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Spring BeanPostFactoryProcessor研

Spring BeanPostFactoryProcessor研

作者: IceBins | 来源:发表于2017-08-03 21:31 被阅读0次

在实际开发中,我们可能会遇到这样的场景,数据库的用户名和密码明文存储非常不安全,一种可行的方法是:运行时候通过访问一个安全性高的内部服务去获取用户名和密码.

配置如下:
application-Context.xml

<!--配置PropertyPlaceholderConfigurer -->
<bean class="xxxx.DBConnectionConfigurer">
        <property name="fetcher" ref="fetcher"></property>
        <property name="locations">
            <list>
                <value>classpath:db.properties</value>
            </list>
        </property>
    </bean>

<!-- 配置数据源 -->
<bean id="dataSource.master"
          class="org.apache.commons.dbcp.BasicDataSource"
          destroy-method="close">
        <property name="driverClassName" value="com.mysql.jdbc.Driver"></property>
        <property name="url" value="${master_jdbc_url}"></property>
        <property name="username" value="${master_jdbc_username}"></property>
        ...............
    </bean>

db.properties

master_jdbc_url=database_master_jdbc_url
master_jdbc_username=database_master_jdbc_username
  1. 原理
    通过DBConnectionConfigurer来获取指定的key的value, 然后注入给数据源<bean id="dataSource.master". DBConnectionConfigurer继承自PropertyPlaceholderConfigurer, PropertyPlaceholderConfigurer向上追溯发现实现接口BeanFactoryPostProcessor,这个接口在Spring中有特殊的作用

  2. 分析Spring代码
    首先看AbstractApplicationContext, 这个类实现代码如下

    public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // Prepare this context for refreshing.
            prepareRefresh();

            // Tell the subclass to refresh the internal bean factory.
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

            // Prepare the bean factory for use in this context.
            prepareBeanFactory(beanFactory);

            try {
                // Allows post-processing of the bean factory in context subclasses.
                postProcessBeanFactory(beanFactory);

                // Invoke factory processors registered as beans in the context.
                invokeBeanFactoryPostProcessors(beanFactory);

                // Register bean processors that intercept bean creation.
                registerBeanPostProcessors(beanFactory);

                // Initialize message source for this context.
                initMessageSource();

                // Initialize event multicaster for this context.
                initApplicationEventMulticaster();

                // Initialize other special beans in specific context subclasses.
                onRefresh();

                // Check for listener beans and register them.
                registerListeners();

                // Instantiate all remaining (non-lazy-init) singletons.
                finishBeanFactoryInitialization(beanFactory);

                // Last step: publish corresponding event.
                finishRefresh();
            }

            catch (BeansException ex) {
                if (logger.isWarnEnabled()) {
                    logger.warn("Exception encountered during context initialization - " +
                            "cancelling refresh attempt: " + ex);
                }

                // Destroy already created singletons to avoid dangling resources.
                destroyBeans();

                // Reset 'active' flag.
                cancelRefresh(ex);

                // Propagate exception to caller.
                throw ex;
            }

            finally {
                // Reset common introspection caches in Spring's core, since we
                // might not ever need metadata for singleton beans anymore...
                resetCommonCaches();
            }
        }
    }

在refresh函数中调用了函数invokeBeanFactoryPostProcessors, 这个函数会调用PostProcessorRegistrationDelegate类的invokeBeanFactoryPostProcessors函数

public static void invokeBeanFactoryPostProcessors(
            ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {

        // Invoke BeanDefinitionRegistryPostProcessors first, if any.
        Set<String> processedBeans = new HashSet<String>();

        if (beanFactory instanceof BeanDefinitionRegistry) {
            BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
            List<BeanFactoryPostProcessor> regularPostProcessors = new LinkedList<BeanFactoryPostProcessor>();
            List<BeanDefinitionRegistryPostProcessor> registryPostProcessors =
                    new LinkedList<BeanDefinitionRegistryPostProcessor>();

                        // 硬编码注册的后处理器
            for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {
                if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
                    BeanDefinitionRegistryPostProcessor registryPostProcessor =
                            (BeanDefinitionRegistryPostProcessor) postProcessor;
                    registryPostProcessor.postProcessBeanDefinitionRegistry(registry);
                    registryPostProcessors.add(registryPostProcessor);
                }
                else {
                    regularPostProcessors.add(postProcessor);
                }
            }

