Retrofit源码简要分析

一、Retrofit的使用

本文使用的Retrofit 2.1.0版本。

在gradle中加入：

compile 'com.squareup.retrofit2:retrofit:2.1.0'

如果要是要解析json，还需加入：

compile 'com.squareup.retrofit2:converter-gson:2.1.0'

如果要结合rxjava

需要加入：

compile 'io.reactivex:rxjava:1.1.6'
compile 'io.reactivex:rxandroid:1.2.1'
compile 'com.squareup.retrofit2:adapter-rxjava:2.1.0'

Retrofit 2.1.0 依赖 okhttp 3发送网络请求。

所以Retrofit的源码其实并不多



http包下的是注解类。

接下来我们看看Retrofit是怎样创建的：

new Retrofit.Builder()
.baseUrl(Constants.BASE_URL)
.addConverterFactory(GsonConverterFactory.create())
.client(okHttpClient)
.build();

okHttpClient是okhttp3.0的okHttpClient,Retrofit将会使用它来发送网络请求。

当我们需要调用一个后台接口时，我们只需新建一个接口，然后根据请求类型以及参数写一个抽象方法

例如:

public interface Api {
@FormUrlEncoded
@POST("/api/test")
Call<LoginResp> getTest(@Field("test")String test);
}

其中getTest方法就可以向后台发起一个请求，这个请求类型为Post 表单提交，参数名为test.

使用的话，可以这样：

Api api = new Retrofit.Builder()
.baseUrl("http://www.test.com/")
.addConverterFactory(GsonConverterFactory.create())
.client(okHttpClient)
.build()
.create(Api.class);
Call<LoginResp> call = api.getTest("test");

得到 call 然后使用方法就和okhttp差不多了，可以使用call.enqueue()或call.execute()方法

call.enqueue(new Callback<LoginResp>() {
@Override
public void onResponse(Call<LoginResp> call, retrofit2.Response<LoginResp> response) {

}

@Override
public void onFailure(Call<LoginResp> call, Throwable t) {

}
});

//或者
try {
call.execute();
} catch (IOException e) {
e.printStackTrace();
}

他们分别代表了同步，和异步发起请求。

具体使用方法还请参考官网文档，这里就不再说了，接下来我们进入正题。

二、源码分析

我们在使用Retrofit的使用使用了这么 一段代码来创建retrofit

new Retrofit.Builder()
.baseUrl(Constants.BASE_URL)
.addConverterFactory(GsonConverterFactory.create())
.client(okHttpClient)
.build()

那么我们就先分析retrofit怎么被创建出来

这里使用了一个建造者模式，Build是Retrofit中的一个嵌套类

public static final class Builder {
private Platform platform;
private okhttp3.Call.Factory callFactory;
private HttpUrl baseUrl;
private List<Converter.Factory> converterFactories = new ArrayList<>();
private List<CallAdapter.Factory> adapterFactories = new ArrayList<>();
private Executor callbackExecutor;
private boolean validateEagerly;

Builder(Platform platform) {
this.platform = platform;
// Add the built-in converter factory first. This prevents overriding its behavior but also
// ensures correct behavior when using converters that consume all types.
converterFactories.add(new BuiltInConverters());
}

public Builder() {
this(Platform.get());
}

可以看到这个类中几个重要的域 Platform、okhttp3.Call.Factory、List< Converter.Factory >、List< CallAdapter.Factory > 、Executor

从类名上大致可以猜想到他们的作用，platform 代表目前使用的平台，可以是android、ios、java8

okhttp3.Call.Factory 的实现实际是我们传入的 okhttpClient,它的作用是发送网络请求

List

public Retrofit build() {
if (baseUrl == null) {
throw new IllegalStateException("Base URL required.");
}

okhttp3.Call.Factory callFactory = this.callFactory;
if (callFactory == null) {
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);
}

