public void run() throws Exception {
    Request request = new Request.Builder()
        .url("http://publicobject.com/helloworld.txt")
        .build();

    String response1Body;
    try (Response response1 = client.newCall(request).execute()) {
      if (!response1.isSuccessful()) throw new IOException("Unexpected code " + response1);

      response1Body = response1.body().string();
      System.out.println("Response 1 response:          " + response1);
      System.out.println("Response 1 cache response:    " + response1.cacheResponse());
      System.out.println("Response 1 network response:  " + response1.networkResponse());
    }

    String response2Body;
    try (Response response2 = client.newCall(request).execute()) {
      if (!response2.isSuccessful()) throw new IOException("Unexpected code " + response2);

      response2Body = response2.body().string();
      System.out.println("Response 2 response:          " + response2);
      System.out.println("Response 2 cache response:    " + response2.cacheResponse());
      System.out.println("Response 2 network response:  " + response2.networkResponse());
    }

    System.out.println("Response 2 equals Response 1? " + response1Body.equals(response2Body));
  }

  public static void main(String... args) throws Exception {
    new CacheResponse(new File("CacheResponse.tmp")).run();
  }
}

要防止响应使用缓存，请使用CacheControl.FORCE_NETWORK。为了防止它使用网络，使用CacheControl.FORCE_CACHE。注意:如果您使用FORCE_CACHE，并且响应需要网络，那么OkHttp将返回一个504不可满足的请求响应。

11.取消一个调用（Call）

使用call.cancel()方法立即停止正在进行的调用。如果线程当前正在编写请求或读取响应，它将收到IOException。当不再需要调用时，使用此方法节省网络资源；例如，当用户从应用程序导航离开时。同步和异步调用都可以取消。

import java.io.IOException;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
import okhttp3.Call;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

public class CancelCall {
  private final ScheduledExecutorService executor = Executors.newScheduledThreadPool(1);
  private final OkHttpClient client = new OkHttpClient();

  public void run() throws Exception {
    Request request = new Request.Builder()
        .url("http://httpbin.org/delay/2") // This URL is served with a 2 second delay.
        .build();

    final long startNanos = System.nanoTime();
    final Call call = client.newCall(request);

    // Schedule a job to cancel the call in 1 second.
    executor.schedule(new Runnable() {
      @Override public void run() {
        System.out.printf("%.2f Canceling call.%n", (System.nanoTime() - startNanos) / 1e9f);
        call.cancel();
        System.out.printf("%.2f Canceled call.%n", (System.nanoTime() - startNanos) / 1e9f);
      }
    }, 1, TimeUnit.SECONDS);

    System.out.printf("%.2f Executing call.%n", (System.nanoTime() - startNanos) / 1e9f);
    try (Response response = call.execute()) {
      System.out.printf("%.2f Call was expected to fail, but completed: %s%n",
          (System.nanoTime() - startNanos) / 1e9f, response);
    } catch (IOException e) {
      System.out.printf("%.2f Call failed as expected: %s%n",
          (System.nanoTime() - startNanos) / 1e9f, e);
    }
  }

  public static void main(String... args) throws Exception {
    new CancelCall().run();
  }
}

12.超时（Timeouts）

当调用的对等端不可到达时，使用timeouts来结束一次调用。网络分区可能是由于客户端连接问题、服务器可用性问题或两者之间的任何问题。OkHttp支持连接、读和写超时。

import java.util.concurrent.TimeUnit;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

public final class ConfigureTimeouts {
  private final OkHttpClient client;

  public ConfigureTimeouts() throws Exception {
    client = new OkHttpClient.Builder()
        .connectTimeout(10, TimeUnit.SECONDS)
        .writeTimeout(10, TimeUnit.SECONDS)
        .readTimeout(30, TimeUnit.SECONDS)
        .build();
  }

  public void run() throws Exception {
    Request request = new Request.Builder()
        .url("http://httpbin.org/delay/2") // This URL is served with a 2 second delay.
        .build();

    try (Response response = client.newCall(request).execute()) {
      System.out.println("Response completed: " + response);
    }
  }

  public static void main(String... args) throws Exception {
    new ConfigureTimeouts().run();
  }
}

