拆Retrofit2.0 的create()方法流程

滚滚长江东逝水，浪花也要乘风迎浪。一直在心里感激在网上写博客的各路大神们，在开发过程中帮助很大，自己也有心向先辈们学习，一来对自己也是一种督促，写博客也是巩固知识的过程，二来也希望能将自己开发中的所学记录下来，说不定能帮助到某些程序猿呢？这也是我的第一篇博客，内容如果有误还请指出来。好了，废话少说，进入正文。

Retrofit不用多介绍了，谁用谁知道。我在使用过程中会产生疑问：

1.声明的接口NetService怎么经过create就能调用发起网络请求？

2.接口中的方法可以返回Call，也可以返回其他类型，比如Observable，这之间有什么关系，或者经过怎样的转化过程？

带着这两个疑问去探索下create()方法的执行流程。下面是该方法的源码，比较短我们直接粘贴过来：

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);
                    }
                });
    }

我们的第一个疑问有解了，在create中会通过Java动态代理的方式来处理对方法的调用问题。这样说可能比较抽象，简单举个栗子，通过动态代理就会返回Call对象。

public interface MyService{
  @Get("info")
  Call getUser();
}

重点是最后的三行代码。

1. ServiceMethod serviceMethod =(ServiceMethod)loadServiceMethod(method);

ServiceMethod官方解释：

Adapts an invocation of an interface method into an HTTP call

有四个重要的成员：

//封装call，默认是okHttpCall
this.callFactory = builder.retrofit.callFactory();
//最著名的RxJava，默认是不支持，通过添加.addCallAdapterFactory(RxJavaCallAdapterFactory.create())增加对Obsevable的支持
this.callAdapter = builder.callAdapter;
//addConverterFactory(GsonConverterFactory.create())将服务器返回的response，提取出ResponseBody(json)转化为User
this.responseConverter = builder.responseConverter;
// 负责解析API定义时每个方法的参数
this.parameterHandlers = builder.parameterHandlers;

2. okHttpCall = new OkHttpCall(serviceMethod, args);

OkHttpCall实现Call接口，主要会用到的方法execute和enqueue,一个同步一个异步。

@Override
public Response execute() throws IOException {
    okhttp3.Call call;
    synchronized (this) {
     if (executed) throw new IllegalStateException("Already executed.");
        executed = true;
        if (creationFailure != null) {
            if (creationFailure instanceof IOException) {
                throw (IOException) creationFailure;
            } else {
                throw (RuntimeException) creationFailure;
            }
        }
        call = rawCall;
        if (call == null) {
            try {
                call = rawCall = createRawCall();
            } catch (IOException | RuntimeException e) {
                creationFailure = e;
                throw e;
            }
        }
    }
        if (canceled) {
            call.cancel();
        }
        return parseResponse(call.execute());
    }

可以看出，会通过createRawCall创建okhttp,创建过程先通过serviceMethod创建request，request传入callFactory创建okhttp3.Call。最后通过okhttpCall调用execute,enqueue的调用过程类似，最后也是通过okhttp调用enqueue。

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

3. serviceMethod.callAdapter.adapt(okHttpCall)

有几个概念需要注意下：

responseType:这个返回的是关注的类型，比如Call ,返回的是User的类型

returnType:这个是接口中方法的返回类型，比如Call那么返回的就是Call类型

public interface CallAdapter {
        Type responseType();
         T adapt(Call call);
        public abstract CallAdapter get(Type returnType, Annotation[] annotations, Retrofit retrofit);
protected static Type getParameterUpperBound(int index, ParameterizedType type) {
            return Utils.getParameterUpperBound(index, type);
        }
        protected static Class getRawType(Type type) {
            return Utils.getRawType(type);
        }
    }

callAdapter是接口public interface CallAdapter的实现类

serviceMethod中的callAdapter通过callAdapter = createCallAdapter()创建

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);
        }
    }

在retrofit中根据returnType从adapterFactories(List adapterFactories = new ArrayList();)中得到callAdapter