在Builder 的构造方法中可以看出它获取了当前的平台：

public Builder() {
this(Platform.get());
}

class Platform {
private static final Platform PLATFORM = findPlatform();

static Platform get() {
return PLATFORM;
}

private static Platform findPlatform() {
try {
Class.forName("android.os.Build");
if (Build.VERSION.SDK_INT != 0) {
return new Android();
}
} catch (ClassNotFoundException ignored) {
}
try {
Class.forName("java.util.Optional");
return new Java8();
} catch (ClassNotFoundException ignored) {
}
try {
Class.forName("org.robovm.apple.foundation.NSObject");
return new IOS();
} catch (ClassNotFoundException ignored) {
}
return new Platform();
}

我们只看Android平台：

static class Android extends Platform {
@Override public Executor defaultCallbackExecutor() {
return new MainThreadExecutor();
}

@Override CallAdapter.Factory defaultCallAdapterFactory(Executor callbackExecutor) {
return new ExecutorCallAdapterFactory(callbackExecutor);
}

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

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

他有2个方法，一个是获得一个 CallAdapter.Factory ，它是一个制造CallAdapter的工厂

一个是获得一个Executor ，用来执行运行调用完成的结果。

接下来我们看，当我们调用了 retrofit.create(Api.class); 之后发生了什么

public <T> T create(final Class<T> service) {
Utils.validateServiceInterface(service);
if (validateEagerly) {
eagerlyValidateMethods(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, 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 serviceMethod = loadServiceMethod(method);
OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);
return serviceMethod.callAdapter.adapt(okHttpCall);
}
});
}

看以看到这个方法使用了java的动态代理，关于动态代理可以参考我的上一篇文章http://blog.csdn.net/a992036795/article/details/52101685

可以知道执行的将是invoke(Object proxy, Method method, Object… args) 这个方法，也就是执行：

// 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 serviceMethod = loadServiceMethod(method);
OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);
return serviceMethod.callAdapter.adapt(okHttpCall);

如果我们传入参数正确的话，它将走最后这三行代码

我们就先看 ServiceMethod serviceMethod = loadServiceMethod(method); 这句代码的作用

ServiceMethod loadServiceMethod(Method method) {
ServiceMethod result;
synchronized (serviceMethodCache) {
result = serviceMethodCache.get(method);
if (result == null) {
result = new ServiceMethod.Builder(this, method).build();
serviceMethodCache.put(method, result);
}
}
return result;
}

serviceMethodCache 是一个Map，这里估计是考虑到了反射的效率问题，所以加了一个缓存机制，我们考到它使用new ServiceMethod.Builder(this, method).build(); 来创建一个

ServiceMethod这里又是一个建造者模式。我们点进去看一下

public ServiceMethod build() {
callAdapter = createCallAdapter();
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);
}

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);
}

这里执行了一些方法，我们看些主要的：

先看第一个createCallAdapter

private CallAdapter<?> createCallAdapter() {
Type returnType = method.getGenericReturnType();
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();
try {
return 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);
}
}

它调用了retrofit.callAdapter(returnType, annotations);

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

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

int start = adapterFactories.indexOf(skipPast) + 1;
for (int i = start, count = adapterFactories.size(); i < count; i++) {
// 这里就调用了我们前文讲的那个 工厂ExecutorCallAdapterFactory，然后掉了它的get方法
CallAdapter<?> adapter = adapterFactories.get(i).get(returnType, annotations, this);
if (adapter != null) {
return adapter;
}
}

StringBuilder builder = new StringBuilder("Could not locate call adapter for ")
.append(returnType)
.append(".\n");
if (skipPast != null) {
builder.append("  Skipped:");
for (int i = 0; i < start; i++) {
builder.append("\n   * ").append(adapterFactories.get(i).getClass().getName());
}
builder.append('\n');
}
builder.append("  Tried:");
for (int i = start, count = adapterFactories.size(); i < count; i++) {
builder.append("\n   * ").append(adapterFactories.get(i).getClass().getName());
}
throw new IllegalArgumentException(builder.toString());
}

这里调用了 adapterFactories.get(i).get(returnType, annotations, this);

