Retrofit2.5如何傳送請求和處理響應？

Retrofit · 發表 2019-04-08 15:23:49

摘要： Retrofit對請求方法的解析 Retrofit對我們在介面類中定義的方法是如何轉換成了請求，又如何處理響應。這一切都在它的loadServiceMethod 方法中程式碼基於Retrofit2.5.0 Ser...

Retrofit對請求方法的解析

Retrofit對我們在介面類中定義的方法是如何轉換成了請求，又如何處理響應。這一切都在它的loadServiceMethod 方法中

程式碼基於Retrofit2.5.0

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;
}

主要是呼叫ServiceMethod.parseAnnotations

static <T> ServiceMethod<T> parseAnnotations(Retrofit retrofit, Method method) {
RequestFactory requestFactory = RequestFactory.parseAnnotations(retrofit, method);//這裡就是對請求的解析，也就是我們定義的介面方法

Type returnType = method.getGenericReturnType();//獲取返回的型別
//省略無關程式碼

return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);
}

RequestFactory.parseAnnotations 這個方法就是對我們定義的介面方法的解析，在裡面解析註解和使用反射得到了關於請求的所有資訊。

響應的處理

在完成了對請求的處理後構建了RequestFactory類，這個類裡面包含了請求的所有資訊：

final class RequestFactory {
static RequestFactory parseAnnotations(Retrofit retrofit, Method method) {
return new Builder(retrofit, method).build();
}

private final Method method;
private final HttpUrl baseUrl;
final String httpMethod;
private final @Nullable String relativeUrl;
private final @Nullable Headers headers;
private final @Nullable MediaType contentType;
private final boolean hasBody;
private final boolean isFormEncoded;
private final boolean isMultipart;
private final ParameterHandler<?>[] parameterHandlers;

接下來繼續來看ServiceMethod的parseAnnotations剩下的語句

ServiceMethod.parseAnnotations

static <T> ServiceMethod<T> parseAnnotations(Retrofit retrofit, Method method) {
//省略無關程式碼

return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);//對響應的處理
}

HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory)

static <ResponseT, ReturnT> HttpServiceMethod<ResponseT, ReturnT> parseAnnotations(
Retrofit retrofit, Method method, RequestFactory requestFactory) {
CallAdapter<ResponseT, ReturnT> callAdapter = createCallAdapter(retrofit, method);//建立請求介面卡
Type responseType = callAdapter.responseType();
//省略無關程式碼
Converter<ResponseBody, ResponseT> responseConverter =
createResponseConverter(retrofit, method, responseType);//建立響應資料轉換器（Gson）

okhttp3.Call.Factory callFactory = retrofit.callFactory;//Okhttp請求客戶端
return new HttpServiceMethod<>(requestFactory, callFactory, callAdapter, responseConverter);
}

建立請求介面卡CallAdapter

首先進入第一行HttpServiceMethod.createCallAdapter

private static <ResponseT, ReturnT> CallAdapter<ResponseT, ReturnT> createCallAdapter(
Retrofit retrofit, Method method) {
Type returnType = method.getGenericReturnType();//獲取返回的型別
Annotation[] annotations = method.getAnnotations();//獲取註解
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);
}
}

在獲取了方法的返回型別和註解之後，呼叫

retrofit.callAdapter

public CallAdapter<?, ?> callAdapter(Type returnType, Annotation[] annotations) {
return nextCallAdapter(null, returnType, annotations);
}

public CallAdapter<?, ?> nextCallAdapter(@Nullable CallAdapter.Factory skipPast, Type returnType,
Annotation[] annotations) {
//空判斷
checkNotNull(returnType, "returnType == null");
checkNotNull(annotations, "annotations == null");

int start = callAdapterFactories.indexOf(skipPast) + 1;
for (int i = start, count = callAdapterFactories.size(); i < count; i++) {
CallAdapter<?, ?> adapter = callAdapterFactories.get(i).get(returnType, annotations, this);
if (adapter != null) {
return adapter;
}
}
}

這裡面是一個迴圈獲取請求介面卡工廠陣列callAdapterFactories ，在retrofit建立的時候，我們可以知道往這個數組裡面加的是Android 平臺預設的ExecutorCallAdapterFactory

public Retrofit build() {
List<CallAdapter.Factory> callAdapterFactories = new ArrayList<>(this.callAdapterFactories);
callAdapterFactories.addAll(platform.defaultCallAdapterFactories(callbackExecutor));
}
static class Android extends Platform {
@Override List<? extends CallAdapter.Factory> defaultCallAdapterFactories(
@Nullable Executor callbackExecutor) {
if (callbackExecutor == null) throw new AssertionError();
//這裡可以看到返回的類
ExecutorCallAdapterFactory executorFactory = new ExecutorCallAdapterFactory(callbackExecutor);
return Build.VERSION.SDK_INT >= 24
? asList(CompletableFutureCallAdapterFactory.INSTANCE, executorFactory)
: singletonList(executorFactory);
}
}

