学习源码-Retrofit

构建Retrofit对象

简单的使用

Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(Config.serverUrl())
                .client(okHttpClient)
                .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
                .addConverterFactory(GsonConverterFactory.create())
                .build();

逐行分析

我们按照先后顺序逐行分析:

  public static final class Builder {
    private final Platform platform;
    private @Nullable okhttp3.Call.Factory callFactory;
    private HttpUrl baseUrl;
    private final List converterFactories = new ArrayList<>();
    private final List adapterFactories = new ArrayList<>();
    private @Nullable Executor callbackExecutor;
    private boolean validateEagerly;

    Builder(Platform platform) {
      this.platform = platform;
      // 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());
    }

    public Builder() {
      this(Platform.get());
    }

    Builder(Retrofit retrofit) {
      platform = Platform.get();
      callFactory = retrofit.callFactory;
      baseUrl = retrofit.baseUrl;
      converterFactories.addAll(retrofit.converterFactories);
      adapterFactories.addAll(retrofit.adapterFactories);
      // Remove the default, platform-aware call adapter added by build().
      adapterFactories.remove(adapterFactories.size() - 1);
      callbackExecutor = retrofit.callbackExecutor;
      validateEagerly = retrofit.validateEagerly;
    }
    ......
        /**
     * Create the {@link Retrofit} instance using the configured values.
     * 

     * Note: If neither {@link #client} nor {@link #callFactory} is called a default {@link
     * OkHttpClient} will be created and used.
     */
    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 adapterFactories = new ArrayList<>(this.adapterFactories);
      adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));

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

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

创建Builder对象的时候，会自动生成一个Platform对象，而在build()方法中Builder从Platform中得到了的配置对象：defaultCallbackExecutor()。配置了默认的CallFactory:OkHttpClient()，等等......
那么我们接下来查看Platform对象的源码，看看它做了什么。

class Platform {
  private static final Platform PLATFORM = findPlatform();

  static Platform get() {
    return PLATFORM;
  }

  private static Platform findPlatform() {
    try {
      Class.forName("android.os.Build");
      if (Build.VERSION.SDK_INT != 0) {
        return new Android();
      }
    } catch (ClassNotFoundException ignored) {
    }
    try {
      Class.forName("java.util.Optional");
      return new Java8();
    } catch (ClassNotFoundException ignored) {
    }
    return new Platform();
  }

  @Nullable Executor defaultCallbackExecutor() {
    return null;
  }

  CallAdapter.Factory defaultCallAdapterFactory(@Nullable Executor callbackExecutor) {
    if (callbackExecutor != null) {
      return new ExecutorCallAdapterFactory(callbackExecutor);
    }
    return DefaultCallAdapterFactory.INSTANCE;
  }

  boolean isDefaultMethod(Method method) {
    return false;
  }

  @Nullable Object invokeDefaultMethod(Method method, Class declaringClass, Object object,
      @Nullable Object... args) throws Throwable {
    throw new UnsupportedOperationException();
  }

  @IgnoreJRERequirement // Only classloaded and used on Java 8.
  static class Java8 extends Platform {
    @Override boolean isDefaultMethod(Method method) {
      return method.isDefault();
    }

    @Override Object invokeDefaultMethod(Method method, Class declaringClass, Object object,
        @Nullable Object... args) throws Throwable {
      // Because the service interface might not be public, we need to use a MethodHandle lookup
      // that ignores the visibility of the declaringClass.
      Constructor constructor = Lookup.class.getDeclaredConstructor(Class.class, int.class);
      constructor.setAccessible(true);
      return constructor.newInstance(declaringClass, -1 /* trusted */)
          .unreflectSpecial(method, declaringClass)
          .bindTo(object)
          .invokeWithArguments(args);
    }
  }

  static class Android extends Platform {
    @Override public Executor defaultCallbackExecutor() {
      return new MainThreadExecutor();
    }

    @Override CallAdapter.Factory defaultCallAdapterFactory(@Nullable Executor callbackExecutor) {
      if (callbackExecutor == null) throw new AssertionError();
      return new ExecutorCallAdapterFactory(callbackExecutor);
    }

    static class MainThreadExecutor implements Executor {
      private final Handler handler = new Handler(Looper.getMainLooper());

      @Override public void execute(Runnable r) {
        handler.post(r);
      }
    }
  }
}

这是一个单例，它自动检测了使用Retrofit的平台，创建了Android : Platform，其中定义了默认的回调执行器MainThreadExecutor，而MainThreadExecutor在内部使用了Handler作为回调主线程的工具。除此之外，它还提供了默认的ExecutorCallAdapterFactory用来包装Call请求。
我们代码中传入的是RxJava2CallAdapterFactory,我们来跟踪一下它的代码具体做了什么工作。

public final class RxJava2CallAdapterFactory extends CallAdapter.Factory {
  /**
   * Returns an instance which creates synchronous observables that do not operate on any scheduler
   * by default.
   */
  public static RxJava2CallAdapterFactory create() {
    return new RxJava2CallAdapterFactory(null, false);
  }

