Spring原始碼:從FactoryBean獲取Bean的例項
主程式碼
類AbstractBeanFactory
protected <T> T doGetBean(......){
// ......
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
// ......
}
protected Object getObjectForBeanInstance(
Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {
// 若name傳遞的是如“&student”且該bean不是FactoryBean,丟擲異常
if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
}
// 如果beanInstance不是FactoryBean,是一個普通的Bean,直接返回
if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
return beanInstance;
}
Object object = null;
if (mbd == null) {
// 嘗試從快取中獲取Bean
object = getCachedObjectForFactoryBean(beanName);
}
if (object == null) {
// Return bean instance from factory.
FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
// Caches object obtained from FactoryBean if it is a singleton.
if (mbd == null && containsBeanDefinition(beanName)) {
mbd = getMergedLocalBeanDefinition(beanName);
}
boolean synthetic = (mbd != null && mbd.isSynthetic());
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
}
詳細分析
getCachedObjectForFactoryBean這個方法放在FactoryBeanRegistrySupport類中,FactoryBeanRegistrySupport類在DefaultSingletonBeanRegistry類的基礎上(繼承於DefaultSingletonBeanRegistry類)增加了對FactoryBean的特殊處理(從命名上可以看出),下面是從快取中獲取FactoryBean:
private final Map<String, Object> factoryBeanObjectCache = new ConcurrentHashMap<String, Object>(16); protected Object getCachedObjectForFactoryBean(String beanName) { Object object = this.factoryBeanObjectCache.get(beanName); return (object != NULL_OBJECT ? object : null); }
問題來了,FactoryBean什麼時候放到快取中呢?
是由本類(FactoryBeanRegistrySupport)的getObjectFromFactoryBean方法設定進去的,該方法內若從快取中獲取不到Bean,就會呼叫factory.getObject()獲取Bean,並put到factoryBeanObjectCache中(第一次獲取factoryBeanObjectCache一定獲取不到的)。
接下來看下containsBeanDefinition(beanName)方法,定位到DefaultListableBeanFactory類中,該類是bean載入的核心部分,是Spring註冊及載入的預設實現,功能強大。
private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<String, BeanDefinition>(256); public boolean containsBeanDefinition(String beanName) { Assert.notNull(beanName, "Bean name must not be null"); return this.beanDefinitionMap.containsKey(beanName); }
從程式碼中可以看出,beanDefinitionMap是否包含beanName的key,beanDefinitionMap是什麼時候put進去值的時候,分析spring配置檔案的載入方式不難發現,所有被掃描的Bean都會註冊到beanDefinitionMap中,具體參考該類中registerBeanDefinition方法。
再看getMergedLocalBeanDefinition(beanName)方法,該方法的作用是獲取子Bean與父Bean合併後RootBeanDefinition,即如果beanName有父Bean,需要把父Bean與該Bean合併成一個RootBeanDefinition(屬性合併等等,參考AbstractBeanDefinition的overrideFrom方法,同時參考parent標籤的使用)
protected RootBeanDefinition getMergedLocalBeanDefinition(String beanName) throws BeansException { // 嘗試從快取中獲取RootBeanDefinition RootBeanDefinition mbd = this.mergedBeanDefinitions.get(beanName); if (mbd != null) { return mbd; } // 若獲取失敗,呼叫以下方法來獲取RootBeanDefinition return getMergedBeanDefinition(beanName, getBeanDefinition(beanName)); } protected RootBeanDefinition getMergedBeanDefinition(String beanName, BeanDefinition bd) throws BeanDefinitionStoreException { return getMergedBeanDefinition(beanName, bd, null); }
