简介

Retrofit是目前比较流行的网络框架，它是以restful风格的请求方式，将接口请求使用动态代理方式将获取方法名，方法注解，方法参数注解，方法值翻译成http请求，内部封装了以okhttp作为底层的网络请求框架。除此之外有强大可扩展型，以及返回结果支持各种格式转换，以及对rxjava的支持。

gradle引用

implementation 'com.squareup.retrofit2:retrofit:2.5.0'
//rxjava
api 'com.squareup.retrofit2:adapter-rxjava2:2.5.0'
//数据转换
api 'com.squareup.retrofit2:converter-gson:2.5.0'
//过滤器
api 'com.squareup.okhttp3:logging-interceptor:3.8.0'

github地址https://github.com/square/retrofit

使用

官方使用说明http://square.github.io/retrofit/

首先创建api接口

    public interface GitHubService {
    @GET("users/{user}/repos")
    Call<List<Repo>> listRepos(@Path("user") String user);
    }

获取动态代理对象

    Retrofit retrofit = new Retrofit.Builder()
                .baseUrl("")
                .addConverterFactory(GsonConverterFactory.create())
                .build();

     GitHubService gitHubService =  retrofit.create(GitHubService.class);

创建Call对象

       Call<Repo> call = gitHubService.listRepos("");

执行异步网络请求

    call.enqueue(new Callback<Repo>() {
           @Override
           public void onResponse(Call<Repo> call, Response<Repo> response) {

           }

           @Override
           public void onFailure(Call<Repo> call, Throwable t) {

           }
       });

问题

源码解析

     Retrofit retrofit = new Retrofit.Builder()
                .baseUrl("")
                .addConverterFactory(GsonConverterFactory.create())
                .build();

首先retrofit通过一个Builder模式，创建一个retrofit实例。

     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> callAdapterFactories = new ArrayList<>(this.callAdapterFactories);
      callAdapterFactories.addAll(platform.defaultCallAdapterFactories(callbackExecutor));

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

      // Add the built-in converter factory first. This prevents overriding its behavior but also
      // ensures correct behavior when using converters that consume all types.
      converterFactories.add(new BuiltInConverters());
      converterFactories.addAll(this.converterFactories);
      converterFactories.addAll(platform.defaultConverterFactories());

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

通过build方法进行属性校验并初始化一些变量。

     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();
          private final Object[] emptyArgs = new Object[0];

          @Override public Object invoke(Object proxy, Method method, @Nullable 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);
            }
            return loadServiceMethod(method).invoke(args != null ? args : emptyArgs);
          }
        });
    }

然后通过creat创建一个接口实例，creat主要通过Proxy.newProxyInstance方法创建一个动态代理。在newProxyInstance的invoke方法中method可以拿到接口的方法名；方法的注解类型，以及接口方法的注解的key值，以及注解的value值。

     GitHubService gitHubService=(GitHubService) Proxy.newProxyInstance(GitHubService.class.getClassLoader(),
                new Class[]{GitHubService.class}, new InvocationHandler() {
                    @Override
                    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                        System.out.println("方法名"+method.getName());
                        GET get = method.getAnnotation(GET.class);
                        System.out.println("方法的注解值"+get.value());
                        Annotation[] annotations = method.getAnnotations();
                        for (int i = 0; i <annotations.length ; i++) {
                            System.out.println("方法参数的注解"+Arrays.toString(annotations));
                        }
                       System.out.println("方法的参数值"+Arrays.toString(args));
                        return null;
                    }
                });
        gitHubService.listRepos("111");

通过代理获取method相关的一些信息

    ServiceMethod<?> loadServiceMethod(Method method) {
    ServiceMethod<?> result = serviceMethodCache.get(method);
    if (result != null) return result;

    synchronized (serviceMethodCache) {
      result = serviceMethodCache.get(method);
      if (result == null) {
        result = ServiceMethod.parseAnnotations(this, method);
        serviceMethodCache.put(method, result);
      }
    }
    return result;
    }

loadServiceMethod方法返回ServiceMethod对象。如果有缓存就从缓存中取，如果没有直接创建一个放map中。ServiceMethod通过 result = ServiceMethod.parseAnnotations(this, method)获取。

    static <T> ServiceMethod<T> parseAnnotations(Retrofit retrofit, Method method) {
    RequestFactory requestFactory = RequestFactory.parseAnnotations(retrofit, method);

    Type returnType = method.getGenericReturnType();
    if (Utils.hasUnresolvableType(returnType)) {
      throw methodError(method,
          "Method return type must not include a type variable or wildcard: %s", returnType);
    }
    if (returnType == void.class) {
      throw methodError(method, "Service methods cannot return void.");
    }
    return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);
    }



       static <ResponseT, ReturnT> HttpServiceMethod<ResponseT, ReturnT> parseAnnotations(
      Retrofit retrofit, Method method, RequestFactory requestFactory) {
    CallAdapter<ResponseT, ReturnT> callAdapter = createCallAdapter(retrofit, method);
    Type responseType = callAdapter.responseType();
    if (responseType == Response.class || responseType == okhttp3.Response.class) {
      throw methodError(method, "'"
          + Utils.getRawType(responseType).getName()
          + "' is not a valid response body type. Did you mean ResponseBody?");
    }
    if (requestFactory.httpMethod.equals("HEAD") && !Void.class.equals(responseType)) {
      throw methodError(method, "HEAD method must use Void as response type.");
    }

