Android进阶必学retrofit源码解析

源码的分析将从基本的使用方法入手，分析retrofit的实现方案，以及其中涉及到的一些有趣的技巧。

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简单使用

定义HTTP API

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

创建Retrofit并生成API的实现

Retrofit retrofit = new Retrofit.Builder()
    .baseUrl("https://api.github.com/")
    .build();
GitHubService service = retrofit.create(GitHubService.class);

调用API方法，生成Call

Call> repos = service.listRepos("octocat");

Retrofit的创建

retrofit实例的创建，使用了builder模式，从下面的源码中可以看出

public static final class Builder {
    Builder(Platform platform) {
        this.platform = platform;
        converterFactories.add(new BuiltInConverters());
    }
    public Builder() {
        // Platform.get()方法可以用于判断当前的环境
        this(Platform.get());
    }
    public Builder baseUrl(String baseUrl) {
      checkNotNull(baseUrl, "baseUrl == null");
      HttpUrl httpUrl = HttpUrl.parse(baseUrl);
      if (httpUrl == null) {
        throw new IllegalArgumentException("Illegal URL: " + baseUrl);
      }
      return baseUrl(httpUrl);
    }
    
    public Retrofit build() {
      if (baseUrl == null) {
        throw new IllegalStateException("Base URL required.");
      }
      okhttp3.Call.Factory callFactory = this.callFactory;
      if (callFactory == null) {
        callFactory = new OkHttpClient();// 新建Client，留到之后newCall什么的
      }
      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);
    }
}

retrofit.create

好玩的地方开始了，我们先来看看这个方法

public  T create(final Class service) {
  Utils.validateServiceInterface(service);
  if (validateEagerly) {
    eagerlyValidateMethods(service);
  }
  // 动态代理，啦啦啦
  return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class[] { service },
      new InvocationHandler() {
        // platform 可以分辨出你是在android，还是java8，又或者别的
        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.
          // 这里的invoke，Object方法都走这里，比如equals、toString、hashCode什么的
          if (method.getDeclaringClass() == Object.class) {
            return method.invoke(this, args);
          }
          // java8默认方法，1.8的新特性
          if (platform.isDefaultMethod(method)) {
            return platform.invokeDefaultMethod(method, service, proxy, args);
          }
          // 这里是核心代码了
          ServiceMethod serviceMethod =
              (ServiceMethod) loadServiceMethod(method);
          OkHttpCall