BeanDefinition到Bean

BeanDefinition到Bean
转自：http://songzi0206.iteye.com/blog/1430239

当 BeanDefinition 注册完毕以后， Spring Bean 工厂就可以随时根据需要进行实例化了。对于XmlBeanFactory 来说，实例化默认是延迟进行的，也就是说在 getBean 的时候才会；而对于 ApplicationContext来说，实例化会在容器启动后通过 AbstractApplicationContext 中 reflash 方法自动进行，主要经过方法链：reflesh() à finishBeanFactoryInitialization (factory) à DefaultListableBeanFactory.preInstantiateSingletons (), 在这里会根据注册的 BeanDefinition 信息依此调用 getBean(beanName) 。而真正实例化的逻辑和 BeanFactory 是“殊途同归”的，所有有关 Bean 实例化都可以从 getBean(beanName) 入手。

AbstractBeanFactory 有四个 getBean 的重载方法，不管调用哪个方法最终都是会调用另一个 doGetBean 方法：
Java代码
1. public Object getBean(String name) throws BeansException {
2. return getBean(name, null, null);
3. }
5. public Object getBean(String name, Class requiredType) throws BeansException {
6. return getBean(name, requiredType, null);
7. }
9. public Object getBean(String name, Object[] args) throws BeansException {
10. return getBean(name, null, args);
11. }
13. /**
14. * Return an instance, which may be shared or independent, of the specified bean.
15. * @param name the name of the bean to retrieve
16. * @param requiredType the required type of the bean to retrieve
17. * @param args arguments to use if creating a prototype using explicit arguments to a
18. * static factory method. It is invalid to use a non-null args value in any other case.
19. * @return an instance of the bean
20. * @throws BeansException if the bean could not be created
21. */
22. public Object getBean(String name, Class requiredType, Object[] args) throws BeansException {
23. return doGetBean(name, requiredType, args, false);
24. }
doGetBean方法比较长一点，见下面注释：
Java代码
1. protected Object doGetBean(
2. final String name, final Class requiredType, final Object[] args, boolean typeCheckOnly)
3. throws BeansException {
4. //bean name处理，去除FactoryBean前缀等
5. final String beanName = transformedBeanName(name);
6. Object bean = null;
8. //先从singleton缓存中查看是否已经实例化过该Bean，根据是否有缓存分为两个分支分别处理
9. Object sharedInstance = getSingleton(beanName);
10. if (sharedInstance != null && args == null) {
11. // 分支一，若缓存中获取到了并且该BeanDefinition信息表明该bean是singleton的，直接将获取到的缓存Bean
12. //(有可能是半成品)交给getObjectForBeanInstance处理
13. /*.........省略logger部分代码............*/
14. //调用getObjectForBeanInstance处理
15. bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
16. }else {
17. // 分之二：没有缓存，则需要从头实例化该bean
18. // We're assumably within a circular reference.
19. if (isPrototypeCurrentlyInCreation(beanName)) {
20. throw new BeanCurrentlyInCreationException(beanName);}
22. // 检查BeanDefinition是否在当前工厂或父工厂
23. BeanFactory parentBeanFactory = getParentBeanFactory();
24. if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
25. // Not found -> check parent.
26. String nameToLookup = originalBeanName(name);
27. if (args != null) {
28. // 父工厂getBean
29. return parentBeanFactory.getBean(nameToLookup, args);
30. }
31. else {
32. // No args -> delegate to standard getBean method.
33. return parentBeanFactory.getBean(nameToLookup, requiredType);
34. }
35. }
36. //将bean加入“正在创建”的集合，完成后会remove,对应afterSingletonCreation/afterPrototypeCreation方法
37. if (!typeCheckOnly) {
38. markBeanAsCreated(beanName);
39. }
41. final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
42. checkMergedBeanDefinition(mbd, beanName, args);
44. // 解决依赖关系，将依赖的bean提前实例化
45. String[] dependsOn = mbd.getDependsOn();
46. if (dependsOn != null) {
47. for (int i = 0; i < dependsOn.length; i++) {
48. String dependsOnBean = dependsOn[i];
49. getBean(dependsOnBean);
50. registerDependentBean(dependsOnBean, beanName);
51. }
52. }
54. // 这里又需要根据bean的类型分为三种情况：singleton、prototype、request/session
55. if (mbd.isSingleton()) {
