1、从Retrofit retrofit = new Retrofit.Builder() .baseUrl(url）.addConverterFactory(GsonConverterFactory.create()) .build()入手：
public Retrofit build() {
if (baseUrl == null) {
throw new IllegalStateException("Base URL required."); //baseUrl为null就抛异常
}
okhttp3.Call.Factory callFactory = this.callFactory;
if (callFactory == null) { //用户没用设置OK就创建默认的
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);
}
}
经过上面Retrofit就创建了，接下来看IpService ipService = retrofit.create(IpService.class)，也就是create：
public <T> T create(final Class<T> service) {
Utils.validateServiceInterface(service); //检查是不是接口，有没有继承接口，是不能继承其他接口的
if (validateEagerly) { //默认是false
eagerlyValidateMethods(service); //作用是将service所有方法加入缓存
}
//返回一个代理类，动态代理，Proxy.newProxyInstance是Android提供的
return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class<?>[] { service },
new InvocationHandler() {
private final Platform platform = Platform.get();
//使用Retrofit当调用service的方法时调用，参数一是代理对象，二是调用的方法，三是参数
@Override public 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);
}
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);
}
});
}
所以调用任何方法，最后会走上面的invoke，那么看loadServiceMethod：
ServiceMethod<?, ?> loadServiceMethod(Method method) {
ServiceMethod<?, ?> result = serviceMethodCache.get(method); //有没缓存
if (result != null) return result;
synchronized (serviceMethodCache) {
result = serviceMethodCache.get(method); //这里理解是保证线程安全
if (result == null) {
result = new ServiceMethod.Builder<>(this, method).build(); //将Method封装到ServiceMethod
serviceMethodCache.put(method, result);
}
}
return result;
}
看result = new ServiceMethod.Builder<>(this, method).build()：
public ServiceMethod build() {
callAdapter = createCallAdapter(); //1，看下面的步骤
responseType = callAdapter.responseType();
if (responseType == Response.class || responseType == okhttp3.Response.class) {
throw methodError("'"
+ Utils.getRawType(responseType).getName()
+ "' is not a valid response body type. Did you mean ResponseBody?");
}
responseConverter = createResponseConverter();
for (Annotation annotation : methodAnnotations) {
parseMethodAnnotation(annotation); //解析请求方式，会对httpMethod 赋值
}
if (httpMethod == null) {
throw methodError("HTTP method annotation is required (e.g., @GET, @POST, etc.).");
}
if (!hasBody) {
if (isMultipart) {
throw methodError(
"Multipart can only be specified on HTTP methods with request body (e.g., @POST).");
}
if (isFormEncoded) {
throw methodError("FormUrlEncoded can only be specified on HTTP methods with "
+ "request body (e.g., @POST).");
}
}
int parameterCount = parameterAnnotationsArray.length;
parameterHandlers = new ParameterHandler<?>[parameterCount];
for (int p = 0; p < parameterCount; p++) {
Type parameterType = parameterTypes[p];
if (Utils.hasUnresolvableType(parameterType)) {
throw parameterError(p, "Parameter type must not include a type variable or wildcard: %s",
parameterType);
}
Annotation[] parameterAnnotations = parameterAnnotationsArray[p];
if (parameterAnnotations == null) {
throw parameterError(p, "No Retrofit annotation found.");
}
parameterHandlers[p] = parseParameter(p, parameterType, parameterAnnotations);
}
if (relativeUrl == null && !gotUrl) {
throw methodError("Missing either @%s URL or @Url parameter.", httpMethod);
}
if (!isFormEncoded && !isMultipart && !hasBody && gotBody) {
throw methodError("Non-body HTTP method cannot contain @Body.");
}
if (isFormEncoded && !gotField) {
throw methodError("Form-encoded method must contain at least one @Field.");
}
if (isMultipart && !gotPart) {
throw methodError("Multipart method must contain at least one @Part.");
}
return new ServiceMethod<>(this);
}
上面的注释1那里，createCallAdapter：
private CallAdapter<T, R> createCallAdapter() {
Type returnType = method.getGenericReturnType(); //获取返回值类型
if (Utils.hasUnresolvableType(returnType)) {
throw methodError(
"Method return type must not include a type variable or wildcard: %s", returnType);
}
if (returnType == void.class) {
throw methodError("Service methods cannot return void.");
}
Annotation[] annotations = method.getAnnotations(); //拿到方法的注解
try {
//noinspection unchecked
//这里其实是ExecutorCallAdapterFactory内部的get方法，具有看下面
return (CallAdapter<T, R>) retrofit.callAdapter(returnType, annotations);
} catch (RuntimeException e) { // Wide exception range because factories are user code.
throw methodError(e, "Unable to create call adapter for %s", returnType);
}
}
ExecutorCallAdapterFactory的get：
@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; //返回类型比如Call<MyData>，返回MyData
}
@Override public Call<Object> adapt(Call<Object> call) {
return new ExecutorCallbackCall<>(callbackExecutor, call); //将原本call的回调转发至UI线程
}
};
}
再回到Retrofit的create方法，最后走这一步serviceMethod.callAdapter.adapt(okHttpCall)，上面分析知道其实是返回了ExecutorCallbackCall，看ExecutorCallbackCall的执行方法：
@Override
public void enqueue(final Callback<T> callback) {
checkNotNull(callback, "callback == null");
//这里相当于OKHttp的newCall.enqueue
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()) {
//主线程回调，callback是在主线程创建的
// 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);
}
});
}
});
}
最后总结下，retrofit通过Java的Proxy类完成动态代理的相关代理，拿到接口的实现类，在invoke方法内部，拿到我们所声明的注解以及实参等，构造ServiceMethod，ServiceMethod中解析了大量的信息，最后通过OKHttp请求，然后处理UI线程回调。
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