                         // 配置注册的后处理器
            // Do not initialize FactoryBeans here: We need to leave all regular beans
            // uninitialized to let the bean factory post-processors apply to them!
            // Separate between BeanDefinitionRegistryPostProcessors that implement
            // PriorityOrdered, Ordered, and the rest.
            String[] postProcessorNames =
                    beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);

            // First, invoke the BeanDefinitionRegistryPostProcessors that implement PriorityOrdered.
            List<BeanDefinitionRegistryPostProcessor> priorityOrderedPostProcessors = new ArrayList<BeanDefinitionRegistryPostProcessor>();
            for (String ppName : postProcessorNames) {
                if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
                    priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
                    processedBeans.add(ppName);
                }
            }
            sortPostProcessors(beanFactory, priorityOrderedPostProcessors);
            registryPostProcessors.addAll(priorityOrderedPostProcessors);
            invokeBeanDefinitionRegistryPostProcessors(priorityOrderedPostProcessors, registry);

            // Next, invoke the BeanDefinitionRegistryPostProcessors that implement Ordered.
            postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
            List<BeanDefinitionRegistryPostProcessor> orderedPostProcessors = new ArrayList<BeanDefinitionRegistryPostProcessor>();
            for (String ppName : postProcessorNames) {
                if (!processedBeans.contains(ppName) && beanFactory.isTypeMatch(ppName, Ordered.class)) {
                    orderedPostProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
                    processedBeans.add(ppName);
                }
            }
            sortPostProcessors(beanFactory, orderedPostProcessors);
            registryPostProcessors.addAll(orderedPostProcessors);
            invokeBeanDefinitionRegistryPostProcessors(orderedPostProcessors, registry);

            // Finally, invoke all other BeanDefinitionRegistryPostProcessors until no further ones appear.
            boolean reiterate = true;
            while (reiterate) {
                reiterate = false;
                postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
                for (String ppName : postProcessorNames) {
                    if (!processedBeans.contains(ppName)) {
                        BeanDefinitionRegistryPostProcessor pp = beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class);
                        registryPostProcessors.add(pp);
                        processedBeans.add(ppName);
                        pp.postProcessBeanDefinitionRegistry(registry);
                        reiterate = true;
                    }
                }
            }

            // Now, invoke the postProcessBeanFactory callback of all processors handled so far.
            invokeBeanFactoryPostProcessors(registryPostProcessors, beanFactory);
            invokeBeanFactoryPostProcessors(regularPostProcessors, beanFactory);
        }

        else {
            // Invoke factory processors registered with the context instance.
            invokeBeanFactoryPostProcessors(beanFactoryPostProcessors, beanFactory);
        }

        // Do not initialize FactoryBeans here: We need to leave all regular beans
        // uninitialized to let the bean factory post-processors apply to them!
        String[] postProcessorNames =
                beanFactory.getBeanNamesForType(BeanFactoryPostProcessor.class, true, false);

        // Separate between BeanFactoryPostProcessors that implement PriorityOrdered,
        // Ordered, and the rest.
        List<BeanFactoryPostProcessor> priorityOrderedPostProcessors = new ArrayList<BeanFactoryPostProcessor>();
        List<String> orderedPostProcessorNames = new ArrayList<String>();
        List<String> nonOrderedPostProcessorNames = new ArrayList<String>();
        for (String ppName : postProcessorNames) {
            if (processedBeans.contains(ppName)) {
                // skip - already processed in first phase above
            }
            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);
            }
        }

        // First, invoke the BeanFactoryPostProcessors that implement PriorityOrdered.
        sortPostProcessors(beanFactory, priorityOrderedPostProcessors);
        invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory);

        // Next, invoke the BeanFactoryPostProcessors that implement Ordered.
        List<BeanFactoryPostProcessor> orderedPostProcessors = new ArrayList<BeanFactoryPostProcessor>();
        for (String postProcessorName : orderedPostProcessorNames) {
            orderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
        }
        sortPostProcessors(beanFactory, orderedPostProcessors);
        invokeBeanFactoryPostProcessors(orderedPostProcessors, beanFactory);

        // Finally, invoke all other BeanFactoryPostProcessors.
        List<BeanFactoryPostProcessor> nonOrderedPostProcessors = new ArrayList<BeanFactoryPostProcessor>();
        for (String postProcessorName : nonOrderedPostProcessorNames) {
            nonOrderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
        }
        invokeBeanFactoryPostProcessors(nonOrderedPostProcessors, beanFactory);

        // Clear cached merged bean definitions since the post-processors might have
        // modified the original metadata, e.g. replacing placeholders in values...
        beanFactory.clearMetadataCache();
    }