protected RootBeanDefinition getMergedBeanDefinition(String beanName, BeanDefinition bd, BeanDefinition containingBd) throws BeanDefinitionStoreException { // 對快取map上鎖 synchronized (this.mergedBeanDefinitions) { RootBeanDefinition mbd = null; // containingBd可以看作是內部Bean if (containingBd == null) { mbd = this.mergedBeanDefinitions.get(beanName); } if (mbd == null) { if (bd.getParentName() == null) { // 沒有父Bean if (bd instanceof RootBeanDefinition) { mbd = ((RootBeanDefinition) bd).cloneBeanDefinition(); } else { mbd = new RootBeanDefinition(bd); } } else { // 有父Bean BeanDefinition pbd; // 獲取父Bean的名稱 String parentBeanName = transformedBeanName(bd.getParentName()); if (!beanName.equals(parentBeanName)) { pbd = getMergedBeanDefinition(parentBeanName); } else { // 獲取父Bean的BeanFactory,並從中獲取父BeanDefinition(流程同子BeanDefinition的獲取過程) BeanFactory parent = getParentBeanFactory(); if (parent instanceof ConfigurableBeanFactory) { pbd = ((ConfigurableBeanFactory)parent).getMergedBeanDefinition(parentBeanName); } else { // 丟擲異常(父Bean工廠不是AbstractBeanFactory型別) } } // 深度拷貝 mbd = new RootBeanDefinition(pbd); // 父子BeanDefinition屬性合併 mbd.overrideFrom(bd); } // Set default singleton scope, if not configured before. if (!StringUtils.hasLength(mbd.getScope())) { mbd.setScope(RootBeanDefinition.SCOPE_SINGLETON); } if (containingBd != null && !containingBd.isSingleton() && mbd.isSingleton()) { mbd.setScope(containingBd.getScope()); } if (containingBd == null && isCacheBeanMetadata()) { this.mergedBeanDefinitions.put(beanName, mbd); } } return mbd; } }
關於synthetic的意義:該BeanDefinition是不是應用(這一點可討論,感覺說是編譯器合成的比較貼切)自己擁有並生成的,一般自定義的Bean都是返回false,由應用合成的返回true,下面程式碼:若取到的Bean是自定義的返回false,若是應用自己合成的返回true
boolean synthetic = (mbd != null && mbd.isSynthetic());
看最後一段程式碼,真正的獲取Bean,直接分析程式碼比較好說,為了分析,省略了異常
// !synthetic = true(自定義的Bean) object = getObjectFromFactoryBean(factory, beanName, !synthetic);
protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) { if (factory.isSingleton() && containsSingleton(beanName)) { synchronized (getSingletonMutex()) { // 從快取中獲取,factoryBeanObjectCache存放<FactoryBean名稱,Object> Object object = this.factoryBeanObjectCache.get(beanName); if (object == null) { // 快取中獲取不到,從factory中獲取 object = doGetObjectFromFactoryBean(factory, beanName); // 有可能當執行doGetObjectFromFactoryBean函式時,Bean正在建立,所以此時再獲取一下 Object alreadyThere = this.factoryBeanObjectCache.get(beanName); if (alreadyThere != null) { object = alreadyThere; } else { if (object != null && shouldPostProcess) { // 後處理Bean,不再分析 object = postProcessObjectFromFactoryBean(object, beanName); } // 放到快取中 this.factoryBeanObjectCache.put(beanName, (object != null ? object :NULL_OBJECT)); } } // 返回結果 return (object != NULL_OBJECT ? object : null); } } else { // 獲取Bean Object object = doGetObjectFromFactoryBean(factory, beanName); if (object != null && shouldPostProcess) { // 後處理 object = postProcessObjectFromFactoryBean(object, beanName); } return object; } }
private Object doGetObjectFromFactoryBean(final FactoryBean<?> factory, final String beanName) throws BeanCreationException { Object object; if (System.getSecurityManager() != null) { AccessControlContext acc = getAccessControlContext(); object = AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() { @Override public Object run() throws Exception { return factory.getObject(); } }, acc); } else { object = factory.getObject(); } // Do not accept a null value for a FactoryBean that's not fully initialized yet: Many FactoryBeans just return null then. if (object == null && isSingletonCurrentlyInCreation(beanName)) { throw new BeanCurrentlyInCreationException(beanName, "FactoryBean which is currently in creation returned null from getObject"); } return object; }
至此,整個從FactoryBean中獲取BeanDefinition完成。