Spring Cloud OpenFeign工作原理
一 、简介
OpenFeign是Spring Cloud 在Feign的基础上支持了Spring MVC的注解，如@RequesMapping等等。
OpenFeign的@FeignClient可以解析SpringMVC的@RequestMapping注解下的接口，并通过动态代理的方式产生实现类，实现类中做负载均衡并调用其他服务。
二、OpenFeign的使用
1. 引入依赖
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
<groupId>com.alibaba.cloud</groupId>
<artifactId>spring-cloud-starter-alibaba-nacos-discovery</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-starter-openfeign</artifactId>
</dependency>
2. 启动类添加@EnableFeignClients注解
@EnableDiscoveryClient
@SpringBootApplication
@EnableFeignClients(basePackages = {"xx.xx"})
@ComponentScan(basePackages = {"xx.xx"})
public class TagApplication {
public static void main(String[] args) {
SpringApplication application = new SpringApplication(TagApplication.class);
}
}
3. 编写业务类
@FeignClient(name = "user-service")
public interface BaseFeignApi {
@GetMapping("/list")
List<User> getList();
}
三、OpenFeign实现原理
1.@EnableFeignClients(basePackages = {"xx.xx"})注解
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(FeignClientsRegistrar.class)
public @interface EnableFeignClients {
String[] value() default {};
String[] basePackages() default {};
Class<?>[] basePackageClasses() default {};
Class<?>[] defaultConfiguration() default {};
Class<?>[] clients() default {};
}
2.FeignClientsRegistrar类
2.1 FeignClientsRegistrar
class FeignClientsRegistrar
implements ImportBeanDefinitionRegistrar, ResourceLoaderAware, EnvironmentAware {
....
}
2.2ImportBeanDefinitionRegistrar函数
public interface ImportBeanDefinitionRegistrar {
default void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry, BeanNameGenerator importBeanNameGenerator) {
this.registerBeanDefinitions(importingClassMetadata, registry);
}
default void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
}
}
FeignClientsRegistrar本质是一个ImportBeanDefinitionRegistrar,并且持有环境变量和资源加载器的能力,FeignClientsRegistrar重写了registerBeanDefinitions方法,该方法在容器上下文刷新(启动时调用refresh)时被调用,调用时机此处不展开分析,我们看一下.
2.3FeignClientsRegistrar#registerBeanDefinitions实现
@Override
public void registerBeanDefinitions(AnnotationMetadata metadata,
BeanDefinitionRegistry registry) {
// 处理@EnableFeignClients注解上的属性配置，将配置注册到容器仲
registerDefaultConfiguration(metadata, registry);
// 核心方法：注册@FeignClient对应的接口，奖@FeignClient注册到容器中
registerFeignClients(metadata, registry);
}
2.4FeignClientsRegistrar#registerFeignClients方法
public void registerFeignClients(AnnotationMetadata metadata,
BeanDefinitionRegistry registry) {
// 创建Spring内置的扫描器
ClassPathScanningCandidateComponentProvider scanner = getScanner();
scanner.setResourceLoader(this.resourceLoader);
Set<String> basePackages;
Map<String, Object> attrs = metadata
.getAnnotationAttributes(EnableFeignClients.class.getName());
AnnotationTypeFilter annotationTypeFilter = new AnnotationTypeFilter(
FeignClient.class);
// 获取 EnableFeignClients 注解中的 clients 属性值
final Class<?>[] clients = attrs == null ? null
: (Class<?>[]) attrs.get("clients");
if (clients == null || clients.length == 0) {
// 添加过滤器：过滤器所有被 @FeignClient 标记接口
scanner.addIncludeFilter(annotationTypeFilter);
basePackages = getBasePackages(metadata);
}
else {
final Set<String> clientClasses = new HashSet<>();
basePackages = new HashSet<>();
for (Class<?> clazz : clients) {
basePackages.add(ClassUtils.getPackageName(clazz));
clientClasses.add(clazz.getCanonicalName());