在剛才的迴圈裡，呼叫了它的get 方法，實際就是呼叫

ExecutorCallAdapterFactory.get

@Override public @Nullable CallAdapter<?, ?> get(
Type returnType, Annotation[] annotations, Retrofit retrofit) {
if (getRawType(returnType) != Call.class) {
return null;
}
final Type responseType = Utils.getCallResponseType(returnType);//根據返回型別獲取泛型裡面的想要獲取的響應型別
return new CallAdapter<Object, Call<?>>() {
@Override public Type responseType() {
return responseType;
}

@Override public Call<Object> adapt(Call<Object> call) {
return new ExecutorCallbackCall<>(callbackExecutor, call);
}
};
}

我們定義的返回型別一般是泛型的形式，如Call<Response>或者Observable<Response> ，所以Utils.getCallResponseType就是取出泛型裡面的型別。最後將得到的型別傳入新建的CallAdapter

return new CallAdapter<Object, Call<?>>() {
@Override public Type responseType() {
return responseType;
}

@Override public Call<Object> adapt(Call<Object> call) {
return new ExecutorCallbackCall<>(callbackExecutor, call);
}
};

自此就完成了CallAdapter 的建立，這裡記住它的adapt 方法，裡面是返回的ExecutorCallbackCall ，這個後面會呼叫，再來繼續看。

HttpServiceMethod的parseAnnotations第一行程式碼到此就完了，繼續往下執行：

建立響應資料轉換器（如Gson）和請求客戶端

HttpServiceMethod.parseAnnotations

static <ResponseT, ReturnT> HttpServiceMethod<ResponseT, ReturnT> parseAnnotations(
Retrofit retrofit, Method method, RequestFactory requestFactory) {
CallAdapter<ResponseT, ReturnT> callAdapter = createCallAdapter(retrofit, method);
Type responseType = callAdapter.responseType();//獲取具體的響應型別
//省略無關程式碼
Converter<ResponseBody, ResponseT> responseConverter =
createResponseConverter(retrofit, method, responseType);

okhttp3.Call.Factory callFactory = retrofit.callFactory;
return new HttpServiceMethod<>(requestFactory, callFactory, callAdapter, responseConverter);
}

注意這裡傳給responseConverter的responseType是我們剛才獲取Call泛型裡面的具體型別，也就是我們想要拿到的實體類型別。

這個過程跟剛才建立CallAdapter的過程很像，就連方法名都很像，所以來看看createResponseConverter 方法：

HttpServiceMethod.createResponseConverter

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);
}
}

通過method的getAnnotations拿到註解列表，然後跟返回型別傳入responseBodyConverter方法：

retrofit.responseBodyConverter

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) {
checkNotNull(type, "type == null");
checkNotNull(annotations, "annotations == null");

int start = converterFactories.indexOf(skipPast) + 1;
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;
}
}

這個過程簡直跟剛才建立CallAdapter的過程一模一樣，也是迴圈從converterFactories取出Converter，根據之前Retrofit的建立時我們傳入的是GsonConverterFactory ，所以是呼叫的它的responseBodyConverter 方法：

GsonConverterFactory.responseBodyConverter

public Converter<ResponseBody, ?> responseBodyConverter(Type type, Annotation[] annotations,
Retrofit retrofit) {
TypeAdapter<?> adapter = gson.getAdapter(TypeToken.get(type));
return new GsonResponseBodyConverter<>(gson, adapter);
}

這裡的程式碼就很面熟了，通過TypeToken 獲取型別，之後返回一個Gson解析器，它的convert就是呼叫Gson進行泛型的解析。

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 {
T result = adapter.read(jsonReader);
if (jsonReader.peek() != JsonToken.END_DOCUMENT) {
throw new JsonIOException("JSON document was not fully consumed.");
}
return result;
} finally {
value.close();
}
}
}

到這裡就完成了響應字串的解析器的建立，接下來會執行

okhttp3.Call.Factory callFactory = retrofit.callFactory;
return new HttpServiceMethod<>(requestFactory, callFactory, callAdapter, responseConverter);

建立okhttp請求的客戶端，這裡命名為callFactory，在Retrofit建立的時候我們傳入的okhttpClient就是賦值給了callFactory，最後將建立好的物件傳入HttpServiceMethod返回。

到此為止就執行完了HttpServiceMethod的parseAnnotations方法，在這個方法裡面，建立了幾個重要的東西

對請求和響應做具體處理的CallAdapter類，具體為ExecutorCallAdapterFactory ，這裡要注意它的adapt 方法返回ExecutorCallbackCall 類
響應資料解析工廠Converter，具體為GsonResponseBodyConverter
okhttp請求客戶端

