背景
在系统中我们经常需要使用到配置，而在springboot里加载配置到bean中，通常采用ConfigurationProperties或value注解，这里两种注解有什么异同和需要注意的地方呢？
ConfigurationProperties
我们都知道使用ConfigurationProperties通常要指定prefix属性，同时应用入口Application类需要增加@ConfigurationPropertiesScan。类似下面
@ConfigurationProperties(prefix="weixin")
class WeChatConfig {
private String host;
private String templateMessageUrl;
public String getHost(){
return this.host;
}
public void setHost(String host){
this.host = host
}
public String getTemplateMessageUrl(){
return this.templateMessageUrl;
}
public String setTemplateMessageUrl(String templateMessageUrl){
this.templateMessageUrl = templateMessageUrl;
}
}
@SpringBootApplication
@ConfigurationPropertiesScan
public class SpringConfigApplication {
}
对应的yaml配置如下：
weixin:
host: "https://api.weixin.qq.com",
templateMessageUrl: "/cgi-bin/message/template/send"
同时得益于ConfigurationProperties松散绑定的机制，templateMessageUrl可以是template-message-url。同时我们知道，在spring cloud项目里修改nacos的yaml配置后，是即时生效的。那这个配置的即时更新以及配置到java bean字段的绑定具体是怎么做的呢？这就要看ConfigurationProperties的配置绑定机制了。
ConfigurationProperties配置绑定机制
spring中关于配置绑定都是ConfigurationPropertiesBindingPostProcessor来做的，先来看postProcessBeforeInitialization方法，bind方法会先判断该Bean是否已在容器中或构造函数绑定的。如果不在容器中或是构造函数绑定的，则直接返回，接着调用binder.bind()方法。
public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
bind(ConfigurationPropertiesBean.get(this.applicationContext, bean, beanName));
return bean;
}
private void bind(ConfigurationPropertiesBean bean) {
if (bean == null || hasBoundValueObject(bean.getName())) {
return;
}
Assert.state(bean.getBindMethod() == BindMethod.JAVA_BEAN, "Cannot bind @ConfigurationProperties for bean '"
+ bean.getName() + "'. Ensure that @ConstructorBinding has not been applied to regular bean");
try {
this.binder.bind(bean);
}
catch (Exception ex) {
throw new ConfigurationPropertiesBindException(bean, ex);
}
}
这里的binder是在afterPropertiesSet方法设置进来的，核心逻辑在ConfigurationPropertiesBinder.get()方法中。
public void afterPropertiesSet() throws Exception {
// We can't use constructor injection of the application context because
// it causes eager factory bean initialization
this.registry = (BeanDefinitionRegistry) this.applicationContext.getAutowireCapableBeanFactory();
this.binder = ConfigurationPropertiesBinder.get(this.applicationContext);
}
ConfigurationPropertiesBinder.get()就是从容器获取BEAN_NAME=org.springframework.boot.context.internalConfigurationPropertiesBinder的Bean。而这个bean是通过ConfigurationPropertiesBinder.Factory.create方法创建出来的。这是怎么做到的呢？因为在ConfigurationPropertiesBindingPostProcessor.register方法中调用了ConfigurationPropertiesBinder.register方法，ConfigurationPropertiesBinder.register创建了GenericBeanDefinition指定了factoryBean就是ConfigurationPropertiesBinder.Factory对应的BeanDefinition，factoryMethodName就是create方法。
static ConfigurationPropertiesBinder get(BeanFactory beanFactory) {
return beanFactory.getBean(BEAN_NAME, ConfigurationPropertiesBinder.class);
}
static void register(BeanDefinitionRegistry registry) {
if (!registry.containsBeanDefinition(FACTORY_BEAN_NAME)) {
GenericBeanDefinition definition = new GenericBeanDefinition();
definition.setBeanClass(ConfigurationPropertiesBinder.Factory.class);
definition.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
registry.registerBeanDefinition(ConfigurationPropertiesBinder.FACTORY_BEAN_NAME, definition);
}
if (!registry.containsBeanDefinition(BEAN_NAME)) {
GenericBeanDefinition definition = new GenericBeanDefinition();
definition.setBeanClass(ConfigurationPropertiesBinder.class);
definition.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
definition.setFactoryBeanName(FACTORY_BEAN_NAME);
definition.setFactoryMethodName("create");
registry.registerBeanDefinition(ConfigurationPropertiesBinder.BEAN_NAME, definition);
}
}
static ConfigurationPropertiesBinder get(BeanFactory beanFactory) {
return beanFactory.getBean(BEAN_NAME, ConfigurationPropertiesBinder.class);
}
/**
* Factory bean used to create the {@link ConfigurationPropertiesBinder}. The bean
* needs to be {@link ApplicationContextAware} since we can't directly inject an
* {@link ApplicationContext} into the constructor without causing eager
* {@link FactoryBean} initialization.
