前言
retrofit作為最近非常火的一個(gè)網(wǎng)絡(luò)請(qǐng)求框架冀偶，已經(jīng)被很多同學(xué)應(yīng)用到了項(xiàng)目中醒第。但是同學(xué)們都知道它運(yùn)作的一個(gè)流程嗎？我們不僅要知其然进鸠，更要知其所以然稠曼。看過(guò)很多人都寫(xiě)過(guò)了對(duì)retrofit的解析客年，而且都寫(xiě)的特別不錯(cuò)霞幅。但是別人寫(xiě)的始終是別人寫(xiě)的（包括我這篇）漠吻，同學(xué)們一定要自己跟蹤一遍執(zhí)行的流程，相信一定會(huì)有很大的收獲司恳。之前總是看別人對(duì)源碼的分析途乃，過(guò)不了幾天就忘了，但是這次把流程過(guò)了一遍扔傅，對(duì)我產(chǎn)生了很大的幫助耍共。畢竟最快的學(xué)習(xí)方法出了多敲代碼之外就是閱讀優(yōu)秀的開(kāi)源代碼了。
retrofit簡(jiǎn)介
retrofit官方介紹：“A type-safe REST client for Android and Java.”它可以通過(guò)注解來(lái)描述Http請(qǐng)求猎塞，URL參數(shù)试读，查詢(xún)參數(shù)，同時(shí)荠耽，它還支持多個(gè)請(qǐng)求體和文件上傳钩骇。
使用retrofit的好處：
1.底層進(jìn)行網(wǎng)絡(luò)請(qǐng)求的是okhttp，retrofit是對(duì)okhttp進(jìn)行了一個(gè)上層的封裝铝量，okhttp在android 4.4已經(jīng)被加入源碼倘屹，所以它的強(qiáng)大就不用我多說(shuō)了吧
2.使用靈活，支持多種數(shù)據(jù)的解析慢叨，無(wú)間配合rxjaxa
3.功能模塊高度封裝唐瀑，解耦徹底
retrofit網(wǎng)絡(luò)請(qǐng)求流程
從圖中可以看出retrofit起到的是一個(gè)中間者的作用，app應(yīng)用層通過(guò)retrofit請(qǐng)求網(wǎng)絡(luò)插爹，實(shí)際上是retrofit對(duì)請(qǐng)求的url哄辣、請(qǐng)求頭等信息進(jìn)行一層封裝之后交給okhttp去請(qǐng)求服務(wù)器，之后服務(wù)器返回原始數(shù)據(jù)給okhttp赠尾，okhttp再交給retrofit進(jìn)行數(shù)據(jù)轉(zhuǎn)換力穗，最終在應(yīng)用層就看到了我們想要的結(jié)果。
Retrofit使用建造者模式創(chuàng)建所需對(duì)象气嫁，接下來(lái)結(jié)合源碼分析：
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() {
//通過(guò)構(gòu)造函數(shù)找到運(yùn)行平臺(tái)
this(Platform.get());
}
private static final Platform PLATFORM = findPlatform();
static Platform get() {
return PLATFORM;
}
private static Platform findPlatform() {
try {
//要求jvm查找并加載指定類(lèi)当窗，jvm會(huì)執(zhí)行改類(lèi)的靜態(tài)代碼塊
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();
}
上面代碼是通過(guò)Retrofit.Builder的構(gòu)造方法找到運(yùn)行平臺(tái)，在這里肯定返回的是Android平臺(tái);
public Builder baseUrl(String baseUrl) {
//對(duì)baseUrl進(jìn)行非空檢查寸宵，為空拋出一個(gè)異常
checkNotNull(baseUrl, "baseUrl == null");
//將baseUrl轉(zhuǎn)換為HttpUrl對(duì)象崖面，這里會(huì)驗(yàn)證baseUrl是否為一個(gè)合法的Url地址
HttpUrl httpUrl = HttpUrl.parse(baseUrl);
if (httpUrl == null) {
throw new IllegalArgumentException("Illegal URL: " + baseUrl);
}
return baseUrl(httpUrl);
}
public Builder baseUrl(HttpUrl baseUrl) {
checkNotNull(baseUrl, "baseUrl == null");
List<String> pathSegments = baseUrl.pathSegments();
//baseUrl是否以"/"結(jié)尾
if (!"".equals(pathSegments.get(pathSegments.size() - 1))) {
throw new IllegalArgumentException("baseUrl must end in /: " + baseUrl);
}
this.baseUrl = baseUrl;
return this;
}
static <T> T checkNotNull(T object, String message) {
if (object == null) {
throw new NullPointerException(message);
}
return object;
}
上面代碼是build.baseUrl()方法的源碼，主要進(jìn)行的工作是檢查url地址是否為空且是否合法
接下來(lái)是添加轉(zhuǎn)換器和rajaxa適配器梯影，代碼很簡(jiǎn)單
/** Add converter factory for serialization and deserialization of objects. */
public Builder addConverterFactory(Converter.Factory factory) {
converterFactories.add(checkNotNull(factory, "factory == null"));
return this;
}
/**
* Add a call adapter factory for supporting service method return types other than {@link
* Call}.
