Spring原始碼學習(-)別怕,老外點中餐與AbstractBeanFactory.getBean的主流程差不多
引言
AbstractBeanFactory.getBean的流程,有點像老外點餐,不信咱們往下看。
入口
AbstractBeanFactory中有getBean的通用邏輯
//AbstractBeanFactory 中getBean方法第原始碼
@Override
public Object getBean(String name, Object... args) throws BeansException {
return doGetBean(name, null, args, false);
}
核心邏輯在:doGetBean中。
邏輯說明:
- 獲取BeanName,BeanFactory去掉&,根據別名找到對應的BeanName。(就是老闆翻譯老外說的
- 嘗試從快取中獲取單例,如果存在就返回 (就是餐館老闆看看有沒有做好的,有就端出來給老外)。
- 檢查是否是原型型別Bean在建立中,如果是,假設存在迴圈引用,丟擲異常 (就是老闆看看是不是正在做,如果說是就不管了)
- 找到定義bean對應的BeanFactory(老闆找到張大廚)
- 將多個GernericBeanDefinition合併為RootBeanDefinition,如果Bean有父Bean,時會合並父類的相關屬性。(老闆記錄客人的忌口)
- 保證bean的依賴先初始化,對DependOn註解的支援(完成做這道菜的必須工作,例如洗菜)
- 按不同作用域建立Bean (張大廚做飯
- 如果需要進行型別裝換
玩笑過後讓我們看點實在的。
原始碼註釋:
/** * 獲得一個例項,可以是共享(原型的),獨立(單例的),指定的bean * @param name the name of the bean to retrieve * @param requiredType the required type of the bean to retrieve * @param args arguments to use when creating a bean instance * using explicit arguments * (only applied when creating a new instance as opposed to retrieving an existing one) * args (僅在建立一個新例項時使用,而不是獲取一個已經存在的bean) * @param typeCheckOnly whether the instance is obtained for a type check,not for actual use * 是否僅僅是型別檢查 * @return an instance of the bean * @throws BeansException if the bean could not be created */ @SuppressWarnings("unchecked") protected <T> T doGetBean( final String name, @Nullable final Class<T> requiredType, @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException { //1.獲取對應的beanName ,BeanFactory去掉&,別名找到對應的BeanName final String beanName = transformedBeanName(name); Object bean; // Eagerly check singleton cache for manually registered singletons. //2. 嘗試從快取中獲取單例 Spring會,在Bean未建立完成的情況下,建立Bean的ObjectFactory物件,提早曝光,以方便解決迴圈依賴。 Object sharedInstance = getSingleton(beanName); if (sharedInstance != null && args == null) { if (logger.isTraceEnabled()) { if (isSingletonCurrentlyInCreation(beanName)) { logger.trace("Returning eagerly cached instance of singleton bean '" + beanName + "' that is not fully initialized yet - a consequence of a circular reference"); } else { logger.trace("Returning cached instance of singleton bean '" + beanName + "'"); } } //根據bean例項獲取物件,如果是FactoryBean 獲取它建立的物件 bean = getObjectForBeanInstance(sharedInstance, name, beanName, null); } else { // Fail if we're already creating this bean instance: // We're assumably within a circular reference. //3.原型依賴的檢查,如果是原型,假設存在迴圈引用,丟擲異常 if (isPrototypeCurrentlyInCreation(beanName)) { throw new BeanCurrentlyInCreationException(beanName); } // Check if bean definition exists in this factory. // 4.檢查bean,是否定義在了BeanFactory中, BeanFactory parentBeanFactory = getParentBeanFactory(); // 如果父BeanFactory不為空&&當前並沒有beanDefinitionMap不包含, // 委託給父BeanFactory if (parentBeanFactory != null && !containsBeanDefinition(beanName)) { // Not found -> check parent. String nameToLookup = originalBeanName(name); if (parentBeanFactory instanceof AbstractBeanFactory) { return ((AbstractBeanFactory) parentBeanFactory).doGetBean( nameToLookup, requiredType, args, typeCheckOnly); } else if (args != null) { // Delegation to parent with explicit args. //委託給,需要引數的 getBean方法 return (T) parentBeanFactory.getBean(nameToLookup, args); } else if (requiredType != null) { // No args -> delegate to standard getBean method. // 委託給標準的getBean方法 return parentBeanFactory.getBean(nameToLookup, requiredType); } else { return (T) parentBeanFactory.getBean(nameToLookup); } } if (!typeCheckOnly) { //不僅是,型別檢查,標記bean為建立 // 允許bean可以重新合併 markBeanAsCreated(beanName); } try { //5.將GernericBeanDefinition轉換為RootBeanDefinition, //如果Bean有父Bean,時會合並父類的相關屬性。 final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName); checkMergedBeanDefinition(mbd, beanName, args); // Guarantee initialization of beans that // the current bean depends on. // 6.保證bean的依賴先初始化了 String[] dependsOn = mbd.getDependsOn(); if (dependsOn != null) { for (String dep : dependsOn) { // 對應@DependsOn 註解, /** * 檢測是否存在 depends-on 迴圈依賴,若存在則拋異常。 * 比如 A 依賴 B, * B 又依賴 A,他們的配置如下: @Bean(name="a") @DependsOn("b")public A a () { return new A();} @Bean(name = "b") @DependsOn("a") public B b () { return new B(); } * a 要求 b 在其之前被建立,但 b 又要求 a 先於它 * 建立。