Spring之properties解析

Spring Properties使用的几种方式

在Java中使用这个 @Value(“${ }”)注解 读取
在配置文件中使用 ${ } 读取

解析器注册

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;
	}

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

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
@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
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：

"来瞅瞅Spring怎么解析properties的吧"

Spring Properties使用的几种方式

解析器注册

@Value 源码解析

CATALOG

FEATURED TAGS

LINKS