retrofit2+rxjava+mockserver使用和理解

特点概要

Square的retrofit作为目前android比较火的网络框架，具有以下特点：

1、rest api 风格
2、网络核心使用优秀开源框架okttp，其本身致力于组装request和便捷转换response。
3、代码简洁，拓展性高，开发包只有90k左右
4、支持RxJava、Guava、Java8等适配器。

简单使用

案例是一个简单的登陆请求案例。使用name和pwd参数登陆接口，返回获取response并直接转换成User对象，User有两个成员变量name，age。

使用步骤分解为：
1、创建retrofit

private void initRetrofit(){
         retrofit = new Retrofit.Builder()
                .baseUrl("http://192.168.56.1:12306/")
                .addConverterFactory(GsonConverterFactory.create())
                .build();
}

2、定义接口

public interface ApiServicce {

    @FormUrlEncoded
    @POST("/user/login/")
    Call<User> login(@Field("name")String name, @Field("pwd")String pwd);
}

3、创建Call代理对象

 ApiServicce apiService = retrofit.create(ApiServicce.class);
 Call<User> userCall = apiService.login("name","retrofit");

4、发起请求并获取回调内容

    @Override
    public void onResponse(Call<User> call, Response<User> response) {
              User user = response.body();
          }

            @Override
            public void onFailure(Call<User> call, Throwable t) {
                System.err.println(call.toString());
            }
   });

以上四个步骤就完成了一个完成的数据请求和回复的解析。高度解耦的特点只能用简洁优雅来形容。那么他怎么做到的呢我们稍后分析。这里涉及到一个的接口是使用mockserver来模拟完成的，有关于mockserver的工作可以从github mockserver了解。这里我使用的是简易的moco，只需要是简单使用其jar包和定义一个配置便可以轻松的完成一次模拟过程。详情可以参考这里。

服务启动命令：
java -jar moco-runner-0.11.0-standalone.jar http -p 12306 -c foo.json

foo.json内容：

[
  {
    "request" :
    {
      "uri" :"/user/login/",
      "method" :"post"
    },
   "response" :
      {
        "json" :
        {
            "name" : "jerry",
            "age" :"24"
        }         
      }
  },
  {
    "request":
    {
        "uri" : "/"
    },
    "response" :
    {
        "text" : "hello"
    }
  }
]

可以看出调用
http://192.168.56.1:12306/返回hello的文本内容，调用http://192.168.56.1:12306/user/login 返回上述的json串再经由retrofit转换成对应的java对象。ok，大体了解了使用方法，开始剖析他的工作原理。

原理剖析

可以看出其工作流程主要分为下面四步

下面对其源码进行分析：
结合我们使用过程 retrofit build

retrofit build

public Retrofit build() {
      if (baseUrl == null) {
        throw new IllegalStateException("Base URL required.");
      }

      okhttp3.Call.Factory callFactory = this.callFactory;
      if (callFactory == null) {
        callFactory = new OkHttpClient();
      }

      Executor callbackExecutor = this.callbackExecutor;
      if (callbackExecutor == null) {
        callbackExecutor = platform.defaultCallbackExecutor();
      }

      // Make a defensive copy of the adapters and add the default Call adapter.
      List<CallAdapter.Factory> adapterFactories = new ArrayList<>(this.adapterFactories);
      adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));

      // Make a defensive copy of the converters.
      List<Converter.Factory> converterFactories = new ArrayList<>(this.converterFactories);

      return new Retrofit(callFactory, baseUrl, converterFactories, adapterFactories,
          callbackExecutor, validateEagerly);
    }

实例CallFactory，CallFactory决定调用的网络模块使用的是哪个,这里默认使用的是OkHttpClient。成员callbackExecutor为执行回调的任务或线程。成员adapterFactories存放CallAdapter的一个列表，并且add一个默认的AdapterFactory,AdapterFactory中get方法用来获取到指定的CallAdapter。先来看下默认的AdapterFactory是什么，位置ExecutorCallAdapterFactory类。

