[TOC]
前篇我们详细地分析了OKHttp的核心源码，如果对OKHttp内部机制不了解的可以看看[Android主流三方库源码分析（一、深入理解OKHttp源码）](https://jsonchao.github.io/2018/12/01/Android%E4%B8%BB%E6%B5%81%E4%B8%89%E6%96%B9%E5%BA%93%E6%BA%90%E7%A0%81%E5%88%86%E6%9E%90%EF%BC%88%E4%B8%80%E3%80%81%E6%B7%B1%E5%85%A5%E7%90%86%E8%A7%A3OKHttp%E6%BA%90%E7%A0%81%EF%BC%89/)。这篇，将会来深入地分析下目前Android最优秀的网络封装框架Retrofit的源码流程。
### 一、基本使用流程
#### 1、定义HTTP API，用于描述请求
~~~
public interface GitHubService {
@GET("users/{user}/repos")
Call<List<Repo>> listRepos(@Path("user") String user);
}
~~~
#### 2、创建Retrofit并生成API的实现（注意：方法上面的注解表示请求的接口部分，返回类型是请求的返回值类型，方法的参数即是请求的参数）
~~~
// 1.Retrofit构建过程
Retrofit retrofit = new Retrofit.Builder()
.baseUrl("https://api.github.com/")
.build();
// 2.创建网络请求接口类实例过程
GitHubService service = retrofit.create(GitHubService.class);
~~~
#### 3、调用API方法，生成Call，执行请求
~~~
// 3.生成并执行请求过程
Call<List<Repo>> repos = service.listRepos("octocat");
repos.execute() or repos.enqueue()
~~~
Retrofit的基本使用流程很简洁，但是简洁并不代表简单，Retrofit为了实现这种简洁的使用流程，内部使用了优秀的架构设计和大量的设计模式，在我分析过Retrofit最新版的源码和大量优秀的Retrofit源码分析文章后，我发现，要想真正理解Retrofit内部的核心源码流程和设计思想，首先，需要对这九大设计模式有一定的了解，如下：
~~~
1.Retrofit构建过程
建造者模式、工厂方法模式
2.创建网络请求接口实例过程
外观模式、代理模式、单例模式、策略模式、装饰模式（建造者模式）
3.生成并执行请求过程
适配器模式（代理模式、装饰模式）
~~~
其次，需要对OKHttp源码有一定的了解，如果不了解的可以看看这篇[Android主流三方库源码分析（一、深入理解OKHttp源码）](https://jsonchao.github.io/2018/12/01/Android%E4%B8%BB%E6%B5%81%E4%B8%89%E6%96%B9%E5%BA%93%E6%BA%90%E7%A0%81%E5%88%86%E6%9E%90%EF%BC%88%E4%B8%80%E3%80%81%E6%B7%B1%E5%85%A5%E7%90%86%E8%A7%A3OKHttp%E6%BA%90%E7%A0%81%EF%BC%89/)。最后，让我们按以上流程去深入Retrofit源码内部，领悟它带给我们的**设计之美**。
### 二、Retrofit构建过程
#### 1、Retrofit核心对象解析
首先Retrofit中有一个全局变量非常关键，在V2.5之前的版本，使用的是LinkedHashMap()，它是一个网络请求配置对象，是由网络请求接口中方法注解进行解析后得到的。
~~~
public final class Retrofit {
// 网络请求配置对象，存储网络请求相关的配置，如网络请求的方法、数据转换器、网络请求适配器、网络请求工厂、基地址等
private final Map<Method, ServiceMethod<?>> serviceMethodCache = new ConcurrentHashMap<>();
~~~
Retrofit使用了建造者模式通过内部类Builder类建立一个Retrofit实例，如下：
~~~
public static final class Builder {
// 平台类型对象（Platform -> Android)
private final Platform platform;
// 网络请求工厂，默认使用OkHttpCall（工厂方法模式）
private @Nullable okhttp3.Call.Factory callFactory;
// 网络请求的url地址
private @Nullable HttpUrl baseUrl;
// 数据转换器工厂的集合
private final List<Converter.Factory> converterFactories = new ArrayList<>();
// 网络请求适配器工厂的集合，默认是ExecutorCallAdapterFactory
private final List<CallAdapter.Factory> callAdapterFactories = new ArrayList<>();
// 回调方法执行器，在 Android 上默认是封装了 handler 的 MainThreadExecutor, 默认作用是：切换线程（子线程 -> 主线程）
private @Nullable Executor callbackExecutor;
// 一个开关，为true则会缓存创建的ServiceMethod
private boolean validateEagerly;
~~~
#### 2、Builder内部构造
下面看看Builder内部构造做了什么。
~~~
public static final class Builder {
...