    Converter<ResponseBody, ResponseT> responseConverter =
        createResponseConverter(retrofit, method, responseType);

    okhttp3.Call.Factory callFactory = retrofit.callFactory;
    return new HttpServiceMethod<>(requestFactory, callFactory, callAdapter, responseConverter);
    }

ServiceMethod是一个抽象类，调用的是它子类HttpServiceMethod的parseAnnotations方法，在该方法中直接new HttpServiceMethod对象。

     private HttpServiceMethod(RequestFactory requestFactory, okhttp3.Call.Factory callFactory,
      CallAdapter<ResponseT, ReturnT> callAdapter,
      Converter<ResponseBody, ResponseT> responseConverter) {
    this.requestFactory = requestFactory;
    this.callFactory = callFactory;
    this.callAdapter = callAdapter;
    this.responseConverter = responseConverter;
    }

HttpServiceMethod类中的相关属性和方法。

    return loadServiceMethod(method).invoke(args != null ? args : emptyArgs);

回到creat方法，最后调用的是invoke方法。

    @Override ReturnT invoke(Object[] args) {
    return callAdapter.adapt(
        new OkHttpCall<>(requestFactory, args, callFactory, responseConverter));
      }

invoke的具体实现，该方法就会返回一个Call对象，allAdapter.adapt()方法就是将OkHttpCall对象转换成其他平台能用的Call对象，比如：RxJava。这里返回的是OkHttpCall的对象，该类是Retrofit中定义的，是对Okhttp中的Call对象的封装。

    OkHttpCall(RequestFactory requestFactory, Object[] args,
      okhttp3.Call.Factory callFactory, Converter<ResponseBody, T> responseConverter) {
    this.requestFactory = requestFactory;
    this.args = args;
    this.callFactory = callFactory;
    this.responseConverter = responseConverter;
    }

    ……
    ……

     @Override public synchronized Request request() {
    okhttp3.Call call = rawCall;
    if (call != null) {
      return call.request();
    }
    if (creationFailure != null) {
      if (creationFailure instanceof IOException) {
        throw new RuntimeException("Unable to create request.", creationFailure);
      } else if (creationFailure instanceof RuntimeException) {
        throw (RuntimeException) creationFailure;
      } else {
        throw (Error) creationFailure;
      }
    }
    try {
      return (rawCall = createRawCall()).request();
    } catch (RuntimeException | Error e) {
      throwIfFatal(e); // Do not assign a fatal error to creationFailure.
      creationFailure = e;
      throw e;
    } catch (IOException e) {
      creationFailure = e;
      throw new RuntimeException("Unable to create request.", e);
    }
    }

    @Override public void enqueue(final Callback<T> callback) {
    checkNotNull(callback, "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) {
          throwIfFatal(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) {
        Response<T> response;
        try {
          response = parseResponse(rawResponse);
        } catch (Throwable e) {
          throwIfFatal(e);
          callFailure(e);
          return;
        }

        try {
          callback.onResponse(OkHttpCall.this, response);
        } catch (Throwable t) {
          t.printStackTrace();
        }
      }

      @Override public void onFailure(okhttp3.Call call, IOException e) {
        callFailure(e);
      }

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

以上是OkHttpCall对象部分代码，其实内部主要是调用okhttp3的call对象的网络同步或者异步请求等。

    private okhttp3.Call createRawCall() throws IOException {
    okhttp3.Call call = callFactory.newCall(requestFactory.create(args));
    if (call == null) {
      throw new NullPointerException("Call.Factory returned null.");
    }
    return call;
    }

通过requestFactory的create方法创建了okhttp3.Call对象。

    final class RequestFactory {

      ………………

     Builder(Retrofit retrofit, Method method) {
      this.retrofit = retrofit;
      this.method = method;
      this.methodAnnotations = method.getAnnotations();
      this.parameterTypes = method.getGenericParameterTypes();
      this.parameterAnnotationsArray = method.getParameterAnnotations();
      }

    ………………

    okhttp3.Request create(Object[] args) throws IOException {
    @SuppressWarnings("unchecked") // It is an error to invoke a method with the wrong arg types.
    ParameterHandler<Object>[] handlers = (ParameterHandler<Object>[]) parameterHandlers;

    int argumentCount = args.length;
    if (argumentCount != handlers.length) {
      throw new IllegalArgumentException("Argument count (" + argumentCount
          + ") doesn't match expected count (" + handlers.length + ")");
    }

    RequestBuilder requestBuilder = new RequestBuilder(httpMethod, baseUrl, relativeUrl,
        headers, contentType, hasBody, isFormEncoded, isMultipart);

    List<Object> argumentList = new ArrayList<>(argumentCount);
    for (int p = 0; p < argumentCount; p++) {
      argumentList.add(args[p]);
      handlers[p].apply(requestBuilder, args[p]);
    }

    return requestBuilder.get()
        .tag(Invocation.class, new Invocation(method, argumentList))
        .build();
    }
    }

在RequestFactory类中可以看出该方法就是根据ParameterHandler解析的参数以及传入的参数值args创建Request对象.

总结

调用Retrofit.creat方法创建接口实例。内部使用动态代理，ServiceMethod调用invoke方法，ServiceMethod是抽象类，实际是它的子类HttpServiceMethod调用。HttpServiceMethod是对请求方法的封装，主要有requestFactory、callFactory、callAdapter、responseConverter成员变量。