04、Android--Retrofit原理解析

Retrofit原理

Retrofit的创建过程

当我们使用Retrofit请求网络时,首先要写请求接口:

public interface ApiService {
    @GET("getInfo.php?ip=59.105.23.12")
    Call<IpModel> getIpMsg();
}

接着,我们通过调用如下代码来创建Retrofit:

Retrofit retrofit = new Retrofit.Builder()
        .baseUrl(url)
        .addConverterFactory(GsonConverterFactory.create())
        .build();

Retrofit是通过建造者模式构建出来的。接下来查看Builder方法:

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

查看Platform的get方法,如下所示:

private static final Platform PLATFORM = findPlatform();

static Platform get() {
    return PLATFORM;
}

private static Platform findPlatform() {
    try {
        Class.forName("android.os.Build");
        if (Build.VERSION.SDK_INT != 0) {
            return new Platform.Android();
        }
    } catch (ClassNotFoundException ignored) {
    }
    try {
        Class.forName("java.util.Optional");
        return new Platform.Java8();
    } catch (ClassNotFoundException ignored) {
    }
    try {
        Class.forName("org.robovm.apple.foundation.NSObject");
        return new Platform.IOS();
    } catch (ClassNotFoundException ignored) {
    }
    return new Platform();
}

Platform的get方法最终调用的是findPlatform方法,根据不同的运行平台来提供不同的线程池。

接下来查看build方法,代码如下所示

public Retrofit build() {
    // baseUrl 是必须指定的
    if (baseUrl == null) {
        throw new IllegalStateException("Base URL required.");
    }
	// callFactory 默认为this.callFactory
    okhttp3.Call.Factory callFactory = this.callFactory;
    if (callFactory == null) {
        // 如果没有设置callFactory,则直接创建 OkHttpClient
        callFactory = new OkHttpClient();
    }

    Executor callbackExecutor = this.callbackExecutor;
    if (callbackExecutor == null) {
        // 重点标记
        callbackExecutor = platform.defaultCallbackExecutor();
    }

    // adapterFactories主要用于存储对Call进行转化的对象
    List<CallAdapter.Factory> adapterFactories = new ArrayList<>(this.adapterFactories);
    adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));

    // 的converterFactories主要用于存储转化数据对象
    List<Converter.Factory> converterFactories = new ArrayList<>(this.converterFactories);

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

this.callFactory就是我们在构建Retrofit时调用callFactory方法所传进来的,如下所示:

public Retrofit.Builder callFactory(okhttp3.Call.Factory factory) {
    this.callFactory = checkNotNull(factory, "factory == null");
    return this;
}

因此,如果需要对 OkHttpClient 进行设置,则可以构建 OkHttpClient 对象,然后调用callFactory方法将

设置好的OkHttpClient传进去。

Call的创建过程

下面我们创建Retrofit实例并调用如下代码来生成接口的动态代理对象:

ApiService apiService = retrofit.create(ApiService.class);

接下来看Retrofit的create方法,代码如下所示:

public <T> T create(final Class<T> 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, Object... args)
                        throws Throwable {
                    // method就是我们定义的getIpMsg方法。
                    if (method.getDeclaringClass() == Object.class) {
                        return method.invoke(this, args);
                    }
                    if (platform.isDefaultMethod(method)) {
                        return platform.invokeDefaultMethod(method, service, proxy, args);
                    }
                    // 创建OkHttpCall
                    ServiceMethod serviceMethod = loadServiceMethod(method);
                    OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);
                    // 创建 ExecutorCallbackCall,并传入OkHttpCall
                    return serviceMethod.callAdapter.adapt(okHttpCall);
                }
            });
}

可以看到 create 方法返回了一个 Proxy.newProxyInstance 动态代理对象。

当我们调用IpService的 getIpMsg方法时,最终会调用InvocationHandler的invoke方法。它有三个参数:第一个是代理对象,第二个是调用的方法,第三个是方法的参数。

下面查看loadServiceMethod方法:

ServiceMethod loadServiceMethod(Method method) {
    ServiceMethod result;
    synchronized (serviceMethodCache) {
        result = serviceMethodCache.get(method);
        if (result == null) {
            result = new ServiceMethod.Builder(this, method).build();
            serviceMethodCache.put(method, result);
        }
    }
    return result;
}