Builder(Platform platform) {
this.platform = platform;
}
public Builder() {
this(Platform.get());
}
...
}
class Platform {
private static final Platform PLATFORM = findPlatform();
static Platform get() {
return PLATFORM;
}
private static Platform findPlatform() {
try {
// 使用JVM加载类的方式判断是否是Android平台
Class.forName("android.os.Build");
if (Build.VERSION.SDK_INT != 0) {
return new Android();
}
} catch (ClassNotFoundException ignored) {
}
try {
// 同时支持Java平台
Class.forName("java.util.Optional");
return new Java8();
} catch (ClassNotFoundException ignored) {
}
return new Platform();
}
static class Android extends Platform {
...
@Override public Executor defaultCallbackExecutor() {
//切换线程（子线程 -> 主线程）
return new MainThreadExecutor();
}
// 创建默认的网络请求适配器工厂，如果是Android7.0或Java8上，则使
// 用了并发包中的CompletableFuture保证了回调的同步
// 在Retrofit中提供了四种CallAdapterFactory(策略模式)：
// ExecutorCallAdapterFactory（默认）、GuavaCallAdapterFactory、
// va8CallAdapterFactory、RxJavaCallAdapterFactory
@Override List<? extends CallAdapter.Factory> defaultCallAdapterFactories(
@Nullable Executor callbackExecutor) {
if (callbackExecutor == null) throw new AssertionError();
ExecutorCallAdapterFactory executorFactory = new ExecutorCallAdapterFactory(callbackExecutor);
return Build.VERSION.SDK_INT >= 24
? asList(CompletableFutureCallAdapterFactory.INSTANCE, executorFactory)
: singletonList(executorFactory);
}
...
@Override List<? extends Converter.Factory> defaultConverterFactories() {
return Build.VERSION.SDK_INT >= 24
? singletonList(OptionalConverterFactory.INSTANCE)
: Collections.<Converter.Factory>emptyList();
}
...
static class MainThreadExecutor implements Executor {
// 获取Android 主线程的Handler
private final Handler handler = new Handler(Looper.getMainLooper());
@Override public void execute(Runnable r) {
// 在UI线程对网络请求返回数据处理
handler.post(r);
}
}
}
~~~
可以看到，在Builder内部构造时设置了默认Platform、callAdapterFactories和callbackExecutor。
#### 3、添加baseUrl
很简单，就是将String类型的url转换为OkHttp的HttpUrl过程如下：
~~~
/**
* Set the API base URL.
*
* @see #baseUrl(HttpUrl)
*/
public Builder baseUrl(String baseUrl) {
checkNotNull(baseUrl, "baseUrl == null");
return baseUrl(HttpUrl.get(baseUrl));
}
public Builder baseUrl(HttpUrl baseUrl) {
checkNotNull(baseUrl, "baseUrl == null");
List<String> pathSegments = baseUrl.pathSegments();
if (!"".equals(pathSegments.get(pathSegments.size() - 1))) {
throw new IllegalArgumentException("baseUrl must end in /: " + baseUrl);
}
this.baseUrl = baseUrl;
return this;
}
~~~
#### 4、添加GsonConverterFactory
首先，看到GsonConverterFactory.creat()的源码。
~~~
public final class GsonConverterFactory extends Converter.Factory {
public static GsonConverterFactory create() {
return create(new Gson());
}
public static GsonConverterFactory create(Gson gson) {
if (gson == null) throw new NullPointerException("gson == null");
return new GsonConverterFactory(gson);
}
private final Gson gson;
// 创建了一个含有Gson对象实例的GsonConverterFactory
private GsonConverterFactory(Gson gson) {
this.gson = gson;
}
~~~
然后，看看addConverterFactory()方法内部。
~~~
public Builder addConverterFactory(Converter.Factory factory) {
converterFactories.add(checkNotNull(factory, "factory null"));
return this;
}
~~~
可知，这一步是将一个含有Gson对象实例的GsonConverterFactory放入到了数据转换器工厂converterFactories里。
#### 5、build过程
~~~
public Retrofit build() {
if (baseUrl == null) {
throw new IllegalStateException("Base URL required.");
}
okhttp3.Call.Factory callFactory = this.callFactory;
if (callFactory == null) {
// 默认使用okhttp
callFactory = new OkHttpClient();
}
Executor callbackExecutor = this.callbackExecutor;
if (callbackExecutor == null) {
// Android默认的callbackExecutor
callbackExecutor = platform.defaultCallbackExecutor();
}
// Make a defensive copy of the adapters and add the defaultCall adapter.