CallAdapter

 @Override
  public CallAdapter<Call<?>> get(Type returnType, Annotation[] annotations, Retrofit retrofit) {
    if (getRawType(returnType) != Call.class) {
      return null;
    }
    final Type responseType = Utils.getCallResponseType(returnType);
    return new CallAdapter<Call<?>>() {
      @Override public Type responseType() {
        return responseType;
      }

      @Override public <R> Call<R> adapt(Call<R> call) {
        return new ExecutorCallbackCall<>(callbackExecutor, call);
      }
    };
 }

return new ExecutorCallbackCall，那么看下这个ExecutorCallbackCall是什么

static final class ExecutorCallbackCall<T> implements Call<T> {
    final Executor callbackExecutor;
    final Call<T> delegate;

    ExecutorCallbackCall(Executor callbackExecutor, Call<T> delegate) {
      this.callbackExecutor = callbackExecutor;
      this.delegate = delegate;
    }

    @Override public void enqueue(final Callback<T> callback) {
      if (callback == null) throw new NullPointerException("callback == null");

      delegate.enqueue(new Callback<T>() {
        @Override public void onResponse(Call<T> call, final Response<T> response) {
          callbackExecutor.execute(new Runnable() {
            @Override public void run() {
              if (delegate.isCanceled()) {
                // Emulate OkHttp‘s behavior of throwing/delivering an IOException on cancellation.
                callback.onFailure(ExecutorCallbackCall.this, new IOException("Canceled"));
              } else {
                callback.onResponse(ExecutorCallbackCall.this, response);
              }
            }
          });
        }

        @Override public void onFailure(Call<T> call, final Throwable t) {
          callbackExecutor.execute(new Runnable() {
            @Override public void run() {
              callback.onFailure(ExecutorCallbackCall.this, t);
            }
          });
        }
      });
    }

    @Override public boolean isExecuted() {
      return delegate.isExecuted();
    }

    @Override public Response<T> execute() throws IOException {
      return delegate.execute();
    }

    @Override public void cancel() {
      delegate.cancel();
    }

    @Override public boolean isCanceled() {
      return delegate.isCanceled();
    }

    @SuppressWarnings("CloneDoesntCallSuperClone") // Performing deep clone.
    @Override public Call<T> clone() {
      return new ExecutorCallbackCall<>(callbackExecutor, delegate.clone());
    }

    @Override public Request request() {
      return delegate.request();
    }
  }

ok，很简单。只是一个Call的代理类,不了解Call对象的可以去研究下okhttp。delegate.enqueue回复之后使用callbackExecutor将回复处理放回到一个子线程交由callback处理。做好这些准备工作之后。需要看下他是如果做到create代理对象的解耦和连接整个的工作流程。

retrofit create

retrofit.create(申明的接口class)

public <T> T create(final Class<T> service) {
    Utils.validateServiceInterface(service);
    if (validateEagerly) {
      eagerlyValidateMethods(service);
    }
    return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class<?>[] { service },
        new InvocationHandler() {
          private final Platform platform = Platform.get();

          @Override public Object invoke(Object proxy, Method method, Object... args)
              throws Throwable {
            // If the method is a method from Object then defer to normal invocation.
            if (method.getDeclaringClass() == Object.class) {
              return method.invoke(this, args);
            }
            if (platform.isDefaultMethod(method)) {
              return platform.invokeDefaultMethod(method, service, proxy, args);
            }
            ServiceMethod serviceMethod = loadServiceMethod(method);
            OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);
            return serviceMethod.callAdapter.adapt(okHttpCall);
          }
        });
  }

