Spring之properties解析

原文来自个人小站 ( Ariescat's Blog ) ：Spring之properties解析

Spring Properties使用的几种方式

1. 在Java中使用这个 @Value("${ }")注解 读取

2. 在配置文件中使用 ${ } 读取

解析器注册

1. context:property-placeholder标签

   <context:property-placeholder location="classpath:*.properties"/>
   

   源码解析：

   public class ContextNamespaceHandler extends NamespaceHandlerSupport {
    @Override
    public void init() {
        registerBeanDefinitionParser("property-placeholder", new PropertyPlaceholderBeanDefinitionParser());
        ...省略代码
    }
   }
   

   这个Parser实质上注册了一个org.springframework.context.support.PropertySourcesPlaceholderConfigurer（实现了BeanFactoryPostProcessor的处理器）。

   // PropertySourcesPlaceholderConfigurer.java
    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
        if (this.propertySources == null) {
            this.propertySources = new MutablePropertySources();
            if (this.environment != null) {
                this.propertySources.addLast(
                    // 注意看这里this.environment里包含了propertySources
                    // spring boot的配置都在这里
                    new PropertySource<Environment>(ENVIRONMENT_PROPERTIES_PROPERTY_SOURCE_NAME, this.environment) {
                        @Override
                        @Nullable
                        public String getProperty(String key) {
                            // this.source就是this.environment
                            return this.source.getProperty(key);
                        }
                    }
                );
            }
            try {
                // 这一步的mergeProperties()也很重要
                // xml里的location配置会在这里获取
                PropertySource<?> localPropertySource =
                        new PropertiesPropertySource(LOCAL_PROPERTIES_PROPERTY_SOURCE_NAME, mergeProperties());
                if (this.localOverride) {
                    this.propertySources.addFirst(localPropertySource);
                }
                else {
                    this.propertySources.addLast(localPropertySource);
                }
            }
            catch (IOException ex) {
                throw new BeanInitializationException("Could not load properties", ex);
            }
        }
   
        processProperties(beanFactory, new PropertySourcesPropertyResolver(this.propertySources));
        this.appliedPropertySources = this.propertySources;
    }
   

2. PropertyPlaceholderConfigurer类

   <bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
       <property name="locations">
           <list>
               <value>classpath:resource1.properties</value>
               <value>classpath:config/resource2.properties</value>
               <value>classpath*:resource3.properties</value>
           </list>
       </property>
   </bean>
   

   原理和 @1 差不多

3. SpringBoot自动装配

   @EnableAutoConfiguration自动装配中spring.factories文件里包含PropertyPlaceholderAutoConfiguration类，

   @Bean
   @ConditionalOnMissingBean(search = SearchStrategy.CURRENT)
   public static PropertySourcesPlaceholderConfigurer propertySourcesPlaceholderConfigurer() {
    return new PropertySourcesPlaceholderConfigurer();
   }
   

   把PropertySourcesPlaceholderConfigurer这个实现了BeanFactoryPostProcessor的处理器注册进去。

   这个类就是 @1 里面的类

4. @PropertySource

   上面@EnableAutoConfiguration的自动装配只是加载了默认的 ./ (config/) application.properties（.yml，.xml）配置文件，可见@PropertySource灵活一点

   @PropertySource(value={"classpath:mail.properties"})
   public class ReadProperties {
   　　@Value(value="${mail.username}")
    　 private String USER_NAME;
   }
   

   区别@ImportResource，这个是可以引入带有<bean>标签的xml文件的（也就是普通的spring配置文件），曾经一度把这个和properties配置搞混。

   源码解析：

   org.springframework.context.annotation.ConfigurationClassParser#doProcessConfigurationClass

5. spring boot特有的@ConfigurationProperties

   源码解析：

   ConfigurationPropertiesBindingPostProcessor#postProcessBeforeInitialization

   这里面有空再研究吧，看着有点头大

@Value 源码解析

org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor#postProcessProperties

此处是Spring统一处理注入依赖的地方，接下来会调用到org.springframework.beans.factory.support.DefaultListableBeanFactory#doResolveDependency：

    // 这里的AutowireCandidateResolver是QualifierAnnotationAutowireCandidateResolver，
    // 其实就是获取@Value上的注解值
    Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor);
    if (value != null) {
        if (value instanceof String) {
            String strVal = resolveEmbeddedValue((String) value);
            BeanDefinition bd = (beanName != null && containsBean(beanName) ?
                    getMergedBeanDefinition(beanName) : null);
            value = evaluateBeanDefinitionString(strVal, bd);
        }
        TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
        try {
            return converter.convertIfNecessary(value, type, descriptor.getTypeDescriptor());
        } catch (UnsupportedOperationException ex) {
            // A custom TypeConverter which does not support TypeDescriptor resolution...
            return (descriptor.getField() != null ?
                    converter.convertIfNecessary(value, type, descriptor.getField()) :
                    converter.convertIfNecessary(value, type, descriptor.getMethodParameter()));
        }
    }

QualifierAnnotationAutowireCandidateResolver就是我们要找的处理类，它负责处理@Qualifier和@Value两个注解的取值操作。取值结果一般为“${xxx}”，然后执行resolveEmbeddedValue((String) value)，走进：

// PropertySourcesPlaceholderConfigurer.java
    protected void processProperties(ConfigurableListableBeanFactory beanFactoryToProcess,
            final ConfigurablePropertyResolver propertyResolver) throws BeansException {

        propertyResolver.setPlaceholderPrefix(this.placeholderPrefix);
        propertyResolver.setPlaceholderSuffix(this.placeholderSuffix);
        propertyResolver.setValueSeparator(this.valueSeparator);

        StringValueResolver valueResolver = strVal -> {
            // 走进这里！！propertyResolver就是上面postProcessBeanFactory()里面创建出来的
            // 通过这一步就可以获取到properties文件的值
            String resolved = (this.ignoreUnresolvablePlaceholders ?
                    propertyResolver.resolvePlaceholders(strVal) :
                    propertyResolver.resolveRequiredPlaceholders(strVal));
            if (this.trimValues) {
                resolved = resolved.trim();
            }
            return (resolved.equals(this.nullValue) ? null : resolved);
        };

        doProcessProperties(beanFactoryToProcess, valueResolver);
    }

参考：

1. spring- properties 读取的五种方式
2. SpringBoot 中 @Value 源码解析