List<CallAdapter.Factory> callAdapterFactories = new ArrayList<>(this.callAdapterFactories);
// 添加默认适配器工厂在集合尾部
callAdapterFactories.addAll(platform.defaultCallAdapterFactorisca llbackExecutor));
// Make a defensive copy of the converters.
List<Converter.Factory> converterFactories = new ArrayList<>(
1 + this.converterFactories.size() + platform.defaultConverterFactoriesSize());
// Add the built-in converter factory first. This prevents overriding its behavior but also
// ensures correct behavior when using converters thatconsumeall types.
converterFactories.add(new BuiltInConverters());
converterFactories.addAll(this.converterFactories);
converterFactories.addAll(platform.defaultConverterFactories();
return new Retrofit(callFactory, baseUrl, unmodifiableList(converterFactories),
unmodifiableList(callAdapterFactories), callbackExecutor, validateEagerly);
}
~~~
可以看到，最终我们在Builder类中看到的6大核心对象都已经配置到Retrofit对象中了。
### 三、创建网络请求接口实例过程
retrofit.create()使用了外观模式和代理模式创建了网络请求的接口实例，我们分析下create方法。
~~~
public <T> T create(final Class<T> service) {
Utils.validateServiceInterface(service);
if (validateEagerly) {
// 判断是否需要提前缓存ServiceMethod对象
eagerlyValidateMethods(service);
}
// 使用动态代理拿到请求接口所有注解配置后，创建网络请求接口实例
return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class<?>[] { service },
new InvocationHandler() {
private final Platform platform = Platform.get();
private final Object[] emptyArgs = new Object[0];
@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);
}
return loadServiceMethod(method).invoke(args != null ? args : emptyArgs);
}
});
}
private void eagerlyValidateMethods(Class<?> service) {
Platform platform = Platform.get();
for (Method method : service.getDeclaredMethods()) {
if (!platform.isDefaultMethod(method)) {
loadServiceMethod(method);
}
}
}
~~~
继续看看loadServiceMethod的内部流程
~~~
ServiceMethod<?> loadServiceMethod(Method method) {
ServiceMethod<?> result = serviceMethodCache.get(method);
if (result != null) return result;
synchronized (serviceMethodCache) {
result = serviceMethodCache.get(method);
if (result == null) {
// 解析注解配置得到了ServiceMethod
result = ServiceMethod.parseAnnotations(this, method);
// 可以看到，最终加入到ConcurrentHashMap缓存中
serviceMethodCache.put(method, result);
}
}
return result;
}
abstract class ServiceMethod<T> {
static <T> ServiceMethod<T> parseAnnotations(Retrofit retrofit, Method method) {
// 通过RequestFactory解析注解配置（工厂模式、内部使用了建造者模式）
RequestFactory requestFactory = RequestFactory.parseAnnotations(retrofit, method);
Type returnType = method.getGenericReturnType();
if (Utils.hasUnresolvableType(returnType)) {
throw methodError(method,
"Method return type must not include a type variable or wildcard: %s", returnType);
}
if (returnType == void.class) {
throw methodError(method, "Service methods cannot return void.");
}
// 最终是通过HttpServiceMethod构建的请求方法
return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);
}
abstract T invoke(Object[] args);
}
~~~
#### 以下为请求构造核心流程
根据RequestFactory#Builder构造方法和parseAnnotations方法的源码，可知的它的作用就是用来解析注解配置的。
~~~
Builder(Retrofit retrofit, Method method) {
this.retrofit = retrofit;
this.method = method;
// 获取网络请求接口方法里的注释
this.methodAnnotations = method.getAnnotations();
// 获取网络请求接口方法里的参数类型
this.parameterTypes = method.getGenericParameterTypes();
// 获取网络请求接口方法里的注解内容
this.parameterAnnotationsArray = method.getParameterAnnotations();
}
~~~
接着看HttpServiceMethod.parseAnnotations()的内部流程。
~~~
static <ResponseT, ReturnT> HttpServiceMethod<ResponseT, ReturnT> parseAnnotations(
Retrofit retrofit, Method method, RequestFactory requestFactory) {
//1.根据网络请求接口方法的返回值和注解类型，
// 从Retrofit对象中获取对应的网络请求适配器
CallAdapter<ResponseT, ReturnT> callAdapter = createCallAdapter(retrofit,method);
// 得到响应类型
Type responseType = callAdapter.responseType();
...
