【Spring源碼分析】非懶載入的Bean實例化過程（上篇）

代碼入口上文【Spring源碼分析】Bean載入流程概覽，比較詳細地分析了Spring上下文載入的代碼入口，並且在AbstractApplicationContext的refresh方法中，點出了finishBeanFactoryInitialization方法完成了對於所有非懶載入的Bean的初 ...

代碼入口

上文【Spring源碼分析】Bean載入流程概覽，比較詳細地分析了Spring上下文載入的代碼入口，並且在AbstractApplicationContext的refresh方法中，點出了finishBeanFactoryInitialization方法完成了對於所有非懶載入的Bean的初始化。

finishBeanFactoryInitialization方法中調用了DefaultListableBeanFactory的preInstantiateSingletons方法，本文針對preInstantiateSingletons進行分析，解讀一下Spring是如何構造Bean實例對象出來的。

DefaultListableBeanFactory的preInstantiateSingletons方法

DefaultListableBeanFactory的preInstantiateSingletons方法，顧名思義，初始化所有的單例Bean，看一下方法的定義：

 1 public void preInstantiateSingletons() throws BeansException {
 2     if (this.logger.isInfoEnabled()) {
 3         this.logger.info("Pre-instantiating singletons in " + this);
 4     }
 5     synchronized (this.beanDefinitionMap) {
 6         // Iterate over a copy to allow for init methods which in turn register new bean definitions.
 7         // While this may not be part of the regular factory bootstrap, it does otherwise work fine.
 8         List<String> beanNames = new ArrayList<String>(this.beanDefinitionNames);
 9         for (String beanName : beanNames) {
10             RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
11             if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
12                 if (isFactoryBean(beanName)) {
13                     final FactoryBean factory = (FactoryBean) getBean(FACTORY_BEAN_PREFIX + beanName);
14                     boolean isEagerInit;
15                     if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
16                         isEagerInit = AccessController.doPrivileged(new PrivilegedAction<Boolean>() {
17                             public Boolean run() {
18                                 return ((SmartFactoryBean) factory).isEagerInit();
19                             }
20                         }, getAccessControlContext());
21                     }
22                     else {
23                         isEagerInit = (factory instanceof SmartFactoryBean &&
24                                 ((SmartFactoryBean) factory).isEagerInit());
25                     }
26                     if (isEagerInit) {
27                         getBean(beanName);
28                     }
29                 }
30                 else {
31                     getBean(beanName);
32                 }
33             }
34         }
35     }
36 }

前面的代碼比較簡單，根據beanName拿到BeanDefinition（即Bean的定義）。由於此方法實例化的是所有非懶載入的單例Bean，因此要實例化Bean，必須滿足11行的三個定義：

（1）不是抽象的

（2）必須是單例的

（3）必須是非懶載入的

接著簡單看一下第12行~第29行的代碼，這段代碼主要做的是一件事情：首先判斷一下Bean是否FactoryBean的實現，接著判斷Bean是否SmartFactoryBean的實現，假如Bean是SmartFactoryBean的實現並且eagerInit（這個單詞字面意思是渴望載入，找不到一個好的詞語去翻譯，意思就是定義了這個Bean需要立即載入的意思）的話，會立即實例化這個Bean。Java開發人員不需要關註這段代碼，因為SmartFactoryBean基本不會用到，我翻譯一下Spring官網對於SmartFactoryBean的定義描述：

FactoryBean介面的擴展介面。介面實現並不表示是否總是返回單獨的實例對象，比如FactoryBean.isSingleton()實現返回false的情況並不清晰地表示每次返回的都是單獨的實例對象
不實現這個擴展介面的簡單FactoryBean的實現，FactoryBean.isSingleton()實現返回false總是簡單地告訴我們每次返回的都是單獨的實例對象，暴露出來的對象只能夠通過命令訪問
註意：這個介面是一個有特殊用途的介面，主要用於框架內部使用與Spring相關。通常，應用提供的FactoryBean介面實現應當只需要實現簡單的FactoryBean介面即可，新方法應當加入到擴展介面中去

