注意：这篇文章不是一篇系统的文章，如果查看完整系统文章可以参考文末的相关文章。
这篇文章只对自己觉得重要的地方进行了分析。
使用方法：官方使用教程
主要有以下步骤：
- 编写Api接口
- 创建Retrofit实例
- 获得ApiService
- 获取请求
- 执行请求（同步或者异步）
类似下面的代码：
//1.编写Api接口：
public interface GitHubService {
@GET("users/{user}/repos")
Call<List<Repo>> listRepos(@Path("user") String user);
}
//2.创建Retrofit实例
Retrofit retrofit = new Retrofit.Builder()
.baseUrl("https://api.github.com/")
.addConverterFactory(GsonConverterFactory.create())
.build();
//3.获得ApiService
GitHubService service = retrofit.create(GitHubService.class);
//4.获取请求
Call<List<Repo>> repos = service.listRepos("octocat");
//5.执行请求
repos.enqueue(new Callback<List<Repo>>()
{
@Override
public void onResponse(Call<List<Repo>> call, Response<List<Repo>> response)
{
}
@Override
public void onFailure(Call<List<User>> call, Throwable t)
{
}
});
源码解析：
首先给出Retrofit类中非常重要成员变量的解释：
final okhttp3.Call.Factory callFactory; //okhttpClient，用于产生Call请求
final List<Converter.Factory> converterFactories; //数据解析器，将ResponseBody和定义的数据类型进行转换
final List<CallAdapter.Factory> adapterFactories; //对Call进行转化，默认为 ExecutorCallbackCall
final @Nullable Executor callbackExecutor; // 请求返回的线程池
下面进行源码分析（基于上面提到的使用方式中的代码进行分析）：
- 跟踪Retrofit的创建
Retrofit retrofit = new Retrofit.Builder()
.baseUrl("https://api.github.com/")
.addConverterFactory(GsonConverterFactory.create())
.build();
底层实现（Retrofit.class）
//采用Build模式，通过传入的参数构建实例
public Retrofit build() {
okhttp3.Call.Factory callFactory = this.callFactory;
if (callFactory == null) {
//所以可以看出callFactory其实是OkHttpClient，并且如果外部没有设定就自己new一个
callFactory = new OkHttpClient();
}
//请求返回的线程执行者
Executor callbackExecutor = this.callbackExecutor;
if (callbackExecutor == null) {
callbackExecutor = platform.defaultCallbackExecutor();
}
List<CallAdapter.Factory> adapterFactories = new ArrayList<>(this.adapterFactories);
adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));
//添加的GsonConverterFactory.create()存储converterFactories里
List<Converter.Factory> converterFactories = new ArrayList<>(this.converterFactories);
return new Retrofit(callFactory, baseUrl, converterFactories, adapterFactories,
callbackExecutor, validateEagerly);
}
}
- ApiService的创建
GitHubService service = retrofit.create(GitHubService.class);
底层实现（Retrofit.class）
//使用动态代理的方式进行对象构造。
//之所以选择接口的形式，个人认为是因为比较简洁
public <T> T create(final Class<T> service) {
return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class<?>[] { service },
new InvocationHandler() {
private final Platform platform = Platform.get();
@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);
}
//重要的地方。
ServiceMethod<Object, Object> serviceMethod =
(ServiceMethod<Object, Object>) loadServiceMethod(method);
OkHttpCall<Object> okHttpCall = new OkHttpCall<>(serviceMethod, args);
return serviceMethod.callAdapter.adapt(okHttpCall);
}
});
}
接下来对下面代码进行分析：
//将apiService接口中的方法传入，生成ServiceMethod
1. ServiceMethod<Object, Object> serviceMethod =
(ServiceMethod<Object, Object>) loadServiceMethod(method);
//根据serviceMethod和args构建出Call.（类似Okhttp3中的请求Call）
2. OkHttpCall<Object> okHttpCall = new OkHttpCall<>(serviceMethod, args);
//返回apiService中的返回类型对应的Call<T>
3. return serviceMethod.callAdapter.adapt(okHttpCall);
特别说明
ServiceMethod：主要存储api接口中的所有请求参数。根据前面提到的动态代理技术以及注解，获取到一系列参数。
- loadServiceMethod（）：
//从缓存中取，没有就新生成一个
ServiceMethod<?, ?> loadServiceMethod(Method method) {
ServiceMethod<?, ?> result = serviceMethodCache.get(method);
if (result != null) return result;
synchronized (serviceMethodCache) {
result = serviceMethodCache.get(method);
if (result == null) {
//使用Build模式生成一个ServiceMethod
result = new ServiceMethod.Builder<>(this, method).build();
//加入到Map集合
serviceMethodCache.put(method, result);
}
}
return result;
}
- OkHttpCall：
封装了Okhttp，有自己实现的enqueue（）和execute（）方法，桥接模式？
//构造函数
OkHttpCall(ServiceMethod<T, ?> serviceMethod, @Nullable Object[] args) {
this.serviceMethod = serviceMethod;
this.args = args;
}
enqueue方法：
@Override public void enqueue(final Callback<T> callback) {
...
}
//可见还是使用了Okhttp
call.enqueue(new okhttp3.Callback() {
@Override public void onResponse(okhttp3.Call call, okhttp3.Response rawResponse)
throws IOException {
Response<T> response;
try {
response = parseResponse(rawResponse);
} catch (Throwable e) {
callFailure(e);
return;
}
callSuccess(response);
}
@Override public void onFailure(okhttp3.Call call, IOException e) {
try {
callback.onFailure(OkHttpCall.this, e);
} catch (Throwable t) {
t.printStackTrace();
}
}
private void callFailure(Throwable e) {
try {
callback.onFailure(OkHttpCall.this, e);
} catch (Throwable t) {
t.printStackTrace();
}
}
private void callSuccess(Response<T> response) {
try {
callback.onResponse(OkHttpCall.this, response);
} catch (Throwable t) {
t.printStackTrace();
}
}
});
}
- serviceMethod.callAdapter.adapt(okHttpCall);
//默认的callAdapter其实是在Retroft的build()方法中传入
//的"platform.defaultCallAdapterFactory(callbackExecutor)"，具体细节跟下源码
List<CallAdapter.Factory> adapterFactories = new ArrayList<>(this.adapterFactories);
adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));
查看Platform
CallAdapter.Factory defaultCallAdapterFactory(@Nullable Executor callbackExecutor) {
if (callbackExecutor != null) {
//走这个逻辑
return new ExecutorCallAdapterFactory(callbackExecutor);
}
return DefaultCallAdapterFactory.INSTANCE;
}
ExecutorCallAdapterFactory：
对Call对象进行封装，回调时通过callbackExecutor进行回调到UI线程中去。也对应着Retrofit的回调在主线程
final class ExecutorCallAdapterFactory extends CallAdapter.Factory {
final Executor callbackExecutor;
ExecutorCallAdapterFactory(Executor callbackExecutor) {
this.callbackExecutor = callbackExecutor;
}
@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;
}
@Override public Call<Object> adapt(Call<Object> call) {
return new ExecutorCallbackCall<>(callbackExecutor, call);
}
};
}
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");
//将回调放在主线程中执行
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();
}
}
}
总结：
- 使用动态代理和注解的技术，将api方法中的参数解析到ServiceMethod中去。
- 内部封装okhttp，使用converterFactoriy将ResponseBody和对象，对象和RequestBody进行转换
- 使用converterAdapter进行线程切换
下面给出一些文章地址：
Retrofit2 完全解析 探索与okhttp之间的关系
Retrofit2 源码解析