}
AbstractClassTestingTypeFilter filter = new AbstractClassTestingTypeFilter() {
@Override
protected boolean match(ClassMetadata metadata) {
String cleaned = metadata.getClassName().replaceAll("\\$", ".");
return clientClasses.contains(cleaned);
}
};
scanner.addIncludeFilter(
new AllTypeFilter(Arrays.asList(filter, annotationTypeFilter)));
}
// 处理@FeignClient类，解析
for (String basePackage : basePackages) {
Set<BeanDefinition> candidateComponents = scanner
.findCandidateComponents(basePackage);
for (BeanDefinition candidateComponent : candidateComponents) {
if (candidateComponent instanceof AnnotatedBeanDefinition) {
// verify annotated class is an interface
AnnotatedBeanDefinition beanDefinition = (AnnotatedBeanDefinition) candidateComponent;
AnnotationMetadata annotationMetadata = beanDefinition.getMetadata();
Assert.isTrue(annotationMetadata.isInterface(),
"@FeignClient can only be specified on an interface");
Map<String, Object> attributes = annotationMetadata
.getAnnotationAttributes(
FeignClient.class.getCanonicalName());
String name = getClientName(attributes);
registerClientConfiguration(registry, name,
attributes.get("configuration"));
// 重点方法，注入FeignClient对象
registerFeignClient(registry, annotationMetadata, attributes);
}
}
}
}
第一部分是为了找到@Feignclient标识的接口类，第二部分就是对找出的接口类进行处理了处理，主要关注registerClientConfiguration和registerFeignClient函数。
其中registerClientConfiguration是为了处理@FeignClient#configuration属性的，在这个函数会往spirng容器中添加#{serviceName}.FeignClientSpecification作为名字的FeignClientSpecification类对象，例如user-center.FeignClientSpecification。
而registerFeignClient函数则是处理接口类的主要方法了。我们在之前考虑到，我们在接口上填写了@FeignClient注解，在之后程序中我们可以直接引用这个接口对象来调用接口上的函数，理论分析一波：接口如果没有实现类，是不能直接在spring中直接进行注入并调用相应的方法的，一定需要我们去实现这个接口，那么我们可以想到，OpenFeign中一定做了这样的操作。
接下来我们看一下registerFeignClient函数
2.5registerFeignClient函数
private void registerFeignClient(BeanDefinitionRegistry registry,
AnnotationMetadata annotationMetadata, Map<String, Object> attributes) {
String className = annotationMetadata.getClassName();
// 创建一个 BeanDefinitionBuilder 对象，用于构建并注入 FeignClientFactoryBean
BeanDefinitionBuilder definition = BeanDefinitionBuilder
.genericBeanDefinition(FeignClientFactoryBean.class);
// 参数校验
validate(attributes);
// 设置BeanDefinition参数
definition.addPropertyValue("url", getUrl(attributes));
definition.addPropertyValue("path", getPath(attributes));
String name = getName(attributes);
definition.addPropertyValue("name", name);
String contextId = getContextId(attributes);
definition.addPropertyValue("contextId", contextId);
definition.addPropertyValue("type", className);
definition.addPropertyValue("decode404", attributes.get("decode404"));
definition.addPropertyValue("fallback", attributes.get("fallback"));
definition.addPropertyValue("fallbackFactory", attributes.get("fallbackFactory"));
definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
String alias = contextId + "FeignClient";
AbstractBeanDefinition beanDefinition = definition.getBeanDefinition();
boolean primary = (Boolean) attributes.get("primary"); // has a default, won't be
// null
beanDefinition.setPrimary(primary);
String qualifier = getQualifier(attributes);
if (StringUtils.hasText(qualifier)) {
alias = qualifier;
}
BeanDefinitionHolder holder = new BeanDefinitionHolder(beanDefinition, className,
new String[] { alias });