代碼示例

為了後面的代碼分析方便，事先我定義一個Bean：

 1 package org.xrq.action;
 2 
 3 import org.springframework.beans.factory.BeanClassLoaderAware;
 4 import org.springframework.beans.factory.BeanNameAware;
 5 import org.springframework.beans.factory.InitializingBean;
 6 
 7 public class MultiFunctionBean implements InitializingBean, BeanNameAware, BeanClassLoaderAware {
 8 
 9     private int    propertyA;
10     
11     private int    propertyB;
12     
13     public int getPropertyA() {
14         return propertyA;
15     }
16 
17     public void setPropertyA(int propertyA) {
18         this.propertyA = propertyA;
19     }
20 
21     public int getPropertyB() {
22         return propertyB;
23     }
24 
25     public void setPropertyB(int propertyB) {
26         this.propertyB = propertyB;
27     }
28     
29     public void initMethod() {
30         System.out.println("Enter MultiFunctionBean.initMethod()");
31     }
32 
33     @Override
34     public void setBeanClassLoader(ClassLoader classLoader) {
35         System.out.println("Enter MultiFunctionBean.setBeanClassLoader(ClassLoader classLoader)");
36     }
37 
38     @Override
39     public void setBeanName(String name) {
40         System.out.println("Enter MultiFunctionBean.setBeanName(String name)");
41     }
42 
43     @Override
44     public void afterPropertiesSet() throws Exception {
45         System.out.println("Enter MultiFunctionBean.afterPropertiesSet()");
46     }
47     
48     @Override
49     public String toString() {
50         return "MultiFunctionBean [propertyA=" + propertyA + ", propertyB=" + propertyB + "]";
51     }
52     
53 }

定義對應的spring.xml：

1 <?xml version="1.0" encoding="UTF-8"?>
2 <beans xmlns="http://www.springframework.org/schema/beans"
3     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
4     xsi:schemaLocation="http://www.springframework.org/schema/beans
5     http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">
6     
7     <bean id="multiFunctionBean" class="org.xrq.action.MultiFunctionBean" init-method="initMethod" />
8     
9 </beans>

利用這個MultiFunctionBean，我們可以用來探究Spring載入Bean的多種機制。

doGetBean方法構造Bean流程

上面把getBean之外的代碼都分析了一下，看代碼就可以知道，獲取Bean對象實例，都是通過getBean方法，getBean方法最終調用的是DefaultListableBeanFactory的父類AbstractBeanFactory類的doGetBean方法，因此這部分重點分析一下doGetBean方法是如何構造出一個單例的Bean的。

