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<List<Repo>> 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<List<Repo>> 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<CallAdapter.Factory> adapterFactories = new ArrayList<>(this.adapterFactories);
adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));
// Make a defensive copy of the converters.
List<Converter.Factory> 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> T create(final Class<T> 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<Object, Object> serviceMethod =
(ServiceMethod<Object, Object>) loadServiceMethod(method);
OkHttpCall<Object> okHttpCall = new OkHttpCall<>(serviceMethod, args);
return serviceMethod.callAdapter.adapt(okHttpCall);
}
});
}可以看出创建API使用了动态代理，根据接口动态生成的代理类，将接口的都转发给了负责连接代理类和委托类的InvocationHandler实例，接口方法也都通过其invoke方法来处理。
在invoke方法中，首先会通过Platform.get()方法判断出当前代码的执行环境，之后会先把Object和Java8的默认方法进行一个处理，也是在进行后续处理之前进行去噪。其中的关键代码其实就是最后三句，这也是这篇文章将要分析的。
创建ServiceMethod
erviceMethod<?, ?> loadServiceMethod(Method method) {
// 从缓存里面取出，如果有的话，直接返回好了
ServiceMethod<?, ?> result = serviceMethodCache.get(method);
if (result != null) return result;
synchronized (serviceMethodCache) {
result = serviceMethodCache.get(method);
if (result == null) {
// 为null的话，解析方法的注解和返回类型、参数的注解he参数类型，新建一个ServiceMethod
result = new ServiceMethod.Builder<>(this, method).build();// ->
// 新建的ServiceMethod加到缓存列表里面
serviceMethodCache.put(method, result);
}
}
return result;
}注解的解析
CallAdapter和Converter等到后面再分析，这里先看看parseMethodAnnotation(annotation)，功能和其名字一样，其对方法注解进行了解析。
/**
* 解析方法注解，呜啦啦
* 通过判断注解类型来解析
* @param annotation
*/
private void parseMethodAnnotation(Annotation annotation) {
if (annotation instanceof DELETE) {
parseHttpMethodAndPath("DELETE", ((DELETE) annotation).value(), false);
} else if (annotation instanceof GET) {
parseHttpMethodAndPath("GET", ((GET) annotation).value(), false);
}
// 其他的一些方法注解的解析
...
}
private void parseHttpMethodAndPath(String httpMethod, String value, boolean hasBody) {
if (this.httpMethod != null) {// 已经赋值过了
throw methodError("Only one HTTP method is allowed. Found: %s and %s.",
this.httpMethod, httpMethod);
}
this.httpMethod = httpMethod;
this.hasBody = hasBody;
// value为设置注解方法时候，设置的值，官方例子中的users/{user}/repos or user
if (value.isEmpty()) {
return;
}
// 查询条件的一些判断
...
this.relativeUrl = value;
this.relativeUrlParamNames = parsePathParameters(value);
}
`在解析注解时，先通过instanceof判断出注解的类型，之后调用parseHttpMethodAndPath方法解析注解参数值，并设置httpMethod、relativeUrl、relativeUrlParamNames等属性。
上面说了API中方法注解的解析，现在来看看方法参数注解的解析，这是通过调用parseParameterAnnotation方法生成ParameterHandler实例来实现的，代码比较多，这里挑选@Query来看看。
else if (annotation instanceof Query) {
Query query = (Query) annotation;
String name = query.value();
boolean encoded = query.encoded();
Class<?> rawParameterType = Utils.getRawType(type);// 返回基础的类
gotQuery = true;
// 可以迭代，Collection
if (Iterable.class.isAssignableFrom(rawParameterType)) {
if (!(type instanceof ParameterizedType)) {
throw parameterError(p, rawParameterType.getSimpleName()
+ " must include generic type (e.g., "
+ rawParameterType.getSimpleName()
+ "<String>)");
}
ParameterizedType parameterizedType = (ParameterizedType) type;
Type iterableType = Utils.getParameterUpperBound(0, parameterizedType);// 返回基本类型
Converter<?, String> converter =
retrofit.stringConverter(iterableType, annotations);
return new ParameterHandler.Query<>(name, converter, encoded).iterable();
} else if (rawParameterType.isArray()) {// Array
Class<?> arrayComponentType = boxIfPrimitive(rawParameterType.getComponentType());// 如果是基本类型，自动装箱
Converter<?, String> converter =
retrofit.stringConverter(arrayComponentType, annotations);
return new ParameterHandler.Query<>(name, converter, encoded).array();
} else {// Other
Converter<?, String> converter =
retrofit.stringConverter(type, annotations);
return new ParameterHandler.Query<>(name, converter, encoded);
}在@Query中，将分成Collection、array、other三种情况处理参数，之后根据这些参数，调用ParameterHandler中的Query静态类，创建出一个ParameterHandler实例。这样循环直到解析了所有的参数注解，组合成为全局变量parameterHandlers，之后构建请求时会用到
OkHttpCall
ServiceMethod创建完成之后，我们来看看下一行代码中的OkHttpCall类，里面的包含了请求的执行和响应处理，我们来看看异步请求的做法
OkHttpCall(ServiceMethod<T, ?> serviceMethod, Object[] args) {
this.serviceMethod = serviceMethod;
this.args = args;
}
@Override public void enqueue(final Callback<T> callback) {