// 注入接口实例
BeanDefinitionReaderUtils.registerBeanDefinition(holder, registry);
}
3. 接口代理对象的构建
OpenFeign 接口代理对象的构建，主要是通过 Spring 的扩展接口 FactoryBean 来实现的。在上面的代码中，通过解析 FeignClient 对象，构建成一个 FeignClientFactoryBean 对象，Spring 在注入对应接口是，会调用 FeignClientFactoryBean 对象中的 getObject() 方法，返回注入对应的代理对象。
3.1FeignClientFactoryBean类
class FeignClientFactoryBean
implements FactoryBean<Object>, InitializingBean, ApplicationContextAware {
@Override
public Object getObject() throws Exception {
// 获取目标对象
return getTarget();
}
/**
* 获取目标对象
* @param <T> the target type of the Feign client
* @return a {@link Feign} client created with the specified data and the context
* information
*/
<T> T getTarget() {
FeignContext context = this.applicationContext.getBean(FeignContext.class);
Feign.Builder builder = feign(context);
// 判断当前 FeignClient 注解中的url是否为空，如果不为空，直接通过url的调用
if (!StringUtils.hasText(this.url)) {
if (!this.name.startsWith("http")) {
this.url = "http://" + this.name;
}
else {
this.url = this.name;
}
this.url += cleanPath();
// 返回目标对象
return (T) loadBalance(builder, context,
new HardCodedTarget<>(this.type, this.name, this.url));
}
if (StringUtils.hasText(this.url) && !this.url.startsWith("http")) {
this.url = "http://" + this.url;
}
String url = this.url + cleanPath();
Client client = getOptional(context, Client.class);
if (client != null) {
if (client instanceof LoadBalancerFeignClient) {
// not load balancing because we have a url,
// but ribbon is on the classpath, so unwrap
client = ((LoadBalancerFeignClient) client).getDelegate();
}
if (client instanceof FeignBlockingLoadBalancerClient) {
// not load balancing because we have a url,
// but Spring Cloud LoadBalancer is on the classpath, so unwrap
client = ((FeignBlockingLoadBalancerClient) client).getDelegate();
}
builder.client(client);
}
Targeter targeter = get(context, Targeter.class);
// 返回目标对象
return (T) targeter.target(this, builder, context,
new HardCodedTarget<>(this.type, this.name, url));
}
}
3.2 Targeter#target函数
HystrixTargeter类实现了Targeter。
class HystrixTargeter implements Targeter {
@Override
public <T> T target(FeignClientFactoryBean factory, Feign.Builder feign,
FeignContext context, Target.HardCodedTarget<T> target) {
if (!(feign instanceof feign.hystrix.HystrixFeign.Builder)) {
return feign.target(target);
}
feign.hystrix.HystrixFeign.Builder builder = (feign.hystrix.HystrixFeign.Builder) feign;
String name = StringUtils.isEmpty(factory.getContextId()) ? factory.getName()
: factory.getContextId();
SetterFactory setterFactory = getOptional(name, context, SetterFactory.class);
if (setterFactory != null) {
builder.setterFactory(setterFactory);
}
Class<?> fallback = factory.getFallback();
if (fallback != void.class) {
return targetWithFallback(name, context, target, builder, fallback);
}
Class<?> fallbackFactory = factory.getFallbackFactory();
if (fallbackFactory != void.class) {
return targetWithFallbackFactory(name, context, target, builder,
fallbackFactory);
}
// 返回目标对象
return feign.target(target);
}
}
3.3 Feign对象
feign.target(target)函数
public abstract class Feign {
public <T> T target(Target<T> target) {
// 创建代理对象
return this.build().newInstance(target);
}
public abstract <T> T newInstance(Target<T> target);
/**
* 构建 Feign对象
*/
public Feign build() {