看一下doGetBean方法的代碼實現，比較長：

  1 protected <T> T doGetBean(
  2         final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
  3         throws BeansException {
  4 
  5     final String beanName = transformedBeanName(name);
  6     Object bean;
  7 
  8     // Eagerly check singleton cache for manually registered singletons.
  9     Object sharedInstance = getSingleton(beanName);
 10     if (sharedInstance != null && args == null) {
 11         if (logger.isDebugEnabled()) {
 12             if (isSingletonCurrentlyInCreation(beanName)) {
 13                 logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
 14                         "' that is not fully initialized yet - a consequence of a circular reference");
 15             }
 16             else {
 17                 logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
 18             }
 19         }
 20         bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
 21     }
 22 
 23     else {
 24         // Fail if we're already creating this bean instance:
 25         // We're assumably within a circular reference.
 26         if (isPrototypeCurrentlyInCreation(beanName)) {
 27             throw new BeanCurrentlyInCreationException(beanName);
 28         }
 29 
 30         // Check if bean definition exists in this factory.
 31         BeanFactory parentBeanFactory = getParentBeanFactory();
 32         if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
 33             // Not found -> check parent.
 34             String nameToLookup = originalBeanName(name);
 35             if (args != null) {
 36                 // Delegation to parent with explicit args.
 37                 return (T) parentBeanFactory.getBean(nameToLookup, args);
 38             }
 39             else {
 40                 // No args -> delegate to standard getBean method.
 41                 return parentBeanFactory.getBean(nameToLookup, requiredType);
 42             }
 43         }
 44 
 45         if (!typeCheckOnly) {
 46             markBeanAsCreated(beanName);
 47         }
 48 
 49         final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
 50         checkMergedBeanDefinition(mbd, beanName, args);
 51 
 52         // Guarantee initialization of beans that the current bean depends on.
 53         String[] dependsOn = mbd.getDependsOn();
 54         if (dependsOn != null) {
 55             for (String dependsOnBean : dependsOn) {
 56                 getBean(dependsOnBean);
 57                 registerDependentBean(dependsOnBean, beanName);
 58             }
 59         }
 60 
 61         // Create bean instance.
 62         if (mbd.isSingleton()) {
 63             sharedInstance = getSingleton(beanName, new ObjectFactory() {
 64                 public Object getObject() throws BeansException {
 65                     try {
 66                         return createBean(beanName, mbd, args);
 67                     }
 68                     catch (BeansException ex) {
 69                         // Explicitly remove instance from singleton cache: It might have been put there
 70                         // eagerly by the creation process, to allow for circular reference resolution.
 71                         // Also remove any beans that received a temporary reference to the bean.
 72                         destroySingleton(beanName);
 73                         throw ex;
 74                     }
 75                 }
 76             });
 77             bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
 78         }
 79 
 80         else if (mbd.isPrototype()) {
 81             // It's a prototype -> create a new instance.
 82             Object prototypeInstance = null;
 83             try {
 84                 beforePrototypeCreation(beanName);
 85                 prototypeInstance = createBean(beanName, mbd, args);
 86             }
 87             finally {
 88                 afterPrototypeCreation(beanName);
 89             }
 90             bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
 91         }
 92 
 93         else {
 94             String scopeName = mbd.getScope();
 95             final Scope scope = this.scopes.get(scopeName);
 96             if (scope == null) {
 97                 throw new IllegalStateException("No Scope registered for scope '" + scopeName + "'");
 98             }
 99             try {
100                 Object scopedInstance = scope.get(beanName, new ObjectFactory() {
101                     public Object getObject() throws BeansException {
102                             beforePrototypeCreation(beanName);
103                         try {
104                             return createBean(beanName, mbd, args);
105                         }
106                         finally {
107                             afterPrototypeCreation(beanName);
108                         }
109                     }
110                 });
111                 bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
112             }
113             catch (IllegalStateException ex) {
114                 throw new BeanCreationException(beanName,
115                         "Scope '" + scopeName + "' is not active for the current thread; " +
116                         "consider defining a scoped proxy for this bean if you intend to refer to it from a singleton",
117                         ex);
118             }
119         }
120     }
121 
122     // Check if required type matches the type of the actual bean instance.
123     if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
124         try {
125             return getTypeConverter().convertIfNecessary(bean, requiredType);
126         }
127         catch (TypeMismatchException ex) {
128             if (logger.isDebugEnabled()) {
129                 logger.debug("Failed to convert bean '" + name + "' to required type [" +
130                         ClassUtils.getQualifiedName(requiredType) + "]", ex);
131             }
132             throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
133         }
134     }
135     return (T) bean;
136 }

首先第9行~第21行的代碼，第9行的代碼就不進去看了，簡單說一下：首先檢查一下本地的單例緩存是否已經載入過Bean，沒有的話再檢查earlySingleton緩存是否已經載入過Bean（又是early，不好找到詞語翻譯），沒有的話執行後面的邏輯。

接著第26行~第50行，這裡執行的都是一些基本的檢查和簡單的操作，包括bean是否是prototype的（prototype的Bean當前創建會拋出異常）、是否抽象的、將beanName加入alreadyCreated這個Set中等。

