Retrofit adaptation process

Retrofit adaptation to Coroutine

When using Retrofit, an interface class will be defined. When accessing the network, the methods in the interface will be called to access the network. This process is realized through dynamic proxy. When we use the getApi() method of the Retrofit instance, we will actually use proxy Newproxyinstance() to create a dynamic proxy object of the interface class. When Paging3 is used, the interface is defined as:

//interface HomeHttpApiReplace
interface HomeHttpApiReplace {
    suspend fun getHomePageConfig(
        @Path("id") pageId: Int, @Query("uid") uid: Long?,
        @Query("categoryList") categoryList: String
    ): HomePageDataConfig

//class HomeDataPagingSource
override suspend fun load(params: LoadParams<Int>): LoadResult<Int, BaseItemData> {
	val homePageConfig = api.getHomePageConfig(tabId, getUid(),
                StringUtils.join(RecommendCore.instance.queryCacheCategory(), ","))

Dynamic proxy process of Api interface class

According to the principle of dynamic proxy, calling getHomePageConfig() method will call invoke() method of InvocationHandler class:

  public <T> T create(final Class<T> service) {
    return (T)
            new Class<?>[] {service},
            new InvocationHandler() {
              private final Platform platform = Platform.get();
              private final Object[] emptyArgs = new Object[0];

              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);

Here, the platform is Android, and the method is public abstract Java lang.Object com. yy. onepiece. home. view. LivingHomeViewReplace. HomeHttpApiReplace. Gethomepageconfig (int, java.lang.long, java.lang.string, kotlin. Coroutines. Continuation), you can see that there is an additional parameter of type continuation in its function parameters.

  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;

The method will not be resolved the first time it is called.

Process of parsing method annotation in Api interface

//abstract class ServiceMethod
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 return type must not include a type variable or wildcard: %s",
    if (returnType == void.class) {
      throw methodError(method, "Service methods cannot return void.");

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

First, use the Builder design pattern to store the information contained in the method in the created RequestFactory object. Take a look at this process:

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

//class RequestFactory.Builder
Builder(Retrofit retrofit, Method method) {
      this.retrofit = retrofit;
      this.method = method;
      this.methodAnnotations = method.getAnnotations();
      this.parameterTypes = method.getGenericParameterTypes();
      this.parameterAnnotationsArray = method.getParameterAnnotations();

    RequestFactory build() {
      for (Annotation annotation : methodAnnotations) {
	//Omit security check code
      int parameterCount = parameterAnnotationsArray.length;
      parameterHandlers = new ParameterHandler<?>[parameterCount];
      for (int p = 0, lastParameter = parameterCount - 1; p < parameterCount; p++) {
        parameterHandlers[p] =
            parseParameter(p, parameterTypes[p], parameterAnnotationsArray[p], p == lastParameter);
      return new RequestFactory(this);

There are two steps to parse the annotation. The first step is to parse the method annotation, i.e. @ GET, etc., and the second step is to parse the Parameter annotation, i.e. @ Path, etc.

1. Process of parsing method annotation

The process of parsing method annotation is as follows:

    private void parseMethodAnnotation(Annotation annotation) {
      if (annotation instanceof DELETE) {
        parseHttpMethodAndPath("DELETE", ((DELETE) annotation).value(), false);
      } else if (annotation instanceof GET) {
        parseHttpMethodAndPath("GET", ((GET) annotation).value(), false);
      } else if (annotation instanceof HEAD) {
        parseHttpMethodAndPath("HEAD", ((HEAD) annotation).value(), false);
      } else if (annotation instanceof PATCH) {
        parseHttpMethodAndPath("PATCH", ((PATCH) annotation).value(), true);
      } else if (annotation instanceof POST) {
        parseHttpMethodAndPath("POST", ((POST) annotation).value(), true);
      } else if (annotation instanceof PUT) {
        parseHttpMethodAndPath("PUT", ((PUT) annotation).value(), true);
      } else if (annotation instanceof OPTIONS) {
        parseHttpMethodAndPath("OPTIONS", ((OPTIONS) annotation).value(), false);
      } else if (annotation instanceof HTTP) {
        HTTP http = (HTTP) annotation;
        parseHttpMethodAndPath(http.method(), http.path(), http.hasBody());
      } else if (annotation instanceof retrofit2.http.Headers) {
        String[] headersToParse = ((retrofit2.http.Headers) annotation).value();
        if (headersToParse.length == 0) {
          throw methodError(method, "@Headers annotation is empty.");
        headers = parseHeaders(headersToParse);
      } else if (annotation instanceof Multipart) {
        if (isFormEncoded) {
          throw methodError(method, "Only one encoding annotation is allowed.");
        isMultipart = true;
      } else if (annotation instanceof FormUrlEncoded) {
        if (isMultipart) {
          throw methodError(method, "Only one encoding annotation is allowed.");
        isFormEncoded = true;

