BeanFactoryBeanPostProcessor 是当 BeanDefinition 读取完元数据（也就是从任意资源中定义的 bean 数据）后还未实例化之前可以进行修改
抄录并翻译官方的语句
BeanFactoryPostProcessor 操作 bean 的元数据配置. 也就是说,Spring IoC 容器允许 BeanFactoryPostProcessor 读取配置元数据, 并可能在容器实例化除 BeanFactoryPostProcessor 实例之外的任何 bean 之前 更改它
tip:
在 BeanFactoryPostProcessor (例如使用 BeanFactory.getBean()) 中使用这些 bean 的实例虽然在技术上是可行的,但这么来做会将 bean 过早实例化, 这违反了标准的容器生命周期. 同时也会引发一些副作用,例如绕过 bean 的后置处理。
public interface BeanFactoryPostProcessor { /** *通过ConfigurableListableBeanFactory这个可配置的BeanFactory对我们的bean原数据进行修改 */ void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException; }Copy to clipboardErrorCopied
BeanFactoryPostProcessor 执行时期的探究
ApplicationContext 的 refresh() 中的 invokeBeanFactoryPostProcessors 方法就开始创建我们的 BFPP(BeanFactoryPostProcessor)了
具体执行方法 invokeBeanFactoryPostProcessors，虽然一百多行代码，其实只需要特别了解的地方就几处。
public static void invokeBeanFactoryPostProcessors( ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) { Set<String> processedBeans = new HashSet<>(); // 由于我们的beanFactory是DefaultListableBeanFactory实例是BeanDefinitionRegistry的子类所以可以进来 if (beanFactory instanceof BeanDefinitionRegistry) { BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory; List<BeanFactoryPostProcessor> regularPostProcessors = new ArrayList<>(); List<BeanDefinitionRegistryPostProcessor> registryProcessors = new ArrayList<>(); for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) { if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) { BeanDefinitionRegistryPostProcessor registryProcessor = (BeanDefinitionRegistryPostProcessor) postProcessor; registryProcessor.postProcessBeanDefinitionRegistry(registry); registryProcessors.add(registryProcessor); } else { regularPostProcessors.add(postProcessor); } } // BeanDefinitionRegistryPostProcessor是BFPP的子类但是比BFPP提前执行 // 顺序实现PriorityOrdered接口先被执行，然后是Ordered接口，最后是什么都没实现的BeanDefinitionRegistryPostProcessor /** *都有beanFactory.getBean方法，证明BeanDefinitionRegistryPostProcessor这个bean现在已经被创建了 */ List<BeanDefinitionRegistryPostProcessor> currentRegistryProcessors = new ArrayList<>(); String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false); for (String ppName : postProcessorNames) { if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) { currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class)); processedBeans.add(ppName); } } sortPostProcessors(currentRegistryProcessors, beanFactory); registryProcessors.addAll(currentRegistryProcessors); invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry, beanFactory.getApplicationStartup()); currentRegistryProcessors.clear(); postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false); for (String ppName : postProcessorNames) { if (!processedBeans.contains(ppName) && beanFactory.isTypeMatch(ppName, Ordered.class)) { currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class)); processedBeans.add(ppName); } } sortPostProcessors(currentRegistryProcessors, beanFactory); registryProcessors.addAll(currentRegistryProcessors); invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry, beanFactory.getApplicationStartup()); currentRegistryProcessors.clear(); boolean reiterate = true; while (reiterate) { reiterate = false; postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false); for (String ppName : postProcessorNames) { if (!processedBeans.contains(ppName)) { currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class)); processedBeans.add(ppName); reiterate = true; } } sortPostProcessors(currentRegistryProcessors, beanFactory); registryProcessors.addAll(currentRegistryProcessors); invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry, beanFactory.getApplicationStartup()); currentRegistryProcessors.clear(); } invokeBeanFactoryPostProcessors(registryProcessors, beanFactory); invokeBeanFactoryPostProcessors(regularPostProcessors, beanFactory); } else { invokeBeanFactoryPostProcessors(beanFactoryPostProcessors, beanFactory); } // BFPP的执行顺序与上一样 /** *都有beanFactory.getBean方法，证明BFPP这个bean现在已经被创建了 */ String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanFactoryPostProcessor.class, true, false); List<BeanFactoryPostProcessor> priorityOrderedPostProcessors = new ArrayList<>(); List<String> orderedPostProcessorNames = new ArrayList<>(); List<String> nonOrderedPostProcessorNames = new ArrayList<>(); for (String ppName : postProcessorNames) { if (processedBeans.contains(ppName)) { } else if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) { priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanFactoryPostProcessor.class)); } else if (beanFactory.isTypeMatch(ppName, Ordered.class)) { orderedPostProcessorNames.add(ppName); } else { nonOrderedPostProcessorNames.add(ppName); } } sortPostProcessors(priorityOrderedPostProcessors, beanFactory); invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory); List<BeanFactoryPostProcessor> orderedPostProcessors = new ArrayList<>(orderedPostProcessorNames.size()); for (String postProcessorName : orderedPostProcessorNames) { orderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class)); } sortPostProcessors(orderedPostProcessors, beanFactory); invokeBeanFactoryPostProcessors(orderedPostProcessors, beanFactory); List<BeanFactoryPostProcessor> nonOrderedPostProcessors = new ArrayList<>(nonOrderedPostProcessorNames.size()); for (String postProcessorName : nonOrderedPostProcessorNames) { nonOrderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class)); } invokeBeanFactoryPostProcessors(nonOrderedPostProcessors, beanFactory); beanFactory.clearMetadataCache(); }Copy to clipboardErrorCopied
我们可以具体分析一下 BeanFactoryPostProcessor 的子类 CustomEditorConfigurer 自定义属性编辑器来巩固一下执行流程
所谓属性编辑器是当你要自定义更改配置文件中的属性属性时，如 String 类型转为 Date 或者其他，下面的一个小例子展示如何 String 类型的属性怎么转化为 Address 属性
简单工程（Spring-version-5.3.18)
Person 类
package cn.demo1; import lombok.Getter; import lombok.Setter; @Setter @Getter @ToString public class Person { private String name; private Address address; }Copy to clipboardErrorCopied
Address 类
package cn.demo1; @Setter @Getter @ToString public class Address { private String city; private String town; } Copy to clipboardErrorCopied
AddressParse 类
package cn.demo1; import java.beans.PropertyEditorSupport; public class AddressParse extends PropertyEditorSupport { @Override public void setAsText(String text) throws IllegalArgumentException { final String[] vals = text.split(","); Address addr = new Address(); addr.setProvince(vals[0]); addr.setCity(vals[1]); setValue(addr); } }Copy to clipboardErrorCopied
MyCustomEditor 类
package cn.demo1; import org.springframework.beans.PropertyEditorRegistrar; import org.springframework.beans.PropertyEditorRegistry; public class MyCustomEditor implements PropertyEditorRegistrar { @Override public void registerCustomEditors(PropertyEditorRegistry registry) { registry.registerCustomEditor(Address.class, new AddressParse()); } }Copy to clipboardErrorCopied
配置文件 test1.xml