//2.根据网络请求接口方法的返回值和注解类型从Retrofit对象中获取对应的数据转换器
Converter<ResponseBody, ResponseT>responseConverter =
createResponseConverter(retrofit,method, responseType);
okhttp3.Call.Factory callFactory = retrofit.callFactory;
return newHttpServiceMethod<>(requestFactory, callFactory, callAdapter,responseConverter);
}
~~~
##### 1.createCallAdapter(retrofit, method)
~~~
private static <ResponseT, ReturnT> CallAdapter<ResponseT, ReturnT> createCallAdapter(
Retrofit retrofit, Method method) {
// 获取网络请求接口里方法的返回值类型
Type returnType = method.getGenericReturnType();
// 获取网络请求接口接口里的注解
Annotation[] annotations = method.getAnnotations();
try {
//noinspection unchecked
return (CallAdapter<ResponseT, ReturnT>) retrofit.callAdapter(returnType, annotations);
} catch (RuntimeException e) { // Wide exception range because factories are user code.
throw methodError(method, e, "Unable to create call adapter for %s", returnType);
}
}
public CallAdapter<?, ?> callAdapter(Type returnType, Annotation[] annotations) {
return nextCallAdapter(null, returnType, annotations);
}
public CallAdapter<?, ?> nextCallAdapter(@Nullable CallAdapter.Factory skipPast, Type returnType,
Annotation[] annotations) {
...
int start = callAdapterFactories.indexOf(skipPast) + 1;
// 遍历 CallAdapter.Factory 集合寻找合适的工厂
for (int i = start, count = callAdapterFactories.size(); i <count; i++) {
CallAdapter<?, ?> adapter = callAdapterFactories.get(i).get(returnType, annotations, this);
if (adapter != null) {
return adapter;
}
}
}
~~~
##### 2.createResponseConverter(Retrofit retrofit, Method method, Type responseType)
~~~
private static <ResponseT> Converter<ResponseBody, ResponseT> createResponseConverter(
Retrofit retrofit, Method method, Type responseType) {
Annotation[] annotations = method.getAnnotations();
try {
return retrofit.responseBodyConverter(responseType,annotations);
} catch (RuntimeException e) { // Wide exception range because factories are user code.
throw methodError(method, e, "Unable to create converter for%s", responseType);
}
}
public <T> Converter<ResponseBody, T> responseBodyConverter(Type type, Annotation[] annotations) {
return nextResponseBodyConverter(null, type, annotations);
}
public <T> Converter<ResponseBody, T> nextResponseBodyConverter(
@Nullable Converter.Factory skipPast, Type type, Annotation[] annotations) {
...
int start = converterFactories.indexOf(skipPast) + 1;
// 遍历 Converter.Factory 集合并寻找合适的工厂, 这里是GsonResponseBodyConverter
for (int i = start, count = converterFactories.size(); i < count; i++) {
Converter<ResponseBody, ?> converter =
converterFactories.get(i).responseBodyConverter(type, annotations, this);
if (converter != null) {
//noinspection unchecked
return (Converter<ResponseBody, T>) converter;
}
}
~~~
#### 最终，执行HttpServiceMethod的invoke方法
~~~
@Override ReturnT invoke(Object[] args) {
return callAdapter.adapt(
new OkHttpCall<>(requestFactory, args, callFactory, responseConverter));
}
~~~
最终在adapt中创建了一个ExecutorCallbackCall对象，它是一个装饰者，而在它内部真正去执行网络请求的还是OkHttpCall。
### 四、创建网络请求接口类实例并执行请求过程
#### 1、service.listRepos()
~~~
1、Call<List<Repo>> repos = service.listRepos("octocat");
~~~
service对象是动态代理对象Proxy.newProxyInstance()，当调用getCall()时会被 它拦截，然后调用自身的InvocationHandler#invoke()，得到最终的Call对象。
#### 2、同步执行流程 repos.execute()
~~~
@Override public Response<T> execute() throws IOException {
okhttp3.Call call;
synchronized (this) {
if (executed) throw new IllegalStateException("Already executed.");
executed = true;
if (creationFailure != null) {
if (creationFailure instanceof IOException) {
throw (IOException) creationFailure;
} else if (creationFailure instanceof RuntimeException) {
throw (RuntimeException) creationFailure;
} else {
throw (Error) creationFailure;
}
}
call = rawCall;
if (call == null) {
try {
// 创建一个OkHttp的Request对象请求
call = rawCall = createRawCall();
} catch (IOException | RuntimeException | Error e) {
throwIfFatal(e); // Do not assign a fatal error to creationFailure.