checkNotNull(callback, "callback == null");
okhttp3.Call call;
Throwable failure;
synchronized (this) {
if (executed) throw new IllegalStateException("Already executed.");
executed = true;
call = rawCall;
failure = creationFailure;
if (call == null && failure == null) {
try {
call = rawCall = createRawCall();// 创建OkHttp3.Call
} catch (Throwable t) {
failure = creationFailure = t;
}
}
}
if (failure != null) {
callback.onFailure(this, failure);
return;
}
if (canceled) {
call.cancel();
}
call.enqueue(new okhttp3.Callback() {
@Override public void onResponse(okhttp3.Call call, okhttp3.Response rawResponse)
throws IOException {
Response<T> response;
try {
response = parseResponse(rawResponse);// ->
} catch (Throwable e) {
callFailure(e);
return;
}
callSuccess(response);
}
@Override public void onFailure(okhttp3.Call call, IOException e) {
try {
callback.onFailure(OkHttpCall.this, e);
} catch (Throwable t) {
t.printStackTrace();
}
}
private void callFailure(Throwable e) {
try {
callback.onFailure(OkHttpCall.this, e);
} catch (Throwable t) {
t.printStackTrace();
}
}
private void callSuccess(Response<T> response) {
try {
callback.onResponse(OkHttpCall.this, response);
} catch (Throwable t) {
t.printStackTrace();
}
}
});
}
private okhttp3.Call createRawCall() throws IOException {
Request request = serviceMethod.toRequest(args);// 根据ParameterHandler组装Request.Builder，生成Request
okhttp3.Call call = serviceMethod.callFactory.newCall(request);// Retrofit中创建的new OkHttpClient().newCall(request)
...
return call;
}CallAdapter
现在来看看enqueue传入的参数callback,这个参数可能和很多人心中想的并不一样，它并不是用户在使用时传入的那个Callback对象。那么他是从哪里来的呢？不知道你还记不记得我之前在Retrofit.Builder.build()方法中提到过一句代码Platform.get()。在不使用addCallAdapterFactory的情况下。将会使用Platform的一种内部类，在Android环境下将会使用到Android类（这其实是个策略模式）
static class Android extends Platform {
@Override public Executor defaultCallbackExecutor() {
return new MainThreadExecutor();
}
@Override CallAdapter.Factory defaultCallAdapterFactory(Executor callbackExecutor) {
return new ExecutorCallAdapterFactory(callbackExecutor);
}
static class MainThreadExecutor implements Executor {
// Looper.getMainLooper()就是为嘛响应会在主线程的原因
private final Handler handler = new Handler(Looper.getMainLooper());
@Override public void execute(Runnable r) {
handler.post(r);
}
}
}上面的代码先稍微放一下，我们继续看retrofit.Bulider.build,其中有几句比较关键的代码
callFactory = new OkHttpClient();
callbackExecutor = platform.defaultCallbackExecutor();
adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));
结合Android类中的代码可以看出，其最后生成了ExecutorCallAdapterFactory类。虽然看到了CallAdapter.Factory，但是到底是哪里执行了enqueue方法呢？现在我们来看看retrofit.create的最后一句代码serviceMethod.callAdapter.adapt(okHttpCall)。
Converter
现在回到OkhttpCall.enqueue方法中，在其中还有一句重要的代码没有看，那就是response = parseResponse(rawResponse);,我们来看看这其中做了什么。
Response<T> parseResponse(okhttp3.Response rawResponse) throws IOException
ResponseBody rawBody = rawResponse.body();
// Remove the body's source (the only stateful object) so we can pass th
rawResponse = rawResponse.newBuilder()
.body(new NoContentResponseBody(rawBody.contentType(), rawBody.conte
.build();
...
ExceptionCatchingRequestBody catchingBody = new ExceptionCatchingRequestBody(rawBody);
try {
T body = serviceMethod.toResponse(catchingBody);// 解析body，比如Gson解析
return Response.success(body, rawResponse);
} catch (RuntimeException e) {
// If the underlying source threw an exception, propagate that rather
// a runtime exception.
catchingBody.throwIfCaught();
throw e;
}
}
### ServiceMethod
R toResponse(ResponseBody body) throws IOException {
return responseConverter.convert(body);
}可以看出parseResponse最终调用了Converter.convert方法。这里以常用的GsonConverterFactory为例。
# GsonConverterFactory
@Override
public Converter<ResponseBody, ?> responseBodyConverter(Type type, Annotation[] annotations,
Retrofit retrofit) {
TypeAdapter<?> adapter = gson.getAdapter(TypeToken.get(type));
return new GsonResponseBodyConverter<>(gson, adapter);
}
# 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();
}
}
}responseBodyConverter方法中用到的type参数就是之前我在CallAdapter中提到的responseType方法的返回值。生成adapter方法，用于convert方法使用。OkHttpCall在这之后的代码就比较简单了，通过回调将转换后得响应数据发送出去即可
本文分析了Retrofit的执行流程，其实包含了Retrofit、ServiceMethod、OkHttpCall、CallAdapter、Converter等方面。Retrofit的代码相对是比较少，也比较容易理解的，不过却是很好的架构实例。
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