這個時候形成了迴圈,對於 depends-on 迴圈,Spring 會直接 * 丟擲異常 * *@see org.springframework.context.annotation.ClassPathBeanDefinitionScanner#doScan */ if (isDependent(beanName, dep)) { throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'"); } //註冊,Dependent和Bean的關係 registerDependentBean(dep, beanName); try { //先建立Dependent getBean(dep); } catch (NoSuchBeanDefinitionException ex) { throw new BeanCreationException(mbd.getResourceDescription(), beanName, "'" + beanName + "' depends on missing bean '" + dep + "'", ex); } } } // Create bean instance. //7.建立Bean的例項 if (mbd.isSingleton()) { sharedInstance = getSingleton(beanName, () -> { try { return createBean(beanName, mbd, args); } catch (BeansException ex) { // Explicitly remove instance from singleton cache: It might have been put there // eagerly by the creation process, to allow for circular reference resolution. // Also remove any beans that received a temporary reference to the bean. destroySingleton(beanName); throw ex; } }); bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd); } else if (mbd.isPrototype()) { // It's a prototype -> create a new instance. Object prototypeInstance = null; try { beforePrototypeCreation(beanName); prototypeInstance = createBean(beanName, mbd, args); } finally { afterPrototypeCreation(beanName); } bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd); } else { String scopeName = mbd.getScope(); final Scope scope = this.scopes.get(scopeName); if (scope == null) { throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'"); } try { Object scopedInstance = scope.get(beanName, () -> { beforePrototypeCreation(beanName); try { return createBean(beanName, mbd, args); } finally { afterPrototypeCreation(beanName); } }); bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd); } catch (IllegalStateException ex) { throw new BeanCreationException(beanName, "Scope '" + scopeName + "' is not active for the current thread; consider " + "defining a scoped proxy for this bean if you intend to refer to it from a singleton", ex); } } } catch (BeansException ex) { cleanupAfterBeanCreationFailure(beanName); throw ex; } } // Check if required type matches the type of the actual bean instance. // 8.進行型別轉換 // requiredType.isInstance(bean) 類似 bean instanceof requiredType // 為true表示可以直接返回,或強轉 //clazz.isAssignableFrom(obj.getClass()) == clazz.isInstance(obj) //僅當obj not null if (requiredType != null && !requiredType.isInstance(bean)) { try { T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType); if (convertedBean == null) { throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass()); } return convertedBean; } catch (TypeMismatchException ex) { if (logger.isTraceEnabled()) { logger.trace("Failed to convert bean '" + name + "' to required type '" + ClassUtils.getQualifiedName(requiredType) + "'", ex); } throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass()); } } return (T) bean; }
一圖勝千言(時序圖)
當看完主流程時,我有如下疑問:
- getObjectForBeanInstance的作用?
- markBeanAsCreated 和getMergedLocalBeanDefinition的作用
- bean的DependsOn是什麼?
- createBean的邏輯
getObjectForBeanInstance和createBean的邏輯下一篇再聊,預計20190921前上傳。 這裡先說下2,4。
補充說明:
markBeanAsCreated 和getMergedLocalBeanDefinition的作用
簡單說,就是清除原有的RootBeanDefinition,再通過當前的多個GenericBeanDefinition合併成新的RootBeanDefinition,供BeanFactory使用。
什麼是BeanDefinition
- BeanDefinition 是一個極簡介面,主要目的是允許BeanFactoryPostProcessor(例如:PropertyPlaceholderConfigurer)去反射和修改屬性值和其他的bean元資料。
- GenericBeanDefinition 用於一站式定義標準的bean。和其他beanDefinition一樣,它允許指定一個類,屬性值,可選的構造器引數值,並且可以通過配置parentName,來支援派生關係。
- RootBeanDefinition 代表合併的beanDefinition,Spring的BeanFactory使用RootBeanDefinition建立指定的bean它可能繼承了多個原始的BeanDefinition,這些源BeanDefinition通常是GenericBeanDefinition,RootBeanDefinition本質上是執行時的“統一”bean定義檢視。
如果有興趣可以搜尋
org.springframework.beans.factory.config.BeanDefinition org.springframework.beans.factory.support.GenericBeanDefinition org.springframework.beans.factory.support.RootBeanDefinition
bean的DependOn是什麼?
我最開始以為是bean需要依賴,處理Autowired註解,然而不是。 利用IDEA查找了呼叫BeanDefinition的setDependsOn的地方,
發現了AnnotationConfigUtils.processCommonDefinitionAnnotations方法中如如下程式碼:
AnnotationAttributes dependsOn = attributesFor(metadata, DependsOn.class);
if (dependsOn != null) {
abd.setDependsOn(dependsOn.getStringArray("value"));
}
也就是說doGetBean中的
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
.....
}
是對@DependOn註解的支援,我孤陋寡聞,工作中重來沒有使用過@DependOn註解。從網上查閱了資料。
當時我查下的資料的url:https://blog.csdn.net/qq_30257149/article/details/88350320
@DependOn註解用來表示一個bean A的例項化依賴另一個bean B的例項化, 但是A並不需要持有一個B的物件
囉嗦幾句
讀原始碼不容易,最開始我總是揪住一個方法不放,個把小時下來,已經不知道自己讀到哪裡了,後來總是渾淪吞棗,一味求快,好像看了很多,實際上什麼也不知道。今年9月初,不知道是北京的天氣變涼快了,還是什麼別的原因,我自己也不清楚,竟然能靜下心來,不求快,一層一層讀,一點一點翻譯,每天僅讀40分鐘,總算感覺是明白了點,不禁感慨如下:
只抓細節太糊塗
渾淪吞棗淨瞎看
靜心慢讀未必快
反正我是入門了