Client client = (Client)Capability.enrich(this.client, this.capabilities);
Retryer retryer = (Retryer)Capability.enrich(this.retryer, this.capabilities);
List<RequestInterceptor> requestInterceptors = (List)this.requestInterceptors.stream().map((ri) -> {
return (RequestInterceptor)Capability.enrich(ri, this.capabilities);
}).collect(Collectors.toList());
Logger logger = (Logger)Capability.enrich(this.logger, this.capabilities);
Contract contract = (Contract)Capability.enrich(this.contract, this.capabilities);
Options options = (Options)Capability.enrich(this.options, this.capabilities);
Encoder encoder = (Encoder)Capability.enrich(this.encoder, this.capabilities);
Decoder decoder = (Decoder)Capability.enrich(this.decoder, this.capabilities);
// 创建代理对象的 InvocationHandler 工厂实例
InvocationHandlerFactory invocationHandlerFactory = (InvocationHandlerFactory)Capability.enrich(this.invocationHandlerFactory, this.capabilities);
QueryMapEncoder queryMapEncoder = (QueryMapEncoder)Capability.enrich(this.queryMapEncoder, this.capabilities);
Factory synchronousMethodHandlerFactory = new Factory(client, retryer, requestInterceptors, logger, this.logLevel, this.decode404, this.closeAfterDecode, this.propagationPolicy, this.forceDecoding);
ParseHandlersByName handlersByName = new ParseHandlersByName(contract, options, encoder, decoder, queryMapEncoder, this.errorDecoder, synchronousMethodHandlerFactory);
return new ReflectiveFeign(handlersByName, invocationHandlerFactory, queryMapEncoder);
}
}
点击进入newInstance()函数
3.4ReflectiveFeign 对象
ReflectiveFeign继承了Feign
public class ReflectiveFeign extends Feign {
/**
* 创建代理对象
*/
@Override
public <T> T newInstance(Target<T> target) {
Map<String, MethodHandler> nameToHandler = targetToHandlersByName.apply(target);
Map<Method, MethodHandler> methodToHandler = new LinkedHashMap<Method, MethodHandler>();
List<DefaultMethodHandler> defaultMethodHandlers = new LinkedList<DefaultMethodHandler>();
// 解析模板：将方法解析，封装为MethodHandler
for (Method method : target.type().getMethods()) {
if (method.getDeclaringClass() == Object.class) {
continue;
} else if (Util.isDefault(method)) {
DefaultMethodHandler handler = new DefaultMethodHandler(method);
defaultMethodHandlers.add(handler);
methodToHandler.put(method, handler);
} else {
methodToHandler.put(method, nameToHandler.get(Feign.configKey(target.type(), method)));
}
}
InvocationHandler handler = factory.create(target, methodToHandler);
// 创建代理对象
T proxy = (T) Proxy.newProxyInstance(target.type().getClassLoader(),
new Class<?>[] {target.type()}, handler);
for (DefaultMethodHandler defaultMethodHandler : defaultMethodHandlers) {
defaultMethodHandler.bindTo(proxy);
}
return proxy;
}
}
看到这里就比较清晰了,把接口中的方法和默认实现放到Map<Method, MethodHandler>中然后使用InvocationHandlerFactory.Default()创建InvocationHandler,然后使用jdk动态代理生成接口的代理并返回,这里主要看一下Feign的InvocationHandler实现:
static class FeignInvocationHandler implements InvocationHandler {
private final Target target;
private final Map<Method, MethodHandler> dispatch;
FeignInvocationHandler(Target target, Map<Method, MethodHandler> dispatch) {
this.target = checkNotNull(target, "target");
this.dispatch = checkNotNull(dispatch, "dispatch for %s", target);
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
if ("equals".equals(method.getName())) {
try {
Object otherHandler =