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/aop https://www.springframework.org/schema/aop/spring-aop.xsd http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd"> <!-- 待属性编辑的bean，value代表的就是string类型--> <bean class="cn.demo1.Person" id="person"> <property name="name" value="李华"/> <property name="address" value="四川,成都"/> </bean> <!-- 注册属性编辑器--> <bean class="org.springframework.beans.factory.config.CustomEditorConfigurer" id="configurer"> <property name="propertyEditorRegistrars"> <list> <bean class="cn.demo1.MyCustomEditor"/> </list> </property> </bean> </beans>Copy to clipboardErrorCopied
测试类 EdT
package cn.test1; import cn.demo1.Person; import org.junit.Test; import org.springframework.context.ApplicationContext; import org.springframework.context.support.ClassPathXmlApplicationContext; public class EdT { @Test public void test1() { ApplicationContext context = new ClassPathXmlApplicationContext("test1.xml"); final Person bean = context.getBean(Person.class); System.out.println(bean); } } =====================测试结果 Person(name=李华, address=Address(province=四川, city=成都))Copy to clipboardErrorCopied
可以看见我们成功的将 String 类型转化为 Address 类型，让我们来看看实现流程，
- 首先实现 PropertyEditorSupport 来自定义属性编辑规则
- 其次将你的编辑规则给到 PropertyEditorRegistrar 子类里进行注册
- 最后在 Spring 中配置 CustomEditorConfigurer 类然后注入你的 PropertyEditorRegistrar 注册器
让我们 debug 走一遍
如果你已经耐心看完上面的BeanFactoryPostProcessor执行时期的探究那么你应该可以知道接下来我们的步骤应该是进入 invokeBeanFactoryPostProcessors 这个方法里了
private static void invokeBeanFactoryPostProcessors( Collection<? extends BeanFactoryPostProcessor> postProcessors, ConfigurableListableBeanFactory beanFactory) { for (BeanFactoryPostProcessor postProcessor : postProcessors) { StartupStep postProcessBeanFactory = beanFactory.getApplicationStartup().start("spring.context.bean-factory.post-process") .tag("postProcessor", postProcessor::toString); postProcessor.postProcessBeanFactory(beanFactory); postProcessBeanFactory.end(); } }Copy to clipboardErrorCopied
很明显它执行 postProcessBeanFactory 这个方法
我们探究的 BFPP 正是 CustomEditorConfigurer，所以这个是 CustomEditorConfigurer 对 BFPP 的 postProcessBeanFactory 实现
// 必然有个set方法让我们进行注入 public void setPropertyEditorRegistrars(PropertyEditorRegistrar[] propertyEditorRegistrars) { this.propertyEditorRegistrars = propertyEditorRegistrars; } @Override public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException { if (this.propertyEditorRegistrars != null) { for (PropertyEditorRegistrar propertyEditorRegistrar : this.propertyEditorRegistrars) { // 把它加入Bean工厂里后面可以进行调用 private final Set<PropertyEditorRegistrar> propertyEditorRegistrars = new LinkedHashSet<>(4); beanFactory.addPropertyEditorRegistrar(propertyEditorRegistrar); } } if (this.customEditors != null) { this.customEditors.forEach(beanFactory::registerCustomEditor); } }Copy to clipboardErrorCopied
关于这个注册器使用要到后面填充属性的时候才会用到，
我其实觉得这个有点瑕疵，因为 BFPP 作用影响应该是当 Spring 还未创建 bean 的时候，可以用 BFPP 进行修改操作，可是这个属性编辑却影响了 bean 创建过后的修改操作，那么它就替代了 BPP（BeanPostProcessor)的作用发挥了。（以上仅仅代表个人的观点，有可能是我想错了）
当我们 debug 到 AbstractAutowireCapableBeanFactory 的 populateBean 这个方法填充 bean 的属性的时候，
让我们看看它的方法，其中我省略了大部分无关代码
protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) { // 这个是如果你配置的bean中有属性值的话 // 也就是如下的配置，那么pvs不会为空的 /** <bean class="cn.demo1.Person" id="person"> <property name="name" value="李华"/> <property name="address" value="四川,成都"/> </bean> */ PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null); if (pvs != null) { // 属性操作 applyPropertyValues(beanName, mbd, bw, pvs); } }Copy to clipboardErrorCopied
