Spring Ioc 之 Bean的載入(二)
在上篇文章中Spring Ioc 之 Bean的載入(一),我們分析了Spring Ioc中Bean的載入 doGetBean() 方法的
2.2從快取中獲取單例bean
和2.3獲取最終的bean例項物件
兩個步驟,我們接著分析餘下幾個步驟。
直接上程式碼:
//真正實現向IOC容器獲取Bean的功能,也是觸發依賴注入功能的地方 protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType, @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException { //根據指定的名稱獲取被管理Bean的名稱,剝離指定名稱中對容器的相關依賴 // 如果指定的是別名,將別名轉換為規範的Bean名稱 <1> final String beanName = transformedBeanName(name); Object bean; // Eagerly check singleton cache for manually registered singletons. // 從快取中獲取已被建立過的單例Bean <2> Object sharedInstance = getSingleton(beanName); //如果快取中有 if (sharedInstance != null && args == null) { if (logger.isDebugEnabled()) { if (isSingletonCurrentlyInCreation(beanName)) { logger.debug("Returning eagerly cached instance of singleton bean '" + beanName + "' that is not fully initialized yet - a consequence of a circular reference"); } else { logger.debug("Returning cached instance of singleton bean '" + beanName + "'"); } } //注意:BeanFactory是管理容器中Bean的工廠 // FactoryBean是建立建立物件的工廠Bean,兩者之間有區別 //獲取給定Bean的例項物件,該物件要麼是 bean 例項本身,要麼就是 FactoryBean 建立的 Bean 物件 //(為什麼要再次獲取呢,因為上面獲取的sharedInstance不一定是完整的) <3> bean = getObjectForBeanInstance(sharedInstance, name, beanName, null); } else { // Fail if we're already creating this bean instance: // We're assumably within a circular reference. // 因為 Spring 只解決單例模式下的迴圈依賴,在原型模式下如果存在迴圈依賴則會丟擲異常。 <4> if (isPrototypeCurrentlyInCreation(beanName)) { throw new BeanCurrentlyInCreationException(beanName); } // Check if bean definition exists in this factory. //對IOC容器中是否存在指定名稱的BeanDefinition進行檢查,首先檢查是否 //能在當前的BeanFactory中獲取的所需要的Bean,如果不能則委託當前容器 //的父級容器去查詢,如果還是找不到則沿著容器的繼承體系向父級容器查詢 BeanFactory parentBeanFactory = getParentBeanFactory(); //當前容器的父級容器存在,且當前容器中不存在指定名稱的Bean if (parentBeanFactory != null && !containsBeanDefinition(beanName)) { // Not found -> check parent. //解析指定Bean名稱的原始名稱 String nameToLookup = originalBeanName(name); // 若為 AbstractBeanFactory 型別,委託父類處理 if (parentBeanFactory instanceof AbstractBeanFactory) { return ((AbstractBeanFactory) parentBeanFactory).doGetBean( nameToLookup, requiredType, args, typeCheckOnly); } else if (args != null) { // Delegation to parent with explicit args. //委派父級容器根據指定名稱和顯式的引數查詢 return (T) parentBeanFactory.getBean(nameToLookup, args); } else { // No args -> delegate to standard getBean method. //委派父級容器根據指定名稱和型別查詢 return parentBeanFactory.getBean(nameToLookup, requiredType); } } // 建立的Bean是否需要進行型別驗證,一般不需要 <5> if (!typeCheckOnly) { //向容器標記指定的Bean已經被建立 markBeanAsCreated(beanName); } try { //從容器中獲取 beanName 相應的 GenericBeanDefinition 物件,並將其轉換為 RootBeanDefinition 物件 // 主要解決Bean繼承時子類合併父類公共屬性問題 <6> final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName); // 檢查給定的合併的 BeanDefinition (是否為抽象類) checkMergedBeanDefinition(mbd, beanName, args); // Guarantee initialization of beans that the current bean depends on. // 處理所依賴的 bean @DependsOn() // 獲取當前Bean所有依賴Bean的名稱 <7> String[] dependsOn = mbd.getDependsOn(); //如果有依賴 if (dependsOn != null) { for (String dep : dependsOn) { //校驗該依賴是否已經註冊過給當前 