*/
static class Factory implements ApplicationContextAware {
private ApplicationContext applicationContext;
@Override
public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
this.applicationContext = applicationContext;
}
ConfigurationPropertiesBinder create() {
return new ConfigurationPropertiesBinder(this.applicationContext);
}
}
回到ConfigurationPropertiesBindingPostProcessor.bind方法，这里实际调用的就是ConfigurationPropertiesBinder.bind方法。
BindResult<?> bind(ConfigurationPropertiesBean propertiesBean) {
Bindable<?> target = propertiesBean.asBindTarget();
ConfigurationProperties annotation = propertiesBean.getAnnotation();
BindHandler bindHandler = getBindHandler(target, annotation);
return getBinder().bind(annotation.prefix(), target, bindHandler);
}
这里的target实际就是Bindable对象，包含待处理对应class及该对象上的annotation信息，类似下面这样。
image-20240303164536468.png
接着回到getBindHandler方法，源码如下
private <T> BindHandler getBindHandler(Bindable<T> target, ConfigurationProperties annotation) {
List<Validator> validators = getValidators(target);
BindHandler handler = getHandler();
if (annotation.ignoreInvalidFields()) {
handler = new IgnoreErrorsBindHandler(handler);
}
if (!annotation.ignoreUnknownFields()) {
UnboundElementsSourceFilter filter = new UnboundElementsSourceFilter();
handler = new NoUnboundElementsBindHandler(handler, filter);
}
if (!validators.isEmpty()) {
handler = new ValidationBindHandler(handler, validators.toArray(new Validator[0]));
}
for (ConfigurationPropertiesBindHandlerAdvisor advisor : getBindHandlerAdvisors()) {
handler = advisor.apply(handler);
}
return handler;
可以看到该方法主要获取一些验证器和绑定的advisor，用于在绑定前做一些数据校验等前置工作。然后回调用Binder.bind方法来做数据绑定。
private <T> T bind(ConfigurationPropertyName name, Bindable<T> target, BindHandler handler, Context context,
boolean allowRecursiveBinding, boolean create) {
try {
Bindable<T> replacementTarget = handler.onStart(name, target, context);
if (replacementTarget == null) {
return handleBindResult(name, target, handler, context, null, create);
}
target = replacementTarget;
Object bound = bindObject(name, target, handler, context, allowRecursiveBinding);
return handleBindResult(name, target, handler, context, bound, create);
}
catch (Exception ex) {
return handleBindError(name, target, handler, context, ex);
}
Binder.bind方法的入口是调用bindObject方法，bindObject方法是处理对像的数据绑定。
private <T> Object bindObject(ConfigurationPropertyName name, Bindable<T> target, BindHandler handler,
Context context, boolean allowRecursiveBinding) {
ConfigurationProperty property = findProperty(name, context);
if (property == null && context.depth != 0 && containsNoDescendantOf(context.getSources(), name)) {
return null;
}
AggregateBinder<?> aggregateBinder = getAggregateBinder(target, context);
if (aggregateBinder != null) {
return bindAggregate(name, target, handler, context, aggregateBinder);
}
if (property != null) {
try {
return bindProperty(target, context, property);
}
catch (ConverterNotFoundException ex) {
// We might still be able to bind it using the recursive binders
Object instance = bindDataObject(name, target, handler, context, allowRecursiveBinding);
if (instance != null) {
return instance;
}
throw ex;
}
}
return bindDataObject(name, target, handler, context, allowRecursiveBinding);
}
bindProperty负责对象属性的绑定，bindDataObject负责整个bean数据的绑定。
private Object bindDataObject(ConfigurationPropertyName name, Bindable<?> target, BindHandler handler,
Context context, boolean allowRecursiveBinding) {
if (isUnbindableBean(name, target, context)) {
return null;
}
Class<?> type = target.getType().resolve(Object.class);
if (!allowRecursiveBinding && context.isBindingDataObject(type)) {
return null;
}
DataObjectPropertyBinder propertyBinder = (propertyName, propertyTarget) -> bind(name.append(propertyName),
propertyTarget, handler, context, false, false);
return context.withDataObject(type, () -> {
for (DataObjectBinder dataObjectBinder : this.dataObjectBinders) {
Object instance = dataObjectBinder.bind(name, target, context, propertyBinder);
if (instance != null) {
return instance;
}
}
return null;
});
真正数据的绑定在context.withDataObject方法的lamda语句中实现的。lamda里面实际上就是调用不同的dataObjectBinder的bind方法。
public Binder(Iterable<ConfigurationPropertySource> sources, PlaceholdersResolver placeholdersResolver,
ConversionService conversionService, Consumer<PropertyEditorRegistry> propertyEditorInitializer,
BindHandler defaultBindHandler, BindConstructorProvider constructorProvider) {
Assert.notNull(sources, "Sources must not be null");
this.sources = sources;
this.placeholdersResolver = (placeholdersResolver != null) ? placeholdersResolver : PlaceholdersResolver.NONE;
this.conversionService = (conversionService != null) ? conversionService
: ApplicationConversionService.getSharedInstance();
this.propertyEditorInitializer = propertyEditorInitializer;
this.defaultBindHandler = (defaultBindHandler != null) ? defaultBindHandler : BindHandler.DEFAULT;
if (constructorProvider == null) {
constructorProvider = BindConstructorProvider.DEFAULT;
}
ValueObjectBinder valueObjectBinder = new ValueObjectBinder(constructorProvider);
JavaBeanBinder javaBeanBinder = JavaBeanBinder.INSTANCE;
this.dataObjectBinders = Collections.unmodifiableList(Arrays.asList(valueObjectBinder, javaBeanBinder));
}
从Binder类的构造函数可以看出，dataObjectBinders实际上就ValueObjectBinder、JavaBeanBinder。ValueObjectBinder实际就是调用构造函数去构建值对象，JavaBeanBinder实际上就是调用对象的set方法去设置对象的属性。
同时从下面JavaBeanBinder的binder方法可以看到，当配置文件中属性不存在配置的时候，会直接返回，并不会报错。