    public CallAdapter callAdapter(Type returnType, Annotation[] annotations) {
        return nextCallAdapter(null, returnType, annotations);
    }
    public CallAdapter nextCallAdapter(CallAdapter.Factory skipPast, Type returnType, Annotation[] annotations) {
        checkNotNull(returnType, "returnType == null");
        checkNotNull(annotations, "annotations == null");
        int start = adapterFactories.indexOf(skipPast) + 1;
        for (int i = start, count = adapterFactories.size(); i < count; i++) {
            CallAdapter adapter = adapterFactories.get(i).get(returnType, annotations, this);
            if (adapter != null) {
                return adapter;
            }
        }
        …….
    }

factories可以在初始化retrofit的时候添加，我们来看下默认的CallAdapter.Factory, 工厂模式，这个也是retrofit中大量使用的设计模式，在public interface Converter 接口中也会使用。DefaultCallAdapterFactory没有多少代码，重点看下get方法

1.默认的模式只支持Call这种returnType,所以一进来会判断是不是Call.class。

2.记下来会根据renturnType来获得responseType，也就是上面例子中Call中的User类型。

3.最后会new一个CallAdapter，因为默认的是支持Call这种returnType,因此在adapt方法中直接返回call。

    final class DefaultCallAdapterFactory extends CallAdapter.Factory {
        static final CallAdapter.Factory INSTANCE = new DefaultCallAdapterFactory();
        @Override
        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 call;
                }
            } ;
        }
    }

4.我们再简单看看RxJava2CallAdapterFactory

    public final class RxJava2CallAdapterFactory extends CallAdapter.Factory {
        ......
        @Override
        public CallAdapter get(Type returnType, Annotation[] annotations, Retrofit retrofit) {
            Class rawType = getRawType(returnType);
            ......
            return new RxJava2CallAdapter(responseType, scheduler, isAsync, isResult, isBody, isFlowable,isSingle, isMaybe, false);
        }
    }

在RxJava2CallAdapter中，看到熟悉的adapt方法，方法会将call封装成Observable，最后我们就可以快乐的使用RxJava

    final class RxJava2CallAdapter implements CallAdapter {
        @Override
        public Type responseType() {
            return responseType;
        }
        @Override
        public Object adapt(Call call) {
            Observable > responseObservable = isAsync
                    ? new CallEnqueueObservable(call)
                    : new CallExecuteObservable(call);
            ……
            return observable;
        }
    }

当然还没完，我们在深入到Observable中一探究竟，我们看下异步CallEnqueueObservable的例子。可以看到实际执行还是通过call.enqueue(callback)执行。

    final class CallEnqueueObservable extends Observable {
        private final Call originalCall;
        CallEnqueueObservable(Call originalCall) {
            this.originalCall = originalCall;
        }
        @Override
        protected void subscribeActual(Observer>observer) {
            // Since Call is a one-shot type, clone it for each new observer.
            Call call = originalCall.clone();
            CallCallback callback = new CallCallback(call, observer);
            observer.onSubscribe(callback);
            call.enqueue(callback);
        }
        private static final class CallCallback implements Disposable, Callback {
            ......
        }
    }

到这里我们对create()方法的分析过程就结束了，Retrofit是通过Java动态代理实现对接口方法的调用处理，通过解析方法的注解和参数封装request，通过request去构造HttpCall，再根据CallAdapter Factory对Call进行代理，其实内部还是通过OkHttp的Call模式做实际工作。Retrofit中比较多的使用工厂模式，实现很好的解耦，很值得借鉴。这方面的文章可以借鉴http://www.jianshu.com/p/45cb536be2f4。
谢谢！

扩展阅读：

https://blog.piasy.com/2016/06/25/Understand-Retrofit/

http://blog.csdn.net/lmj623565791/article/details/51304204

转自：http://www.jianshu.com/p/4d0eae0bc40c