Bean if (isDependent(beanName, dep)) { //已註冊,丟擲異常 throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'"); } //沒有,則先註冊依賴的bean registerDependentBean(dep, beanName); //遞迴呼叫getBean(),先生成依賴的bean getBean(dep); } } // Create bean instance. //建立單例Bean <8> if (mbd.isSingleton()) { //這裡使用了一個匿名內部類,建立Bean例項物件,並且註冊給所依賴的物件 sharedInstance = getSingleton(beanName, () -> { try { //建立一個指定Bean例項物件,如果有父級繼承,則合併子類和父類的定義 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. //顯式地從容器單例模式Bean快取中清除例項物件 destroySingleton(beanName); throw ex; } }); //獲取給定Bean的例項物件 bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd); } //建立多例Bean else if (mbd.isPrototype()) { //原型模式(Prototype)是每次都會建立一個新的物件 Object prototypeInstance = null; try { //載入前置處理,預設的功能是註冊當前建立的原型物件 beforePrototypeCreation(beanName); //建立指定Bean物件例項 prototypeInstance = createBean(beanName, mbd, args); } finally { //載入後置處理,預設的功能告訴IOC容器指定Bean的原型物件不再建立 afterPrototypeCreation(beanName); } //獲取給定Bean的例項物件 bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd); } //要建立的Bean既不是Singleton也不是Prototype //如:request、session、application等生命週期 else { String scopeName = mbd.getScope(); final Scope scope = this.scopes.get(scopeName); //Bean定義資源中沒有配置生命週期範圍,則Bean定義不合法 if (scope == null) { throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'"); } try { //這裡又使用了一個匿名內部類,獲取一個指定生命週期範圍的例項 Object scopedInstance = scope.get(beanName, () -> { //前置處理 beforePrototypeCreation(beanName); try { return createBean(beanName, mbd, args); } finally { //後置處理 afterPrototypeCreation(beanName); } }); //獲取給定Bean的例項物件 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. //對建立的Bean例項物件進行型別檢查 <9> 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.isDebugEnabled()) { logger.debug("Failed to convert bean '" + name + "' to required type '" + ClassUtils.getQualifiedName(requiredType) + "'", ex); } throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass()); } } return (T) bean; }
程式碼很長,需要一些耐心,下面我們來逐步分析這段程式碼:
-
<1>處:具體分析,見
2.1獲取原始beanName
-
<2>處: 具體分析,見
2.2從快取中獲取單例bean
-
<3>處: 具體分析,見
2.3獲取最終的bean例項物件
-
<4>處: 具體分析,見
2.4原型模式依賴檢查(Prototype)和從 parentBeanFactory 獲取 Bean
-
<5>處: 具體分析,見
2.5標記bean為已建立或即將建立
-
<6>處: 具體分析,見
2.6獲取BeanDefinition
-
<7>處: 具體分析,見
2.7bean依賴處理
-
<8>處: 具體分析,見
2.8不同作用域bean的例項化
-
<9>處: 具體分析,見
2.9型別轉換
2.4、原型模式依賴檢查(Prototype)和從 parentBeanFactory 獲取 Bean
原型模式依賴檢查,對應程式碼如下:
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
跟蹤進去:
/** Names of beans that are currently in creation */ private final ThreadLocal<Object> prototypesCurrentlyInCreation = new NamedThreadLocal<>("Prototype beans currently in creation"); protected boolean isPrototypeCurrentlyInCreation(String beanName) { //從ThreadLocal中取出正在建立的prototype Object curVal = this.prototypesCurrentlyInCreation.get(); return (curVal != null && (curVal.equals(beanName) || (curVal instanceof Set && ((Set<?>) curVal).contains(beanName)))); }
Spring 只處理單例模式下得迴圈依賴,對於原型模式的迴圈依賴直接丟擲異常。
Spring會把正在建立的原型模式Bean存入ThreadLoacl
,在這裡通過ThreadLoacl來判斷當前Bean是否已經建立。
從 parentBeanFactory 獲取 Bean,對應程式碼如下:
// Check if bean definition exists in this factory.
