序章
首先引入依赖
implementation 'com.squareup.retrofit2:retrofit:2.9.0'
在原理之前，我们先来回忆一下Retrofit的基本使用
1、定义接口
interface MyService {
@GET("gallery/{imageType}/response")
fun getImages(@Path("imageType") imageType: String): Call<List<String>>
}
2、构建Retrofit，创建网络请求接口类实例
val retrofit = Retrofit.Builder()
.baseUrl(BASE_URL)
.build()
val myService = retrofit.create(MyService::class.java)
3、生成Call，执行请求
val resp = myService.getImages("banner")
resp.enqueue(object : Callback<List<String>> {
override fun onResponse(call: Call<List<String>>, response: Response<List<String>>) {
TODO("Not yet implemented")
}
override fun onFailure(call: Call<List<String>>, t: Throwable) {
TODO("Not yet implemented")
}
})这样一个基本的网络请求就搞定了，使用很简洁，正是因为其内部使用了大量的设计模式和优秀的架构设计，才得以使其如此方便地进行网络请求，下面我们就一起来探索探索Retrofit的设计之美。
Retrofit构建过程
使用了建造者模式通过内部静态类Builder构建一个Retrofit实例，这里只列出了部分方法，其他类似
public static final class Builder {
private final Platform platform;
// 网络请求工厂，工厂方法模式
private @Nullable okhttp3.Call.Factory callFactory;
// 网络请求地址
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，用于切换线程
private @Nullable Executor callbackExecutor;
// 一个开关，为true则会缓存创建的ServiceMethod
private boolean validateEagerly;
//......
public Builder baseUrl(String baseUrl) {
Objects.requireNonNull(baseUrl, "baseUrl == null");
return baseUrl(HttpUrl.get(baseUrl));
}
public Builder baseUrl(HttpUrl baseUrl) {
Objects.requireNonNull(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;
}
// 将一个含有Gson对象实例的GsonConverterFactory放入数据转换器工厂
public Builder addConverterFactory(Converter.Factory factory) {
converterFactories.add(Objects.requireNonNull(factory, "factory == null"));
return this;
}
//......
}通过build，我们上面Builder类中的参数对象都配置到了Retrofit对象中。
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> callAdapterFactories = new ArrayList<>(this.callAdapterFactories);
callAdapterFactories.addAll(platform.defaultCallAdapterFactories(callbackExecutor));
// 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 that consume all types.
converterFactories.add(new BuiltInConverters());
converterFactories.addAll(this.converterFactories);
converterFactories.addAll(platform.defaultConverterFactories());
return new Retrofit(
callFactory,
baseUrl,
unmodifiableList(converterFactories),
unmodifiableList(callAdapterFactories),
callbackExecutor,
validateEagerly);
}创建网络请求接口实例过程
使用动态代理的方式拿到所有注解配置后，创建网络请求接口实例。
public <T> T create(final Class<T> service) {
validateServiceInterface(service);
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 @Nullable 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);
}
args = args != null ? args : emptyArgs;
return platform.isDefaultMethod(method)
? platform.invokeDefaultMethod(method, service, proxy, args)
: loadServiceMethod(method).invoke(args);
}
});
}跟踪 loadServiceMethod，parseAnnotations解析注解配置得到ServiceMethod，然后加入到serviceMethodCache缓存中，是一个ConcurrentHashMap。
ServiceMethod<?> loadServiceMethod(Method method) {
ServiceMethod<?> result = serviceMethodCache.get(method);
if (result != null) return result;
synchronized (serviceMethodCache) {
result = serviceMethodCache.get(method);
if (result == null) {
result = ServiceMethod.parseAnnotations(this, method);
serviceMethodCache.put(method, result);
}
}
return result;
}abstract class ServiceMethod<T> {
static <T> ServiceMethod<T> parseAnnotations(Retrofit retrofit, Method method) {
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.");
}
return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);
}
abstract @Nullable T invoke(Object[] args);
}执行请求过程
public void enqueue(final Callback<T> callback) {
Objects.requireNonNull(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 {
// 创建一个OkHttp的Request对象请求
call = rawCall = createRawCall();
} catch (Throwable t) {
throwIfFatal(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) {
Response<T> response;
try {
// 解析网络请求返回的数据
response = parseResponse(rawResponse);
} catch (Throwable e) {
throwIfFatal(e);
callFailure(e);
return;
}
try {
callback.onResponse(OkHttpCall.this, response);
} catch (Throwable t) {
throwIfFatal(t);
t.printStackTrace(); // TODO this is not great
}
}
@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) {
throwIfFatal(t);
t.printStackTrace(); // TODO this is not great
}
}
});
} 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();
int code = rawResponse.code();
// 根据响应返回的状态码进行处理
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;
}
}总结
首先，使用建造者模式通过Builder构建一个Retrofit实例，Builder类中的参数对象都配置到Retrofit对象中，然后使用动态代理的方式拿到所有注解配置后，创建网络请求接口实例，生成OkHttp请求，通过callAdapterFactory找到对应的执行器，比如RxJava2CallAdapterFactory，最后通过ConverterFactory将返回数据解析成JavaBean，使用者只需关心请求参数，内部实现由Retrofit封装完成，底层请求还是基于okhttp实现的。
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