args.length > 0 && args[0] != null ? Proxy.getInvocationHandler(args[0]) : null;
return equals(otherHandler);
} catch (IllegalArgumentException e) {
return false;
}
} else if ("hashCode".equals(method.getName())) {
return hashCode();
} else if ("toString".equals(method.getName())) {
return toString();
}
return dispatch.get(method).invoke(args);
}
}
四、服务调用
正如我们前边所说,通过@Autowired或者@Resource注入的时候,注入的是被封装之后的代理类实现,
jdk动态代理持有的是ReflectiveFeign.FeignInvocationHandler类型的InvocationHandler,那么具体调用的时候,会调用FeignInvocationHandler#invoke
4.1 FeignInvocationHandler对象
static class FeignInvocationHandler implements InvocationHandler {
private final Target target;
private final Map<Method, MethodHandler> dispatch;
FeignInvocationHandler(Target target, Map<Method, MethodHandler> dispatch) {
this.target = checkNotNull(target, "target");
this.dispatch = checkNotNull(dispatch, "dispatch for %s", target);
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
if ("equals".equals(method.getName())) {
try {
Object otherHandler =
args.length > 0 && args[0] != null ? Proxy.getInvocationHandler(args[0]) : null;
return equals(otherHandler);
} catch (IllegalArgumentException e) {
return false;
}
} else if ("hashCode".equals(method.getName())) {
return hashCode();
} else if ("toString".equals(method.getName())) {
return toString();
}
// 执行
return dispatch.get(method).invoke(args);
}
}
前几个判断分支都是调用了Object的基本方法,最后dispatch.get(method).invoke(args)才是接口的业务方法调用,dispatch的类型是Map<Method, MethodHandler>,是接口中方法与MethodHandler的映射关系,而MethodHandler又被SynchronousMethodHandler.Factory封装成SynchronousMethodHandler(实现了MethodHandler):
4.2 SynchronousMethodHandler类
SynchronousMethodHandler类实现了MethodHandler。那么dispatch.get(method).invoke(args)最终调用的就是SynchronousMethodHandler#invoke:
final class SynchronousMethodHandler implements MethodHandler {
@Override
public Object invoke(Object[] argv) throws Throwable {
// 构建请求的一个模版
RequestTemplate template = buildTemplateFromArgs.create(argv);
Options options = findOptions(argv);
Retryer retryer = this.retryer.clone();
while (true) {
try {
// 可以看到调用的时候默认是带有重试能力,默认是5次,具体调用交给executeAndDecode来实现:
return executeAndDecode(template, options);
} catch (RetryableException e) {
try {
retryer.continueOrPropagate(e);
} catch (RetryableException th) {
Throwable cause = th.getCause();
if (propagationPolicy == UNWRAP && cause != null) {
throw cause;
} else {
throw th;
}
}
if (logLevel != Logger.Level.NONE) {
logger.logRetry(metadata.configKey(), logLevel);
}
continue;
}
}
}
}
可以看到调用的时候默认是带有重试能力,默认是5次,具体调用交给executeAndDecode来实现:
Object executeAndDecode(RequestTemplate template, Options options) throws Throwable {
Request request = targetRequest(template);
if (logLevel != Logger.Level.NONE) {
logger.logRequest(metadata.configKey(), logLevel, request);
}
Response response;
long start = System.nanoTime();
try {
// 通过 Client执行，进行远程通信
response = client.execute(request, options);
} catch (IOException e) {
if (logLevel != Logger.Level.NONE) {
logger.logIOException(metadata.configKey(), logLevel, e, elapsedTime(start));
}
throw errorExecuting(request, e);
}
long elapsedTime = TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - start);
boolean shouldClose = true;
try {
if (logLevel != Logger.Level.NONE) {
response =
logger.logAndRebufferResponse(metadata.configKey(), logLevel, response, elapsedTime);
