Spring源码：Spring IoC容器加载过程（2）

Spring源码版本：4.3.23.RELEASE

一、加载XML配置

通过XML配置创建Spring，创建入口是使用org.springframework.context.support.ClassPathXmlApplicationContext类，创建容器的代码如下：

 package hello;

 import org.springframework.context.ApplicationContext;
 import org.springframework.context.support.ClassPathXmlApplicationContext;

 public class HelloWorld
 {
     public static void main(String[] args) {
         ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
         System.out.println(context.getBean("hello.Dog", Dog.class).name);
         System.out.println(context.getBean("hello.Dog#0", Dog.class).name);
     }
 }

 class Dog {
     String name = "PiPi";
 }

 <?xml version="1.0" encoding="UTF-8"?>
 <beans xmlns="http://www.springframework.org/schema/beans"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">

     <bean class="hello.Dog"/>

 </beans>

1.1 如果bean没有配置id或name属性，那Spring会取class属性值（也就是全限定类名）加＃index（index为bean对象在容器里的序号）作为name，取类全限定名作为别名aliases。具体看下面源码：

org/springframework/beans/factory/xml/BeanDefinitionParserDelegate.java#parseBeanDefinitionElement

     /**
      * Parses the supplied {@code <bean>} element. May return {@code null}
      * if there were errors during parse. Errors are reported to the
      * {@link org.springframework.beans.factory.parsing.ProblemReporter}.
      */
     public BeanDefinitionHolder parseBeanDefinitionElement(Element ele, BeanDefinition containingBean) {
         String id = ele.getAttribute(ID_ATTRIBUTE);
         String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);

         List<String> aliases = new ArrayList<String>();
         if (StringUtils.hasLength(nameAttr)) {
             String[] nameArr = StringUtils.tokenizeToStringArray(nameAttr, MULTI_VALUE_ATTRIBUTE_DELIMITERS);
             aliases.addAll(Arrays.asList(nameArr));
         }

         String beanName = id;
         if (!StringUtils.hasText(beanName) && !aliases.isEmpty()) {
             beanName = aliases.remove(0);
             if (logger.isDebugEnabled()) {
                 logger.debug("No XML 'id' specified - using '" + beanName +
                         "' as bean name and " + aliases + " as aliases");
             }
         }

         if (containingBean == null) {
             checkNameUniqueness(beanName, aliases, ele);
         }

         AbstractBeanDefinition beanDefinition = parseBeanDefinitionElement(ele, beanName, containingBean);
         if (beanDefinition != null) {
             if (!StringUtils.hasText(beanName)) {
                 try {
                     if (containingBean != null) {
                         beanName = BeanDefinitionReaderUtils.generateBeanName(
                                 beanDefinition, this.readerContext.getRegistry(), true);
                     }
                     else {
                         beanName = this.readerContext.generateBeanName(beanDefinition);  //获取到的bean名称为com.example.Hello#0这样的格式，#后面的数字为该类对象在进程中的编号。
                         // Register an alias for the plain bean class name, if still possible,
                         // if the generator returned the class name plus a suffix.
                         // This is expected for Spring 1.2/2.0 backwards compatibility.
                         String beanClassName = beanDefinition.getBeanClassName(); //获取class属性配置的bean的类名（全限定类名）
                         if (beanClassName != null &&
                                 beanName.startsWith(beanClassName) && beanName.length() > beanClassName.length() &&
                                 !this.readerContext.getRegistry().isBeanNameInUse(beanClassName)) {
                             aliases.add(beanClassName);  //使用全限定类名作为别名
                         }
                     }
                     if (logger.isDebugEnabled()) {
                         logger.debug("Neither XML 'id' nor 'name' specified - " +
                                 "using generated bean name [" + beanName + "]");
                     }
                 }
                 catch (Exception ex) {
                     error(ex.getMessage(), ele);
                     return null;
                 }
             }
             String[] aliasesArray = StringUtils.toStringArray(aliases);
             return new BeanDefinitionHolder(beanDefinition, beanName, aliasesArray);  //创建bean定义对象
         }

         return null;
     }

1.2 Bean的定义加载到一个ConcurrentHashMap，key为bean名称，value为org.springframework.beans.factory.config.BeanDefinition：

org.springframework.beans.factory.support.DefaultListableBeanFactory#beanDefinitionMap

    /** Map of bean definition objects, keyed by bean name */
    private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<String, BeanDefinition>(256);

org.springframework.beans.factory.support.DefaultListableBeanFactory#registerBeanDefinition

     //---------------------------------------------------------------------
     // Implementation of BeanDefinitionRegistry interface
     //---------------------------------------------------------------------

     @Override
     public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
             throws BeanDefinitionStoreException {

         Assert.hasText(beanName, "Bean name must not be empty");
         Assert.notNull(beanDefinition, "BeanDefinition must not be null");

         if (beanDefinition instanceof AbstractBeanDefinition) {
             try {
                 ((AbstractBeanDefinition) beanDefinition).validate();
             }
             catch (BeanDefinitionValidationException ex) {
                 throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
                         "Validation of bean definition failed", ex);
             }
         }

         BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
         if (existingDefinition != null) {
             if (!isAllowBeanDefinitionOverriding()) {
                 throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
                         "Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
                         "': There is already [" + existingDefinition + "] bound.");
             }
             else if (existingDefinition.getRole() < beanDefinition.getRole()) {
                 // e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
                 if (logger.isWarnEnabled()) {
                     logger.warn("Overriding user-defined bean definition for bean '" + beanName +
                             "' with a framework-generated bean definition: replacing [" +
                             existingDefinition + "] with [" + beanDefinition + "]");
                 }
             }
             else if (!beanDefinition.equals(existingDefinition)) {
                 if (logger.isInfoEnabled()) {
                     logger.info("Overriding bean definition for bean '" + beanName +
                             "' with a different definition: replacing [" + existingDefinition +
                             "] with [" + beanDefinition + "]");
                 }
             }
             else {
                 if (logger.isDebugEnabled()) {
                     logger.debug("Overriding bean definition for bean '" + beanName +
                             "' with an equivalent definition: replacing [" + existingDefinition +
                             "] with [" + beanDefinition + "]");
                 }
             }
             this.beanDefinitionMap.put(beanName, beanDefinition);
         }
         else {
             if (hasBeanCreationStarted()) {
                 // Cannot modify startup-time collection elements anymore (for stable iteration)
                 synchronized (this.beanDefinitionMap) {
                     this.beanDefinitionMap.put(beanName, beanDefinition);
                     List<String> updatedDefinitions = new ArrayList<String>(this.beanDefinitionNames.size() + 1);
                     updatedDefinitions.addAll(this.beanDefinitionNames);
                     updatedDefinitions.add(beanName);
                     this.beanDefinitionNames = updatedDefinitions;
                     if (this.manualSingletonNames.contains(beanName)) {
                         Set<String> updatedSingletons = new LinkedHashSet<String>(this.manualSingletonNames);
                         updatedSingletons.remove(beanName);
                         this.manualSingletonNames = updatedSingletons;
                     }
                 }
             }
             else {
                 // Still in startup registration phase
                 this.beanDefinitionMap.put(beanName, beanDefinition);
                 this.beanDefinitionNames.add(beanName);
                 this.manualSingletonNames.remove(beanName);
             }
             this.frozenBeanDefinitionNames = null;
         }

         if (existingDefinition != null || containsSingleton(beanName)) {
             resetBeanDefinition(beanName);
         }
     }