Retrofit的出现让Android的网络请求变得异常简单，同时可以很好的配合后台的REST接口。非常值得我们去探究一下它的原理。
Retrofit的使用
通常我们是Retrofit是和Rxjava配合使用，这里我们不做用法上的过多研究，主要看原理，所以下面的代码都是Retrofit的自身API，没有用Rxjava。
下面是一个普通get请求
1.新建接口
新建接口API.java文件：
public interface API {
@GET("请求地址，但是不包括服务器的地址")
Call<Response> get(
@Query("param1") String param1,//第一个参数
@Query("param2") int param2);//第二个参数
}
在@GET注解里面加上除去服务器链接的请求地址，@Query注解里面是请求的参数名。
2.创建Retrofit服务和请求客户端
新建一个单例类，RetrofitService.java(名字随意)，在里面定义一个静态的OkHttpClient
private RetrofitService() {//构造方法私有化
}
public static RetrofitService getInstance() {//双重校验
if (instance == null) {
synchronized (RetrofitService.class) {
if (instance == null) {
instance = new RetrofitService();
}
}
}
return instance;
}
private static OkHttpClient mOkHttpClient;
private static void initOkHttpClient() {
HttpLoggingInterceptor logInterceptor = new HttpLoggingInterceptor();//日志拦截器
mOkHttpClient = new OkHttpClient.Builder().cache(cache)
.addInterceptor(logInterceptor)//日志拦截器，按照需要添加
.connectTimeout(10, TimeUnit.SECONDS).build();//连接超时时间
}
向外提供一个方法，用于获取刚才的API接口
private volatile static API aPI = null;
public static API createAPI() {
if (aPI == null) {
synchronized (RetrofitService.class) {
if (aPI == null) {
initOkHttpClient();
aPI = new Retrofit.Builder()
.client(mOkHttpClient)
.baseUrl("服务器地址")
.addConverterFactory(GsonConverterFactory.create())//指定json处理库
.build().create(API.class);//将第一步创建的API接口传入
}
}
}
return aPI;
}
3.开始发送请求
Call<Response> = call = RetrofitService.getInstance()
.createShowAPI()
.get("参数1", "参数2");
call.enqueue(new Callback<Response>() {
@Override
public void onResponse(Call<Response> call, Response<Response> response) {
//请求成功的处理
}
@Override
public void onFailure(Call<ShowApiResponse<ShowApiNews>> call, Throwable t) {
//请求失败的处理
}
});
Retrofit的核心-动态代理
Retrofit是如何将我们定义的接口方法最后转化成请求发送出去呢，这里就到源码去看看
创建者模式
首先来看Retrofit的创建，这里使用了创建者模式
new Retrofit.Builder()
.client(mOkHttpClient)
.baseUrl("服务器地址")
.addConverterFactory(GsonConverterFactory.create())//指定json处理库
.build().create(API.class);//将第一步创建的API接口传入
首先来看Retrofit.Builder()这个类的构造方法
public static final class Builder {
public Builder() {
this(Platform.get());
}
}
这里调用了一个带参数的构造方法，先看看Platform.get()是什么
private static final Platform PLATFORM = findPlatform();
static Platform get() {
return PLATFORM;
}
Platform.findPlatform
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) {
}
return new Platform();
}
可以看到，这个Platform类，顾名思义其实就是平台。在Retrofit中，内置了两种平台，一种是Android，一种是Java8。不同的平台，处理的方式不同。继续往下看代码就明白了
static class Android extends Platform {
@Override public Executor defaultCallbackExecutor() {
return new MainThreadExecutor();
}
@Override CallAdapter.Factory defaultCallAdapterFactory(@Nullable Executor callbackExecutor) {
if (callbackExecutor == null) throw new AssertionError();
return new ExecutorCallAdapterFactory(callbackExecutor);
}
static class MainThreadExecutor implements Executor {
private final Handler handler = new Handler(Looper.getMainLooper());
@Override public void execute(Runnable r) {
handler.post(r);
}
}
}
这里先不深入探究，后面还会再回来，不过我们已经可以看到，Executor类在Android平台里是返回了MainThreadExecutor，里面提供了一个handler，并且这个handler是传入的主线程的Looper，也就是说在execute方法里面，handler.post实际上是在主线程（UI线程）执行的。
这里再回到Retrofit.Builder()，看看那个带参数的构造方法：
public Builder() {
this(Platform.get());
}
Builder(Retrofit retrofit) {
platform = Platform.get();
callFactory = retrofit.callFactory;
baseUrl = retrofit.baseUrl;
converterFactories.addAll(retrofit.converterFactories);
adapterFactories.addAll(retrofit.adapterFactories);
// Remove the default, platform-aware call adapter added by build().
