android Retrofit简单使用及源码分析

简介

Retrofit是Square公司开发的一款针对Android网络请求的框架，Retrofit2底层基于OkHttp实现的。
在gradle中添加

compile 'com.squareup.retrofit2:retrofit:2.3.0'
    compile 'com.squareup.retrofit2:converter-gson:2.3.0'

实现简单的POST请求

1. 首先创建业务接口，代码如下

public interface LoginService {
    @POST("user/auth")
    Call login(@Query("username") String username, @Query("password") String password);
}

这里对几种类型的注解做下说明：
@GET 和@POST分别对应get和post请求
@Query代表请求参数，另外还有@Body，@Field，@Part，@Path，@Header等注解

2.完成LoginService的配置

Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(baseUrl)
                .addConverterFactory(GsonConverterFactory.create())
                .build();
   LoginService loginService = retrofit.create(LoginService.class);

3.调用接口方法，完成请求

Call<User> call = loginService.login("nanguangtailang","123");
        call.enqueue(new Callback<User>() {
            @Override
            public void onResponse(Call<User> call, Response<User> response) {
                Log.d("d",response.body().toString());
            }
           @Override
            public void onFailure(Call<User> call, Throwable t) {
                Log.d("f",t.getMessage());
            }
        });

到这里我们的第一次简单使用Retrofit之旅已经结束了，接下来我们看看Retrofit关键实现的源码(主要关注第二步中出现的类和方法)
在创建Retrofit实例使用了builder模式，Builder是Retrofit中的一个嵌套类，代码如下

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

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

可以看到这个类中的几个重要属性 Platform、okhttp3.Call.Factory、List< Converter.Factory >、List< CallAdapter.Factory > 、Executor
platform 代表我们目前使用的平台，Android、iOS等 ，okhttp3.Call.Factory 是我们传入的 okhttpClient,作用是代理Retrofit发出http请求

接下来我们看看

 LoginService loginService = retrofit.create(LoginService.class);

这行代码执行时发生了什么
这是create()方法的内部实现

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, @Nullable 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 =
                (ServiceMethod) loadServiceMethod(method);
            OkHttpCall