//對IOC容器中是否存在指定名稱的BeanDefinition進行檢查,首先檢查是否
//能在當前的BeanFactory中獲取的所需要的Bean,如果不能則委託當前容器
//的父級容器去查詢,如果還是找不到則沿著容器的繼承體系向父級容器查詢
BeanFactory parentBeanFactory = getParentBeanFactory();
//當前容器的父級容器存在,且當前容器中不存在指定名稱的Bean
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
//解析指定Bean名稱的原始名稱
String nameToLookup = originalBeanName(name);
// 若為 AbstractBeanFactory 型別,委託父類處理
if (parentBeanFactory instanceof AbstractBeanFactory) {
return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
nameToLookup, requiredType, args, typeCheckOnly);
}
else if (args != null) {
// Delegation to parent with explicit args.
//委派父級容器根據指定名稱和顯式的引數查詢
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
// No args -> delegate to standard getBean method.
//委派父級容器根據指定名稱和型別查詢
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
如果當前容器快取中沒有相對應的 BeanDefinition 物件,則會嘗試從父類工廠(parentBeanFactory)中載入,然後再去遞迴呼叫 getBean(...) 方法
2.5、標記bean為已建立或即將建立
對應程式碼如下:
//建立的Bean是否需要進行型別驗證,一般不需要
if (!typeCheckOnly) {
//向容器標記指定的Bean已經被建立
markBeanAsCreated(beanName);
}
typeCheckOnly
是doGetBean(final String name, @Nullable final Class<T> requiredType,@Nullable final Object[] args, boolean typeCheckOnly)方法中的一個引數,一般這個引數傳的都是false
接著追蹤markBeanAsCreated()
方法:
protected void markBeanAsCreated(String beanName) {
// 沒有建立
if (!this.alreadyCreated.contains(beanName)) {
synchronized (this.mergedBeanDefinitions) {
// 再次檢查一次:DCL 雙重校驗
if (!this.alreadyCreated.contains(beanName)) {
clearMergedBeanDefinition(beanName);
// 新增到已建立 bean 集合中
this.alreadyCreated.add(beanName);
}
}
}
}
這裡用到了單例模式中耳熟能詳的雙重校驗
2.6、獲取BeanDefinition
對應程式碼如下:
//從容器中獲取 beanName 相應的 GenericBeanDefinition 物件,並將其轉換為 RootBeanDefinition 物件
//主要解決Bean繼承時子類合併父類公共屬性問題
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
// 檢查給定的合併的 BeanDefinition (是否為抽象類)
checkMergedBeanDefinition(mbd, beanName, args);
這段程式碼註釋很詳細,就不多解釋了。
2.7、bean依賴處理
對應程式碼如下:
// Guarantee initialization of beans that the current bean depends on.
