Retrofit使用与解析

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Retrofit是一个基于OkHttp的网络请求工具，其功能与Volley很相似，但是使用起来就很不一样。
Retrofit不用你去创建Request对象，每次指定Get还是Post等，它请求一个api只需你去调用一个Java方法。

使用

1.首先需要创建一个Retrofit对象,并指定域名：

//使用Builder模式构建对象private static final String BASE_URL = "http://xxx";//域名Retrofit retrofit = new Retrofit.Builder()    .baseUrl(BASE_URL)    .addConverterFactory(GsonConverterFactory.create())    .build();

Retrofit中Bulider类的build方法

 public Retrofit build() {      if (baseUrl == null) {        throw new IllegalStateException("Base URL required.");      }      okhttp3.Call.Factory callFactory = this.callFactory;      if (callFactory == null) {        callFactory = new OkHttpClient();//默认使用OkHttp      }      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);    }  }

2.之后，你只要根据api新建一个Java接口，并使用注解进行相关描述：

//Get请求public interface ElectricAPI {        @GET("/api/v1/get/power/{lou}/{hao}")        Call<Electric> electricData(@Path("lou") String lou,@Path("hao") String hao);}//Post请求public interface FeedBackAPI {    @FormUrlEncoded    @POST("/home/msg/0")    Call<ResponseBody> feed(@Field("email") String email,                                      @Field("content") String content);}//文件上传public interface FileUploadAPI {    @Multipart    @POST("api/v1/stuff/upload")    Call<HttpResult<String>> uploadImage(@Part MultipartBody.Part file);}

3.最后只要通过retrofit对象创建Api对象了,并使用call对象进行请求和处理：

ElectricAPI api = retrofit.create(ElectricAPI.class);Call<Electric> call = api.electricData("12","122");// 请求数据，并且处理responsecall.enqueue(new Callback<Electric>() {    @Override    public void onResponse(Response<Electric> r) {        System.out.println("请求成功");    }    @Override    public void onFailure(Throwable t) {    }});

原理

从使用上可以看出，Retrofit其实是把一个Java接口对象翻译成一个Http请求，然后使用OkHttp请求。Retrofit使用的技术就是动态代理。
先看create方法：

public <T> T create(final Class<T> service) {    Utils.validateServiceInterface(service);//检查是否是接口    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)) {//默认返回false              return platform.invokeDefaultMethod(method, service, proxy, args);            }            ServiceMethod serviceMethod = loadServiceMethod(method);//处理注解            OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);            return serviceMethod.callAdapter.adapt(okHttpCall);          }        });  }

接着是create方法的使用：

ElectricAPI api = retrofit.create(ElectricAPI.class);Call<Electric> call = api.electricData("12","122");

这里的api对象其实是一个动态代理，并不是实现了相关接口的对象。当api对象调用eletricData方法时，会被动态代理拦截，然后调用Proxy.newProxyInstance方法中的InvocationHandler对象的invoke方法。这里invoke方法传入三个对象，依次为代理对象（不用管）、method（这里是electricData方法），方法的参数（这里是”12”，”122”）。

这里Retrofit关心的就是method和它的参数args，接下去Retrofit就会用Java反射获取到electricData方法的注解信息，配合args参数，创建一个ServiceMethod对象。

ServiceMethod就像是一个处理器，传入Retrofit对象和Method对象，调用各个接口和解析器，最终生成一个Request，包含api 的域名、path、http请求方法、请求头、是否有body、是否是multipart等等。最后返回一个Call对象，Retrofit2中Call接口的默认实现是OkHttpCall，它默认使用OkHttp3作为底层http请求client。

接着看下上面的loadServiceMethod(Method method)方法

ServiceMethod loadServiceMethod(Method method) {    ServiceMethod result;    synchronized (serviceMethodCache) {      result = serviceMethodCache.get(method);      if (result == null) {        result = new ServiceMethod.Builder(this, method).build();        serviceMethodCache.put(method, result);      }    }    return result;  } private final Map<Method, ServiceMethod> serviceMethodCache = new LinkedHashMap<>();

这里对解析请求进行了缓存，用于避免解析过慢。这里同样使用Builder模式构建ServiceMethod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();      //处理注解      for (Annotation annotation : methodAnnotations) {        parseMethodAnnotation(annotation);      }      if (httpMethod == null) {        throw methodError("HTTP method annotation is required (e.g., @GET, @POST, etc.).");      }      //检查错误      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).");        }      }      //处理api中的参数{ }占位符，真实参数在Java方法中传入，这里Retrofit会使用一个ParameterHandler来进行替换      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);    }

执行Http请求

之前讲到，OkHttpCall是实现了Call接口的，并且是真正调用OkHttp3发送Http请求的类。OkHttp3发送一个Http请求需要一个Request对象，而这个Request对象就是从ServiceMethod的toRequest返回的。

 Request toRequest(Object... args) throws IOException {    RequestBuilder requestBuilder = new RequestBuilder(httpMethod, baseUrl, relativeUrl, headers,        contentType, hasBody, isFormEncoded, isMultipart);    @SuppressWarnings("unchecked") // It is an error to invoke a method with the wrong arg types.    ParameterHandler<Object>[] handlers = (ParameterHandler<Object>[]) parameterHandlers;    //参数替换    int argumentCount = args != null ? args.length : 0;    if (argumentCount != handlers.length) {      throw new IllegalArgumentException("Argument count (" + argumentCount          + ") doesn't match expected count (" + handlers.length + ")");    }    for (int p = 0; p < argumentCount; p++) {      handlers[p].apply(requestBuilder, args[p]);    }    return requestBuilder.build();  }

总的来说，OkHttpCall就是调用ServiceMethod获得一个可以执行的Request对象，然后等到Http请求返回后，再将response body传入ServiceMethod中，ServiceMethod就可以调用Converter接口将response body转成一个Java对象。

结合上面说的就可以看出，ServiceMethod中几乎保存了一个api请求所有需要的数据，OkHttpCall需要从ServiceMethod中获得一个Request对象，然后得到response后，还需要传入ServiceMethod用Converter转换成Java对象。