    private void parseHttpMethodAndPath(String httpMethod, String value, boolean hasBody) {
     //Omit code security check
      this.httpMethod = httpMethod;
      this.hasBody = hasBody;

      if (value.isEmpty()) {
	//Omit code security check
      this.relativeUrl = value;
      this.relativeUrlParamNames = parsePathParameters(value);

	static Set<String> parsePathParameters(String path) {
      Matcher m = PARAM_URL_REGEX.matcher(path);
      Set<String> patterns = new LinkedHashSet<>();
      while (m.find()) {
      return patterns;

The annotations of DELETE, GET and other types are parsed respectively. The parsing results are saved in the httpMethod field, the value corresponding to the annotation is saved in the relativeUrl field, and the value is further parsed in the Set form in the relativeUrlParamNames field.

2. Parsing Parameter annotation process

The process of parsing Parameter annotation is as follows:

    private @Nullable ParameterHandler<?> parseParameter(
        int p, Type parameterType, @Nullable Annotation[] annotations, boolean allowContinuation) {
      ParameterHandler<?> result = null;
      if (annotations != null) {
        for (Annotation annotation : annotations) {
          ParameterHandler<?> annotationAction =
              parseParameterAnnotation(p, parameterType, annotations, annotation);

          if (annotationAction == null) {

          if (result != null) {
            throw parameterError(
                method, p, "Multiple Retrofit annotations found, only one allowed.");

          result = annotationAction;

      if (result == null) {
        if (allowContinuation) {
          try {
            if (Utils.getRawType(parameterType) == Continuation.class) {
              isKotlinSuspendFunction = true;
              return null;
          } catch (NoClassDefFoundError ignored) {
        throw parameterError(method, p, "No Retrofit annotation found.");

      return result;

The parseParameter() function is used to parse each function parameter. Each parameter may contain multiple annotations. According to the number of annotations for each parameter, further call the parseParameterAnnotation() method to parse. The parsing result is a parameterhandler object result. According to the parsed annotation objects such as @ path, @ query, etc., it will be parsed to generate subclass objects of parameterhandler, such as parameterhandler Path,ParameterHandler.Query, etc. If a parameter has no annotation or the annotation is not within the given type, result = null. The method defined according to the beginning Api has a suspend ID, so the method will include a parameter of Continuation type in the parameter, so result == null will be true. Therefore, when allowContinuation is true, iskotlinsuspundfunction will be set to true.

Adaptation process of Call

Next, go back to the parseannotations () function of ServiceMethod and call httpservicemethod parseAnnotations().

   * Inspects the annotations on an interface method to construct a reusable service method that
   * speaks HTTP. This requires potentially-expensive reflection so it is best to build each service
   * method only once and reuse it.
  static <ResponseT, ReturnT> HttpServiceMethod<ResponseT, ReturnT> parseAnnotations(
      Retrofit retrofit, Method method, RequestFactory requestFactory) {
    boolean isKotlinSuspendFunction = requestFactory.isKotlinSuspendFunction;
    boolean continuationWantsResponse = false;
    boolean continuationBodyNullable = false;