// 處理所依賴的 bean @DependsOn()
//獲取當前Bean所有依賴Bean的名稱
<1> String[] dependsOn = mbd.getDependsOn();
//如果有依賴
if (dependsOn != null) {
for (String dep : dependsOn) {
//校驗該依賴是否已經註冊過給當前 Bean
<2> if (isDependent(beanName, dep)) {
//已註冊,丟擲異常
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
//沒有,則先註冊依賴的bean
<3> registerDependentBean(dep, beanName);
//遞迴呼叫getBean(),先生成依賴的bean
<4> getBean(dep);
}
}
在spring中有一個@DependsOn
註解,它的作用是依賴載入,比如A物件要在B物件載入之後才能載入,那麼可以在A上面加@DependsOn(value = "B")
註解,就可以達到我們的要求。
其實@DependsOn
實現的原理就是上面這段程式碼。
-
<1>、通過我們前面從IoC容器中拿到的
BeanDefinition
,呼叫mbd.getDependsOn()
方法,獲取當前bean所有的依賴。 -
<2>、遍歷這些依賴,判斷此依賴是否已註冊給當前的Bean
-
<3>、沒有,則先註冊依賴的Bean
-
<4>、遞迴呼叫getBean(),先生成依賴的bean
<2>、遍歷這些依賴,判斷此依賴是否已註冊給當前的Bean
程式碼:
// 儲存的是bean與其依賴的對映關係:B - > A
private final Map<String, Set<String>> dependentBeanMap = new ConcurrentHashMap<>(64);
//儲存的是bean與其依賴的對映關係:A - > B
private final Map<String, Set<String>> dependenciesForBeanMap = new ConcurrentHashMap<>(64);
private boolean isDependent(String beanName, String dependentBeanName, @Nullable Set<String> alreadySeen) {
if (alreadySeen != null && alreadySeen.contains(beanName)) {
return false;
}
// 獲取當前原始 beanName
String canonicalName = canonicalName(beanName);
// 獲取該bean依賴的其他bean集合
Set<String> dependentBeans = this.dependentBeanMap.get(canonicalName);
if (dependentBeans == null) {
return false;
}
// 存在,則證明該依賴已經註冊到bean中
if (dependentBeans.contains(dependentBeanName)) {
return true;
}
// 遞迴檢測依賴
for (String transitiveDependency : dependentBeans) {
if (alreadySeen == null) {
alreadySeen = new HashSet<>();
}
alreadySeen.add(beanName);
if (isDependent(transitiveDependency, dependentBeanName, alreadySeen)) {
return true;
}
}
return false;
}
這段程式碼很簡單,主要就是通過dependentBeanMap
獲取當前bean對應的所有依賴dependentBeans
,然後判斷是否已註冊,接著遞迴檢查依賴的Bean有沒有依賴,如果有,就遞迴呼叫isDependent()
檢查
<3>、沒有,則先註冊依賴的Bean
如果沒有註冊依賴的Bean到該 Bean,則執行註冊registerDependentBean(dep, beanName)
:
// 儲存的是bean與其依賴的對映關係:B - > A
private final Map<String, Set<String>> dependentBeanMap = new ConcurrentHashMap<>(64);
//儲存的是bean與其依賴的對映關係:A - > B
private final Map<String, Set<String>> dependenciesForBeanMap = new ConcurrentHashMap<>(64);
//為指定的Bean注入依賴的Bean
public void registerDependentBean(String beanName, String dependentBeanName) {
// A quick check for an existing entry upfront, avoiding synchronization...
//獲取原始beanName
String canonicalName = canonicalName(beanName);
Set<String> dependentBeans = this.dependentBeanMap.get(canonicalName);
if (dependentBeans != null && dependentBeans.contains(dependentBeanName)) {
return;
}
// No entry yet -> fully synchronized manipulation of the dependentBeans Set
//先從容器中:bean名稱-->全部依賴Bean名稱集合找查詢給定名稱Bean的依賴Bean
synchronized (this.dependentBeanMap) {
//獲取給定名稱Bean的所有依賴Bean名稱
dependentBeans = this.dependentBeanMap.get(canonicalName);
if (dependentBeans == null) {
//為Bean設定依賴Bean資訊
dependentBeans = new LinkedHashSet<>(8);
this.dependentBeanMap.put(canonicalName, dependentBeans);
}
//把對映關係存入集合
dependentBeans.add(dependentBeanName);
}
//從容器中:bean名稱-->指定名稱Bean的依賴Bean集合找查詢給定名稱Bean的依賴Bean
synchronized (this.dependenciesForBeanMap) {
Set<String> dependenciesForBean = this.dependenciesForBeanMap.get(dependentBeanName);
if (dependenciesForBean == null) {
dependenciesForBean = new LinkedHashSet<>(8);
this.dependenciesForBeanMap.put(dependentBeanName, dependenciesForBean);
}
//把對映關係存入集合
dependenciesForBean.add(canonicalName);
}
}
套用上面的例子,如果 A @DependsOn(value = "B")
,也就是說A依賴於B,那麼該方法registerDependentBean(dep, beanName)
中,引數 dep 就是B,beanName 就是A。
這段程式碼中其實就是把bean之間的依賴關係註冊到兩個map中。
-
dependentBeanMap 存入(B,A)
-
dependenciesForBeanMap 存入(A,B)
<4>、遞迴呼叫getBean(dep),先生成依賴的bean
到了這一步,遞迴呼叫getBean(beanName)方法也就是doGetBean(beanName)重走當前流程,來先例項化依賴的Bean。等依賴的Bean例項化之後,當前bean再接著往下執行。
2.8、不同作用域bean的例項化
程式碼:
// Create bean instance.
