Retrofit2.0源码解析

怎么使用就不详述了，我们主要来看原理。

1. 得到retrofit对象
   我们首先通过建造者模式获得retrofit实例

    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());
    }

    public Builder baseUrl(HttpUrl baseUrl) {
      checkNotNull(baseUrl, "baseUrl == null");
      List pathSegments = baseUrl.pathSegments();
      if (!"".equals(pathSegments.get(pathSegments.size() - 1))) {
        throw new IllegalArgumentException("baseUrl must end in /: " + baseUrl);
      }
      this.baseUrl = baseUrl;
      return this;
    }

    /** Add converter factory for serialization and deserialization of objects. */
    public Builder addConverterFactory(Converter.Factory factory) {
      converterFactories.add(checkNotNull(factory, "factory == null"));
      return this;
    }
 public Builder client(OkHttpClient client) {
      return callFactory(checkNotNull(client, "client == null"));
    }
        /**
     * Add a call adapter factory for supporting service method return types other than {@link
     * Call}.
     */
    public Builder addCallAdapterFactory(CallAdapter.Factory factory) {
      adapterFactories.add(checkNotNull(factory, "factory == null"));
      return this;
    }


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);
    }
  }
} 

在构造器中，首先获得当前设备的的平台信息，并把内置的BuiltInConverters添加到工厂集合，主要作用是使用多种converters时能够找到可以消耗该类型的转化器；
我们来看Platform这个类：

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) {
    }
    try {
      Class.forName("org.robovm.apple.foundation.NSObject");
      return new IOS();
    } catch (ClassNotFoundException ignored) {
    }
    return new Platform();
  }

  Executor defaultCallbackExecutor() {
    return null;
  }

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

  boolean isDefaultMethod(Method method) {
    return false;
  }

  Object invokeDefaultMethod(Method method, Class declaringClass, Object object, 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,
        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(Executor callbackExecutor) {
      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);
      }
    }
  }

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

    @Override CallAdapter.Factory defaultCallAdapterFactory(Executor callbackExecutor) {
      return new ExecutorCallAdapterFactory(callbackExecutor);
    }

    static class MainThreadExecutor implements Executor {
      private static Object queue;
      private static Method addOperation;

      static {
        try {
          // queue = NSOperationQueue.getMainQueue();
          Class operationQueue = Class.forName("org.robovm.apple.foundation.NSOperationQueue");
          queue = operationQueue.getDeclaredMethod("getMainQueue").invoke(null);
          addOperation = operationQueue.getDeclaredMethod("addOperation", Runnable.class);
        } catch (Exception e) {
          throw new AssertionError(e);
        }
      }

      @Override public void execute(Runnable r) {
        try {
          // queue.addOperation(r);
          addOperation.invoke(queue, r);
        } catch (IllegalArgumentException | IllegalAccessException e) {
          throw new AssertionError(e);
        } catch (InvocationTargetException e) {
          Throwable cause = e.getCause();
          if (cause instanceof RuntimeException) {
            throw (RuntimeException) cause;
          } else if (cause instanceof Error) {
            throw (Error) cause;
          }
          throw new RuntimeException(cause);
        }
      }
    }
  }
}

我们可以看到，如果检测到Android平台，会首先创建一个默认MainThreadExecutor的方法，(会在retrofit的build方法中调用，然后加到adapterFactories中)，它会获取ui线程的looper生成一个对应的handler，执行execute方法就是用主线程的handler执行handler.post(runnable)事件，然后用这个MainThreadExector生成一个默认的ExecutorCallAdapterFactory。Retrofit 需要 Java7 版本或 Android2.3 以上。

在build方法中我们可以看到必须传入baseurl，当不指定client的时候为默认的okhttpclient，在Platform中生成的MainThreadExecutor是回调执行者，在子线程获得网络数据之后转换到ui线程中，CallAdapterFactory通过工厂模式生成CallAdapter用静态代理的方式将网络请求返回的call进行包装，比如RxJava,ConverterFactory通过工厂模式生成Converter对网络请求数据进行转化,比如Gson。最后通过建造者模式生成retrofit对象 把定义的接口转化为实例

CallAdapter是Call的适配器，作用就是创建/转换Call对象，把Call转换成预期的格式。CallAdatper创建是通过CallAdapter.factory工厂类进行的，本身只有一个转换方法adapt()。创建CallAdatper Factory需要实现Factory接口，DefaultCallAdapter为Retrofit2自带默认Call转换器，用来生成OKHTTP的call请求调用。

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, 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);
          }
        });
  }

可以看出我们采用动态代理的方式，返回了我们传入的接口的代理实例，并和InvocationHandler关联起来，当我们调用代理类的方法时，invoke方法都会被执行，
invoke方法中首先把method转化为servicemethod

  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;
  }

通过建造者模式将method构建为ServiceMethod,

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

    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);
    }

在build方法中，构建calladapter对象，

private CallAdapter createCallAdapter() {
      Type returnType = method.getGenericReturnType();
      if (Utils.hasUnresolvableType(returnType)) {
        throw methodError(
            "Method return type must not include a type variable or wildcard: %s", returnType);
      }
      if (returnType == void.class) {
        throw methodError("Service methods cannot return void.");
      }
      Annotation[] annotations = method.getAnnotations();
      try {
        return retrofit.callAdapter(returnType, annotations);
      } catch (RuntimeException e) { // Wide exception range because factories are user code.
        throw methodError(e, "Unable to create call adapter for %s", returnType);
      }
    }