private <T> boolean bind(BeanSupplier<T> beanSupplier, DataObjectPropertyBinder propertyBinder,
BeanProperty property) {
String propertyName = property.getName();
ResolvableType type = property.getType();
Supplier<Object> value = property.getValue(beanSupplier);
Annotation[] annotations = property.getAnnotations();
Object bound = propertyBinder.bindProperty(propertyName,
Bindable.of(type).withSuppliedValue(value).withAnnotations(annotations));
if (bound == null) {
return false;
}
if (property.isSettable()) {
property.setValue(beanSupplier, bound);
}
else if (value == null || !bound.equals(value.get())) {
throw new IllegalStateException("No setter found for property: " + property.getName());
}
return true;
}
ConfigurationProperties配置动态更新机制
Spring cloud配置的刷新是ConfigurationPropertiesRebinder来实现的。
从下面的代码可以看到ConfigurationPropertiesRebinder是通过applicationEvent的触发来做配置更新的。这里有两个问题：1、谁来触发这个applicationEvent的呢？。2、rebind里具体是怎做的呢？
@Override
public void onApplicationEvent(EnvironmentChangeEvent event) {
if (this.applicationContext.equals(event.getSource())
// Backwards compatible
|| event.getKeys().equals(event.getSource())) {
rebind();
}
}
@ManagedOperation
public void rebind() {
this.errors.clear();
for (String name : this.beans.getBeanNames()) {
rebind(name);
}
}
@ManagedOperation
public boolean rebind(String name) {
if (!this.beans.getBeanNames().contains(name)) {
return false;
}
ApplicationContext appContext = this.applicationContext;
while (appContext != null) {
if (appContext.containsLocalBean(name)) {
return rebind(name, appContext);
}
else {
appContext = appContext.getParent();
}
}
return false;
}
private boolean rebind(String name, ApplicationContext appContext) {
try {
Object bean = appContext.getBean(name);
if (AopUtils.isAopProxy(bean)) {
bean = ProxyUtils.getTargetObject(bean);
}
if (bean != null) {
// TODO: determine a more general approach to fix this.
// see
// https://github.com/spring-cloud/spring-cloud-commons/issues/571
if (getNeverRefreshable().contains(bean.getClass().getName())) {
return false; // ignore
}
appContext.getAutowireCapableBeanFactory().destroyBean(bean);
appContext.getAutowireCapableBeanFactory().initializeBean(bean, name);
return true;
}
}
catch (RuntimeException e) {
this.errors.put(name, e);
throw e;
}
catch (Exception e) {
this.errors.put(name, e);
throw new IllegalStateException("Cannot rebind to " + name, e);
}
return false;
}
我们先来看看第一个问题，就nacos的配置场景，来说明applicationEvent是怎么触发的。
要弄清楚配置更新的applicationEvent是怎么触发的，就需要弄清楚nocas的配置是怎么更新。nocas的配置是通过NacosConfigManager类实现的。
public NacosConfigManager(NacosConfigProperties nacosConfigProperties) {
this.nacosConfigProperties = nacosConfigProperties;
// Compatible with older code in NacosConfigProperties,It will be deleted in the
// future.
createConfigService(nacosConfigProperties);
}
/**
* Compatible with old design,It will be perfected in the future.
*/
static ConfigService createConfigService(
NacosConfigProperties nacosConfigProperties) {
if (Objects.isNull(service)) {
synchronized (NacosConfigManager.class) {
try {
if (Objects.isNull(service)) {
service = NacosFactory.createConfigService(
nacosConfigProperties.assembleConfigServiceProperties());
}
}
catch (NacosException e) {
log.error(e.getMessage());
throw new NacosConnectionFailureException(
nacosConfigProperties.getServerAddr(), e.getMessage(), e);
}
}
}
return service;
}
从上面代码可是看到NacosConfigManager在实例化的时候，会调用NacosFactory.createConfigService创建一个NocasConfigService实例。那configService里又具体做了什么呢？
从下面的代码可以看到了实例了一个ClientWorker对象，配置更新的事情就是这个对象来负责的。
public ClientWorker(final ConfigFilterChainManager configFilterChainManager, ServerListManager serverListManager,
final NacosClientProperties properties) throws NacosException {
this.configFilterChainManager = configFilterChainManager;
init(properties);
agent = new ConfigRpcTransportClient(properties, serverListManager);
ScheduledExecutorService executorService = Executors.newScheduledThreadPool(initWorkerThreadCount(properties),
new NameThreadFactory("com.alibaba.nacos.client.Worker"));
agent.setExecutor(executorService);
agent.start();
从下面的代码来看ConfigTransportClient的start方法最终会调用ClientWorker的startInterval方法，而startInterval方法会异步调用executeConfigListen方法。
public void start() throws NacosException {
securityProxy.login(this.properties);
this.executor.scheduleWithFixedDelay(() -> securityProxy.login(properties), 0,
this.securityInfoRefreshIntervalMills, TimeUnit.MILLISECONDS);
startInternal();
}
public void startInternal() {
executor.schedule(() -> {
while (!executor.isShutdown() && !executor.isTerminated()) {
try {
listenExecutebell.poll(5L, TimeUnit.SECONDS);
if (executor.isShutdown() || executor.isTerminated()) {
continue;
}
executeConfigListen();
} catch (Throwable e) {
LOGGER.error("[rpc listen execute] [rpc listen] exception", e);
try {
Thread.sleep(50L);
} catch (InterruptedException interruptedException) {
//ignore
}
notifyListenConfig();
}
}
}, 0L, TimeUnit.MILLISECONDS);
}
从下面的代码可以看到会循环调用CacheData的checkListenerMd5方法，
public void executeConfigListen() throws NacosException {
Map<String, List<CacheData>> listenCachesMap = new HashMap<>(16);
Map<String, List<CacheData>> removeListenCachesMap = new HashMap<>(16);
long now = System.currentTimeMillis();
boolean needAllSync = now - lastAllSyncTime >= ALL_SYNC_INTERNAL;
for (CacheData cache : cacheMap.get().values()) {
synchronized (cache) {
checkLocalConfig(cache);
// check local listeners consistent.