接著第53行~第59行，我們經常在bean標簽中看到depends-on這個屬性，就是通過這段保證了depends-on依賴的Bean會優先於當前Bean被載入。

接著第62行~第78行、第80行~第91行、第93行~第120行有三個判斷，顯然上面的MultiFunctionBean是一個單例的Bean也是本文探究的重點，因此執行第62行~第78行的邏輯。getSingleton方法不貼了，有一些前置的判斷，很簡單的邏輯，重點就是調用了ObjectFactory的getObject()方法來獲取到單例Bean對象，方法的實現是調用了createBean方法，createBean方法是AbstractBeanFactory的子類AbstractAutowireCapableBeanFactory的一個方法，看一下它的方法實現：

 1 protected Object createBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
 2         throws BeanCreationException {
 3 
 4     if (logger.isDebugEnabled()) {
 5         logger.debug("Creating instance of bean '" + beanName + "'");
 6     }
 7     // Make sure bean class is actually resolved at this point.
 8     resolveBeanClass(mbd, beanName);
 9 
10     // Prepare method overrides.
11     try {
12         mbd.prepareMethodOverrides();
13     }
14     catch (BeanDefinitionValidationException ex) {
15         throw new BeanDefinitionStoreException(mbd.getResourceDescription(),
16                 beanName, "Validation of method overrides failed", ex);
17     }
18 
19     try {
20         // Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
21         Object bean = resolveBeforeInstantiation(beanName, mbd);
22         if (bean != null) {
23             return bean;
24         }
25     }
26     catch (Throwable ex) {
27         throw new BeanCreationException(mbd.getResourceDescription(), beanName,
28                 "BeanPostProcessor before instantiation of bean failed", ex);
29     }
30 
31     Object beanInstance = doCreateBean(beanName, mbd, args);
32     if (logger.isDebugEnabled()) {
33         logger.debug("Finished creating instance of bean '" + beanName + "'");
34     }
35     return beanInstance;
36 }

前面的代碼都沒什麼意義，代碼執行到第31行：

 1 protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
 2     // Instantiate the bean.
 3     BeanWrapper instanceWrapper = null;
 4     if (mbd.isSingleton()) {
 5         instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
 6     }
 7     if (instanceWrapper == null) {
 8         instanceWrapper = createBeanInstance(beanName, mbd, args);
 9     }
10     final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
11     Class beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
12 
13     // Allow post-processors to modify the merged bean definition.
14     synchronized (mbd.postProcessingLock) {
15         if (!mbd.postProcessed) {
16             applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
17             mbd.postProcessed = true;
18         }
19     }
20 
21     // Eagerly cache singletons to be able to resolve circular references
22     // even when triggered by lifecycle interfaces like BeanFactoryAware.
23     boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
24             isSingletonCurrentlyInCreation(beanName));
25     if (earlySingletonExposure) {
26         if (logger.isDebugEnabled()) {
27             logger.debug("Eagerly caching bean '" + beanName +
28                     "' to allow for resolving potential circular references");
29         }
30         addSingletonFactory(beanName, new ObjectFactory() {
31             public Object getObject() throws BeansException {
32                 return getEarlyBeanReference(beanName, mbd, bean);
33             }
34         });
35     }
36 
37     // Initialize the bean instance.
38     Object exposedObject = bean;
39     try {
40         populateBean(beanName, mbd, instanceWrapper);
41         if (exposedObject != null) {
42             exposedObject = initializeBean(beanName, exposedObject, mbd);
43         }
44     }
45     catch (Throwable ex) {
46         if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
47             throw (BeanCreationException) ex;
48         }
49         else {
50             throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
51         }
52     }
53 
54     if (earlySingletonExposure) {
55         Object earlySingletonReference = getSingleton(beanName, false);
56         if (earlySingletonReference != null) {
57             if (exposedObject == bean) {
58                 exposedObject = earlySingletonReference;
59             }
60             else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
61                 String[] dependentBeans = getDependentBeans(beanName);
62                 Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length);
63                 for (String dependentBean : dependentBeans) {
64                     if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
65                         actualDependentBeans.add(dependentBean);
66                     }
67                 }
68                 if (!actualDependentBeans.isEmpty()) {
69                     throw new BeanCurrentlyInCreationException(beanName,
70                             "Bean with name '" + beanName + "' has been injected into other beans [" +
71                                 StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
72                             "] in its raw version as part of a circular reference, but has eventually been " +
73                             "wrapped. This means that said other beans do not use the final version of the " +
74                             "bean. This is often the result of over-eager type matching - consider using " +
75                             "'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
76                 }
77             }
78         }
79     }
80 
81     // Register bean as disposable.
82     try {
83         registerDisposableBeanIfNecessary(beanName, bean, mbd);
84     }
85     catch (BeanDefinitionValidationException ex) {
86         throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
87     }
88 
89     return exposedObject;
90 }