13.更改单个Call的配置

所有HTTP客户端配置都存在于OkHttpClient中，包括代理设置、超时和缓存。当您需要更改单个Call的配置时，请调用OkHttpClient.newBuilder()。这将返回一个构建器，该构建器与原始客户端共享相同的连接池、调度程序和配置。在下面的示例中，我们提出一个请求，超时时间为500毫秒，另一个请求超时时间为3000毫秒。

import java.io.IOException;
import java.util.concurrent.TimeUnit;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

public final class PerCallSettings {
  private final OkHttpClient client = new OkHttpClient();

  public void run() throws Exception {
    Request request = new Request.Builder()
        .url("http://httpbin.org/delay/1") // This URL is served with a 1 second delay.
        .build();

    // Copy to customize OkHttp for this request.
    OkHttpClient client1 = client.newBuilder()
        .readTimeout(500, TimeUnit.MILLISECONDS)
        .build();
    try (Response response = client1.newCall(request).execute()) {
      System.out.println("Response 1 succeeded: " + response);
    } catch (IOException e) {
      System.out.println("Response 1 failed: " + e);
    }

    // Copy to customize OkHttp for this request.
    OkHttpClient client2 = client.newBuilder()
        .readTimeout(3000, TimeUnit.MILLISECONDS)
        .build();
    try (Response response = client2.newCall(request).execute()) {
      System.out.println("Response 2 succeeded: " + response);
    } catch (IOException e) {
      System.out.println("Response 2 failed: " + e);
    }
  }

  public static void main(String... args) throws Exception {
    new PerCallSettings().run();
  }
}

14.处理身份验证

OkHttp可以自动重试未经身份验证的请求。如果没有授权的401响应，则要求Authenticator提供证书。具体实现应该构建一个包含丢失证书的新请求。如果没有证书可用，返回null以跳过重试。

使用Response.challenges()获得任何身份验证质询的方案和领域。当完成一个基本的身份验证质询时，使用Credentials.basic(username, password)方法编码请求Header。

import java.io.IOException;
import okhttp3.Authenticator;
import okhttp3.Credentials;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;
import okhttp3.Route;

public final class Authenticate {
  private final OkHttpClient client;

  public Authenticate() {
    client = new OkHttpClient.Builder()
        .authenticator(new Authenticator() {
          @Override public Request authenticate(Route route, Response response) throws IOException {
            if (response.request().header("Authorization") != null) {
              return null; // Give up, we've already attempted to authenticate.
            }

            System.out.println("Authenticating for response: " + response);
            System.out.println("Challenges: " + response.challenges());
            String credential = Credentials.basic("jesse", "password1");
            return response.request().newBuilder()
                .header("Authorization", credential)
                .build();
          }
        })
        .build();
  }

  public void run() throws Exception {
    Request request = new Request.Builder()
        .url("http://publicobject.com/secrets/hellosecret.txt")
        .build();

    try (Response response = client.newCall(request).execute()) {
      if (!response.isSuccessful()) throw new IOException("Unexpected code " + response);

      System.out.println(response.body().string());
    }
  }

  public static void main(String... args) throws Exception {
    new Authenticate().run();
  }
}

为了避免在身份验证不起作用时进行多次重试，您可以返回null以放弃。例如，当已经尝试了这些确切的证书时，您应该可能想要跳过重试:

 if (credential.equals(response.request().header("Authorization"))) {
    return null; // If we already failed with these credentials, don't retry.
   }

当达到应用程序定义的尝试限制次数时，也可以跳过重试:

  if (responseCount(response) >= 3) {
    return null; // If we've failed 3 times, give up.
  }

  private int responseCount(Response response) {
   int result = 1;
    while ((response = response.priorResponse()) != null) {
      result++;
    }
    return result;
  }

本文为翻译文章：原文地址：https://github.com/square/okhttp/wiki/Recipes