if (cache.isConsistentWithServer()) {
cache.checkListenerMd5();
if (!needAllSync) {
continue;
}
}
// If local configuration information is used, then skip the processing directly.
if (cache.isUseLocalConfigInfo()) {
continue;
}
if (!cache.isDiscard()) {
List<CacheData> cacheDatas = listenCachesMap.computeIfAbsent(String.valueOf(cache.getTaskId()),
k -> new LinkedList<>());
cacheDatas.add(cache);
} else {
List<CacheData> cacheDatas = removeListenCachesMap.computeIfAbsent(
String.valueOf(cache.getTaskId()), k -> new LinkedList<>());
cacheDatas.add(cache);
}
}
}
重点看这里的CacheData.checkListenerMd5方法，最终会调用到safeNotifyListener方法。该方法会调用listener.receiveConfigInfo(contentTmp)。
private void safeNotifyListener(final String dataId, final String group, final String content, final String type,
final String md5, final String encryptedDataKey, final ManagerListenerWrap listenerWrap) {
final Listener listener = listenerWrap.listener;
if (listenerWrap.inNotifying) {
LOGGER.warn(
"[{}] [notify-currentSkip] dataId={}, group={},tenant={}, md5={}, listener={}, listener is not finish yet,will try next time.",
envName, dataId, group, tenant, md5, listener);
return;
}
NotifyTask job = new NotifyTask() {
@Override
public void run() {
long start = System.currentTimeMillis();
ClassLoader myClassLoader = Thread.currentThread().getContextClassLoader();
ClassLoader appClassLoader = listener.getClass().getClassLoader();
ScheduledFuture<?> timeSchedule = null;
try {
if (listener instanceof AbstractSharedListener) {
AbstractSharedListener adapter = (AbstractSharedListener) listener;
adapter.fillContext(dataId, group);
LOGGER.info("[{}] [notify-context] dataId={}, group={},tenant={}, md5={}", envName, dataId,
group, tenant, md5);
}
// Before executing the callback, set the thread classloader to the classloader of
// the specific webapp to avoid exceptions or misuses when calling the spi interface in
// the callback method (this problem occurs only in multi-application deployment).
Thread.currentThread().setContextClassLoader(appClassLoader);
ConfigResponse cr = new ConfigResponse();
cr.setDataId(dataId);
cr.setGroup(group);
cr.setContent(content);
cr.setEncryptedDataKey(encryptedDataKey);
configFilterChainManager.doFilter(null, cr);
String contentTmp = cr.getContent();
timeSchedule = getNotifyBlockMonitor().schedule(
new LongNotifyHandler(listener.getClass().getSimpleName(), dataId, group, tenant, md5,
notifyWarnTimeout, Thread.currentThread()), notifyWarnTimeout,
TimeUnit.MILLISECONDS);
listenerWrap.inNotifying = true;
listener.receiveConfigInfo(contentTmp);
// compare lastContent and content
if (listener instanceof AbstractConfigChangeListener) {
Map<String, ConfigChangeItem> data = ConfigChangeHandler.getInstance()
.parseChangeData(listenerWrap.lastContent, contentTmp, type);
ConfigChangeEvent event = new ConfigChangeEvent(data);
((AbstractConfigChangeListener) listener).receiveConfigChange(event);
listenerWrap.lastContent = contentTmp;
}
listenerWrap.lastCallMd5 = md5;
LOGGER.info(
"[{}] [notify-ok] dataId={}, group={},tenant={}, md5={}, listener={} ,job run cost={} millis.",
envName, dataId, group, tenant, md5, listener, (System.currentTimeMillis() - start));
} catch (NacosException ex) {
LOGGER.error(
"[{}] [notify-error] dataId={}, group={},tenant={},md5={}, listener={} errCode={} errMsg={},stackTrace :{}",
envName, dataId, group, tenant, md5, listener, ex.getErrCode(), ex.getErrMsg(),
getTrace(ex.getStackTrace(), 3));
} catch (Throwable t) {
LOGGER.error("[{}] [notify-error] dataId={}, group={},tenant={}, md5={}, listener={} tx={}",
envName, dataId, group, tenant, md5, listener, getTrace(t.getStackTrace(), 3));
} finally {
listenerWrap.inNotifying = false;
Thread.currentThread().setContextClassLoader(myClassLoader);
if (timeSchedule != null) {
timeSchedule.cancel(true);
}
}
}
};
这里的listener是从NacosContextRefresher的applicationReadyEvent事件中注册进来的，上面调用Listener.receiveConfigInfo方法最终会调用innerReceive方法，
public void onApplicationEvent(ApplicationReadyEvent event) {
// many Spring context
if (this.ready.compareAndSet(false, true)) {
this.registerNacosListenersForApplications();
}
}
@Override
public void setApplicationContext(ApplicationContext applicationContext) {
this.applicationContext = applicationContext;
}
/**
* register Nacos Listeners.
