Spring占位符Placeholder的实现原理解析

占位符Placeholder的使用

xml中的配置：

<?xml version="1.0" encoding="utf-8" ?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="
http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context.xsd"
default-lazy-init="false">
<context:property-placeholder location="classpath:application.properties"/>
<bean id="user" class="com.morris.spring.entity.Author">
<property name="name" value="${author.name}" />
</bean>
</beans>

实现原理

前面在Spring中自定义标签的解析中分析到context:property-placeholder这种自定义标签的解析流程如下：

基于SPI机制，扫描所有类路径下jar中/META-INFO/spring.handlers文件，并将这些文件读取为一个key为namespace，value为具体NameSpaceHandler的Map结构。
根据bean标签名获得xml上方的namespace，然后根据namespace从第一步中的map中获得具体的NameSpaceHandler。
调用NameSpaceHandler的init()方法进行初始化，此方法一般会将负责解析各种localName的BeanDefinitionParser解析器注册到一个map中。
根据localName=property-placeholder从上一步中获得具体的BeanDefinitionParser解析器，并调用其parse()方法进行解析。

在这里NameSpaceHandler为ContextNamespaceHandler，而BeanDefinitionParser解析器为PropertyPlaceholderBeanDefinitionParser，所以我们观察的重点为PropertyPlaceholderBeanDefinitionParser的parse()方法。

注册PropertySourcesPlaceholderConfigurer

parse()方法位于父类AbstractBeanDefinitionParser，先来看下继承关系，后面的代码使用了大量的模板方法模式，将会在这几个类中来回切换：

org.springframework.beans.factory.xml.AbstractBeanDefinitionParser#parse

public final BeanDefinition parse(Element element, ParserContext parserContext) {
// 调用子类org.springframework.beans.factory.xml.AbstractSingleBeanDefinitionParser.parseInternal
AbstractBeanDefinition definition = parseInternal(element, parserContext);
if (definition != null && !parserContext.isNested()) {
try {
// 生成一个ID
String id = resolveId(element, definition, parserContext);
if (!StringUtils.hasText(id)) {
parserContext.getReaderContext().error(
"Id is required for element '" + parserContext.getDelegate().getLocalName(element)
+ "' when used as a top-level tag", element);
}
String[] aliases = null;
if (shouldParseNameAsAliases()) {
String name = element.getAttribute(NAME_ATTRIBUTE);
if (StringUtils.hasLength(name)) {
aliases = StringUtils.trimArrayElements(StringUtils.commaDelimitedListToStringArray(name));
}
}
BeanDefinitionHolder holder = new BeanDefinitionHolder(definition, id, aliases);
// 注册BD
registerBeanDefinition(holder, parserContext.getRegistry());
if (shouldFireEvents()) {
BeanComponentDefinition componentDefinition = new BeanComponentDefinition(holder);
postProcessComponentDefinition(componentDefinition);
parserContext.registerComponent(componentDefinition);
}
}
catch (BeanDefinitionStoreException ex) {
String msg = ex.getMessage();
parserContext.getReaderContext().error((msg != null ? msg : ex.toString()), element);
return null;
}
}
return definition;
}

org.springframework.beans.factory.xml.AbstractSingleBeanDefinitionParser#parseInternal