  /**
   * Returns an instance which creates asynchronous observables. Applying
   * {@link Observable#subscribeOn} has no effect on stream types created by this factory.
   */
  public static RxJava2CallAdapterFactory createAsync() {
    return new RxJava2CallAdapterFactory(null, true);
  }

  /**
   * Returns an instance which creates synchronous observables that
   * {@linkplain Observable#subscribeOn(Scheduler) subscribe on} {@code scheduler} by default.
   */
  @SuppressWarnings("ConstantConditions") // Guarding public API nullability.
  public static RxJava2CallAdapterFactory createWithScheduler(Scheduler scheduler) {
    if (scheduler == null) throw new NullPointerException("scheduler == null");
    return new RxJava2CallAdapterFactory(scheduler, false);
  }

  private final @Nullable Scheduler scheduler;
  private final boolean isAsync;

  private RxJava2CallAdapterFactory(@Nullable Scheduler scheduler, boolean isAsync) {
    this.scheduler = scheduler;
    this.isAsync = isAsync;
  }

  @Override
  public CallAdapter get(Type returnType, Annotation[] annotations, Retrofit retrofit) {
    Class rawType = getRawType(returnType);

    if (rawType == Completable.class) {
      // Completable is not parameterized (which is what the rest of this method deals with) so it
      // can only be created with a single configuration.
      return new RxJava2CallAdapter(Void.class, scheduler, isAsync, false, true, false, false,
          false, true);
    }

    boolean isFlowable = rawType == Flowable.class;
    boolean isSingle = rawType == Single.class;
    boolean isMaybe = rawType == Maybe.class;
    if (rawType != Observable.class && !isFlowable && !isSingle && !isMaybe) {
      return null;
    }

    boolean isResult = false;
    boolean isBody = false;
    Type responseType;
    if (!(returnType instanceof ParameterizedType)) {
      String name = isFlowable ? "Flowable"
          : isSingle ? "Single"
          : isMaybe ? "Maybe" : "Observable";
      throw new IllegalStateException(name + " return type must be parameterized"
          + " as " + name + " or " + name + "");
    }

    Type observableType = getParameterUpperBound(0, (ParameterizedType) returnType);
    Class rawObservableType = getRawType(observableType);
    if (rawObservableType == Response.class) {
      if (!(observableType instanceof ParameterizedType)) {
        throw new IllegalStateException("Response must be parameterized"
            + " as Response or Response");
      }
      responseType = getParameterUpperBound(0, (ParameterizedType) observableType);
    } else if (rawObservableType == Result.class) {
      if (!(observableType instanceof ParameterizedType)) {
        throw new IllegalStateException("Result must be parameterized"
            + " as Result or Result");
      }
      responseType = getParameterUpperBound(0, (ParameterizedType) observableType);
      isResult = true;
    } else {
      responseType = observableType;
      isBody = true;
    }

    return new RxJava2CallAdapter(responseType, scheduler, isAsync, isResult, isBody, isFlowable,
        isSingle, isMaybe, false);
  }
}

我们主要关注一下它实现的get方法，该方法经过一系列的参数初始化，返回了一个新的RxJava2CallAdapter对象。我们继续查看一下这个类。

final class RxJava2CallAdapter implements CallAdapter {
  private final Type responseType;
  private final @Nullable Scheduler scheduler;
  private final boolean isAsync;
  private final boolean isResult;
  private final boolean isBody;
  private final boolean isFlowable;
  private final boolean isSingle;
  private final boolean isMaybe;
  private final boolean isCompletable;

  RxJava2CallAdapter(Type responseType, @Nullable Scheduler scheduler, boolean isAsync,
      boolean isResult, boolean isBody, boolean isFlowable, boolean isSingle, boolean isMaybe,
      boolean isCompletable) {
    this.responseType = responseType;
    this.scheduler = scheduler;
    this.isAsync = isAsync;
    this.isResult = isResult;
    this.isBody = isBody;
    this.isFlowable = isFlowable;
    this.isSingle = isSingle;
    this.isMaybe = isMaybe;
    this.isCompletable = isCompletable;
  }

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

  @Override public Object adapt(Call call) {
    Observable> responseObservable = isAsync
        ? new CallEnqueueObservable<>(call)
        : new CallExecuteObservable<>(call);

