spring IOC之篇五 bean的加载

Posted 2020-11-02 烟尘

我们终于结束了对XML配置文件的解析，接下来我们要分析bean的加载，即对代码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 {
 // 根据名称获取对应的beanName。如果传入的别名A指向bean B则返回bean B的beanName，如果别名A指向别名B 然后别名B指向bean c 则返回bean C，如果传入的是FactoryBean，则则去掉修饰符 ，比如 name=”&aa” 则变为 name=”aa”
        final String beanName = transformedBeanName(name);
        Object bean;

        // Eagerly check singleton cache for manually registered singletons.
       // 尝试从缓存中加载单例。首次从单例缓存中加载，如果不成功则从singletonFactories中加载。因为在创建单例的时候存在着循环依赖，所以不等bean创建完成就就将创建bean的ObjectFactory提早曝光加入到缓存中，一旦下一个bean依赖上一个bean 则直接使用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 + "‘");
                }
            }
 // 返回对应的实例，缓存中记录的只是bean最初始化的状态，不一定是最终的bean，有时间需要返回时的指定方法实例化的实例，例如 FactoryBean的getObject的方法的实例
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }

        else {
            // Fail if we‘re already creating this bean instance:
            // We‘re assumably within a circular reference.
       // 只有在单例的情况下面才去解决循环依赖，如果原型的情况下面则直接抛出异常
            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 不为空，且当前的beanFactory不包含beanName则递归到他的上一层去加载beanName
                String nameToLookup = originalBeanName(name);
                if (args != null) {
                    // Delegation to parent with explicit args.
                // args为传入的构造函数的参数
                    return (T) parentBeanFactory.getBean(nameToLookup, args);
                }
                else {
                     // 没有构造参数则使用标准的方法获取bean
                    // No args -> delegate to standard getBean method.
                    return parentBeanFactory.getBean(nameToLookup, requiredType);
                }
            }

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

            try {
               // 将GenericBeanDefinition转换为 RootBeanDefinition
                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
               // 如果父类bean不为空的话，合并一下父类的属性
                checkMergedBeanDefinition(mbd, beanName, args);

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

                // Create bean instance.
     // 根据不同的scope进行初始化策略
                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;
            }
        }
// 根据传入的参数 requiredType和返回的bean类型不一致的时候，进行装换
        // 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;
    }

进一步分析 FactoryBean的使用

FactoryBean接口对于Spring框架来说占有重要的地位，Spring自身就提供了70多种实现类。他们隐藏了一下实例化的复杂的细节，给上层应用带来便利。

 

public interface FactoryBean<T> {
   // 返回由FactoryBean创建的Bean实例，如果isSingleton（）方法返回的是true，则该实例会放入到Spring容器的单例缓存池当中
    T getObject() throws Exception;

    // 返回FactoryBean创建的bean类型
    Class<?> getObjectType();
   // 返回由factoryBean创建实例的Bean是singleton还是prototype
    boolean isSingleton();

}

 

创建一个实体用Factory进行实例化

 

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

 

创建一个自定义的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> 

即可使用自定义的逻辑进行实例化，当调用getBean的时候，Spring通过反射机制发现PeopleFactoryBean实现了FactoryBean，这时候Spring容器就调用接口方法PeopleFactoryBean#getObject()方法返回。如果希望获取PeopleFactoryBean实例，则在beanName前面显示的加&