Spring Bean生命周期-getBean方法（十）

前面提的基本上都是ApplicationContext的refresh方法，在分析的过程中bean容器的基本启动过程心中有了大体的了解，分析过程中，我们发现bean的实例化及加工相关的处理都在getBean方法中，一起看下getBean方法。

@Override
public Object getBean(String name) throws BeansException {
    return doGetBean(name, null, null, false);
}

doGetBean的方法比较长，拆开分析

分析

1. 从缓存中，或者获取手动注册的bean，也就是单例的bean已经被创建过了，那么直接获取。

final String beanName = transformedBeanName(name);
        Object bean;
 
        // Eagerly check singleton cache for manually registered singletons.
        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 + "'");
                }
            }
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }

getObjectForBeanInstance主要做了如下事情：

• 如果sharedInstance不是FactoryBean的实例，也不是Factory的引用，直接返回sharedInstance

• 从缓存中获取FactoryBean创建的Bean实例

• 如果换成中没有，那么使用FactoryBean的getObject方法获取Bean，如果需要postProcess，调用BeanPostProcessor处理bean

第一步一般都是在bean已经创建过的情况下，也不算是完整的bean创建过程

2.在缓存中没有获取的bean的情况下，也就是首次创建bean的时候，判断beanFactory是否有父级beanFactory如果有的话，交给父亲beanFactory完成。

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.
                String nameToLookup = originalBeanName(name);
                if (args != null) {
                    // Delegation to parent with explicit args.
                    return (T) parentBeanFactory.getBean(nameToLookup, args);
                }
                else {
                    // No args -> delegate to standard getBean method.
                    return parentBeanFactory.getBean(nameToLookup, requiredType);
                }
            }

3.bean的创建过程开始，根据之前解析的bean的依赖，首先创建当前bean实例依赖的bean

if (!typeCheckOnly) {
                markBeanAsCreated(beanName);
            }
 
            try {
                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                checkMergedBeanDefinition(mbd, beanName, args);
 
                // Guarantee initialization of beans that the current bean depends on.
                // 保证当前创建bean依赖的bean都已经创建了
                String[] dependsOn = mbd.getDependsOn();
                if (dependsOn != null) {
                    for (String dependsOnBean : dependsOn) {
                        if (isDependent(beanName, dependsOnBean)) {
                            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                    "Circular depends-on relationship between '" + beanName + "' and '" + dependsOnBean + "'");
                        }
                        registerDependentBean(dependsOnBean, beanName);
                        getBean(dependsOnBean);
                    }
                }

4.创建bean实例，根据bean的scope执行不同的创建方式。singleton，prototype，其他的scope

4.1 我们看单例的bean创建方式。其内部调用的是createBean方法。createBean比较重要，放在下次再说

if (mbd.isSingleton()) {
              // 获取单例，getSingleton方法内部掉的是回调中的getObject方法
                    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);
                }

4.2 scope为prototype类型的bean创建方式，其在调用createBean前后调用了beforePrototypeCreation与afterPrototypeCreation方法，也和singleton类似了。

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);
                }

4.3 不同的socpe创建的bean。只需要实现Scope接口即可。实现scope的get方法，其内部也是使用传入的匿名工厂类getObject

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);
                    }
                }

5.判断是否需要将bean转换为我们指定类型的class。

// 调用函数时传递进来的requiredType，bean，并且bean的class类型能转换为requiredType。然后调用TypeConverter进行转换。

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());
            }
        }

6.返回bean。

最后

这次我们知道了bean的创建过程其实是在调用getBean的时候创建的。配置的scope也是在这时生效的。