*/
private void registerNacosListenersForApplications() {
if (isRefreshEnabled()) {
for (NacosPropertySource propertySource : NacosPropertySourceRepository
.getAll()) {
if (!propertySource.isRefreshable()) {
continue;
}
String dataId = propertySource.getDataId();
registerNacosListener(propertySource.getGroup(), dataId);
}
}
}
private void registerNacosListener(final String groupKey, final String dataKey) {
String key = NacosPropertySourceRepository.getMapKey(dataKey, groupKey);
Listener listener = listenerMap.computeIfAbsent(key,
lst -> new AbstractSharedListener() {
@Override
public void innerReceive(String dataId, String group,
String configInfo) {
refreshCountIncrement();
nacosRefreshHistory.addRefreshRecord(dataId, group, configInfo);
NacosSnapshotConfigManager.putConfigSnapshot(dataId, group,
configInfo);
applicationContext.publishEvent(
new RefreshEvent(this, null, "Refresh Nacos config"));
if (log.isDebugEnabled()) {
log.debug(String.format(
"Refresh Nacos config group=%s,dataId=%s,configInfo=%s",
group, dataId, configInfo));
}
}
});
try {
configService.addListener(dataKey, groupKey, listener);
log.info("[Nacos Config] Listening config: dataId={}, group={}", dataKey,
groupKey);
}
catch (NacosException e) {
log.warn(String.format(
"register fail for nacos listener ,dataId=[%s],group=[%s]", dataKey,
groupKey), e);
}
}
}
即会发布Spring的RefreshEvent事件，最终会触发bean的重新初始化，实现配置的更新。
至此applicationEvent的触发机制了解了，现在来看看上面的第二个问题ConfigurationPropertiesRebinder的rebind是怎么来实现配置的更新的呢？
rebind方法很简单，可以先destroyBean，然后再initializeBean，这怎么实现nacos配置的重新绑定呢？还记得开始提到的ConfigurationPropertiesBindingPostProcessor么，bean的初始化会回调这个processor的postProcessBeforeInitialization方法进而完成配置的重新绑定。
public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
if (!hasBoundValueObject(beanName)) {
bind(ConfigurationPropertiesBean.get(this.applicationContext, bean, beanName));
}
return bean;
}
private boolean hasBoundValueObject(String beanName) {
return BindMethod.VALUE_OBJECT.equals(BindMethodAttribute.get(this.registry, beanName));
}
private void bind(ConfigurationPropertiesBean bean) {
if (bean == null) {
return;
}
Assert.state(bean.asBindTarget().getBindMethod() != BindMethod.VALUE_OBJECT,
"Cannot bind @ConfigurationProperties for bean '" + bean.getName()
+ "'. Ensure that @ConstructorBinding has not been applied to regular bean");
try {
this.binder.bind(bean);
}
catch (Exception ex) {
throw new ConfigurationPropertiesBindException(bean, ex);
}
}
小结：可以看到配置的刷新的整个过程是在应用启动时注册好listener，回调listener来发布applicationEvent来触发bean的重新初始化，进而完成配置的重新绑定，整个过程充分使用了观察者模式。
Value
value注解标注的bean注入是在AutowiredAnnotationBeanPostProcessor中处理的。AutowiredAnnotationBeanPostProcessor中的postProcessMergedBeanDefinition方法代码如下。可以看到value标注的字段最终被封装成AutowiredFieldElement放在InjectionMetadata对象中。
@Override
public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName) {
InjectionMetadata metadata = findAutowiringMetadata(beanName, beanType, null);
metadata.checkConfigMembers(beanDefinition);
}
private InjectionMetadata findAutowiringMetadata(String beanName, Class<?> clazz, @Nullable PropertyValues pvs) {
String cacheKey = (StringUtils.hasLength(beanName) ? beanName : clazz.getName());
// Quick check on the concurrent map first, with minimal locking.