56. //通过自定义ObjectFactory实例化Bean，此结果可能是半成品(是FactoryBean等)
57. sharedInstance = getSingleton(beanName, new ObjectFactory() {
58. public Object getObject() throws BeansException {
59. try {
60. //真正实例化装配的逻辑在createBean方法中
61. return createBean(beanName, mbd, args);
62. }
63. catch (BeansException ex) {
64. // Explicitly remove instance from singleton cache: It might have been put there
65. // eagerly by the creation process, to allow for circular reference resolution.
66. // Also remove any beans that received a temporary reference to the bean.
67. destroySingleton(beanName);
68. throw ex;
69. }
70. }
71. });
72. //上一步半成品的Bean交给getObjectForBeanInstance方法处理
73. bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
74. }
76. else if (mbd.isPrototype()) {
77. // It's a prototype -> create a new instance.
78. Object prototypeInstance = null;
79. try {
80. beforePrototypeCreation(beanName);
81. //真正实例化装配的逻辑在createBean方法中
82. prototypeInstance = createBean(beanName, mbd, args);
83. }
84. finally {
85. afterPrototypeCreation(beanName);
86. }
87. //上一步半成品的Bean交给getObjectForBeanInstance方法处理
88. bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
89. }
91. else {
92. //request、session 的bean
93. String scopeName = mbd.getScope();
94. final Scope scope = (Scope) this.scopes.get(scopeName);
95. if (scope == null) {
96. throw new IllegalStateException("No Scope registered for scope '" + scopeName + "'");
97. }
98. try {
99. Object scopedInstance = scope.get(beanName, new ObjectFactory() {
100. public Object getObject() throws BeansException {
101. beforePrototypeCreation(beanName);
102. try {
103. //真正实例化装配的逻辑在createBean方法中
104. return createBean(beanName, mbd, args);
105. }
106. finally {
107. afterPrototypeCreation(beanName);
108. }
109. }
110. });
111. //上一步半成品的Bean交给getObjectForBeanInstance方法处理
112. bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
113. }
114. catch (IllegalStateException ex) {
115. throw new BeanCreationException(beanName,
116. "Scope '" + scopeName + "' is not active for the current thread; " +
117. "consider defining a scoped proxy for this bean if you intend to refer to it from a singleton",
118. ex);
119. }
120. }
121. }
123. // Check if required type matches the type of the actual bean instance.
124. if (requiredType != null && bean != null &&
125. !requiredType.isAssignableFrom(bean.getClass())) {
126. throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
127. }
128. return bean;
129. }
通过注释，可以整理出较清晰的逻辑：检查Bean缓存，已经有缓存的Bean对象(有可能是半成品)则交给getObjectForBeanInstance方法处理；否则先根据Bean的生命周期类型分别实例化，每种情况大致都分两步,第一步都交给createBean方法生产一个半成品的bean对象，然后同样是将半成品的bean交给getObjectForBeanInstance方法处理。所以关键的逻辑就在这两个方法了，下面以singleton为例看看这两个方法，具体代码：
Java代码
1. sharedInstance = getSingleton(beanName, new ObjectFactory() {
2. public Object getObject() throws BeansException {
3. try {
4. return createBean(beanName, mbd, args);
5. }
6. catch (BeansException ex) {
7. // Explicitly remove instance from singleton cache: It might have been put there
8. // eagerly by the creation process, to allow for circular reference resolution.
9. // Also remove any beans that received a temporary reference to the bean.
10. destroySingleton(beanName);
11. throw ex;
12. }
13. }
14. });
15. bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
       getSingleton方法做的工作主要是实例化bean之前和之后回调beforeSingletonCreation/afterSingletonCreation、实例化bean、以及将bean对象缓存起来，具体实例化bean是通过回调匿名对象ObjectFactory的getObject方法实现的，从代码中明显看到主要是createBean方法。这里要特别注意下，缓存的bean对象是createBean生产的，这个方法生产的bean只是“半成品”，有可能是个factoryBean，真正返回给客户端使用的bean还必须进行下一步getObjectBeanInstance处理。所以缓存的bean对象可以认为是“半成品”，这就和前文代码中的注释相呼应(缓存中若取到了bean，必须进行getObjectBeanInstance处理)。