因为在Retrofit.Builder.build的时候 向adapterFactories 加入了我们ExecutorCallAdapterFactory，所以它最终调用的是这个类的get方法

@Override
public CallAdapter<Call<?>> get(Type returnType, Annotation[] annotations, Retrofit retrofit) {
if (getRawType(returnType) != Call.class) {
return null;
}
final Type responseType = Utils.getCallResponseType(returnType);
return new CallAdapter<Call<?>>() {
@Override public Type responseType() {
return responseType;
}

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

可以看它的get方法，它返回了一个匿名类 CallAdapter< Call< ? > >

也就是说serviceMethod中的这个callAdapter 实际指向的就是这个匿名类

build中还有一个方法就是 parameterHandlers[p] = parseParameter(p, parameterType, parameterAnnotations);

它最终解析了方法的注解，来获得请求的类型。

然后我们看retrofit.create的最后 2句

OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);
return serviceMethod.callAdapter.adapt(okHttpCall);

我们看到它创建出了一个OkHttpCall 然后调用serviceMethod.callAdapter.adapt 的adapter方法，并将okhttpCall传了进去，上文我们说到serviceMethod.callAdapter就是我们看的那个匿名类

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

@Override public <R> Call<R> adapt(Call<R> 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) {
if (callback == null) throw new NullPointerException("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();
}
}

这里看到 adapter 返回的是一个 ExecutorCallbackCall，我们看这里是一个装饰者模式

它将 OkHttpCall 装饰成了一个ExecutorCallbackCall 然后返回给了调用程序

实际他是将请求委托给了 OkHttpCall。然后我们来继续看OkHttpCall

final class OkHttpCall<T> implements Call<T> {
private final ServiceMethod<T> serviceMethod;
private final Object[] args;

private volatile boolean canceled;

// All guarded by this.
private okhttp3.Call rawCall;
private Throwable creationFailure; // Either a RuntimeException or IOException.
private boolean executed;

OkHttpCall(ServiceMethod<T> serviceMethod, Object[] args) {
this.serviceMethod = serviceMethod;
this.args = args;
}

@SuppressWarnings("CloneDoesntCallSuperClone") // We are a final type & this saves clearing state.
@Override public OkHttpCall<T> clone() {
return new OkHttpCall<>(serviceMethod, args);
}

@Override public synchronized Request request() {
okhttp3.Call call = rawCall;
if (call != null) {
return call.request();
}
if (creationFailure != null) {
if (creationFailure instanceof IOException) {
throw new RuntimeException("Unable to create request.", creationFailure);
} else {
throw (RuntimeException) creationFailure;
}
}
try {
return (rawCall = createRawCall()).request();
} catch (RuntimeException e) {
creationFailure = e;
throw e;
} catch (IOException e) {
creationFailure = e;
throw new RuntimeException("Unable to create request.", e);
}
}

@Override public void enqueue(final Callback<T> callback) {
if (callback == null) throw new NullPointerException("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) {
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)
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();
}
}
});
}

@Override public synchronized boolean isExecuted() {
return executed;
}

@Override public Response<T> execute() throws IOException {
okhttp3.Call call;

synchronized (this) {
if (executed) throw new IllegalStateException("Already executed.");
executed = true;

if (creationFailure != null) {
if (creationFailure instanceof IOException) {
throw (IOException) creationFailure;
} else {
throw (RuntimeException) creationFailure;
}
}

call = rawCall;
if (call == null) {
try {
call = rawCall = createRawCall();
} catch (IOException | RuntimeException e) {
creationFailure = e;
throw e;
}
}
}

if (canceled) {
call.cancel();
}

return parseResponse(call.execute());
}

它封装了okhttp 的call，最后请求实际是通过okhttp来进行了请求。

也即是说调用程序执行请求的过程是，拿到的实际是一个实现了retrofit2.Call< T > 接口的

ExecutorCallbackCall< T > 然后ExecutorCallbackCall 将请求转给 OkHttpCall

然后 OkHttpCall 调用 okhttp 发起请求