Spring源码阅读——bean的加载过程解析

r囧r小猫 2022-05-20 00:17 274阅读 0赞

### 前言 ###

在上一节中，我们已经了解过Spring对bean从XML中提取，并且以BeanDefinition类型解析注册到Spring的DefaultListableBeanFactory中取了，那么，接下来就应该来看下我们在Spring中获取一个实例过程中，bean是如何加载成为我们需要的对象的了。

### 通过getBean方法了解bean的加载 ###

在Spring中我们是通过getBean方法去获取，那么我们直接打开源码，深入到主要的方法doGetBean中去：

protected <T> T doGetBean(
            final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
            throws BeansException {
        //获取beanName 
        final String beanName = transformedBeanName(name);
        Object bean;
    
        // Eagerly check singleton cache for manually registered singletons.
        //首先检查在缓存单例中是否已经注册过
        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 + "'");
                }
            }
            //真正返回对应的实例，之前得到的其实是工厂bean的初始状态
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }
    
        else {
            // Fail if we're already creating this bean instance:
            // We're assumably within a circular reference.
            //解决循环依赖
            if (isPrototypeCurrentlyInCreation(beanName)) {
                throw new BeanCurrentlyInCreationException(beanName);
            }
    
            // Check if bean definition exists in this factory.
            //尝试从parentBeanFactory中查找
            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);
                }
            }
    
            //如果不仅仅是检查类型，那么标记此bean已经创建
            if (!typeCheckOnly) {
                markBeanAsCreated(beanName);
            }
    
            try {
                //转化为RootBeanDefinition类型
                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                checkMergedBeanDefinition(mbd, beanName, args);
    
                // Guarantee initialization of beans that the current bean depends on.
                //要实例化的bean中是否存在依赖，如果有依赖，那么从中递归实例化历来
                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);
                    }
                }
    
                // Create bean instance.
                //单例模式创建bean
                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);
                }
                //原型模式创建bean
                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 '" + 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;
            }
        }
    
        // Check if required type matches the type of the actual bean instance.
        //检查需要的类型是否是实际的类型
        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;
    }

以上加载的过程是非常多的，每一步都非常的细致。主要可以分为以下几个步骤：

*  获取对应的beanName
 *  尝试从缓存中加载单例
 *  bean的实例化
 *  原型模式的依赖检查
 *  尝试从parentBeanFactory中查找（尝试从父类中加载）
 *  类中依赖的实例化
 *  针对不同的scope创建bean（默认单例，getObjectForBeanInstance才是真正进行实例化的过程）
 *  类型转换