里首先会从 serviceMethodCache 查询传入的方法是否有缓存。如果有,就用缓存的ServiceMethod;

如果没有,就创建一个,并加入 serviceMethodCache 缓存起来。

下面看ServiceMethod是如何构建的,代码如下所示:

public ServiceMethod build() {
    // 最终会得到我们在构建Retrofit调用build方法时 adapterFactories添加的对象的get方法
    callAdapter = createCallAdapter();
    // CallAdapter的responseType得到的是返回数据的真实类型
    responseType = callAdapter.responseType();
    if (responseType == Response.class || responseType == okhttp3.Response.class) {
        throw methodError("'"
                + Utils.getRawType(responseType).getName()
                + "' is not a valid response body type. Did you mean ResponseBody?");
    }
    // 遍历converterFactories列表中存储的Converter.Factory,并返回合适的Converter用来转换对象。
    responseConverter = createResponseConverter();

    // 遍历parseMethodAnnotation方法来对请求方式(比如GET、POST)和请求地址进行解析。
    for (Annotation annotation : methodAnnotations) {
        parseMethodAnnotation(annotation);
    }
	......
    int parameterCount = parameterAnnotationsArray.length;
    parameterHandlers = new ParameterHandler<?>[parameterCount];
    for (int p = 0; p < parameterCount; p++) {
        Type parameterType = parameterTypes[p];
        if (Utils.hasUnresolvableType(parameterType)) {
            throw parameterError(p, "Parameter type must not include a type variable or wildcard: %s",
                    parameterType);
        }
	    // 对方法中的参数注解进行解析(比如@Query、@Part)。
        Annotation[] parameterAnnotations = parameterAnnotationsArray[p];
        if (parameterAnnotations == null) {
            throw parameterError(p, "No Retrofit annotation found.");
        }

        parameterHandlers[p] = parseParameter(p, parameterType, parameterAnnotations);
    }
	......
    // 创建ServiceMethod类并返回。    
    return new ServiceMethod<>(this);
}

在前面Retrofit的build方法中,adapterFactories 列表默认会添加 defaultCallAdapterFactory。

CallAdapter.Factory defaultCallAdapterFactory(Executor callbackExecutor) {
    if (callbackExecutor != null) {
        return new ExecutorCallAdapterFactory(callbackExecutor);
    }
    return DefaultCallAdapterFactory.INSTANCE;
}

defaultCallAdapterFactory指的是ExecutorCallAdapterFactory。 ExecutorCallAdapterFactory的get方法如下所示:

@Override
public CallAdapter<Call<?>> get(Type returnType, Annotation[] annotations, Retrofit retrofit) {
    if (getRawType(returnType) != Call.class) {
        return null;
    }
    final Type responseType = Utils.getCallResponseType(returnType);
    return new CallAdapter<Call<?>>() {
        // 返回真实数据类型ipModel
        @Override public Type responseType() {
            return responseType;
        }
		// adapt 方法会创建ExecutorCallbackCall,它会将call 的回调转发至UI线程。
        @Override public <R> Call<R> adapt(Call<R> call) {
            return new ExecutorCallAdapterFactory.ExecutorCallbackCall<>(callbackExecutor, call);
        }
    };
}

get方法会得到CallAdapter对象,CallAdapter的responseType方法会返回数据的真实类型。

查看Retrofit的create方法,在调用了loadServiceMethod方法后会创建OkHttpCall,OkHttpCall 的构造方法只是进行了赋值操作。紧接着调用 serviceMethod.callAdapter.adapt(okHttpCall)。

其中,ExecutorCallbackCall的部分代码如下所示:

static final class ExecutorCallbackCall<T> implements Call<T> {
    final Executor callbackExecutor;
    final Call<T> delegate;

    ExecutorCallbackCall(Executor callbackExecutor, Call<T> delegate) {
        this.callbackExecutor = callbackExecutor;
        this.delegate = delegate;
    }

    @Override public void enqueue(final Callback<T> callback) {
        if (callback == null) throw new NullPointerException("callback == null");
        delegate.enqueue(new Callback<T>() {
            @Override public void onResponse(Call<T> call, final Response<T> response) {
                callbackExecutor.execute(new Runnable() {
                    @Override public void run() {
                        if (delegate.isCanceled()) {
 callback.onFailure(ExecutorCallAdapterFactory.ExecutorCallbackCall.this, new IOException("Canceled"));
                        } else {
                            callback.onResponse(ExecutorCallAdapterFactory.ExecutorCallbackCall.this, response);
                        }
                    }
                });
            }

