Retrofit2 源码分析

Retrofit介绍

Retrofit是Square公司的一个开源网络访问框架。主要是把网络请求的数据，通过接口来封装。内部通过OKHttpClient来请求。

Retrofit的简单配置，使用

 Retrofit retrofit = new Retrofit
                .Builder()
                .baseUrl("https://api.xxx.com/")
                .build();
        APIService service = retrofit.create(APIService.class);
        Call call = service.getList("PRODUCT");
        try {
            call.enqueue(new Callback() {
                @Override
                public void onResponse(Call call, Response response) {}
                @Override
                public void onFailure(Call call, Throwable t) {}
            });
        } catch (Exception e) {
            e.printStackTrace();
        }

    ....

    @GET("product/list")
    Call getList(@Query String type);

下面，我们就从上面的调用，开始分析下Retrofit的大致流程。

Retrofit 分析

主要属性:
- Platform
- callFactory
- baseUrl
- converterFactories
- adapterFactories
- callbackExecutor

1.首先，从它的配置开始分析。new Retrofit.Builder()

看下源代码

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

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

      converterFactories.add(new BuiltInConverters());
    }

这里主要做了两件事。

• 1，获取了一个Platform.
• 2，给converterFactories集合添加了一个BuiltInConverters(转换器).

我们分别看下Platfor跟BuiltInConverters的内容。

1.1 ，先看下Platform的生成

private static final Platform PLATFORM = findPlatform();

  static Platform get() {
    return PLATFORM;
  }
    //这里我们只看Android平台
  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();
  }
=========================
Android类型的Platform
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();
        //Platform返回的CallAdapter.Factory
      return new ExecutorCallAdapterFactory(callbackExecutor);
    }

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

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

这里我们可以看到
- Platform使用的是 Android 的平台。
- MainThreadExecutor就是把执行的内容放到了主线程。
- platform返回的CallAdapter.Factory是ExecutorCallAdapterFactory

1.2，看下BuiltInConverters

final class BuiltInConverters extends Converter.Factory {
  @Override
  public Converter responseBodyConverter(Type type, Annotation[] annotations,
      Retrofit retrofit) {
    if (type == ResponseBody.class) {
      return Utils.isAnnotationPresent(annotations, Streaming.class)
          ? StreamingResponseBodyConverter.INSTANCE
          : BufferingResponseBodyConverter.INSTANCE;
    }
    if (type == Void.class) {
      return VoidResponseBodyConverter.INSTANCE;
    }
    return null;
  }

如果type是ResponseBody的话
如果是流，就返回StreamingResponseBodyConverter
否则，就返回BufferingResponseBodyConverter
否则，返回null

2，Retrofit的build()方法

 public Retrofit build() {
      if (baseUrl == null) {
        throw new IllegalStateException("Base URL required.");
      }
        //这里默认使用的是OkHttpClient
      okhttp3.Call.Factory callFactory = this.callFactory;
      if (callFactory == null) {
        callFactory = new OkHttpClient();
      }
        //这里没有设置的话，用的就是上面分析的Android里面获取的MainThreadExecutor
      Executor callbackExecutor = this.callbackExecutor;
      if (callbackExecutor == null) {
        callbackExecutor = platform.defaultCallbackExecutor();
      }

      List adapterFactories = new ArrayList<>(this.adapterFactories);
        //添加ExecutorCallAdapterFactory
      adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));

      // converterFactories里面有个默认的BuiltInConverters。
      List converterFactories = new ArrayList<>(this.converterFactories);
        //生成Retrofit
      return new Retrofit(callFactory, baseUrl, converterFactories, adapterFactories,
          callbackExecutor, validateEagerly);
    }
==========================
//Retrofit里面的属性都是不可变的
final okhttp3.Call.Factory callFactory;
  final HttpUrl baseUrl;
  final List converterFactories;
  final List adapterFactories;
  final @Nullable Executor callbackExecutor;
  final boolean validateEagerly;

  Retrofit(okhttp3.Call.Factory callFactory, HttpUrl baseUrl,
      List converterFactories, List adapterFactories,
      @Nullable Executor callbackExecutor, boolean validateEagerly) {
    this.callFactory = callFactory;
    this.baseUrl = baseUrl;
    this.converterFactories = unmodifiableList(converterFactories); // Defensive copy at call site.
    this.adapterFactories = unmodifiableList(adapterFactories); // Defensive copy at call site.
    this.callbackExecutor = callbackExecutor;
    this.validateEagerly = validateEagerly;
  }

到这里，配置就看完了。
Retrofit里面的属性：
- callFactory 是OKHttpClient
- baseUrl就是传入的url
- converterFactories 里面有个默认的BuiltInConverters
- adapterFactories 里面有个ExecutorCallAdapterFactory
- callbackExecutor 就是Android里面的MainThreadExecutor

3，看下他的调用，生成call

//调用方法
ApiService service = retrofit.create(ApiService.class);
Call call = service.getList("product");

源码，看下create()方法

public  T create(final Class service) {
    Utils.validateServiceInterface(service);
    if (validateEagerly) {
      eagerlyValidateMethods(service);
    }
    //动态代理
    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 {
            // 如果是Object的方法，直接过滤
            if (method.getDeclaringClass() == Object.class) {
              return method.invoke(this, args);
            }
            //默认是false
            if (platform.isDefaultMethod(method)) {
              return platform.invokeDefaultMethod(method, service, proxy, args);
            }
            //注释1。获取ServiceMethod对象
            ServiceMethod serviceMethod =
                (ServiceMethod) loadServiceMethod(method);
            //注释2。通过serviceMethod和参数，构建OKHttpCall
            OkHttpCall