代碼跟蹤到這裡，已經到了主流程，接下來分段分析doCreateBean方法的代碼。

創建Bean實例

第8行的createBeanInstance方法，會創建出Bean的實例，並包裝為BeanWrapper，看一下createBeanInstance方法，只貼最後一段比較關鍵的：

 1 // Need to determine the constructor...
 2 Constructor[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
 3 if (ctors != null ||
 4         mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
 5         mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {
 6     return autowireConstructor(beanName, mbd, ctors, args);
 7 }
 8 
 9 // No special handling: simply use no-arg constructor.
10 return instantiateBean(beanName, mbd);

意思是bean標簽使用構造函數註入屬性的話，執行第6行，否則執行第10行。MultiFunctionBean使用預設構造函數，使用setter註入屬性，因此執行第10行代碼：

 1 protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
 2     try {
 3         Object beanInstance;
 4         final BeanFactory parent = this;
 5         if (System.getSecurityManager() != null) {
 6             beanInstance = AccessController.doPrivileged(new PrivilegedAction<Object>() {
 7                 public Object run() {
 8                     return getInstantiationStrategy().instantiate(mbd, beanName, parent);
 9                 }
10             }, getAccessControlContext());
11         }
12         else {
13             beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
14         }
15         BeanWrapper bw = new BeanWrapperImpl(beanInstance);
16         initBeanWrapper(bw);
17         return bw;
18     }
19     catch (Throwable ex) {
20         throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
21     }
22 }

代碼執行到13行：

 1 public Object instantiate(RootBeanDefinition beanDefinition, String beanName, BeanFactory owner) {
 2     // Don't override the class with CGLIB if no overrides.
 3     if (beanDefinition.getMethodOverrides().isEmpty()) {
 4         Constructor<?> constructorToUse;
 5         synchronized (beanDefinition.constructorArgumentLock) {
 6             constructorToUse = (Constructor<?>) beanDefinition.resolvedConstructorOrFactoryMethod;
 7             if (constructorToUse == null) {
 8                 final Class clazz = beanDefinition.getBeanClass();
 9                 if (clazz.isInterface()) {
10                     throw new BeanInstantiationException(clazz, "Specified class is an interface");
11                 }
12                 try {
13                     if (System.getSecurityManager() != null) {
14                         constructorToUse = AccessController.doPrivileged(new PrivilegedExceptionAction<Constructor>() {
15                             public Constructor run() throws Exception {
16                                 return clazz.getDeclaredConstructor((Class[]) null);
17                             }
18                         });
19                     }
20                     else {
21                         constructorToUse = clazz.getDeclaredConstructor((Class[]) null);
22                     }
23                     beanDefinition.resolvedConstructorOrFactoryMethod = constructorToUse;
24                 }
25                 catch (Exception ex) {
26                     throw new BeanInstantiationException(clazz, "No default constructor found", ex);
27                 }
28             }
29         }
30         return BeanUtils.instantiateClass(constructorToUse);
31     }
32