spring ioc

Spring IoC容器的设计

一）从接口BeanFactory到HierarchicalBeanFactory，再到ConfigurableBeanFactory，是一条主要的BeanFactory

设计路径。在这条接口设计路径中，BeanFactory接口定义了基本的IoC容器的规范。在这个接口定义中，包

括了getBean()这样的IoC容器的基本方法。而HierarchicalBeanFactory接口在继承了BeanFactory的基本接口

之后，增加了getParentBeanFactory()的接口功能，使BeanFactory具备了双亲IoC容器的管理功能。在接下来

的ConfigurableBeanFactory接口中，主要定义了一些对BeanFactory的配置功能，比如

通过setParentBeanFactory( )设置双亲IoC容器，通过addBeanPostProcessor( )配置Bean后置处理器，

等等。通过这些接口设计的叠加，定义了BeanFactory就是简单IoC容器的基本功能

二）第二条接口设计主线是，以ApplicationContext应用上下文接口为核心的接口设计，这里涉及的主要接口

设计有，从BenaFactory到ListableBeanFactory，再到ApplicationContext，再到我们常用的WebApplicationContext或者ConfigurableApplicationContext接口。

在这个接口体系中，ListableBeanFactory和HierarchicalBeanFactory两个接口，连接BeanFactory接口定义和ApplicationContext应用上下文的接口定义。

在ListableBeanFactory接口中，细化了许多BeanFactory的接口功能，比如定义了getBeanDefinitionNames()接口方法；对于HierarchicalBeanFactory接口，

我们在前文中已经提到过；对于ApplicationContext接口，它通过继承MessageSource、ResourceLoader、

ApplicationEventPublisher接口，在BeanFactory简单IoC容器的基础上添加了许多对高级容器的特性的支持。

1. BeanFactory

以XmlBeanFactory为例

XmlBeanFactory中

@Deprecated
@SuppressWarnings({"serial", "all"})
public class XmlBeanFactory extends DefaultListableBeanFactory {

    private final XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(this);


    /**
     * Create a new XmlBeanFactory with the given resource,
     * which must be parsable using DOM.
     * @param resource XML resource to load bean definitions from
     * @throws BeansException in case of loading or parsing errors
     */
    public XmlBeanFactory(Resource resource) throws BeansException {
        this(resource, null);
    }

    /**
     * Create a new XmlBeanFactory with the given input stream,
     * which must be parsable using DOM.
     * @param resource XML resource to load bean definitions from
     * @param parentBeanFactory parent bean factory
     * @throws BeansException in case of loading or parsing errors
     */
    public XmlBeanFactory(Resource resource, BeanFactory parentBeanFactory) throws BeansException {
        super(parentBeanFactory);
        this.reader.loadBeanDefinitions(resource);
    }

}

参考XmlBeanFactory 编程式使用IoC容器

ClassPathResource res = new ClassPathResource("bean.xml");
DefaultListableBeanFactory factory = new DefaultListableBeanFactory();
XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(factory);
reader.loadBeanDefinitions(res);

　　这样，我们就可以通过factory对象来使用DefaultListableBeanFactory这个IoC容器了。

在使用IoC容器时，需要如下几个步骤：

1）创建IoC配置文件的抽象资源，这个抽象资源包含了BeanDefinition的定义信息。

2）创建一个BeanFactory，这里使用DefaultListableBeanFactory。

3）创建一个载入BeanDefinition的读取器，这里使用XmlBeanDefinitionReader来载入XML文件形式的

BeanDefinition，通过一个回调配置给BeanFactory。

4）从定义好的资源位置读入配置信息，具体的解析过程由XmlBeanDefinitionReader来完成。完成整个载入

和注册Bean定义之后，需要的IoC容器就建立起来了。这个时候就可以直接使用IoC容器了。

2. ApplicationContext

在ApplicationContext容器中，以常用的FileSystemXmlApplicationContext的实现为例

来说明ApplicationContext容器的设计原理。

在FileSystemXmlApplicationContext的设计中，我们看到ApplicationContext应用上下文的主要功能是在

FileSystemXmlApplicationContext的基类AbstractXmlApplicationContext中实现了，

在FileSystemXmlApplicationContext中，作为一个具体的应用上下文，只需要实现和它自身设计相关的

两个功能。

       一个功能是，如果应用直接使用FileSystemXmlApplicationContext，对于实例化这个应用上下文的

支持，同时启动IoC容器的refresh( )过程。

    /**
     * Create a new FileSystemXmlApplicationContext with the given parent,
     * loading the definitions from the given XML files.
     * @param configLocations array of file paths
     * @param refresh whether to automatically refresh the context,
     * loading all bean definitions and creating all singletons.
     * Alternatively, call refresh manually after further configuring the context.
     * @param parent the parent context
     * @throws BeansException if context creation failed
     * @see #refresh()
     */
    public FileSystemXmlApplicationContext(String[] configLocations, boolean refresh, ApplicationContext parent)
            throws BeansException {

        super(parent);
        setConfigLocations(configLocations);
        if (refresh) {
            refresh();
        }
    }