通过得到method的类型和注解，调用retrofit.callAdapter(returnType, annotations)——nextCallAdapter方法中会通过for循环从adapterFactories中获取calladAdapter对象,这会得到一个ExecutorCallAdapter对象；同理我们类似的得到responseConverter对象，主要是对数据进行转化。除此之外还会对方法注解进行解析

private void parseMethodAnnotation(Annotation annotation) {
      if (annotation instanceof DELETE) {
        parseHttpMethodAndPath("DELETE", ((DELETE) annotation).value(), false);
      } else if (annotation instanceof GET) {
        parseHttpMethodAndPath("GET", ((GET) annotation).value(), false);
      } else if (annotation instanceof HEAD) {
        parseHttpMethodAndPath("HEAD", ((HEAD) annotation).value(), false);
        if (!Void.class.equals(responseType)) {
          throw methodError("HEAD method must use Void as response type.");
        }
      } else if (annotation instanceof PATCH) {
        parseHttpMethodAndPath("PATCH", ((PATCH) annotation).value(), true);
      } else if (annotation instanceof POST) {
        parseHttpMethodAndPath("POST", ((POST) annotation).value(), true);
      } else if (annotation instanceof PUT) {
        parseHttpMethodAndPath("PUT", ((PUT) annotation).value(), true);
      } else if (annotation instanceof OPTIONS) {
        parseHttpMethodAndPath("OPTIONS", ((OPTIONS) annotation).value(), false);
      } else if (annotation instanceof HTTP) {
        HTTP http = (HTTP) annotation;
        parseHttpMethodAndPath(http.method(), http.path(), http.hasBody());
      } else if (annotation instanceof retrofit2.http.Headers) {
        String[] headersToParse = ((retrofit2.http.Headers) annotation).value();
        if (headersToParse.length == 0) {
          throw methodError("@Headers annotation is empty.");
        }
        headers = parseHeaders(headersToParse);
      } else if (annotation instanceof Multipart) {
        if (isFormEncoded) {
          throw methodError("Only one encoding annotation is allowed.");
        }
        isMultipart = true;
      } else if (annotation instanceof FormUrlEncoded) {
        if (isMultipart) {
          throw methodError("Only one encoding annotation is allowed.");
        }
        isFormEncoded = true;
      }
    }

以及参数注解进行解析等。
得到ServiceMethod对象后，我们通过接口传入的参数来得到okhttpcall对象

OkHttpCall(ServiceMethod serviceMethod, Object[] args) {
    this.serviceMethod = serviceMethod;
    this.args = args;
  }

然后就是把okhttpcall封装为call对象serviceMethod.callAdapter.adapt(okHttpCall)serviceMethod.callAdapter就是之前提到的ExecutorCallAdapterFactory生成的CallAdapter,在它的adapt方法中

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

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

adapt方法会返回一个ExecutorCallBackCall对象，callbackExecutor就是我们在检测platform时创建的MainThreadExector,
public Executor defaultCallbackExecutor() {
return new MainThreadExecutor();
}，call是我们刚过传入的okhttpcall。
综上就是动态代理的作用，看上去是调用了接口的 方法，其实此时Retrofit把 接口翻译成一个HTTP请求，这个请求中有callAdapter,responseAdapter,okhttpcall三个重要的对象。我们得到接口的代理实例后，调用请求会触发InvocationHandler的invoke方法，并返回一个ExecutorCallBackCall对象，并把okhttpcall传入它的参数中，通过这个call对象完成我们的请求。有两种请求方式，execute和enqueue；

 异步请求enqueue中

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

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

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

      delegate.enqueue(new Callback() {
        @Override public void onResponse(Call call, final Response 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);
              }
            }
          });
        }

这里有两个对象，一个是callbackexecutor，一个是delegate，前者把网络线程切换到主线程中（我们在retrofit的build方法中检测platform时执行的），后者执行真正的网络请求（就是我们之前传入的callback对象）

@Override public void enqueue(final Callback 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 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 response) {
        try {
          callback.onResponse(OkHttpCall.this, response);
        } catch (Throwable t) {
          t.printStackTrace();
        }
      }
    });
  }

在okhttpcall的enque对象中我们直接调用的okhttp进行的网络异步操作；类似的okhttpcall也是应用okhttp进行的同步请求。关于okhttp在这里就不详述了，有兴趣的可以看下 深入解析OkHttp3

注意无论是同步操作还是异步操作每一个call对象实例只能被执行一次，多次执行抛出异常。并且我们通过Clone方法创建一个一模一样的实例，并且它的开销也是很小的。

下面是retrofit与另一款常用的volley库的简单对比：

Volley:
1.缓存处理；Volley自己就提供了一套完整的缓存处理方案，默认使用文件存储到磁盘中
2.代码简单，可读性高。
3.同一个请求如果同时都在发送，那么实际上只会有一个请求真正发出去， 其它的请求会等待这个结果回来
4.请求发送的时候提供了优先级的概念，但是是只保证顺序出去，不保证顺序回来
5.支持不同的Http客户端实现，默认提供了HttpClient和HttpUrlConnection的实现，而Retrofit在2.0版本之后，锁死在OkHttp上了。
6.支持请求取消
Retrofit:
1.较好的可扩展性，Volley中每一个新建一个Request时都需要指定一个父类，告知序列化数据的方式，而Retrofit中只需要在配置时，指定各种转换器即可。CallAdapter的存在，可以使你随意代理调用的Call
2.OkHttpClient自带并发光环，而Volley中的工作线程是自己维护的，那么就有可能存在线程由于异常退出之后，没有下一个工作线程补充的风险(线程池可以弥补这个缺陷)，这在Retrofit中不存在
3.支持请求取消
4.Retrofit没有对缓存提供任何额外支持，缓存不在我们的控制范围之内，而是完全通过OkHttp来管理