Retrofi2源码解析——动态代理和注解的实战应用

Retrofit2源码解析——动态代理和注解的实战应用！
本想年前搞定这篇文章，接下来进入面试题的整理。但是年前确实有些心不在焉，拖拖拉拉就到了现在才完成。唉，这该死的拖延症~
前面讲到了代理和注解的基本应用，感觉有一点枯燥无味。空洞的理论脱离了实际应用就会变得毫无意义。So，今天要写的就是将动态代理和注解完美应用的Android网络请求框架的分析——Retrofit2。

关于OkHttp

稍微了解Retrofit的就会知道Retrofit也是使用的OkHttp作为请求框架，Retrofit只是对其进行封装，更加方便使用。我们看下OkHttp的请求方法：

OkHttpClient client = new OkHttpClient();
//创建一个Request
Request request = new Request.Builder()
        .get()
        .url(url)
        .build();
//通过client发起请求
client.newCall(request).enqueue(new Callback() {
    @Override
    public void onFailure(Call call, IOException e) {

    }
    @Override
    public void onResponse(Call call, Response response) throws IOException {
        if (response.isSuccessful()) {
        // String str = response.body().string();
        }

    }
});

这是关于OkHttp的简单使用，这里面执行了一个get请求。前面也说了Retrofit是对OkHttp的封装，那么Retrofit最后也会调用这些方法。不信？咱往下看。

1. Retrofit的创建

Retrofit通过Builder来创建，下面是简单创建：

new Retrofit.Builder()
    .baseUrl(Constant.BASE_URL)
    .addConverterFactory(GsonConverterFactory.create()) // 添加Gosn转换器
    .addCallAdapterFactory(RxJava2CallAdapterFactory.create()) // 添加RxJava2支持
    .build();

来看下build()过程：

public Builder() {
    // 获取当前的平台，我们这里是Android
    this(Platform.get());
}
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.
    // 默认的转换器
    converterFactories.add(new BuiltInConverters());
}
public Retrofit build() {
    // 检查baseUrl，这里并没有对""这种情况判断
    if (baseUrl == null) {
      throw new IllegalStateException("Base URL required.");
    }
    
    // 如果没有传入OkHttpClient，设置默认的OkHttpClient
    okhttp3.Call.Factory callFactory = this.callFactory;
    if (callFactory == null) {
      callFactory = new OkHttpClient();
    }

    // 也是默认的执行器，根据平台来的，这里的是Android的平台
    Executor callbackExecutor = this.callbackExecutor;
    if (callbackExecutor == null) {
      // MainThreadExecutor
      callbackExecutor = platform.defaultCallbackExecutor();
    }
    
    // Make a defensive copy of the adapters and add the default Call adapter.
    // 添加Call adapter，这个使用RxJava的时候会添加
    List adapterFactories = new ArrayList<>(this.adapterFactories);
    adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));

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

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

Retrofit(okhttp3.Call.Factory callFactory, HttpUrl baseUrl,
    List converterFactories, List adapterFactories,
    @Nullable Executor callbackExecutor, boolean validateEagerly) {
    this.callFactory = callFactory;
    this.baseUrl = baseUrl;
    // 这个是通过Collections.unmodifiableList创建一个只可读的List
    this.converterFactories = unmodifiableList(converterFactories); // Defensive copy at call site.
    this.adapterFactories = unmodifiableList(adapterFactories); // Defensive copy at call site.
    this.callbackExecutor = callbackExecutor;
    // 默认为false
    this.validateEagerly = validateEagerly;
}

2. 动态代理的使用

OK，创建完成。接着我们需要写一个接口：

public interface Api {
    @GET
    Call get(@Url String url, @QueryMap Map params);
}

// 接口创建
Api api = retrofit.create(Api.class)

使用我们刚刚创建的Retrofit来创建我们的Api，会调用Retrofit的create方法：

Retrofit.java:
public  T create(final Class service) {
    // 验证是否是接口并且接口数量是不是一个
    Utils.validateServiceInterface(service);
    if (validateEagerly) {
        eagerlyValidateMethods(service);
    }
    // 动态代理的使用
    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.
                // 该方法是在Object中定义，直接调用
                if (method.getDeclaringClass() == Object.class) {
                    return method.invoke(this, args);
                }
                // 这里返回值一直为false
                if (platform.isDefaultMethod(method)) {
                    return platform.invokeDefaultMethod(method, service, proxy, args);
                }
                
                ServiceMethod serviceMethod =
                    (ServiceMethod) loadServiceMethod(method);
                OkHttpCall