Retrofit2源码分析（一）

/**
   * Build a new {@link Retrofit}.
   * <p>
   * Calling {@link #baseUrl} is required before calling {@link #build()}. All other methods
   * are optional.
   */
  public static final class Builder {
   //支持的平台
    private Platform platform;
    //发起请求的okhttp3的client工厂
    private okhttp3.Call.Factory callFactory;
    //okhttp3里的HttpUrl对象，用来解析和包装url
    private HttpUrl baseUrl;
    //转换器的工厂集合，retrofit构建可以插入多个转换器，比如gson转换器，Jackson转换器等
    private List<Converter.Factory> converterFactories = new ArrayList<>();
    //用来发起request和接收response的Call适配器，retrofit支持rxjava就是通过引入retrofit-adapters支持的，就是这个CallAdapter，这里先不作过多解释
    private List<CallAdapter.Factory> adapterFactories = new ArrayList<>();
    //Executor并发框架，包括线程池等
    private Executor callbackExecutor;
    //是否需要立即生效
    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());
    }

    ...
 }

/**
     * Set the API base URL.
     * <p>
     * The specified endpoint values (such as with {@link GET @GET}) are resolved against this
     * value using {@link HttpUrl#resolve(String)}. The behavior of this matches that of an
     * {@code <a href="">} link on a website resolving on the current URL.
     * <p>
     * <b>Base URLs should always end in {@code /}.</b>
     * <p>
     * A trailing {@code /} ensures that endpoints values which are relative paths will correctly
     * append themselves to a base which has path components.
     * <p>
     * <b>Correct:</b><br>
     * Base URL: http://example.com/api/<br>
     * Endpoint: foo/bar/<br>
     * Result: http://example.com/api/foo/bar/
     * <p>
     * <b>Incorrect:</b><br>
     * Base URL: http://example.com/api<br>
     * Endpoint: foo/bar/<br>
     * Result: http://example.com/foo/bar/
     * <p>
     * This method enforces that {@code baseUrl} has a trailing {@code /}.
     * <p>
     * <b>Endpoint values which contain a leading {@code /} are absolute.</b>
     * <p>
     * Absolute values retain only the host from {@code baseUrl} and ignore any specified path
     * components.
     * <p>
     * Base URL: http://example.com/api/<br>
     * Endpoint: /foo/bar/<br>
     * Result: http://example.com/foo/bar/
     * <p>
     * Base URL: http://example.com/<br>
     * Endpoint: /foo/bar/<br>
     * Result: http://example.com/foo/bar/
     * <p>
     * <b>Endpoint values may be a full URL.</b>
     * <p>
     * Values which have a host replace the host of {@code baseUrl} and values also with a scheme
     * replace the scheme of {@code baseUrl}.
     * <p>
     * Base URL: http://example.com/<br>
     * Endpoint: https://github.com/square/retrofit/<br>
     * Result: https://github.com/square/retrofit/
     * <p>
     * Base URL: http://example.com<br>
     * Endpoint: //github.com/square/retrofit/<br>
     * Result: http://github.com/square/retrofit/ (note the scheme stays 'http')
     */
    public Builder baseUrl(HttpUrl baseUrl) {
      checkNotNull(baseUrl, "baseUrl == null");
      List<String> pathSegments = baseUrl.pathSegments();
      if (!"".equals(pathSegments.get(pathSegments.size() - 1))) {
        throw new IllegalArgumentException("baseUrl must end in /: " + baseUrl);
      }
      this.baseUrl = baseUrl;
      return this;
    }

/**
     * Create the {@link Retrofit} instance using the configured values.
     * <p>
     * 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<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);
    }

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

/** Make a GET request. */
@Documented
@Target(METHOD)
@Retention(RUNTIME)
public @interface GET {
  /**
   * A relative or absolute path, or full URL of the endpoint. This value is optional if the first
   * parameter of the method is annotated with {@link Url @Url}.
   * <p>
   * See {@linkplain retrofit2.Retrofit.Builder#baseUrl(HttpUrl) base URL} for details of how
   * this is resolved against a base URL to create the full endpoint URL.
   */
  String value() default "";
}

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

@SuppressWarnings("unchecked") // Single-interface proxy creation guarded by parameter safety.
  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);
          }
        });
  }

// 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));

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

/**
   * Synchronously send the request and return its response.
   *
   * @throws IOException if a problem occurred talking to the server.
   * @throws RuntimeException (and subclasses) if an unexpected error occurs creating the request
   * or decoding the response.
   */
  Response<T> execute() throws IOException;

  /**
   * Asynchronously send the request and notify {@code callback} of its response or if an error
   * occurred talking to the server, creating the request, or processing the response.
   */
  void enqueue(Callback<T> callback);

final Call<T> delegate;

@Override public void enqueue(final Callback<T> callback) {
    ... //各种判空等代码，这里先省略了

    okhttp3.Call call;
    Throwable failure;

    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();
        }
      }
    });
  }