*/
public Builder addCallAdapterFactory(CallAdapter.Factory factory) {
adapterFactories.add(checkNotNull(factory, "factory == null"));
return this;
}
至于validateEagerly()這個(gè)方法巫员，我覺(jué)得大部分同學(xué)都沒(méi)用過(guò)，我也是看了源碼才知道有這個(gè)方法甲棍，那么這個(gè)方法用什么作用呢简识？
public Builder validateEagerly(boolean validateEagerly) {
this.validateEagerly = validateEagerly;
return this;
}
點(diǎn)進(jìn)去看發(fā)現(xiàn)也只是將validateEagerly的值改變了，那么它的作用是什么，我后面講retrofit.create()方法的時(shí)候再說(shuō)七扰。
參數(shù)都設(shè)置好呢奢赂，接下來(lái)就要build了：
public Retrofit build() {
if (baseUrl == null) {
throw new IllegalStateException("Base URL required.");
}
okhttp3.Call.Factory callFactory = this.callFactory;
if (callFactory == null) {
callFactory = new OkHttpClient();
}
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);
}
從 okhttp3.Call.Factory callFactory = this.callFactory（創(chuàng)建一個(gè)okhttpCilent對(duì)象）可以看出默認(rèn)的網(wǎng)絡(luò)請(qǐng)求是通過(guò)okhttp進(jìn)行的。以上retrofit就把參數(shù)都配置好了颈走，接下來(lái)是retrofit對(duì)接口的提供信息進(jìn)行包裝膳灶。
接下來(lái)就是重點(diǎn)了立由，retrofit通過(guò)create方法對(duì)接口中的注解、方法和參數(shù)進(jìn)行包裝
public <T> T create(final Class<T> service) {
Utils.validateServiceInterface(service);
//是否驗(yàn)證在生成Retrofit實(shí)例時(shí)提供的配置
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);
OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);
return serviceMethod.callAdapter.adapt(okHttpCall);
}
});
}
private void eagerlyValidateMethods(Class<?> service) {
Platform platform = Platform.get();
//遍歷接口中的方法
for (Method method : service.getDeclaredMethods()) {
if (!platform.isDefaultMethod(method)) {
loadServiceMethod(method);
}
}
}
ServiceMethod loadServiceMethod(Method method) {
ServiceMethod result;
synchronized (serviceMethodCache) {
result = serviceMethodCache.get(method);
if (result == null) {
result = new ServiceMethod.Builder(this, method).build();
//將請(qǐng)求添加到緩存
serviceMethodCache.put(method, result);
}
}
return result;
}
其實(shí)validateEagerly為true時(shí)就是提前驗(yàn)證在生成Retrofit實(shí)例時(shí)提供的配置拆吆。可以看到它最終是生成了一個(gè)ServiceMethod對(duì)象脂矫，而動(dòng)態(tài)代理中也是要生成ServiceMethod對(duì)象枣耀。
ServiceMethod就是對(duì)接口中的方法進(jìn)行驗(yàn)證后包裝成了一個(gè)HTTP請(qǐng)求。
來(lái)看一下 Proxy.newProxyInstance(
service.getClassLoader(),
new Class<?>[] { service },
new InvocationHandler())三個(gè)參數(shù)的意義
ClassLoader loader：定義了由哪個(gè)ClassLoader對(duì)象來(lái)對(duì)生成的代理對(duì)象進(jìn)行加載
Class<庭再？>[] interfaces：一個(gè)Interface對(duì)象的數(shù)組捞奕，表示的是我將要給我需要代理的對(duì)象提供一組什么接口，如果我提供了一組接口給它拄轻，那么這個(gè)代理對(duì)象就宣稱(chēng)實(shí)現(xiàn)了該接口(多態(tài))颅围，這樣我就能調(diào)用這組接口中的方法了
InvocationHandler handler：一個(gè)InvocationHandler對(duì)象，表示的是當(dāng)我這個(gè)動(dòng)態(tài)代理對(duì)象在調(diào)用方法的時(shí)候恨搓，會(huì)關(guān)聯(lián)到哪一個(gè)InvocationHandler對(duì)象上
再看一下 invoke(Object proxy, Method method, Object... args)中的三個(gè)參數(shù)
- Object proxy：指代我們所代理的那個(gè)真實(shí)對(duì)象
- Method method：指代的是我們所要調(diào)用真實(shí)對(duì)象的某個(gè)方法的Method對(duì)象
- Object[] args：指代的是調(diào)用真實(shí)對(duì)象某個(gè)方法時(shí)接受的參數(shù)
代理的作用是：當(dāng)我們把包含網(wǎng)絡(luò)請(qǐng)求接口的類(lèi)傳入到Retrofit的create方法后院促，就會(huì)利用動(dòng)態(tài)代理模式，將接口解析包裝成ServiceMethod斧抱，再轉(zhuǎn)化成OkHttpCall供OKhttp調(diào)用常拓。
最后通過(guò)okhttp發(fā)送給服務(wù)器，就完成了一次網(wǎng)絡(luò)請(qǐng)求辉浦。
還有許多細(xì)節(jié)沒(méi)有講到弄抬，比如為什么通過(guò)注解就可以決定post、get等請(qǐng)求及添加head宪郊。這里就交給同學(xué)們自己去摸索啦