//建立單例Bean
if (mbd.isSingleton()) {
//這裡使用了一個匿名內部類,建立Bean例項物件,並且註冊給所依賴的物件
sharedInstance = getSingleton(beanName, () -> {
try {
//建立一個指定Bean例項物件,如果有父級繼承,則合併子類和父類的定義
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.
//顯式地從容器單例模式Bean快取中清除例項物件
destroySingleton(beanName);
throw ex;
}
});
//獲取給定Bean的例項物件
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
//建立多例Bean
else if (mbd.isPrototype()) {
//原型模式(Prototype)是每次都會建立一個新的物件
Object prototypeInstance = null;
try {
//載入前置處理,預設的功能是註冊當前建立的原型物件
beforePrototypeCreation(beanName);
//建立指定Bean物件例項
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
//載入後置處理,預設的功能告訴IOC容器指定Bean的原型物件不再建立
afterPrototypeCreation(beanName);
}
//獲取給定Bean的例項物件
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
//要建立的Bean既不是Singleton也不是Prototype
//如:request、session、application等生命週期
else {
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
//Bean定義資源中沒有配置生命週期範圍,則Bean定義不合法
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
}
try {
//這裡又使用了一個匿名內部類,獲取一個指定生命週期範圍的例項
Object scopedInstance = scope.get(beanName, () -> {
//前置處理
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
//後置處理
afterPrototypeCreation(beanName);
}
});
//獲取給定Bean的例項物件
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);
}
}
這段程式碼很明顯,分成了3個部分:
-
singleton Bean例項化
-
Prototype Bean例項化
-
其他型別 Bean 例項化(session,request等)
我們先來看singleton Bean例項化:
if (mbd.isSingleton()) {
//這裡使用了一個匿名內部類,建立Bean例項物件,並且註冊給所依賴的物件
sharedInstance = getSingleton(beanName, () -> {
try {
//建立一個指定Bean例項物件,如果有父級繼承,則合併子類和父類的定義
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.
//顯式地從容器單例模式Bean快取中清除例項物件
destroySingleton(beanName);
throw ex;
}
});
//獲取給定Bean的例項物件
bean = getObjectForBeanInstance(sharedInstance, name,beanName, mbd);
}
Spring Bean 的作用域預設為 singleton 。還有其他作用域,如 prototype、request、session 等。
不同的作用域會有不同的初始化策略。
詳見spring各個scope的bean建立。
2.9、型別轉換
程式碼:
// Check if required type matches the type of the actual bean instance.
//對建立的Bean例項物件進行型別檢查
if (requiredType != null && !requiredType.isInstance(bean)) {
try {
//執行轉換
T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
// 轉換失敗,拋異常
if (convertedBean == null) {
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
return convertedBean;
}
catch (TypeMismatchException ex) {
if (logger.isDebugEnabled()) {
logger.debug("Failed to convert bean '" + name + "' to required type '" +
ClassUtils.getQualifiedName(requiredType) + "'", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
requiredType
是 getBean() 方法可傳入的一個引數,即可以根據指定的 beanName 和 requiredType 來獲取Bean。
但是一般情況下是不需要型別檢查的,requiredType
一般為null,如getBean(beanName)
當requiredType
不為null的時候走這段邏輯。
總結:
至此,spring載入Bean也就是 getBean() 我們大致分析完了,之後會再寫幾篇文章對其中有些步驟進行詳細介紹。
參考:<