Retrofit2.3.0 源码解析

Retrofit是对OKHttp的封装，简化了网络请求。具体使用参见官方文档。本文分析的代码是 retrofit2.3.0

先看一下retrofit官方的实例：

public final class SimpleService {
  public static final String API_URL = "https://api.github.com";

  public static class Contributor {
    public final String login;
    public final int contributions;

    public Contributor(String login, int contributions) {
      this.login = login;
      this.contributions = contributions;
    }
  }
  // 1
  public interface GitHub {
    @GET("/repos/{owner}/{repo}/contributors")
    Call> contributors(
        @Path("owner") String owner,
        @Path("repo") String repo);
  }

  public static void main(String... args) throws IOException {
    // Create a very simple REST adapter which points the GitHub API.
    //2
    Retrofit retrofit = new Retrofit.Builder()
        .baseUrl(API_URL)
        .addConverterFactory(GsonConverterFactory.create())
        .build();

    // Create an instance of our GitHub API interface.
    //3
    GitHub github = retrofit.create(GitHub.class);

    // Create a call instance for looking up Retrofit contributors.
    //4
    Call> call = github.contributors("square", "retrofit");

    // Fetch and print a list of the contributors to the library.
    //5
    List contributors = call.execute().body();
    //6
    for (Contributor contributor : contributors) {
      System.out.println(contributor.login + " (" + contributor.contributions + ")");
    }
  }
}

其中Retrofit的步骤：

1. 创建一个接口(GitHub 接口)。
2. 创建一个Retrofit对象，并提供baseUrl，ConverterFactory等。
3. 创建一个实现接口的一个代理对象。
4. 调用接口方法并返回一个Call对象。
5. 执行exceute()同步请求。
6. 解析Response。

接下来我们以这个步骤一步一步来看。

1、接口

retrofit2.http包，里面全部是定义HTTP请求的Java注解，比如GET、POST、PUT、DELETE、Headers、Path、Query等等。所以在接口里用注解方式定义请求类型等。

2、Retrofit对象

Retrofit 对象的创建还是用目前流行的Build模式创建。

public static final class Builder {
    private final Platform platform;
    private @Nullable okhttp3.Call.Factory callFactory;
    private HttpUrl baseUrl;
    private final List converterFactories = new ArrayList<>();
    private final List adapterFactories = new ArrayList<>();
    private @Nullable Executor callbackExecutor;
    private boolean validateEagerly;

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

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

    /**
     * Create the {@link Retrofit} instance using the configured values.
     * 

     * Note: If neither {@link #client} nor {@link #callFactory} is called a default {@link
     * OkHttpClient} will be created and used.
     */
    public Retrofit build() {
      if (baseUrl == null) {
        throw new IllegalStateException("Base URL required.");
      }

      okhttp3.Call.Factory callFactory = this.callFactory;
      if (callFactory == null) {
        //1
        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.
      // 2
      List adapterFactories = new ArrayList<>(this.adapterFactories);
      adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));

      // Make a defensive copy of the converters.
      List converterFactories =
          new ArrayList<>(1 + this.converterFactories.size());

      // Add the built-in converter factory first. This prevents overriding its behavior but also
      // ensures correct behavior when using converters that consume all types.
      //3
      converterFactories.add(new BuiltInConverters());
      converterFactories.addAll(this.converterFactories);

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

1. 通过build() 创建 OkHttpClient对象。
2. 设置一个CallAdapterFactory.
3. 添加各种Converter 转换工厂。

这里的AdapterFactory 默认使用DefaultCallAdapterFactory

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