让我们继续看看 applyPropertyValues 这个方法，无关的代码我也给省略了
protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) { // PropertyValues接口的默认实现。允许对属性进行简单操作，并提供构造函数以支持从 Map 进行深度复制和构造。 MutablePropertyValues mpvs = null; List<PropertyValue> original; // 可以进去 if (pvs instanceof MutablePropertyValues) { mpvs = (MutablePropertyValues) pvs; // 默认为false，即我们需要类型转换 if (mpvs.isConverted()) { // Shortcut: use the pre-converted values as-is. try { bw.setPropertyValues(mpvs); return; } catch (BeansException ex) { throw new BeanCreationException( mbd.getResourceDescription(), beanName, "Error setting property values", ex); } } // 把bean的属性以列表的形式展示出来 original = mpvs.getPropertyValueList(); } else { original = Arrays.asList(pvs.getPropertyValues()); } // 默认为空 TypeConverter converter = getCustomTypeConverter(); if (converter == null) { converter = bw; } // 就一个组合类，帮助更好的bean的属性的解析 BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, converter); // 深拷贝 List<PropertyValue> deepCopy = new ArrayList<>(original.size()); boolean resolveNecessary = false; for (PropertyValue pv : original) { if (pv.isConverted()) { deepCopy.add(pv); } else { // 获取bean的属性名字 String propertyName = pv.getName(); //获取bean属性值的包装对象 Object originalValue = pv.getValue(); // 自动装配的事情 if (originalValue == AutowiredPropertyMarker.INSTANCE) { Method writeMethod = bw.getPropertyDescriptor(propertyName).getWriteMethod(); if (writeMethod == null) { throw new IllegalArgumentException("Autowire marker for property without write method: " + pv); } originalValue = new DependencyDescriptor(new MethodParameter(writeMethod, 0), true); } // 把bean的属性值从包装类中分离出来 Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue); Object convertedValue = resolvedValue; // 一般为true boolean convertible = bw.isWritableProperty(propertyName) && !PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName); if (convertible) { // 这个就是重点，对应我们的属性转化 convertedValue = convertForProperty(resolvedValue, propertyName, bw, converter); } }Copy to clipboardErrorCopied
继续追踪
@Nullable private Object convertForProperty( @Nullable Object value, String propertyName, BeanWrapper bw, TypeConverter converter) { // BeanWrapperImpl是继承TypeConverter的 if (converter instanceof BeanWrapperImpl) { // 所以执行下面的方法 return ((BeanWrapperImpl) converter).convertForProperty(value, propertyName); } else { PropertyDescriptor pd = bw.getPropertyDescriptor(propertyName); MethodParameter methodParam = BeanUtils.getWriteMethodParameter(pd); return converter.convertIfNecessary(value, pd.getPropertyType(), methodParam); } }Copy to clipboardErrorCopied
@Nullable public Object convertForProperty(@Nullable Object value, String propertyName) throws TypeMismatchException { CachedIntrospectionResults cachedIntrospectionResults = getCachedIntrospectionResults(); PropertyDescriptor pd = cachedIntrospectionResults.getPropertyDescriptor(propertyName); if (pd == null) { throw new InvalidPropertyException(getRootClass(), getNestedPath() + propertyName, "No property '" + propertyName + "' found"); } TypeDescriptor td = cachedIntrospectionResults.getTypeDescriptor(pd); if (td == null) { td = cachedIntrospectionResults.addTypeDescriptor(pd, new TypeDescriptor(property(pd))); } // 上面的工作不用管，全是一些前戏工作，这个才是主题，至此我们的流程就到这里结束吧 // 后面的流程太多了，大部分都是处理细节，你只需要知道大概的脉络就行，就是最终它肯定会 // 走到AddressParse这个核心处理 return convertForProperty(propertyName, null, value, td); }Copy to clipboardErrorCopied
你可以自己可以尝试 debug 一下，看别人实践真的不如自己动手实践一下，Spring 的包装类实属太多，但是可以抓住核心流程进行 debug。