可以看出这里使用了一个动态代理来代理service类，观察其InvocationHandler代理改变的部分，其Object的方法都自动被原样的执行，当调用service里面的请求方法的时候会被Proxy拦截，loadServiceMethod方法讲原方法包装成ServiceMethod,ServiceMethod类主要工作是解析注解、传参，将他们分装成request，然后再根据具体的返回值，将之前配置的工场Factory生成具体的CallAdapter和ResponseConvert。 OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args)。显式的实例了一个OkhttpCall对象,OkHttpCall封装了Call(okhttp中的对象)，说明retrofit是使用了okttp最为网络的核心模块。最后一步serviceMethod.callAdapter.adapt(okHttpCall)，callAdater是用来做什么的呢，其本质是将call转成一个T型的策略。下面先对CallAdapter如何工作的进行剖析，并且如何整合Rxjava来使用的。

结合Rxjava

如果我们想使用Rxjava 那么接口方法定义得是这样的

    @FormUrlEncoded
    @POST
    Observable<User> rxLogin(@Field("name")String name, @Field("pwd")String pwd);

那么如何才能转换成我们想要的Observable类型呢，前面我们说过CallAdapter旨在转换Call成为我们想要的T类型。那么需要在CallAdapter上做文章了。重点看下RxJavaCallAdapterFactory类

public final class RxJavaCallAdapterFactory implements CallAdapter.Factory

CallAdapter.Factory是Retrofit这个库中的接口，用来给我们自定义去解析我们自己想要的类型用的。

get方法获取CallAdapter的方法

@Override
public CallAdapter<?> get(Type returnType, Annotation[] annotations, Retrofit retrofit) {
    Class<?> rawType = Utils.getRawType(returnType);
    boolean isSingle = "rx.Single".equals(rawType.getCanonicalName());
    if (rawType != Observable.class && !isSingle) {
      return null;
    }
    if (!(returnType instanceof ParameterizedType)) {
      String name = isSingle ? "Single" : "Observable";
      throw new IllegalStateException(name + " return type must be parameterized"
          + " as " + name + "<Foo> or " + name + "<? extends Foo>");
    }

    CallAdapter<Observable<?>> callAdapter = getCallAdapter(returnType);
    if (isSingle) {
      // Add Single-converter wrapper from a separate class. This defers classloading such that
      // regular Observable operation can be leveraged without relying on this unstable RxJava API.
      return SingleHelper.makeSingle(callAdapter);
    }
    return callAdapter;
}

如果不是Observable的类型，则不去处理.Observable类型则调用callAdapter（）方法获取对应的CallAdapter。callAdapter()方法中return SimpleCallAdapter的实例，那么观察SimpleCallAdapter的实现

static final class SimpleCallAdapter implements CallAdapter<Observable<?>> {
    private final Type responseType;
    private final Scheduler scheduler;

    SimpleCallAdapter(Type responseType, Scheduler scheduler) {
      this.responseType = responseType;
      this.scheduler = scheduler;
    }

    @Override public Type responseType() {
      return responseType;
    }

    @Override public <R> Observable<R> adapt(Call<R> call) {
      Observable<R> observable = Observable.create(new CallOnSubscribe<>(call)) //
          .lift(OperatorMapResponseToBodyOrError.<R>instance());
      if (scheduler != null) {
        return observable.subscribeOn(scheduler);
      }
      return observable;
    }
  }

好了，最终我们看到了adapter返回类型就是Observeble了。再回顾下retrofit create的过程中我们代理proxy return的结果是serviceMethod.callAdapter.apater(call)那么我们取得的是不是就一个Observerble对象了！！！那么create拿到的代理对象就是一个observable对象。就可以轻松的使用Rxjava了！！！如果不了解rxjava的可以了解下github rxjava

Request and Response

retrofit的重点是是组装request和回复response类型的转换。serviceMethod类是用来根据注解组装请求参数的。serviceMethod的build方法

public ServiceMethod build() {
      callAdapter = createCallAdapter();
      responseType = callAdapter.responseType();
      if (responseType == Response.class || responseType == okhttp3.Response.class) {
        throw methodError("‘"
            + Utils.getRawType(responseType).getName()
            + "‘ is not a valid response body type. Did you mean ResponseBody?");
      }
      responseConverter = createResponseConverter();

      for (Annotation annotation : methodAnnotations) {
        parseMethodAnnotation(annotation);
      }