　　另一个功能是与FileSystemXmlApplicationContext设计具体相关的功能，这部分与怎样从文件系统

中加载XML的Bean定义资源有关。

　　通过这个过程，可以为在文件系统中读取以XML形式存在的BeanDefinition做准备，因为不同的

应用上下文实现对应着不同的读取BeanDefinition的方式，在FileSystemXmlApplicationContext中的

实现代码如下：

    /**
     * Resolve resource paths as file system paths.
     * <p>Note: Even if a given path starts with a slash, it will get
     * interpreted as relative to the current VM working directory.
     * This is consistent with the semantics in a Servlet container.
     * @param path path to the resource
     * @return Resource handle
     * @see org.springframework.web.context.support.XmlWebApplicationContext#getResourceByPath
     */
    @Override
    protected Resource getResourceByPath(String path) {
        if (path != null && path.startsWith("/")) {
            path = path.substring(1);
        }
        return new FileSystemResource(path);
    }

可以看到，调用这个方法，可以得到FileSystemResource的资源定位。

IoC容器的初始化过程

　　简单来说，IoC容器的初始化是由前面介绍的refresh( )方法来启动的，具体来说，这个启动包括

BeanDefinition的Resource定位、载入和注册三个基本过程。

　　Spring把这三个过程分开，并使用不同的模块来完成，如使用相应的ResourceLoader、

BeanDefinitionReader、BeanDefinitionRegistry等模块，通过这样的设计方式，可以让用户更加灵活

的对这三个过程进行剪裁或扩展，定义出最合适自己的IoC容器的初始化过程。

　　第一个过程是Resource定位过程。这个Resource定位指的是BeanDefinition的资源定位，它由

ResourceLoader通过统一的Resource接口来完成，这个Resource对这种形式的BeanDefinition的使用

都提供了统一接口。比如，在文件系统中的Bean定义信息可以使用FileSystemResource来进行抽象；

在类路径中的Bean定义信息可以使用前面提到的ClassPathResource来使用。

　　第二个过程是BeanDefinition的载入。这个过程是把用户定义好的Bean表示成IoC容器内部的数据结构，

而这个容器内部的数据结构就是BeanDefinition。

　　第三个过程是向IoC容器注册这些BeanDefinition的过程。是通过调用BeanDefinitionRegistry接口的

实现来完成的。这个注册过程把载入过程中解析得到的BeanDefinition向IoC容器进行注册。通过分析，可以

看到，在IoC容器内部将BeanDefinition注入到一个HashMap中，通过HashMap来管理这些BeanDefinition

数据的。

IoC容器的依赖注入

　　当前IoC容器已经载入了用户定义的Bean信息，开始分析依赖注入的原理。首先，注意到依赖注入的过程是

用户第一次向IoC容器索要Bean时触发的，当然也有例外，也就是我们可以在BeanDefinition信息中通过控制

lazy-init属性来让容器完成对Bean的预实例化。下面从DefaultListableBeanFactory的基类AastractBeanFactory

入手去看看getBean的实现

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

    /**
     * Return an instance, which may be shared or independent, of the specified bean.
     * @param name the name of the bean to retrieve
     * @param requiredType the required type of the bean to retrieve
     * @param args arguments to use if creating a prototype using explicit arguments to a
     * static factory method. It is invalid to use a non-null args value in any other case.
     * @param typeCheckOnly whether the instance is obtained for a type check,
     * not for actual use
     * @return an instance of the bean
     * @throws BeansException if the bean could not be created
     */
    @SuppressWarnings("unchecked")
    protected <T> T doGetBean(
            final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
            throws BeansException {

        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);
        }

        else {
            // Fail if we're already creating this bean instance:
            // We're assumably within a circular reference.
            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);
                }
            }

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

            try {
                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                checkMergedBeanDefinition(mbd, beanName, args);

                // Guarantee initialization of beans that the current bean depends on.
                String[] dependsOn = mbd.getDependsOn();
                if (dependsOn != null) {
                    for (String dependsOnBean : dependsOn) {
                        getBean(dependsOnBean);
                        registerDependentBean(dependsOnBean, beanName);
                    }
                }

                // Create bean instance.
                if (mbd.isSingleton()) {
                    sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
                        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 '" + scopeName + "'");
                    }
                    try {
                        Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
                            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.
        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;
    }