注意, 下文中的"后处理器"都是表示BeanFactoryPostProcessor

  1. 流程分析:
  • 处理硬编码的后处理器,

    • 判断硬编码的后处理器类型
      • 如果是BeanDefinitionRegistryPostProcessor类型则加入到registryPostProcessors中,并且同时处理这个registryPostProcessor
      • 否则加入到regularPostProcessors
  • 从beanFactory中获取(不是硬编码的)所有类型为BeanDefinitionRegistryPostProcessor的bean, 并进行处理

    • 1.获得这些bean中具有PriorityOrdered属性的bean, 然后将这些bean添加到priorityOrderedPostProcessors中, 进行排序, 记录在processedBeans中, 并批量执行这些后处理器.

    • 2.获得这些bean中具有Ordered属性的bean, 记录在processedBeans中, 然后将这些bean记录到orderedPostProcessors中, 排序并执行.

    • 3.获得这些bean中不满足上述条件的bean, 记录在processedBeans中, 并依次执行

    • 4.批量处理invokeBeanFactoryPostProcessors(registryPostProcessors, beanFactory); invokeBeanFactoryPostProcessors(regularPostProcessors, beanFactory);

    • 值得注意的是: 上述每个小步骤都会重新获得所有满足条件的bean集合, 即调用beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);,我的理解是在后处理器处理过程中,可能会生成新的BeanDefinitionRegistryPostProcessor类型的后处理器, 所以需要重新加载, 此外, BeanDefinitionRegistryPostProcessor是特殊的后处理器, 在该阶段只执行特殊逻辑, 通用的逻辑在4中批量处理.

  • 接下里处理bean容器中的BeanFactoryPostProcessor.

    • 对于获得的所有的BeanFactoryPostProcessor也同样按照级别去划分为priorityOrderedPostProcessors, orderedPostProcessorNames, nonOrderedPostProcessorNames
      • 执行每个列表的时候都挨个去查找实际的class,然后遍历每个列表并执行
  • 一些特殊的操作

    • PropertyPlaceholderConfigurer的执行过程如下
        Properties mergedProps = mergeProperties();
       // Convert the merged properties, if necessary.
        convertProperties(mergedProps);
      // Let the subclass process the properties.
        processProperties(beanFactory, mergedProps);

首先是从locations指定的properties文件中读取内容, 然后获得调用convertProperties, 最终执行processProperties, convertProperties函数一般由自己完成逻辑,在processProperties中, 会调用BeanDefinitionVisitor, 遍历bean的各个属性用properties填充, BeanDefinitionVisitor会将替换的操作委托给内部的一个StringValueResolver来执行(PlaceholderResolvingStringValueResolver), 而这个StringValueResolver又会将操作委托给PropertyPlaceholderHelper, 这个helper(PropertyPlaceholderHelper)在实际执行的时候会执行内部的parseStringValue函数,解析过程可以参考源代码, 值得注意的是可能会在key里面嵌套新的key,例如${ty${key}}之类的,这种情况也处理了

此外,有一个特殊处理

if (propVal != null) {
                    // Recursive invocation, parsing placeholders contained in the
                    // previously resolved placeholder value.
                    propVal = parseStringValue(propVal, placeholderResolver, visitedPlaceholders);
                    result.replace(startIndex, endIndex + this.placeholderSuffix.length(), propVal);
                    if (logger.isTraceEnabled()) {
                        logger.trace("Resolved placeholder '" + placeholder + "'");
                    }
                    startIndex = result.indexOf(this.placeholderPrefix, startIndex + propVal.length());
                }
                else if (this.ignoreUnresolvablePlaceholders) {
                    // Proceed with unprocessed value.
                    startIndex = result.indexOf(this.placeholderPrefix, endIndex + this.placeholderSuffix.length());
                }
                else {
                    throw new IllegalArgumentException("Could not resolve placeholder '" +
                            placeholder + "'" + " in string value \"" + strVal + "\"");
                }
                visitedPlaceholders.remove(originalPlaceholder);
            }

可见ignoreUnresolvablePlaceholders这个参数有特殊的用途, 当我们定义多个PropertyPlaceholderConfigurer时候, 可能是希望各自解析不同的内容,但是, 如果不指定ignoreUnresolvablePlaceholders的时候,在执行第一个PropertyPlaceholderConfigurer的时候,就会一直使用这个PropertyPlaceholderConfigurer, 遇到不能解析的内容就会报错.这个需要特别注意一下

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