            @Override public void onFailure(Call<T> call, final Throwable t) {
                callbackExecutor.execute(new Runnable() {
                    @Override public void run() {
                        callback.onFailure(ExecutorCallAdapterFactory.ExecutorCallbackCall.this, t);
                    }
                });
            }
        });
    }
	......
}

可以看出ExecutorCallbackCall是对Call的封装,它主要添加了通过callbackExecutor将请求回调到 UI 线

程。当我们得到 Call 对象后会调用它的 enqueue 方法,其实调用的是ExecutorCallbackCall的enqueue方法。

而从上面代码注释处可以看出ExecutorCallbackCall的enqueue方法最终调用的是delegate的enqueue方法。

delegate是传入的OkHttpCall。

Call的enqueue方法

下面我们就来查看OkHttpCall的enqueue方法,代码如下所示:

@Override public void enqueue(final Callback<T> callback) {
    if (callback == null) throw new NullPointerException("callback == null");

    okhttp3.Call call;
    Throwable failure;
	......
    // 调用了okhttp3.Call类型的call的enqueue方法    
    call.enqueue(new okhttp3.Callback() {
        @Override public void onResponse(okhttp3.Call call, okhttp3.Response rawResponse)
                throws IOException {
            Response<T> response;
            try {
                // 重要标记
                response = parseResponse(rawResponse);
            } catch (Throwable e) {
                callFailure(e);
                return;
            }
            callSuccess(response);
        }

	......
    });
}

我们再来看看parseResponse方法:

Response<T> parseResponse(okhttp3.Response rawResponse) throws IOException {
    ResponseBody rawBody = rawResponse.body();
	......
    int code = rawResponse.code();
    if (code < 200 || code >= 300) {
        try {
            // Buffer the entire body to avoid future I/O.
            ResponseBody bufferedBody = Utils.buffer(rawBody);
            return Response.error(bufferedBody, rawResponse);
        } finally {
            rawBody.close();
        }
    }

    if (code == 204 || code == 205) {
        return Response.success(null, rawResponse);
    }

    OkHttpCall.ExceptionCatchingRequestBody catchingBody = new OkHttpCall.ExceptionCatchingRequestBody(rawBody);
    try {
        // 重要标记
        T body = serviceMethod.toResponse(catchingBody);
        return Response.success(body, rawResponse);
    } catch (RuntimeException e) {
        catchingBody.throwIfCaught();
        throw e;
    }
}

根据返回的不同状态码code值来做不同的操作。接下来 看toResponse方法:

T toResponse(ResponseBody body) throws IOException {
    return responseConverter.convert(body);
}

这个responseConverter就是此前讲过在ServiceMethod的build方法调用createResponseConverter方法返回

的 Converter。

在此前的例子中我们传入的是 GsonConverterFactory,因此可以查看GsonConverterFactory的

代码,如下所示:

public final class GsonConverterFactory extends Converter.Factory {
  ......  
  @Override
  public Converter<ResponseBody, ?> responseBodyConverter(Type type, Annotation[] annotations,
      Retrofit retrofit) {
    TypeAdapter<?> adapter = gson.getAdapter(TypeToken.get(type));
    return new GsonResponseBodyConverter<>(gson, adapter);
  }
    ......
}

在GsonConverterFactory中有一个方法responseBodyConverter,它最终会创建GsonResponse- BodyConverter:

final class GsonResponseBodyConverter<T> implements Converter<ResponseBody, T> {
  private final Gson gson;
  private final TypeAdapter<T> adapter;

  GsonResponseBodyConverter(Gson gson, TypeAdapter<T> adapter) {
    this.gson = gson;
    this.adapter = adapter;
  }

  @Override public T convert(ResponseBody value) throws IOException {
    JsonReader jsonReader = gson.newJsonReader(value.charStream());
    try {
      return adapter.read(jsonReader);
    } finally {
      value.close();
    }
  }
}

在GsonResponseBodyConverter的convert方法里会将回调的数据转换为JSON格式。
此前调用responseConverter.convert是为了转换为特定的数据格式。Call的enqueue方法主要做的就是用OkHttp来请求网络,将返回的Response进行数据转换并回调给UI线程。

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