InjectionMetadata metadata = this.injectionMetadataCache.get(cacheKey);
if (InjectionMetadata.needsRefresh(metadata, clazz)) {
synchronized (this.injectionMetadataCache) {
metadata = this.injectionMetadataCache.get(cacheKey);
if (InjectionMetadata.needsRefresh(metadata, clazz)) {
if (metadata != null) {
metadata.clear(pvs);
}
metadata = buildAutowiringMetadata(clazz);
this.injectionMetadataCache.put(cacheKey, metadata);
}
}
}
return metadata;
}
private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) {
if (!AnnotationUtils.isCandidateClass(clazz, this.autowiredAnnotationTypes)) {
return InjectionMetadata.EMPTY;
}
List<InjectionMetadata.InjectedElement> elements = new ArrayList<>();
Class<?> targetClass = clazz;
do {
final List<InjectionMetadata.InjectedElement> currElements = new ArrayList<>();
ReflectionUtils.doWithLocalFields(targetClass, field -> {
MergedAnnotation<?> ann = findAutowiredAnnotation(field);
if (ann != null) {
if (Modifier.isStatic(field.getModifiers())) {
if (logger.isInfoEnabled()) {
logger.info("Autowired annotation is not supported on static fields: " + field);
}
return;
}
boolean required = determineRequiredStatus(ann);
currElements.add(new AutowiredFieldElement(field, required));
}
});
ReflectionUtils.doWithLocalMethods(targetClass, method -> {
Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(method);
if (!BridgeMethodResolver.isVisibilityBridgeMethodPair(method, bridgedMethod)) {
return;
}
MergedAnnotation<?> ann = findAutowiredAnnotation(bridgedMethod);
if (ann != null && method.equals(ClassUtils.getMostSpecificMethod(method, clazz))) {
if (Modifier.isStatic(method.getModifiers())) {
if (logger.isInfoEnabled()) {
logger.info("Autowired annotation is not supported on static methods: " + method);
}
return;
}
if (method.getParameterCount() == 0) {
if (logger.isInfoEnabled()) {
logger.info("Autowired annotation should only be used on methods with parameters: " +
method);
}
}
boolean required = determineRequiredStatus(ann);
PropertyDescriptor pd = BeanUtils.findPropertyForMethod(bridgedMethod, clazz);
currElements.add(new AutowiredMethodElement(method, required, pd));
}
});
elements.addAll(0, currElements);
targetClass = targetClass.getSuperclass();
}
while (targetClass != null && targetClass != Object.class);
return InjectionMetadata.forElements(elements, clazz);
}
那这个方法postProcessMergedBeanDefinition是什么时候调用的，可以看到postProcessMergedBeanDefinition是在AbstractAutowireCapableBeanFactory的applyMergedBeanDefinitionPostProcessors方法中调用的。
protected void applyMergedBeanDefinitionPostProcessors(RootBeanDefinition mbd, Class<?> beanType, String beanName) {
for (MergedBeanDefinitionPostProcessor processor : getBeanPostProcessorCache().mergedDefinition) {
processor.postProcessMergedBeanDefinition(mbd, beanType, beanName);
}
}
从下面的代码可以看到applyMergedBeanDefinitionPostProcessors方法是在AbstractAutowireCapableBeanFactory的doCreateBean方法的22行调用的。就是说在创建bean实例的时候会解析bean属性上的value主键并最终封装成InjectionMetadata。
protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
Object bean = instanceWrapper.getWrappedInstance();
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
// Allow post-processors to modify the merged bean definition.
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Post-processing of merged bean definition failed", ex);
}
mbd.markAsPostProcessed();
}
}
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
if (logger.isTraceEnabled()) {
logger.trace("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
populateBean(beanName, mbd, instanceWrapper);
exposedObject = initializeBean(beanName, exposedObject, mbd);
}
catch (Throwable ex) {
if (ex instanceof BeanCreationException bce && beanName.equals(bce.getBeanName())) {
throw bce;
}
else {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, ex.getMessage(), ex);
}
}
if (earlySingletonExposure) {
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
if (exposedObject == bean) {
exposedObject = earlySingletonReference;
}
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
for (String dependentBean : dependentBeans) {
if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
}
if (!actualDependentBeans.isEmpty()) {
throw new BeanCurrentlyInCreationException(beanName,
"Bean with name '" + beanName + "' has been injected into other beans [" +
StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
"] in its raw version as part of a circular reference, but has eventually been " +
"wrapped. This means that said other beans do not use the final version of the " +
"bean. This is often the result of over-eager type matching - consider using " +
"'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.");
}
}
}
}
// Register bean as disposable.
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
而属性值的绑定是在AutowiredAnnotationBeanPostProcessor的postProcessPropertyValues方法中完成的。
从下面的代码可以看到最终是在AutowiredAnnotationBeanPostProcessor的postProcessPropertyValues(这个方法是在AbstractAutowireCapableBeanFactory的doCreateBean方法的593行调用populateBean方法触发的)方法通过调用AutowiredFieldElement的inject完成value注解注入的。
public PropertyValues postProcessPropertyValues(
PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) {
return postProcessProperties(pvs, bean, beanName);
}
public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
try {
metadata.inject(bean, beanName, pvs);
}
catch (BeanCreationException ex) {
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
}
return pvs;
}
================class InjectionMetadata
public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
Collection<InjectedElement> checkedElements = this.checkedElements;
Collection<InjectedElement> elementsToIterate =
(checkedElements != null ? checkedElements : this.injectedElements);
if (!elementsToIterate.isEmpty()) {
for (InjectedElement element : elementsToIterate) {
element.inject(target, beanName, pvs);
}
}
}
======================class AutowiredFieldElement
protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
Field field = (Field) this.member;
Object value;
if (this.cached) {
try {
value = resolvedCachedArgument(beanName, this.cachedFieldValue);
}
catch (NoSuchBeanDefinitionException ex) {
// Unexpected removal of target bean for cached argument -> re-resolve
value = resolveFieldValue(field, bean, beanName);
}
}
else {
value = resolveFieldValue(field, bean, beanName);
}
if (value != null) {
ReflectionUtils.makeAccessible(field);
field.set(bean, value);
}
}
@Nullable
private Object resolvedCachedArgument(@Nullable String beanName, @Nullable Object cachedArgument) {
if (cachedArgument instanceof DependencyDescriptor) {
DependencyDescriptor descriptor = (DependencyDescriptor) cachedArgument;
Assert.state(this.beanFactory != null, "No BeanFactory available");
return this.beanFactory.resolveDependency(descriptor, beanName, null, null);
}
else {
return cachedArgument;
}
}
而resolvedCachedArgument方法最终会通过DefaultListableBeanFactory的doResolveDependency方法调用到PropertySourcesPlaceholderConfigurer的processProperties方法完成配置的解析。
=========================class DefaultListableBeanFactory===============================
@Nullable
public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName,
@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);
try {
Object shortcut = descriptor.resolveShortcut(this);
if (shortcut != null) {
return shortcut;
}
Class<?> type = descriptor.getDependencyType();
Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor);
if (value != null) {
if (value instanceof String) {
String strVal = resolveEmbeddedValue((String) value);
BeanDefinition bd = (beanName != null && containsBean(beanName) ?
