今天突然想到Retrofit2创建Service需要传递一个接口，但是我们都知道接口是不能直接生成实例的，那么Retrofit2是如何做到的呢？带着疑惑，看一波源码。
//创建实例
retrofit.create(VideoApis.class);
跟进create方法：
//此方法需要一个Class类
public <T> T create(final Class<T> service) {
//对传入的Class进行校验
Utils.validateServiceInterface(service);
if (this.validateEagerly) {
this.eagerlyValidateMethods(service);
}
return Proxy.newProxyInstance(service.getClassLoader(), new Class[]{service}, new InvocationHandler() {
private final Platform platform = Platform.get();
public Object invoke(Object proxy, Method method, Object... args) throws Throwable {
if (method.getDeclaringClass() == Object.class) {
return method.invoke(this, args);
} else if (this.platform.isDefaultMethod(method)) {
return this.platform.invokeDefaultMethod(method, service, proxy, args);
} else {
ServiceMethod serviceMethod = Retrofit.this.loadServiceMethod(method);
OkHttpCall okHttpCall = new OkHttpCall(serviceMethod, args);
return serviceMethod.callAdapter.adapt(okHttpCall);
}
}
});
}
一进入create就调用了一个验证方法 Utils.validateServiceInterface(service);看下源码做了什么事情：
static <T> void validateServiceInterface(Class<T> service) {
//首先验证此类是否是接口，不是接口直接抛出异常。
if (!service.isInterface()) {
throw new IllegalArgumentException("API declarations must be interfaces.");
//继续判断此类是否是独立的接口类，如果有继承其他接口则抛出异常。
} else if (service.getInterfaces().length > 0) {
throw new IllegalArgumentException("API interfaces must not extend other interfaces.");
}
}
返回源码继续分析校验通过过后的代码：
//这里直接return了，后面的实例是使用了动态代理获取了接口的实例。。。
return Proxy.newProxyInstance(service.getClassLoader(), new Class[]{service}, new InvocationHandler() {
private final Platform platform = Platform.get();
public Object invoke(Object proxy, Method method, Object... args) throws Throwable {
if (method.getDeclaringClass() == Object.class) {
return method.invoke(this, args);
} else if (this.platform.isDefaultMethod(method)) {
return this.platform.invokeDefaultMethod(method, service, proxy, args);
} else {
//前面可以忽略，这里会走此方法
ServiceMethod serviceMethod = Retrofit.this.loadServiceMethod(method);
//这里主要是利用源方法的信息，包装在ServiceMethod中，然后连带参数一并传递到OkHttpCall，构建出一个OkHttpCall的实例，通过代理类执行接口中定义的方法。
OkHttpCall okHttpCall = new OkHttpCall(serviceMethod, args);
return serviceMethod.callAdapter.adapt(okHttpCall);
}
}
});
此时我们已经知道Retrofit是通过动态代理，最终使用的okhttp的实例来执行接口中的方法，顺便追踪下ServiceMethod 的出处：
//serviceMethodCache 是定义的一个缓存Map
private final Map<Method, ServiceMethod> serviceMethodCache = new LinkedHashMap();
...
ServiceMethod loadServiceMethod(Method method) {
Map var3 = this.serviceMethodCache;
synchronized(this.serviceMethodCache) {
ServiceMethod result = (ServiceMethod)this.serviceMethodCache.get(method);
if (result == null) {
//主要看result的创建
result = (new retrofit2.ServiceMethod.Builder(this, method)).build();
this.serviceMethodCache.put(method, result);
}
return result;
}
}
通过build构建ServiceMethod的实例，期间做了很多验证操作，包括注解是否合法，等等。
public ServiceMethod build() {
this.callAdapter = this.createCallAdapter();
this.responseType = this.callAdapter.responseType();
if (this.responseType != Response.class && this.responseType != okhttp3.Response.class) {
this.responseConverter = this.createResponseConverter();
Annotation[] var1 = this.methodAnnotations;
int p = var1.length;
//遍历方法上的注解
for(int var3 = 0; var3 < p; ++var3) {
Annotation annotation = var1[var3];
//将注解解析
this.parseMethodAnnotation(annotation);
}
//如果没有指定http请求方式，抛出异常
if (this.httpMethod == null) {
throw this.methodError("HTTP method annotation is required (e.g., @GET, @POST, etc.).");
} else {
if (!this.hasBody) {
if (this.isMultipart) {
throw this.methodError("Multipart can only be specified on HTTP methods with request body (e.g., @POST).");
}
if (this.isFormEncoded) {
throw this.methodError("FormUrlEncoded can only be specified on HTTP methods with request body (e.g., @POST).");
}
}
int parameterCount = this.parameterAnnotationsArray.length;
this.parameterHandlers = new ParameterHandler[parameterCount];
//遍历参数注解
for(p = 0; p < parameterCount; ++p) {
Type parameterType = this.parameterTypes[p];
//校验参数
if (Utils.hasUnresolvableType(parameterType)) {
throw this.parameterError(p, "Parameter type must not include a type variable or wildcard: %s", parameterType);
}
Annotation[] parameterAnnotations = this.parameterAnnotationsArray[p];
if (parameterAnnotations == null) {
throw this.parameterError(p, "No Retrofit annotation found.");
}
//解析参数
this.parameterHandlers[p] = this.parseParameter(p, parameterType, parameterAnnotations);
}
if (this.relativeUrl == null && !this.gotUrl) {
throw this.methodError("Missing either @%s URL or @Url parameter.", this.httpMethod);
} else if (!this.isFormEncoded && !this.isMultipart && !this.hasBody && this.gotBody) {
throw this.methodError("Non-body HTTP method cannot contain @Body.");
} else if (this.isFormEncoded && !this.gotField) {
throw this.methodError("Form-encoded method must contain at least one @Field.");
} else if (this.isMultipart && !this.gotPart) {
throw this.methodError("Multipart method must contain at least one @Part.");
} else {
return new ServiceMethod(this);
}
}
} else {
throw this.methodError("'" + Utils.getRawType(this.responseType).getName() + "' is not a valid response body type. Did you mean ResponseBody?");
}
}
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