     对createBean方法实际上在“Spring IOC之BeanFactory”中已经有过分析，所以流程就不再简单重复，直接按步骤：

1. resolveBeanClass(mbd, beanName);

2. mbd.prepareMethodOverrides();//Spring IOC之BeanFactory已有解释，不重复

3. Object bean = resolveBeforeInstantiation(beanName, mbd);

      解释下，这里主要对于一些代理的bean做处理，我们平常经常会配置一些AOP模块，对于需要aop增强的bean实际上都会经过spring代理织入，而这些bean的代理逻辑应该就在这里处理了，具体看看这部分的代码：
Java代码
1. /**
2. * Apply before-instantiation post-processors, resolving whether there is a
3. * before-instantiation shortcut for the specified bean.
4. * @param beanName the name of the bean
5. * @param mbd the bean definition for the bean
6. * @return the shortcut-determined bean instance, or <code>null</code> if none
7. */
8. protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
9. Object bean = null;
10. if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
11. // Make sure bean class is actually resolved at this point.
12. if (mbd.hasBeanClass() && !mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
13. bean = applyBeanPostProcessorsBeforeInstantiation(mbd.getBeanClass(), beanName);
14. if (bean != null) {
15. bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
16. }
17. }
18. mbd.beforeInstantiationResolved = Boolean.valueOf(bean != null);
19. }
20. return bean;
21. }
23. /**
24. * Apply InstantiationAwareBeanPostProcessors to the specified bean definition
25. * (by class and name), invoking their <code>postProcessBeforeInstantiation</code> methods.
26. * <p>Any returned object will be used as the bean instead of actually instantiating
27. * the target bean. A <code>null</code> return value from the post-processor will
28. * result in the target bean being instantiated.
29. * @param beanClass the class of the bean to be instantiated
30. * @param beanName the name of the bean
31. * @return the bean object to use instead of a default instance of the target bean, or <code>null</code>
32. * @throws BeansException if any post-processing failed
33. * @see InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation
34. */
35. protected Object applyBeanPostProcessorsBeforeInstantiation(Class beanClass, String beanName)
36. throws BeansException {
38. for (Iterator it = getBeanPostProcessors().iterator(); it.hasNext();) {
39. BeanPostProcessor beanProcessor = (BeanPostProcessor) it.next();
40. if (beanProcessor instanceof InstantiationAwareBeanPostProcessor) {
41. InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) beanProcessor;
42. Object result = ibp.postProcessBeforeInstantiation(beanClass, beanName);
43. if (result != null) {
44. return result;
45. }
46. }
47. }
48. return null;
49. }
51. //AbstractAutoProxyCreator
52. public Object postProcessBeforeInstantiation(Class beanClass, String beanName) throws BeansException {
53. Object cacheKey = getCacheKey(beanClass, beanName);
55. if (!this.targetSourcedBeans.contains(cacheKey)) {
56. if (this.advisedBeans.contains(cacheKey) || this.nonAdvisedBeans.contains(cacheKey)) {
57. return null;
58. }
59. if (isInfrastructureClass(beanClass, beanName) || shouldSkip(beanClass, beanName)) {
60. this.nonAdvisedBeans.add(cacheKey);
61. return null;
62. }
63. }
65. // Create proxy here if we have a custom TargetSource.
66. // Suppresses unnecessary default instantiation of the target bean:
67. // The TargetSource will handle target instances in a custom fashion.
68. TargetSource targetSource = getCustomTargetSource(beanClass, beanName);
69. if (targetSource != null) {
70. this.targetSourcedBeans.add(beanName);
71. Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(beanClass, beanName, targetSource);
72. Object proxy = createProxy(beanClass, beanName, specificInterceptors, targetSource);
73. this.proxyTypes.put(cacheKey, proxy.getClass());
74. return proxy;
75. }
77. return null;
78. }
          对于AutoProxy部分代码要放到spring aop部分中去写了，这里只要明白如果是代理bean在postProcessBeforeInstantiation阶段会返回一个proxy，这时候bean已经算是实例化好了，再调用applyBeanPostProcessorsAfterInitialization即BeanPostProcessor.postProcessAfterInitialization回调进行属性的设置。最后的结果就可以返回bean对象了，只是一个Proxy Bean Object.

4. Object beanInstance = doCreateBean(beanName, mbd, args);//如果是代理bean就不会走到这一步

       解释下：对于非代理的bean，基本上实例化的逻辑就在doCreateBean这个方法了，这个方法在“Spring IOC之BeanFactory”中已经有过分析，所以不重复解释了，贴一个bean生命周期的流程图做为createBean方法的结尾：

             上图不是我所画，来源于spring IOC容器介绍。

       半成品出来之后还需要调用getObjectForBeanInstance进一步处理，这个方法：
Java代码
1. protected Object getObjectForBeanInstance(
2. Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {
4. // Don't let calling code try to dereference the factory if the bean isn't a factory.
5. if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
6. throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
7. }
9. // Now we have the bean instance, which may be a normal bean or a FactoryBean.
10. // If it's a FactoryBean, we use it to create a bean instance, unless the
11. // caller actually wants a reference to the factory.
12. if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
13. return beanInstance;
14. }
16. Object object = null;
17. if (mbd == null) {
18. object = getCachedObjectForFactoryBean(beanName);
19. }
20. if (object == null) {
21. // Return bean instance from factory.
22. FactoryBean factory = (FactoryBean) beanInstance;
23. // Caches object obtained from FactoryBean if it is a singleton.
24. if (mbd == null && containsBeanDefinition(beanName)) {
25. mbd = getMergedLocalBeanDefinition(beanName);
26. }
27. boolean synthetic = (mbd != null && mbd.isSynthetic());
28. object = getObjectFromFactoryBean(factory, beanName, !synthetic);
29. }
30. return object;
31. }
开始部分是逻辑检查，是否是FactoryBean,后面是从factoryBean中获取实际的Bean。获取bean也是先检查缓存，没有缓存再getObjectFromFactoryBean获取，详细再doGetObjectFromFactoryBean方法中，最后将获取的bean缓存起来。代码就不贴了。
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原文地址：https://www.cnblogs.com/heyanan/p/6086641.html