    Annotation[] annotations = method.getAnnotations();
    Type adapterType;
    if (isKotlinSuspendFunction) {
      Type[] parameterTypes = method.getGenericParameterTypes();
      Type responseType =
              0, (ParameterizedType) parameterTypes[parameterTypes.length - 1]);
      if (getRawType(responseType) == Response.class && responseType instanceof ParameterizedType) {
        // Unwrap the actual body type from Response<T>.
        responseType = Utils.getParameterUpperBound(0, (ParameterizedType) responseType);
        continuationWantsResponse = true;
      } else {
        // TODO figure out if type is nullable or not
        // Metadata metadata = method.getDeclaringClass().getAnnotation(Metadata.class)
        // Find the entry for method
        // Determine if return type is nullable or not

      adapterType = new Utils.ParameterizedTypeImpl(null, Call.class, responseType);
      annotations = SkipCallbackExecutorImpl.ensurePresent(annotations);
    } else {
      adapterType = method.getGenericReturnType();

    CallAdapter<ResponseT, ReturnT> callAdapter =
        createCallAdapter(retrofit, method, adapterType, annotations);
    Type responseType = callAdapter.responseType();
    if (responseType == okhttp3.Response.class) {
      throw methodError(
              + getRawType(responseType).getName()
              + "' is not a valid response body type. Did you mean ResponseBody?");
    if (responseType == Response.class) {
      throw methodError(method, "Response must include generic type (e.g., Response<String>)");
    // TODO support Unit for Kotlin?
    if (requestFactory.httpMethod.equals("HEAD") && !Void.class.equals(responseType)) {
      throw methodError(method, "HEAD method must use Void as response type.");

    Converter<ResponseBody, ResponseT> responseConverter =
        createResponseConverter(retrofit, method, responseType);

    okhttp3.Call.Factory callFactory = retrofit.callFactory;
    if (!isKotlinSuspendFunction) {
      return new CallAdapted<>(requestFactory, callFactory, responseConverter, callAdapter);
    } else if (continuationWantsResponse) {
      //noinspection unchecked Kotlin compiler guarantees ReturnT to be Object.
      return (HttpServiceMethod<ResponseT, ReturnT>)
          new SuspendForResponse<>(
              (CallAdapter<ResponseT, Call<ResponseT>>) callAdapter);
    } else {
      //noinspection unchecked Kotlin compiler guarantees ReturnT to be Object.
      return (HttpServiceMethod<ResponseT, ReturnT>)
          new SuspendForBody<>(
              (CallAdapter<ResponseT, Call<ResponseT>>) callAdapter,

Although this code is very long, the logic is very clear. First, determine the type of Adapter according to the isKotlinSuspendFunction field. Here, the value is true. It is recorded in the adapterType. It is a ParameterizedTypeImpl instance, and create a CallAdapter object in the factory callAdapterFactories. Secondly, it has a responseType to create a Converter object in the factory converterFactories. Finally, according to isKotlinSuspendFunction.
Next, go back to the invoke() method of InvocationHandler and further call the invoke() virtual method of ServiceMethod. ServiceMethod has only one subclass httpservicemethod, and SuspendForBody is a subclass of httpservicemethod.

  	final @Nullable ReturnT invoke(Object[] args) {
    	Call<ResponseT> call = new OkHttpCall<>(requestFactory, args, 			callFactory, responseConverter);
    	return adapt(call, args);

	//class SuspendForBody
    protected Object adapt(Call<ResponseT> call, Object[] args) {
      call = callAdapter.adapt(call);

      //noinspection unchecked Checked by reflection inside RequestFactory.
      Continuation<ResponseT> continuation = (Continuation<ResponseT>) args[args.length - 1];

      // Calls to OkHttp Call.enqueue() like those inside await and awaitNullable can sometimes
      // invoke the supplied callback with an exception before the invoking stack frame can return.
      // Coroutines will intercept the subsequent invocation of the Continuation and throw the
      // exception synchronously. A Java Proxy cannot throw checked exceptions without them being
      // declared on the interface method. To avoid the synchronous checked exception being wrapped
      // in an UndeclaredThrowableException, it is intercepted and supplied to a helper which will
      // force suspension to occur so that it can be instead delivered to the continuation to
      // bypass this restriction.
      try {
        return isNullable
            ? KotlinExtensions.awaitNullable(call, continuation)
            : KotlinExtensions.await(call, continuation);
      } catch (Exception e) {
        return KotlinExtensions.suspendAndThrow(e, continuation);

