我們終於結束了對XML配置文件的解析，接下來我們要分析bean的加載，即對代碼getBean的分析：

People p = (People)ctx.getBean("cutesource");

@SuppressWarnings("unchecked")
    protected <T> T doGetBean(
            final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
            throws BeansException {
 
 
// 根據名稱獲取對應的beanName。如果傳入的別名A指向bean B則返回bean B的beanName，如果別名A指向別名B 然後別名B指向bean c 則返回bean C，如果傳入的是FactoryBean，則則去掉修飾符 ，比如 name=”&aa” 則變為 name=”aa”
        final String beanName = transformedBeanName(name);
        Object bean;

        // Eagerly check singleton cache for manually registered singletons.
       
 
// 嘗試從緩存中加載單例。首次從單例緩存中加載，如果不成功則從singletonFactories中加載。因為在創建單例的時候存在著循環依賴，所以不等bean創建完成就就將創建bean的ObjectFactory提早曝光加入到緩存中，一旦下一個bean依賴上一個bean 則直接使用ObjectFactory
        Object sharedInstance = getSingleton(beanName);
        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 + "‘");
                }
            }
 // 返回對應的實例，緩存中記錄的只是bean最初始化的狀態，不一定是最終的bean，有時間需要返回時的指定方法實例化的實例，例如 FactoryBean的getObject的方法的實例
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }

        else {
            // Fail if we‘re already creating this bean instance:
            // We‘re assumably within a circular reference.
       // 只有在單例的情況下面才去解決循環依賴，如果原型的情況下面則直接拋出異常
            if (isPrototypeCurrentlyInCreation(beanName)) {
                throw new BeanCurrentlyInCreationException(beanName);
            }

            // Check if bean definition exists in this factory.
             
            BeanFactory parentBeanFactory = getParentBeanFactory();
            if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
                // Not found -> check parent.
               // 如果parentBeanFactory 不為空，且當前的beanFactory不包含beanName則遞歸到他的上一層去加載beanName
                String nameToLookup = originalBeanName(name);
                if (args != null) {
                    // Delegation to parent with explicit args.
                // args為傳入的構造函數的參數
                    return (T) parentBeanFactory.getBean(nameToLookup, args);
                }
                else {
                     // 沒有構造參數則使用標準的方法獲取bean
                    // No args -> delegate to standard getBean method.
                    return parentBeanFactory.getBean(nameToLookup, requiredType);
                }
            }

            if (!typeCheckOnly) {
                markBeanAsCreated(beanName);
            }

            try {
               // 將GenericBeanDefinition轉換為 RootBeanDefinition
                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
               // 如果父類bean不為空的話，合並一下父類的屬性
                checkMergedBeanDefinition(mbd, beanName, args);

                // Guarantee initialization of beans that the current bean depends on.
                String[] dependsOn = mbd.getDependsOn();
                if (dependsOn != null) {
               // 獲取bean的其他依賴，如果依賴其他的bean，則先獲取其他的bean
                    for (String dependsOnBean : dependsOn) {
                        if (isDependent(beanName, dependsOnBean)) {
                            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                    "Circular depends-on relationship between ‘" + beanName + "‘ and ‘" + dependsOnBean + "‘");
                        }
            // 把該bean依賴的其他的bean加入到dependentBeanMap集合中去
                        registerDependentBean(dependsOnBean, beanName);
  // 獲取其依賴的其他bean
                        getBean(dependsOnBean);
                    }
                }

                // Create bean instance.
     // 根據不同的scope進行初始化策略
                if (mbd.isSingleton()) {
                    sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
                        @Override
                        public Object getObject() throws BeansException {
                            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, new ObjectFactory<Object>() {
                            @Override
                            public Object getObject() throws BeansException {
                                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;
            }
        }
// 根據傳入的參數 requiredType和返回的bean類型不一致的時候，進行裝換
        // Check if required type matches the type of the actual bean instance.
        if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
            try {
                return getTypeConverter().convertIfNecessary(bean, requiredType);
            }
            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;
    }

進一步分析 FactoryBean的使用

FactoryBean接口對於Spring框架來說占有重要的地位，Spring自身就提供了70多種實現類。他們隱藏了一下實例化的復雜的細節，給上層應用帶來便利。

public interface FactoryBean<T> {
   // 返回由FactoryBean創建的Bean實例，如果isSingleton（）方法返回的是true，則該實例會放入到Spring容器的單例緩存池當中
    T getObject() throws Exception;

    // 返回FactoryBean創建的bean類型
    Class<?> getObjectType();
   // 返回由factoryBean創建實例的Bean是singleton還是prototype
    boolean isSingleton();

}

創建一個實體用Factory進行實例化

public class People {
        private String id;  
        private String name;  
        private Integer age;
        public String getId() {
            return id;
        }

創建一個自定義的FactoryBean方法

public class PeopleFactoryBean implements FactoryBean<People> {

    private String peopleInfo;
    
    public People getObject() throws Exception {
        
        People people = new People();
        String[] infos = peopleInfo.split(",");
        people.setId(infos[0]);
        people.setName(infos[1]);
        people.setAge(Integer.parseInt(infos[2]));
        return people;
    }

    public Class<People> getObjectType() {
        return People.class;
    }

    public boolean isSingleton() {
        return false;
    }

    public String getPeopleInfo() {
        return peopleInfo;
    }

    public void setPeopleInfo(String peopleInfo) {
        this.peopleInfo = peopleInfo;
    }
    
}

配置文件中配置：

<bean id="people" class="com.spring.test.factory.PeopleFactoryBean">
        <property name="peopleInfo" value="gh2,address2,22"></property>
</bean> 

即可使用自定義的邏輯進行實例化，當調用getBean的時候，Spring通過反射機制發現PeopleFactoryBean實現了FactoryBean，這時候Spring容器就調用接口方法PeopleFactoryBean#getObject()方法返回。如果希望獲取PeopleFactoryBean實例，則在beanName前面顯示的加&

spring IOC之篇五 bean的加載