creationFailure = e;
throw e;
}
}
}
if (canceled) {
call.cancel();
}
// 调用OkHttpCall的execute()发送网络请求（同步），
// 并解析网络请求返回的数据
return parseResponse(call.execute());
}
private okhttp3.Call createRawCall() throws IOException {
// 创建 一个okhttp3.Request
okhttp3.Call call =
callFactory.newCall(requestFactory.create(args));
if (call == null) {
throw new NullPointerException("Call.Factory returned null.");
}
return call;
}
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 the response along.
rawResponse = rawResponse.newBuilder()
.body(new NoContentResponseBody(rawBody.contentType(), rawBody.contentLength()))
.build();
// 根据响应返回的状态码进行处理
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) {
rawBody.close();
return Response.success(null, rawResponse);
}
ExceptionCatchingResponseBody catchingBody = new ExceptionCatchingResponseBody(rawBody);
try {
// 将响应体转为Java对象
T body = responseConverter.convert(catchingBody);
return Response.success(body, rawResponse);
} catch (RuntimeException e) {
// If the underlying source threw an exception, propagate that rather than indicating it was
// a runtime exception.
catchingBody.throwIfCaught();
throw e;
}
}
~~~
#### 3、异步请求流程 reponse.enqueque
~~~
@Override
public void enqueue(final Callback<T> callback) {
// 使用静态代理 delegate进行异步请求
delegate.enqueue(new Callback<T>() {
@Override
public void onResponse(Call<T> call, finalResponse<T>response) {
// 线程切换，在主线程显示结果
callbackExecutor.execute(new Runnable() {
@Override
public void run() {
if (delegate.isCanceled()) {
callback.onFailure(ExecutorCallbackCall.this, newIOException("Canceled"));
} else {
callback.onResponse(ExecutorCallbackCall.this,respons);
}
}
});
}
@Override
public void onFailure(Call<T> call, final Throwable t) {
callbackExecutor.execute(new Runnable() {
@Override public void run() {
callback.onFailure(ExecutorCallbackCall.this, t);
}
});
}
});
}
~~~
看看 delegate.enqueue 内部流程。
~~~
@Override
public void enqueue(final Callback<T> callback) {
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 {
// 创建OkHttp的Request对象，再封装成OkHttp.call
// 方法同发送同步请求，此处上面已分析
call = rawCall = createRawCall();
} catch (Throwable t) {
failure = creationFailure = t;
}
}
@Override public void enqueue(final Callback<T> callback) {
checkNotNull(callback, "callback == null");
okhttp3.Call call;
Throwable failure;
...
call.enqueue(new okhttp3.Callback() {
@Override public void onResponse(okhttp3.Call call, okhttp3.Response rawResponse) {
Response<T> response;
try {
// 此处上面已分析
response = parseResponse(rawResponse);
} catch (Throwable e) {
throwIfFatal(e);
callFailure(e);
return;
}
try {
callback.onResponse(OkHttpCall.this, response);
} catch (Throwable t) {
t.printStackTrace();
}
}
@Override public void onFailure(okhttp3.Call call, IOException e) {
callFailure(e);
}
private void callFailure(Throwable e) {
try {
callback.onFailure(OkHttpCall.this, e);
} catch (Throwable t) {
t.printStackTrace();
}
}
});
}
~~~
如果你看到这里的话，恭喜你，你已经对Retrofit已经有一个比较深入的了解了，但是，笔者还是要建议大家自己主动配合着Retrofit最新版的源码一步步去彻底地认识它，只有这样，你才能看到它真实的内心，最后附上一张Stay的Retrofit源码流程图，要注意的是，这是V2.5之前版本的流程，但是，在看完上面的源码分析后，我们知道，主体流程是没有变化的。
### 五、总结
从本质上来说，Retrofit虽然只是一个RESTful 的HTTP 网络请求框架的封装库。但是，它内部通过 大量的设计模式 封装了 OkHttp，让使用者感到它非常简洁、易懂。它内部主要是用动态代理的方式，动态将网络请求接口的注解解析成HTTP请求，最后执行请求的过程。好了，至此，我们的Android主流三方库源码分析的网络库分析部分已经完毕。接下来，将为大家带来最流行的图片加载框架Glide的源码分析，敬请期待~
##### 参考链接：
* * *
1、Retrofit V2.5.0源码
2、Android进阶之光
3、[Android：手把手带你 深入读懂 Retrofit 2.0 源码](https://www.jianshu.com/p/0c055ad46b6c)
4、[Retrofit分析-漂亮的解耦套路](https://www.jianshu.com/p/45cb536be2f4)
链接：https://juejin.im/post/5e1fb9386fb9a0300a4501a6