The callAdapter here is the httpservicemethod In parseannotations(), a defaultcalladapterfactory is passed in when the SuspendForBody instance is constructed callAdapter instance is to call the get() method of the factory class through the createCallAdapter() method to obtain the initialized anonymous object. Call callAdapter Adapt (call) to adapt the call object.

  public @Nullable CallAdapter<?, ?> get(
      Type returnType, Annotation[] annotations, Retrofit retrofit) {
    if (getRawType(returnType) != Call.class) {
      return null;
    if (!(returnType instanceof ParameterizedType)) {
      throw new IllegalArgumentException(
          "Call return type must be parameterized as Call<Foo> or Call<? extends Foo>");
    final Type responseType = Utils.getParameterUpperBound(0, (ParameterizedType) returnType);

    final Executor executor =
        Utils.isAnnotationPresent(annotations, SkipCallbackExecutor.class)
            ? null
            : callbackExecutor;

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

      public Call<Object> adapt(Call<Object> call) {
        return executor == null ? call : new ExecutorCallbackCall<>(executor, call);

executor = null here, so an ExecutorCallbackCall object will be constructed with call as a parameter, so calladapter is called Adapt (call) returns an ExecutorCallbackCall object.
Go back to the adapt() method of the SuspendForBody class, and use the code of continuation continuation = (continuation < response >) args [args. Length - 1] to obtain the continuation type parameter passed in by the saspend method defined in the Api interface. Next, because isNullable == false, kotlinextensions will be called Await (call, continuation), which is a Java method. It has an ExecutorCallbackCall parameter call of type and a continuation of type. Here, a suspend Kotlin method call is called Await(), these two parameters will be added when the Kotlin is compiled into a Java method.

suspend fun <T : Any> Call<T>.await(): T {
  return suspendCancellableCoroutine { continuation ->
    continuation.invokeOnCancellation {
    enqueue(object : Callback<T> {
      override fun onResponse(call: Call<T>, response: Response<T>) {
        if (response.isSuccessful) {
          val body = response.body()
          if (body == null) {
            val invocation = call.request().tag(!!
            val method = invocation.method()
            val e = KotlinNullPointerException("Response from " +
                '.' +
                " was null but response body type was declared as non-null")
          } else {
        } else {

      override fun onFailure(call: Call<T>, t: Throwable) {

First suspend the current collaboration, record that the current starting point is continuation, and then call the enqueue() method of the ExecutorCallbackCall object:

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;

        public void enqueue(final Callback<T> callback) {
            Objects.requireNonNull(callback, "callback == null");
            this.delegate.enqueue(new Callback<T>() {
                public void onResponse(Call<T> call, Response<T> response) {
                    ExecutorCallbackCall.this.callbackExecutor.execute(() -> {
                        if (ExecutorCallbackCall.this.delegate.isCanceled()) {
                            callback.onFailure(ExecutorCallbackCall.this, new IOException("Canceled"));
                        } else {
                            callback.onResponse(ExecutorCallbackCall.this, response);


                public void onFailure(Call<T> call, Throwable t) {
                    ExecutorCallbackCall.this.callbackExecutor.execute(() -> {
                        callback.onFailure(ExecutorCallbackCall.this, t);

        public boolean isExecuted() {
            return this.delegate.isExecuted();

        public Response<T> execute() throws IOException {
            return this.delegate.execute();

        public void cancel() {

        public boolean isCanceled() {
            return this.delegate.isCanceled();

        public Call<T> clone() {
            return new DefaultCallAdapterFactory.ExecutorCallbackCall(this.callbackExecutor, this.delegate.clone());

        public Request request() {
            return this.delegate.request();

        public Timeout timeout() {
            return this.delegate.timeout();

The proxy mode is used here. ExecutorCallbackCall is a proxy class that uses the delegate object of type Call to handle the work of accessing the network. The enqueue() method of the ExecutorCallbackCall class is actually called by the outside to delegate for further processing, and the network processing result will be returned to the callback processing with the type of callback, that is, it will be processed in the previous section of code. If the onResponse() method is called back and the result is normal, it will be processed through continuation Resume (body) returns to the hanging point to resume execution. If you callback the onFailure() method, you can use continuation Resumewithexception (T) returns to the hanging point to resume execution and return an exception.

Posted by allelopath on Mon, 23 May 2022 15:37:42 +0300