getMergedBeanDefinition(beanName) : null);
value = evaluateBeanDefinitionString(strVal, bd);
}
TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
try {
return converter.convertIfNecessary(value, type, descriptor.getTypeDescriptor());
}
catch (UnsupportedOperationException ex) {
// A custom TypeConverter which does not support TypeDescriptor resolution...
return (descriptor.getField() != null ?
converter.convertIfNecessary(value, type, descriptor.getField()) :
converter.convertIfNecessary(value, type, descriptor.getMethodParameter()));
}
}
Object multipleBeans = resolveMultipleBeans(descriptor, beanName, autowiredBeanNames, typeConverter);
if (multipleBeans != null) {
return multipleBeans;
}
Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);
if (matchingBeans.isEmpty()) {
if (isRequired(descriptor)) {
raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
}
return null;
}
String autowiredBeanName;
Object instanceCandidate;
if (matchingBeans.size() > 1) {
autowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor);
if (autowiredBeanName == null) {
if (isRequired(descriptor) || !indicatesMultipleBeans(type)) {
return descriptor.resolveNotUnique(descriptor.getResolvableType(), matchingBeans);
}
else {
// In case of an optional Collection/Map, silently ignore a non-unique case:
// possibly it was meant to be an empty collection of multiple regular beans
// (before 4.3 in particular when we didn't even look for collection beans).
return null;
}
}
instanceCandidate = matchingBeans.get(autowiredBeanName);
}
else {
// We have exactly one match.
Map.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();
autowiredBeanName = entry.getKey();
instanceCandidate = entry.getValue();
}
if (autowiredBeanNames != null) {
autowiredBeanNames.add(autowiredBeanName);
}
if (instanceCandidate instanceof Class) {
instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);
}
Object result = instanceCandidate;
if (result instanceof NullBean) {
if (isRequired(descriptor)) {
raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
}
result = null;
}
if (!ClassUtils.isAssignableValue(type, result)) {
throw new BeanNotOfRequiredTypeException(autowiredBeanName, type, instanceCandidate.getClass());
}
return result;
}
finally {
ConstructorResolver.setCurrentInjectionPoint(previousInjectionPoint);
}
}
public String resolveEmbeddedValue(@Nullable String value) {
if (value == null) {
return null;
}
String result = value;
for (StringValueResolver resolver : this.embeddedValueResolvers) {
result = resolver.resolveStringValue(result);
if (result == null) {
return null;
}
}
return result;
}
============================class PropertySourcesPlaceholderConfigurer==============
protected void processProperties(ConfigurableListableBeanFactory beanFactoryToProcess,
final ConfigurablePropertyResolver propertyResolver) throws BeansException {
propertyResolver.setPlaceholderPrefix(this.placeholderPrefix);
propertyResolver.setPlaceholderSuffix(this.placeholderSuffix);
propertyResolver.setValueSeparator(this.valueSeparator);
StringValueResolver valueResolver = strVal -> {
String resolved = (this.ignoreUnresolvablePlaceholders ?
propertyResolver.resolvePlaceholders(strVal) :
propertyResolver.resolveRequiredPlaceholders(strVal));
if (this.trimValues) {
resolved = resolved.trim();
}
return (resolved.equals(this.nullValue) ? null : resolved);
};
doProcessProperties(beanFactoryToProcess, valueResolver);
}
而PropertySourcesPlaceholderConfigurer的processProperties方法最终会调用PropertyPlaceholderHelper的
parseStringValue方法。在parseStringValue方法的52行可以看到，如果ignoreUnresolvablePlaceholders=false，在解析到value为null时，会抛错"Could not resolve placeholder"，这就是为什么使用了@value注解，但配置文件中没有配置项时会报该错的原因。
protected String parseStringValue(
String value, PlaceholderResolver placeholderResolver, @Nullable Set<String> visitedPlaceholders) {
int startIndex = value.indexOf(this.placeholderPrefix);
if (startIndex == -1) {
return value;
}
StringBuilder result = new StringBuilder(value);
while (startIndex != -1) {
int endIndex = findPlaceholderEndIndex(result, startIndex);
if (endIndex != -1) {
String placeholder = result.substring(startIndex + this.placeholderPrefix.length(), endIndex);
String originalPlaceholder = placeholder;
if (visitedPlaceholders == null) {
visitedPlaceholders = new HashSet<>(4);
}
if (!visitedPlaceholders.add(originalPlaceholder)) {
throw new IllegalArgumentException(
"Circular placeholder reference '" + originalPlaceholder + "' in property definitions");
}
// Recursive invocation, parsing placeholders contained in the placeholder key.
placeholder = parseStringValue(placeholder, placeholderResolver, visitedPlaceholders);
// Now obtain the value for the fully resolved key...
