spring-PropertyPlaceholderConfiger读取属性

leobhao

发布于 2022-06-28 10:32:23

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概述

spring在读取配置文件的时候，我们时常使用@Value注解来注入配置文件中的配置，在配置文件中也可以通过${}的方式来引用已经申明的配置，这是依靠Spring提供的PropertyPlaceholderConfigure来实现的。

PropertyPlaceholderConfigure调用流程

PropertyPlaceholderConfigure的父类PropertyResourceConfigurer是一个实现了BeanFactoryPostProcessors的类，所以它在spring的生命周期中会被调用。PropertyResourceConfigurer的实现如下：

public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
    try {
        Properties mergedProps = mergeProperties();

        // Convert the merged properties, if necessary.
        convertProperties(mergedProps);

        // Let the subclass process the properties.
        processProperties(beanFactory, mergedProps);
    }
    catch (IOException ex) {
        throw new BeanInitializationException("Could not load properties", ex);
    }
}

mergeProperties主要是从硬盘中加载properties文件
convertProperties用来对PropertyValue做一些自定义对转换，默认是返回原值
processProperties是具体的properties的替换逻辑

PropertyPlaceholderConfigure-PropertyPlaceholderConfigure

PropertyPlaceholderConfigure实现替换properties逻辑如下:

protected void processProperties(ConfigurableListableBeanFactory beanFactoryToProcess, Properties props)
        throws BeansException {

    StringValueResolver valueResolver = new PlaceholderResolvingStringValueResolver(props);
    doProcessProperties(beanFactoryToProcess, valueResolver);
}

PlaceholderResolvingStringValueResolver封装了通过占位符从配置文件中获取对应配置的逻辑。大概的思路是遍历BeanDefinition进行占位符替换，在父类PlaceholderConfigurerSupport的doProcessProperties中实现:

protected void doProcessProperties(ConfigurableListableBeanFactory beanFactoryToProcess,
            StringValueResolver valueResolver) {

        BeanDefinitionVisitor visitor = new BeanDefinitionVisitor(valueResolver);

        String[] beanNames = beanFactoryToProcess.getBeanDefinitionNames();
        for (String curName : beanNames) {
            if (!(curName.equals(this.beanName) && beanFactoryToProcess.equals(this.beanFactory))) {
                BeanDefinition bd = beanFactoryToProcess.getBeanDefinition(curName);
                try {
                    visitor.visitBeanDefinition(bd);
                }
                catch (Exception ex) {
                    throw new BeanDefinitionStoreException(bd.getResourceDescription(), curName, ex.getMessage(), ex);
                }
            }
        }

        // New in Spring 2.5: resolve placeholders in alias target names and aliases as well.
        beanFactoryToProcess.resolveAliases(valueResolver);

        // New in Spring 3.0: resolve placeholders in embedded values such as annotation attributes.
        beanFactoryToProcess.addEmbeddedValueResolver(valueResolver);
    }

visitBeanDefinition对文件中对占位符进行了替换，而@Value注解中占位符，则通过内嵌ValueResolver的方式，创建bean的时候进行替换

替换占位符对逻辑-BeanDefinitionVisitor

BeanDefinitionVisitor封装了操作BeanDefinition的逻辑，对占位符进行了替换:

public void visitBeanDefinition(BeanDefinition beanDefinition) {
    visitParentName(beanDefinition);
    visitBeanClassName(beanDefinition);
    visitFactoryBeanName(beanDefinition);
    visitFactoryMethodName(beanDefinition);
    visitScope(beanDefinition);
    if (beanDefinition.hasPropertyValues()) {
        visitPropertyValues(beanDefinition.getPropertyValues());
    }
    if (beanDefinition.hasConstructorArgumentValues()) {
        ConstructorArgumentValues cas = beanDefinition.getConstructorArgumentValues();
        visitIndexedArgumentValues(cas.getIndexedArgumentValues());
        visitGenericArgumentValues(cas.getGenericArgumentValues());
    }
}

重写PropertyPlaceholderConfigurer

在项目中，我们有时候需要从其他地方(并非项目中的properties文件)读取配置，替换我们在代码中定义的@Value注解标识的变量，比如我们把一些变量定义在了zk，这个时候我们就需要重写PropertyPlaceholderConfigurer的processProperties

例如:

@Component
public class CustomPropertyPlaceholderConfigure extends PropertyPlaceholderConfigurer {


    private static final Logger logger = LoggerFactory.getLogger(CustomPropertyPlaceholderConfigure.class);

    @Override
    protected void processProperties(ConfigurableListableBeanFactory beanFactoryToProcess, Properties props) throws BeansException {
        // 设置为false，将占位符交给其他placeholder来解决
        this.setIgnoreUnresolvablePlaceholders(true);
        // 从此处读取zk，然后put进来
        ........
        super.processProperties(beanFactoryToProcess, props);
    }
}