adapterFactories.remove(adapterFactories.size() - 1);
callbackExecutor = retrofit.callbackExecutor;
validateEagerly = retrofit.validateEagerly;
}
在这个构造方法里，对各种属性进行了初始化，来看看这些属性的定义
private final Platform platform;//刚才看到的平台，这里是Android
private @Nullable okhttp3.Call.Factory callFactory;//后面在分析
private HttpUrl baseUrl;//服务器地址
private final List<Converter.Factory> converterFactories = new ArrayList<>();//json解析工厂列表
private final List<CallAdapter.Factory> adapterFactories = new ArrayList<>();//后面再分析
private @Nullable Executor callbackExecutor;//这里是Android的Executor，在主线程执行回调
private boolean validateEagerly;//是否提前创建的标志
挨个看看这几个属性，这里有些属性的作用我们后面才知道，不过大部分看命名已经可以看到一些蹊跷：主要是callFactory和adapterFactories我们现在暂时不知道作用，继续往下看，在构造方法初始化之后，是调用Builder.build()方法
Builder.build()
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);
}
}
在这里是将Builder的属性，再传给Retrofit的构造方法，来看看我们刚才疑惑的那两个属性怎么赋值：
callFactory
okhttp3.Call.Factory callFactory = this.callFactory;
if (callFactory == null) {
callFactory = new OkHttpClient();
}
callFactory在默认的情况下，其实是一个OkHttpClient，也就是说Retrofit的内部请求原理其实是用的OkHttp。还记得我们最开始创建的时候也传入了一个静态类OkHttpClient么，这之间有什么关系呢？
new Retrofit.Builder().client(mOkHttpClient)
Retrofit.Builder().client
public Builder client(OkHttpClient client) {
return callFactory(checkNotNull(client, "client == null"));
}
public Builder callFactory(okhttp3.Call.Factory factory) {
this.callFactory = checkNotNull(factory, "factory == null");
return this;
}
其实client里面传入的OkHttpClient也是赋值给了callFactory，所以callFactory就是OkHttp的网络请求客户端
adapterFactories
// 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));
创建了一个新的列表，并且加入了默认的CallAdapterFactory，刚才我们知道platform是Android，所以再看看之前的代码：
Platform.Android
static class Android extends Platform {
@Override public Executor defaultCallbackExecutor() {
return new MainThreadExecutor();
}
@Override CallAdapter.Factory defaultCallAdapterFactory(@Nullable Executor callbackExecutor) {
if (callbackExecutor == null) throw new AssertionError();
return new ExecutorCallAdapterFactory(callbackExecutor);
}
static class MainThreadExecutor implements Executor {
private final Handler handler = new Handler(Looper.getMainLooper());
@Override public void execute(Runnable r) {
handler.post(r);
}
}
}
defaultCallAdapterFactory返回的是ExecutorCallAdapterFactory
final class ExecutorCallAdapterFactory extends CallAdapter.Factory {
final Executor callbackExecutor;
ExecutorCallAdapterFactory(Executor callbackExecutor) {
this.callbackExecutor = callbackExecutor;
}
@Override
public CallAdapter<?, ?> get(Type returnType, Annotation[] annotations, Retrofit retrofit) {
if (getRawType(returnType) != Call.class) {
return null;
}
final Type responseType = Utils.getCallResponseType(returnType);
return new CallAdapter<Object, Call<?>>() {
@Override public Type responseType() {
return responseType;
}
@Override public Call<Object> adapt(Call<Object> call) {
return new ExecutorCallbackCall<>(callbackExecutor, call);
}
};
}
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) {
checkNotNull(callback, "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()) {
// Emulate OkHttp's behavior of throwing/delivering an IOException on cancellation.
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);
}
});
}
});
}
@Override public boolean isExecuted() {
return delegate.isExecuted();
}
@Override public Response<T> execute() throws IOException {
return delegate.execute();
}
@Override public void cancel() {
delegate.cancel();
}
@Override public boolean isCanceled() {
return delegate.isCanceled();
}
@SuppressWarnings("CloneDoesntCallSuperClone") // Performing deep clone.
@Override public Call<T> clone() {
return new ExecutorCallbackCall<>(callbackExecutor, delegate.clone());
}
@Override public Request request() {
return delegate.request();
}
}
可以看到构造方法并没有做什么，只是将回调处理类传入。里面的其他方法后面调用的时候再来看。所以到这里我们还是不知道adapterFactories是干嘛的，但是我们看到了这个类里面有enqueue方法，还有一些处理响应的一些方法，所以我们可以知道它的作用是处理请求和响应，具体的用法后面继续看源码可以看到。
Retrofit.creat
在创建者初始化了所有属性之后，来到了Retrofit.creat方法
aPI = new Retrofit.Builder()
.client(mOkHttpClient)
.baseUrl("服务器地址")
.addConverterFactory(GsonConverterFactory.create())//指定json处理库
.build().create(API.class);
Retrofit.creat
public <T> T create(final Class<T> service) {
Utils.validateServiceInterface(service);//验证是否传入的为接口类
if (validateEagerly) {//提前创建，默认为false，这里跳过
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, @Nullable Object[] args)
throws Throwable {
// 方法定义所在的类，这里我们是定义在接口里面，返回false
if (method.getDeclaringClass() == Object.class) {
return method.invoke(this, args);
}
//platform.isDefaultMethod没做任何处理，直接返回false
if (platform.isDefaultMethod(method)) {
return platform.invokeDefaultMethod(method, service, proxy, args);
}
ServiceMethod<Object, Object> serviceMethod =
(ServiceMethod<Object, Object>) loadServiceMethod(method);
OkHttpCall<Object> okHttpCall = new OkHttpCall<>(serviceMethod, args);
return serviceMethod.callAdapter.adapt(okHttpCall);
}
});
}
这里看到了Retrofit的核心，使用动态代理来处理我们在接口中定义的方法。在调用我们定义的接口方法时，会来到动态代理类的invoke方法，然后执行最后的三行，在这里会解析定义的接口方法，并且做相应的处理。下回继续分解。
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