      if (httpMethod == null) {
        throw methodError("HTTP method annotation is required (e.g., @GET, @POST, etc.).");
      }

      if (!hasBody) {
        if (isMultipart) {
          throw methodError(
              "Multipart can only be specified on HTTP methods with request body (e.g., @POST).");
        }
        if (isFormEncoded) {
          throw methodError("FormUrlEncoded can only be specified on HTTP methods with "
              + "request body (e.g., @POST).");
        }
      }

      int parameterCount = parameterAnnotationsArray.length;
      parameterHandlers = new ParameterHandler<?>[parameterCount];
      for (int p = 0; p < parameterCount; p++) {
        Type parameterType = parameterTypes[p];
        if (Utils.hasUnresolvableType(parameterType)) {
          throw parameterError(p, "Parameter type must not include a type variable or wildcard: %s",
              parameterType);
        }

        Annotation[] parameterAnnotations = parameterAnnotationsArray[p];
        if (parameterAnnotations == null) {
          throw parameterError(p, "No Retrofit annotation found.");
        }

        parameterHandlers[p] = parseParameter(p, parameterType, parameterAnnotations);
      }

      if (relativeUrl == null && !gotUrl) {
        throw methodError("Missing either @%s URL or @Url parameter.", httpMethod);
      }
      if (!isFormEncoded && !isMultipart && !hasBody && gotBody) {
        throw methodError("Non-body HTTP method cannot contain @Body.");
      }
      if (isFormEncoded && !gotField) {
        throw methodError("Form-encoded method must contain at least one @Field.");
      }
      if (isMultipart && !gotPart) {
        throw methodError("Multipart method must contain at least one @Part.");
      }

      return new ServiceMethod<>(this);
    }

createCallAdapter创建对应的callAdapter，即从CallAdapterFactory中取出需要的适配器，responseConverter创建对应的回复处理类，从List

@Override public void enqueue(final Callback<T> callback) {
    if (callback == null) throw new NullPointerException("callback == null");

    okhttp3.Call call;
    Throwable failure;

    synchronized (this) {
      if (executed) throw new IllegalStateException("Already executed.");
      executed = true;

      call = rawCall;
      failure = creationFailure;
      if (call == null && failure == null) {
        try {
          call = rawCall = createRawCall();
        } catch (Throwable t) {
          failure = creationFailure = t;
        }
      }
    }

    if (failure != null) {
      callback.onFailure(this, failure);
      return;
    }

    if (canceled) {
      call.cancel();
    }

    call.enqueue(new okhttp3.Callback() {
      @Override public void onResponse(okhttp3.Call call, okhttp3.Response rawResponse)
          throws IOException {
        Response<T> response;
        try {
          response = parseResponse(rawResponse);
        } catch (Throwable e) {
          callFailure(e);
          return;
        }
        callSuccess(response);
      }

      @Override public void onFailure(okhttp3.Call call, IOException e) {
        try {
          callback.onFailure(OkHttpCall.this, e);
        } catch (Throwable t) {
          t.printStackTrace();
        }
      }

      private void callFailure(Throwable e) {
        try {
          callback.onFailure(OkHttpCall.this, e);
        } catch (Throwable t) {
          t.printStackTrace();
        }
      }

      private void callSuccess(Response<T> response) {
        try {
          callback.onResponse(OkHttpCall.this, response);
        } catch (Throwable t) {
          t.printStackTrace();
        }
      }
    });
  }

使用okttp的call对象拿到回复之后调用parseResponse获得到指定的response对象。最终是调用serviceMethod中的toResponse进行转换成对应的T类型。

 /** Builds a method return value from an HTTP response body. */
  T toResponse(ResponseBody body) throws IOException {
    return responseConverter.convert(body);
  }

所以这里就是使用了我们指定的responseConverter进行转换。如GsonConverterFactory。转换的方式根据不同Converter来定，不做详述。由此可见，通过ServiceMethod的协调完成了request的组装和response回复的转换。