傳送請求

終於到了這一步了，在做好了所有的準備工作之後，最終Retrofit怎麼來發送請求呢，首先一步步回到最開始的地方：在HttpServiceMethod.parseAnnotations方法是在ServiceMethod的parseAnnotations呼叫的

ServiceMethod.parseAnnotations

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.");
}

//在這裡呼叫並返回
return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);
}

abstract T invoke(Object[] args);
}

然後ServiceMethod.parseAnnotations方法是在Retrofit的loadServiceMethod 呼叫的

然後loadServiceMethod是在Retrofit的create 方法裡面建立動態代理的invoke呼叫的

Retrofit.create

public <T> T create(final Class<T> service) {
//省略無關程式碼
return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class<?>[] { service },
new InvocationHandler() {
//省略無關程式碼

@Override public Object invoke(Object proxy, Method method, @Nullable Object[] args)
throws Throwable {
//省略無關程式碼
return loadServiceMethod(method).invoke(args != null ? args : emptyArgs);
}
});
}

可以看到，在loadServiceMethod 之後呼叫了invoke 方法，也就是ServiceMethod的invoke方法，點進去之後發現它是個ServiceMethod是個抽象類 ，它的invoke需要實現類來重寫：

abstract class ServiceMethod<T> {
abstract T invoke(Object[] args);
}

ServiceMethod類只有一個實現類，也就是我們剛才看到過的HttpServiceMethod ，所以實際上呼叫的是它的invoke方法：

HttpServiceMethod.invoke

@Override ReturnT invoke(Object[] args) {
return callAdapter.adapt(
new OkHttpCall<>(requestFactory, args, callFactory, responseConverter));
}

在這裡只有一行程式碼，實際上是呼叫callAdapter.adapt，那這個callAdapter是什麼呢？剛才我們已經知道它是ExecutorCallAdapterFactory 建立的，然後在建立的callAdapter的adapt 方法返回ExecutorCallbackCall 類，所以來看看這個類：

ExecutorCallbackCall是ExecutorCallAdapterFactory的靜態內部類

ExecutorCallbackCall

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();
}
}

在這個類裡面，我們就看到了很多很熟悉的方法，enqueue方法傳送請求，實際的請求和處理就是在這裡面，但是還有疑惑就是具體的處理我們還是沒看到，callbackExecutor 和delegate 這兩個熟悉才是關鍵所在：

callbackExecutor

這個是在ExecutorCallAdapterFactory的構造方法中傳入的，它的構造方法又是在Platform的defaultCallAdapterFactories 方法中呼叫的，看到Platform，自然可以想到它的Android 平臺類，所以這個callbackExecutor實際上是MainThreadExecutor

@Override public Executor defaultCallbackExecutor() {
return new MainThreadExecutor();
}

static class MainThreadExecutor implements Executor {
private final Handler handler = new Handler(Looper.getMainLooper());

@Override public void execute(Runnable r) {
handler.post(r);
}
}

其實程式碼很簡單，使用了我們最常用的Handler ，讓程式碼在主執行緒中執行，也就是我們通常做的在子執行緒請求網路，主執行緒中更新UI。

delegate

delegate實際是在HttpServiceMethod的invoke方法裡傳入：

@Override ReturnT invoke(Object[] args) {
return callAdapter.adapt(
new OkHttpCall<>(requestFactory, args, callFactory, responseConverter));
}

實際上是OkHttpCall 這個類，這個類裡面就是傳送請求和處理響應的具體處理，程式碼很多，我們只看看構造方法和enqueue

OkHttpCall(RequestFactory requestFactory, Object[] args,
okhttp3.Call.Factory callFactory, Converter<ResponseBody, T> responseConverter) {
this.requestFactory = requestFactory;//請求工廠，裡面包含了請求的所有資訊
this.args = args;//引數
this.callFactory = callFactory;//請求客戶端，okhttp
this.responseConverter = responseConverter;//響應的json解析器（Gson）
}

OkHttpCall.enqueue

@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();
} 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 {
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();
}
}
});
}

程式碼其實很簡單，就是使用okhttp請求，然後在響應方法onResponse做相應的解析和處理，最後完成網路請求。

總結

Retrofit對響應的解析大概可分為

建立處理介面卡CallAdapter，具體實現類為ExecutorCallbackCall ，在這裡面進行傳送請求和處理響應等相關的操作，最終是呼叫okhttp進行網路請求
建立響應資料轉換器Converter，具體實現類為GsonResponseBodyConverter ，用於對返回json資料的解析，這個是我們自己傳入的Gson解析工廠，Retrofit也支援其他的第三方解析庫
傳送請求，具體的操作類是OkHttpCall 來發送請求和處理響應，MainThreadExecutor 來做回撥的處理，在主執行緒中處理已經轉化好的實體類。