 AbstractAutowireCapableBeanFactory中的createBean

    /**
     * Central method of this class: creates a bean instance,
     * populates the bean instance, applies post-processors, etc.
     * @see #doCreateBean
     */
    @Override
    protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
        if (logger.isDebugEnabled()) {
            logger.debug("Creating instance of bean '" + beanName + "'");
        }
        // Make sure bean class is actually resolved at this point.
        resolveBeanClass(mbd, beanName);

        // Prepare method overrides.
        try {
            mbd.prepareMethodOverrides();
        }
        catch (BeanDefinitionValidationException ex) {
            throw new BeanDefinitionStoreException(mbd.getResourceDescription(),
                    beanName, "Validation of method overrides failed", ex);
        }

        try {
            // Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
            Object bean = resolveBeforeInstantiation(beanName, mbd);
            if (bean != null) {
                return bean;
            }
        }
        catch (Throwable ex) {
            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                    "BeanPostProcessor before instantiation of bean failed", ex);
        }

        Object beanInstance = doCreateBean(beanName, mbd, args);
        if (logger.isDebugEnabled()) {
            logger.debug("Finished creating instance of bean '" + beanName + "'");
        }
        return beanInstance;
    }

    /**
     * Actually create the specified bean. Pre-creation processing has already happened
     * at this point, e.g. checking {@code postProcessBeforeInstantiation} callbacks.
     * <p>Differentiates between default bean instantiation, use of a
     * factory method, and autowiring a constructor.
     * @param beanName the name of the bean
     * @param mbd the merged bean definition for the bean
     * @param args arguments to use if creating a prototype using explicit arguments to a
     * static factory method. This parameter must be {@code null} except in this case.
     * @return a new instance of the bean
     * @throws BeanCreationException if the bean could not be created
     * @see #instantiateBean
     * @see #instantiateUsingFactoryMethod
     * @see #autowireConstructor
     */
    protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
        // Instantiate the bean.
        BeanWrapper instanceWrapper = null;
        if (mbd.isSingleton()) {
            instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
        }
        if (instanceWrapper == null) {
            instanceWrapper = createBeanInstance(beanName, mbd, args);
        }
        final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
        Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);

        // Allow post-processors to modify the merged bean definition.
        synchronized (mbd.postProcessingLock) {
            if (!mbd.postProcessed) {
                applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
                mbd.postProcessed = true;
            }
        }

        // Eagerly cache singletons to be able to resolve circular references
        // even when triggered by lifecycle interfaces like BeanFactoryAware.
        boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
                isSingletonCurrentlyInCreation(beanName));
        if (earlySingletonExposure) {
            if (logger.isDebugEnabled()) {
                logger.debug("Eagerly caching bean '" + beanName +
                        "' to allow for resolving potential circular references");
            }
            addSingletonFactory(beanName, new ObjectFactory<Object>() {
                public Object getObject() throws BeansException {
                    return getEarlyBeanReference(beanName, mbd, bean);
                }
            });
        }

        // Initialize the bean instance.
        Object exposedObject = bean;
        try {
            populateBean(beanName, mbd, instanceWrapper);
            if (exposedObject != null) {
                exposedObject = initializeBean(beanName, exposedObject, mbd);
            }
        }
        catch (Throwable ex) {
            if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
                throw (BeanCreationException) ex;
            }
            else {
                throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
            }
        }

        if (earlySingletonExposure) {
            Object earlySingletonReference = getSingleton(beanName, false);
            if (earlySingletonReference != null) {
                if (exposedObject == bean) {
                    exposedObject = earlySingletonReference;
                }
                else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
                    String[] dependentBeans = getDependentBeans(beanName);
                    Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length);
                    for (String dependentBean : dependentBeans) {
                        if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
                            actualDependentBeans.add(dependentBean);
                        }
                    }
                    if (!actualDependentBeans.isEmpty()) {
                        throw new BeanCurrentlyInCreationException(beanName,
                                "Bean with name '" + beanName + "' has been injected into other beans [" +
                                StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
                                "] in its raw version as part of a circular reference, but has eventually been " +
                                "wrapped. This means that said other beans do not use the final version of the " +
                                "bean. This is often the result of over-eager type matching - consider using " +
                                "'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
                    }
                }
            }
        }

        // Register bean as disposable.
        try {
            registerDisposableBeanIfNecessary(beanName, bean, mbd);
        }
        catch (BeanDefinitionValidationException ex) {
            throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
        }

        return exposedObject;
    }

在这里我们看到，与依赖注入关键的方法有createBeanInstance和populateBean,

在createBeanInstance中生成了Bean所包含的Java对象