}
if (Response.class == metadata.returnType()) {
if (response.body() == null) {
return response;
}
if (response.body().length() == null ||
response.body().length() > MAX_RESPONSE_BUFFER_SIZE) {
shouldClose = false;
return response;
}
// Ensure the response body is disconnected
byte[] bodyData = Util.toByteArray(response.body().asInputStream());
return response.toBuilder().body(bodyData).build();
}
if (response.status() >= 200 && response.status() < 300) {
if (void.class == metadata.returnType()) {
return null;
} else {
Object result = decode(response);
shouldClose = closeAfterDecode;
return result;
}
} else if (decode404 && response.status() == 404 && void.class != metadata.returnType()) {
Object result = decode(response);
shouldClose = closeAfterDecode;
return result;
} else {
throw errorDecoder.decode(metadata.configKey(), response);
}
} catch (IOException e) {
if (logLevel != Logger.Level.NONE) {
logger.logIOException(metadata.configKey(), logLevel, e, elapsedTime);
}
throw errorReading(request, response, e);
} finally {
if (shouldClose) {
ensureClosed(response.body());
}
}
}
核心是client.execute,我们选择性看一下OkHttpClient的实现:
@Override
public feign.Response execute(feign.Request input, feign.Request.Options options)
throws IOException {
okhttp3.OkHttpClient requestScoped;
if (delegate.connectTimeoutMillis() != options.connectTimeoutMillis()
|| delegate.readTimeoutMillis() != options.readTimeoutMillis()
|| delegate.followRedirects() != options.isFollowRedirects()) {
requestScoped = delegate.newBuilder()
.connectTimeout(options.connectTimeoutMillis(), TimeUnit.MILLISECONDS)
.readTimeout(options.readTimeoutMillis(), TimeUnit.MILLISECONDS)
.followRedirects(options.isFollowRedirects())
.build();
} else {
requestScoped = delegate;
}
Request request = toOkHttpRequest(input);
Response response = requestScoped.newCall(request).execute();
return toFeignResponse(response, input).toBuilder().request(input).build();
}
很明显,最终服务调用会委托给okhttp3.OkHttpClient来执行,然后组装结果和状态码返回调用,到这里openfeign的服务调用就分析完了,为了帮助理解和有更直观的概念,我们看一下服务调用时序图:
五、总结
- 通过
@EnableFeignCleints触发 Spring 应用程序对 classpath 中 @FeignClient 修饰类的扫描 - 解析到
@FeignClient修饰类后， Feign 框架通过扩展 Spring BeanDeifinition的注册逻辑， 最终注册一个FeignClientFacotoryBean进入 Spring 容器 - Spring 容器在初始化其他用到
@FeignClient接口的类时， 获得的是FeignClientFacotryBean产生的一个代理对象 Proxy. - 基于 java 原生的动态代理机制， 针对 Proxy 的调用， 都会被统一转发给 Feign 框架所定义的一个
InvocationHandler， 由该Handler完成后续的 HTTP 转换， 发送， 接收， 翻译HTTP响应的工作
感觉这块的逻辑还是有点难度的，我debug源码三四次才搞懂OpenFeign底层是如何创建代理对象，如何实现调用的。
参考：
-
https://blog.csdn.net/weixin_41821642/article/details/129874394?ops_request_misc=%257B%2522request%255Fid%2522%253A%2522171325961116800188518011%2522%252C%2522scm%2522%253A%252220140713.130102334…%2522%257D&request_id=171325961116800188518011&biz_id=0&utm_medium=distribute.pc_search_result.none-task-blog-2allbaidu_landing_v2~default-5-129874394-null-null.142v100pc_search_result_base4&utm_term=openfeign%E5%BA%95%E5%B1%82%E8%B0%83%E7%94%A8%E5%8E%9F%E7%90%86&spm=1018.2226.3001.4187
-
https://blog.csdn.net/bz120413/article/details/122215774?ops_request_misc=&request_id=&biz_id=102&utm_term=openfeign%E5%BA%95%E5%B1%82%E8%B0%83%E7%94%A8%E5%8E%9F%E7%90%86&utm_medium=distribute.pc_search_result.none-task-blog-2allsobaiduweb~default-3-122215774.142v100pc_search_result_base4&spm=1018.2226.3001.4187
