Android网络编程(十一)源码解析Retrofit

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Android网络编程系列

前言

最近博客的产出确实很少,因为博主我正在写一本Android进阶书籍,两头很难兼顾,但是每个月也得至少发一篇博客。上一篇我们介绍了Retrofit的使用方法,这一篇我们照例来学习Retrofit的源码。

1.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 IpService {
@GET("getIpInfo.php?ip=59.108.54.37")
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方法做了什么:
java

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

很简短,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 Android();
}
} catch (ClassNotFoundException ignored) {
}
try {
Class.forName("java.util.Optional");
return new Java8();
} catch (ClassNotFoundException ignored) {
}
try {
Class.forName("org.robovm.apple.foundation.NSObject");
return new 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() {
if (baseUrl == null) {//1
throw new IllegalStateException("Base URL required.");
}
okhttp3.Call.Factory callFactory = this.callFactory;//2
if (callFactory == null) {
callFactory = new OkHttpClient();//3
}
Executor callbackExecutor = this.callbackExecutor;
if (callbackExecutor == null) {
callbackExecutor = platform.defaultCallbackExecutor();//4
}
List<CallAdapter.Factory> adapterFactories = new ArrayList<>(this.adapterFactories);//5
adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));
List<Converter.Factory> converterFactories = new ArrayList<>(this.converterFactories);//6
return new Retrofit(callFactory, baseUrl, converterFactories, adapterFactories,
callbackExecutor, validateEagerly);
}

从注释1处可以看出baseUrl 是必须指定的。注释2处callFactory默认为this.callFactory,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 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。注释3处,如果没有设置callFactory则直接创建OkHttpClient。注释4的callbackExecutor用来将回调传递到UI线程。注释5的adapterFactories主要用于存储对Call进行转化的对象,后面在Call的创建过程会再次提到它。注释6处的converterFactories主要用于存储转化数据对象,后面也会提及到。此前在例子中调用的addConverterFactory(GsonConverterFactory.create()),就是设置返回的数据支持转换为Gson对象。最终会返回配置好的Retrofit类。

2.Call的创建过程

紧接着我们创建Retrofit实例并调用如下代码来生成接口的动态代理对象:

IpService ipService = retrofit.create(IpService.class);

接下来看Retrofit的create方法做了什么,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 {
// 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 = loadServiceMethod(method);//1
OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);
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方法最终会调用InvocationHandler的invoke 方法,它有3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。注释1处的loadServiceMethod(method)中的method就是我们定义的getIpMsg方法。接下来查看loadServiceMethod方法里做了什么:

private final Map<Method, ServiceMethod> serviceMethodCache = new LinkedHashMap<>();
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;
}

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。接下来看ServiceMethod是如何构建的,代码如下所示。

public ServiceMethod build() {
callAdapter = createCallAdapter();//1
responseType = callAdapter.responseType();//2
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?");
}
responseConverter = createResponseConverter();//3
for (Annotation annotation : methodAnnotations) {
parseMethodAnnotation(annotation);//4
}
...
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);
}
Annotation[] parameterAnnotations = parameterAnnotationsArray[p];//5
if (parameterAnnotations == null) {
throw parameterError(p, "No Retrofit annotation found.");
}
parameterHandlers[p] = parseParameter(p, parameterType, parameterAnnotations);
}
...
return new ServiceMethod<>(this);
}

注释1处调用了createCallAdapter方法,它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,Retrofit的build方法部分代码:

List<CallAdapter.Factory> adapterFactories = new ArrayList<>(this.adapterFactories);
adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));

adapterFactories列表默认会添加defaultCallAdapterFactory,defaultCallAdapterFactory指的是ExecutorCallAdapterFactory,ExecutorCallAdapterFactory的get方法如下所示。

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<?>>() {
@Override public Type responseType() {
return responseType;
}
@Override public <R> Call<R> adapt(Call<R> call) {
return new ExecutorCallbackCall<>(callbackExecutor, call);
}
};
}

get方法会得到CallAdapter对象,它的responseType方法会返回数据的真实类型,比如 Call<IpModel>,它就会返回IpModel。adapt方法会创建ExecutorCallbackCall,它会将call的回调转发至UI线程。
接着回到ServiceMethod的 build方法,注释2处调用CallAdapter的responseType得到的是返回数据的真实类型。
注释3处调用createResponseConverter方法来遍历converterFactories列表中存储的Converter.Factory,并返回一个合适的Converter用来转换对象。此前我们在构建Retrofit 调用了addConverterFactory(GsonConverterFactory.create())将GsonConverterFactory(Converter.Factory的子类)添加到converterFactories列表中,表示返回的数据支持转换为Json对象。
注释4处遍历parseMethodAnnotation方法来对请求方式(比如GET、POST)和请求地址进行解析。注释5处对方法中的参数注解进行解析(比如@Query、@Part)。最后创建ServiceMethod类并返回。
接下来回过头来查看Retrofit的create方法,在调用了loadServiceMethod方法后会创建OkHttpCall,OkHttpCall的构造函数只是进行了赋值操作。紧接着调用serviceMethod.callAdapter.adapt(okHttpCall),callAdapter的adapt方法前面讲过,它会创建ExecutorCallbackCall,ExecutorCallbackCall的部分代码如下所示。

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>() {//1
@Override public void onResponse(Call<T> call, final Response<T> response) {
callbackExecutor.execute(new Runnable() {
@Override public void run() {
if (delegate.isCanceled()) {
callback.onFailure(ExecutorCallbackCall.this, new IOException("Canceled"));
} else {
callback.onResponse(ExecutorCallbackCall.this, response);
}
}
});
}
@Override public void onFailure(Call<T> call, final Throwable t) {
callbackExecutor.execute(new Runnable() {
@Override public void run() {
callback.onFailure(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,而从注释1处可以看出ExecutorCallbackCall的enqueue方法最终调用的是delegate的enqueue方法。delegate从Retrofit的create方法的代码中我们知道它其实就是OkHttpCall。

3.Call的enqueue方法

接下来我们就来查看OkHttpCall的enqueue方法,代码如下所示。

public void enqueue(final Callback<T> callback) {
if (callback == null) throw new NullPointerException("callback == null");
okhttp3.Call call;
...
call.enqueue(new okhttp3.Callback() {//1
@Override public void onResponse(okhttp3.Call call, okhttp3.Response rawResponse)
throws IOException {
Response<T> response;
try {
response = parseResponse(rawResponse);//2
} catch (Throwable e) {
callFailure(e);
return;
}
callSuccess(response);
}
...
}

注释1处调用了okhttp3.Call的enqueue方法。注释2处调用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 {
ResponseBody bufferedBody = Utils.buffer(rawBody);
return Response.error(bufferedBody, rawResponse);
} finally {
rawBody.close();
}
}
if (code == 204 || code == 205) {
return Response.success(null, rawResponse);
}
ExceptionCatchingRequestBody catchingBody = new ExceptionCatchingRequestBody(rawBody);
try {
T body = serviceMethod.toResponse(catchingBody);//2
return Response.success(body, rawResponse);
} catch (RuntimeException e) {
catchingBody.throwIfCaught();
throw e;
}
}

根据返回的不同的状态码code值来做不同的操作,如果顺利则会调用注释2处的代码,接下来看toResponse方法里做了什么:

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GsonResponseBodyConverter:

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。因此我们也知道了此前调用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来请求网络并将返回的Response进行数据转换并回调给UI线程。
至此,Retrofit的源码就讲到这里。

刘望舒 wechat
我的新书《Android进阶之光》已出版,更多成体系的Android相关原创技术干货尽在微信公众号「刘望舒」。
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