String propVal = placeholderResolver.resolvePlaceholder(placeholder);
if (propVal == null && this.valueSeparator != null) {
int separatorIndex = placeholder.indexOf(this.valueSeparator);
if (separatorIndex != -1) {
String actualPlaceholder = placeholder.substring(0, separatorIndex);
String defaultValue = placeholder.substring(separatorIndex + this.valueSeparator.length());
propVal = placeholderResolver.resolvePlaceholder(actualPlaceholder);
if (propVal == null) {
propVal = defaultValue;
}
}
}
if (propVal != null) {
// Recursive invocation, parsing placeholders contained in the
// previously resolved placeholder value.
propVal = parseStringValue(propVal, placeholderResolver, visitedPlaceholders);
result.replace(startIndex, endIndex + this.placeholderSuffix.length(), propVal);
if (logger.isTraceEnabled()) {
logger.trace("Resolved placeholder '" + placeholder + "'");
}
startIndex = result.indexOf(this.placeholderPrefix, startIndex + propVal.length());
}
else if (this.ignoreUnresolvablePlaceholders) {
// Proceed with unprocessed value.
startIndex = result.indexOf(this.placeholderPrefix, endIndex + this.placeholderSuffix.length());
}
else {
throw new IllegalArgumentException("Could not resolve placeholder '" +
placeholder + "'" + " in value \"" + value + "\"");
}
visitedPlaceholders.remove(originalPlaceholder);
}
else {
startIndex = -1;
}
}
return result.toString();
}
从上面我们知道value注解的配置是在容器启动时通过AutowiredAnnotationBeanPostProcessor来设置进去的。那修改配置的时候，配置又是怎么重新设置到bean上的呢。
我们知道使用Value注解要让配置更新生效需要配合RefreshScope注解。那spring具体是怎么实现value注解配置的更新呢。
可以看到在RefreshEventListener接收到RefreshEvent事件(在NacosContextRefresher中的listener会发布该事件)后会调用ContextRefresher的refresh方法。
=============================RefreshEventListener===============================
public void onApplicationEvent(ApplicationEvent event) {
if (event instanceof ApplicationReadyEvent) {
handle((ApplicationReadyEvent) event);
}
else if (event instanceof RefreshEvent) {
handle((RefreshEvent) event);
}
}
public void handle(ApplicationReadyEvent event) {
this.ready.compareAndSet(false, true);
}
public void handle(RefreshEvent event) {
if (this.ready.get()) { // don't handle events before app is ready
log.debug("Event received " + event.getEventDesc());
Set<String> keys = this.refresh.refresh();
log.info("Refresh keys changed: " + keys);
}
}
而ContextRefresher的refresh方法最终会调用RefreshScope的refreshAll方法，最终会执行到GenericScope的destroy方法，将bean设置为null。
=====================================ContextRefresher==============================
public synchronized Set<String> refresh() {
Set<String> keys = refreshEnvironment();
this.scope.refreshAll();
return keys;
}
===================================RefreshScope==============================
public void refreshAll() {
super.destroy();
this.context.publishEvent(new RefreshScopeRefreshedEvent());
}
===================================GenericScope==============================
public void destroy() {
List<Throwable> errors = new ArrayList<>();
Collection<BeanLifecycleWrapper> wrappers = this.cache.clear();
for (BeanLifecycleWrapper wrapper : wrappers) {
try {
Lock lock = this.locks.get(wrapper.getName()).writeLock();
lock.lock();
try {
wrapper.destroy();
}
finally {
lock.unlock();
}
}
catch (RuntimeException e) {
errors.add(e);
}
}
if (!errors.isEmpty()) {
throw wrapIfNecessary(errors.get(0));
}
this.errors.clear();
}
清理完bean之后，配置是如何重新设置到bean上的呢。从下面GenericScope的get方法看到，在缓存中bean不存在时会重新创建bean，这样就能将最新的配置设置到bean上了。
public Object get(String name, ObjectFactory<?> objectFactory) {
BeanLifecycleWrapper value = this.cache.put(name, new BeanLifecycleWrapper(name, objectFactory));
this.locks.putIfAbsent(name, new ReentrantReadWriteLock());
try {
return value.getBean();
}
catch (RuntimeException e) {
this.errors.put(name, e);
throw e;
}
}
总结
ConfigurationProperties支持配置的校验、松散绑定&数据转换，默认情况下没有配置项不会报错，而value注解在没有配置项的情况下会报错，报错会导致容器初始化失败。所以大多数场景，建议使用ConfigurationProperties来实现将配置绑定到javaBean上。