    Observable observable;
    if (isResult) {
      observable = new ResultObservable<>(responseObservable);
    } else if (isBody) {
      observable = new BodyObservable<>(responseObservable);
    } else {
      observable = responseObservable;
    }

    if (scheduler != null) {
      observable = observable.subscribeOn(scheduler);
    }

    if (isFlowable) {
      return observable.toFlowable(BackpressureStrategy.LATEST);
    }
    if (isSingle) {
      return observable.singleOrError();
    }
    if (isMaybe) {
      return observable.singleElement();
    }
    if (isCompletable) {
      return observable.ignoreElements();
    }
    return observable;
  }
}

该对象根据传进来的参数，返回了对应的observable结果对象。
综上所述：RxJava2CallAdapterFactory的作用是将Retrofit生成的Call包装成为RxJava2的Observable对象。

现在我们来到了addConverterFactory(GsonConverterFactory.create())，我们依据前面的经验，对它进行分析。

public final class GsonConverterFactory extends Converter.Factory {
  /**
   * Create an instance using a default {@link Gson} instance for conversion. Encoding to JSON and
   * decoding from JSON (when no charset is specified by a header) will use UTF-8.
   */
  public static GsonConverterFactory create() {
    return create(new Gson());
  }

  /**
   * Create an instance using {@code gson} for conversion. Encoding to JSON and
   * decoding from JSON (when no charset is specified by a header) will use UTF-8.
   */
  @SuppressWarnings("ConstantConditions") // Guarding public API nullability.
  public static GsonConverterFactory create(Gson gson) {
    if (gson == null) throw new NullPointerException("gson == null");
    return new GsonConverterFactory(gson);
  }

  private final Gson gson;

  private GsonConverterFactory(Gson gson) {
    this.gson = gson;
  }

  @Override
  public Converter responseBodyConverter(Type type, Annotation[] annotations,
      Retrofit retrofit) {
    TypeAdapter adapter = gson.getAdapter(TypeToken.get(type));
    return new GsonResponseBodyConverter<>(gson, adapter);
  }

  @Override
  public Converter requestBodyConverter(Type type,
      Annotation[] parameterAnnotations, Annotation[] methodAnnotations, Retrofit retrofit) {
    TypeAdapter adapter = gson.getAdapter(TypeToken.get(type));
    return new GsonRequestBodyConverter<>(gson, adapter);
  }
}



final class GsonResponseBodyConverter implements Converter {
  private final Gson gson;
  private final TypeAdapter adapter;

  GsonResponseBodyConverter(Gson gson, TypeAdapter adapter) {
    this.gson = gson;
    this.adapter = adapter;
  }

  @Override public T convert(ResponseBody value) throws IOException {
    JsonReader jsonReader = gson.newJsonReader(value.charStream());
    try {
      return adapter.read(jsonReader);
    } finally {
      value.close();
    }
  }
}




final class GsonRequestBodyConverter implements Converter {
  private static final MediaType MEDIA_TYPE = MediaType.parse("application/json; charset=UTF-8");
  private static final Charset UTF_8 = Charset.forName("UTF-8");

  private final Gson gson;
  private final TypeAdapter adapter;

  GsonRequestBodyConverter(Gson gson, TypeAdapter adapter) {
    this.gson = gson;
    this.adapter = adapter;
  }

  @Override public RequestBody convert(T value) throws IOException {
    Buffer buffer = new Buffer();
    Writer writer = new OutputStreamWriter(buffer.outputStream(), UTF_8);
    JsonWriter jsonWriter = gson.newJsonWriter(writer);
    adapter.write(jsonWriter, value);
    jsonWriter.close();
    return RequestBody.create(MEDIA_TYPE, buffer.readByteString());
  }
}

我们这里不再赘述，直接总结我们得到的结论：GsonConvertFactory使用Gson对RequestBody以及ResponseBody进行Json解析。

创建请求

public interface IIndexApi {

    @GET("/rest/api/index")
    Observable getData(@Header("x-token") String tokenStr,@Query("type") String type);

    @FormUrlEncoded
    @POST("/rest/api/myNewInvest")
    Observable getData(@Header("x-token") String tokenStr, @Field("isStorage") String isStorage);
}

    Observable = retrofit.create(IIndexApi.class).getData(tokenStr,type);

    Observable = retrofit.create(IIndexApi.class).getData(tokenStr,isStorage);

我们使用interface生命接口，用@GET等注解生命请求方式和添加参数，Retrofit通过.create方法创建了这个接口，并在后面调用其包含的具体接口。
我们先从.create方法入手。

 @SuppressWarnings("unchecked") // Single-interface proxy creation guarded by parameter safety.
  public  T create(final Class 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, @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);
            }
            ServiceMethod serviceMethod =
                (ServiceMethod) loadServiceMethod(method);
            OkHttpCall