protected final AbstractBeanDefinition parseInternal(Element element, ParserContext parserContext) {
BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition();
String parentName = getParentName(element);
if (parentName != null) {
builder.getRawBeanDefinition().setParentName(parentName);
}
// 获取子类PropertyPlaceholderBeanDefinitionParser返回的PropertySourcesPlaceholderConfigurer
Class<?> beanClass = getBeanClass(element);
if (beanClass != null) {
builder.getRawBeanDefinition().setBeanClass(beanClass);
}
else {
String beanClassName = getBeanClassName(element);
if (beanClassName != null) {
builder.getRawBeanDefinition().setBeanClassName(beanClassName);
}
}
builder.getRawBeanDefinition().setSource(parserContext.extractSource(element));
BeanDefinition containingBd = parserContext.getContainingBeanDefinition();
if (containingBd != null) {
// Inner bean definition must receive same scope as containing bean.
builder.setScope(containingBd.getScope());
}
if (parserContext.isDefaultLazyInit()) {
// Default-lazy-init applies to custom bean definitions as well.
builder.setLazyInit(true);
}
// 又是一个模板方法模式
/**
* @see org.springframework.context.config.PropertyPlaceholderBeanDefinitionParser#doParse(org.w3c.dom.Element, org.springframework.beans.factory.xml.ParserContext, org.springframework.beans.factory.support.BeanDefinitionBuilder)
*/
doParse(element, parserContext, builder);
return builder.getBeanDefinition();
}

org.springframework.context.config.PropertyPlaceholderBeanDefinitionParser#getBeanClass

protected Class<?> getBeanClass(Element element) {
if (SYSTEM_PROPERTIES_MODE_DEFAULT.equals(element.getAttribute(SYSTEM_PROPERTIES_MODE_ATTRIBUTE))) {
return PropertySourcesPlaceholderConfigurer.class; // 新版返回这个
}
return org.springframework.beans.factory.config.PropertyPlaceholderConfigurer.class;
}

org.springframework.context.config.PropertyPlaceholderBeanDefinitionParser#doParse

protected void doParse(Element element, ParserContext parserContext, BeanDefinitionBuilder builder) {
// 调用父类的doParse
super.doParse(element, parserContext, builder);
builder.addPropertyValue("ignoreUnresolvablePlaceholders",
Boolean.valueOf(element.getAttribute("ignore-unresolvable")));
String systemPropertiesModeName = element.getAttribute(SYSTEM_PROPERTIES_MODE_ATTRIBUTE);
if (StringUtils.hasLength(systemPropertiesModeName) &&
!systemPropertiesModeName.equals(SYSTEM_PROPERTIES_MODE_DEFAULT)) {
builder.addPropertyValue("systemPropertiesModeName", "SYSTEM_PROPERTIES_MODE_" + systemPropertiesModeName);
}
if (element.hasAttribute("value-separator")) {
builder.addPropertyValue("valueSeparator", element.getAttribute("value-separator"));
}
if (element.hasAttribute("trim-values")) {
builder.addPropertyValue("trimValues", element.getAttribute("trim-values"));
}
if (element.hasAttribute("null-value")) {
builder.addPropertyValue("nullValue", element.getAttribute("null-value"));
}
}

org.springframework.context.config.AbstractPropertyLoadingBeanDefinitionParser#doParse

protected void doParse(Element element, ParserContext parserContext, BeanDefinitionBuilder builder) {
// 解析<context:property-placeholder>标签的各种属性
String location = element.getAttribute("location");
if (StringUtils.hasLength(location)) {
location = parserContext.getReaderContext().getEnvironment().resolvePlaceholders(location);
String[] locations = StringUtils.commaDelimitedListToStringArray(location);
builder.addPropertyValue("locations", locations);
}
String propertiesRef = element.getAttribute("properties-ref");
if (StringUtils.hasLength(propertiesRef)) {
builder.addPropertyReference("properties", propertiesRef);
}
String fileEncoding = element.getAttribute("file-encoding");
if (StringUtils.hasLength(fileEncoding)) {
builder.addPropertyValue("fileEncoding", fileEncoding);
}
String order = element.getAttribute("order");
if (StringUtils.hasLength(order)) {
builder.addPropertyValue("order", Integer.valueOf(order));
}
builder.addPropertyValue("ignoreResourceNotFound",
Boolean.valueOf(element.getAttribute("ignore-resource-not-found")));
builder.addPropertyValue("localOverride",
Boolean.valueOf(element.getAttribute("local-override")));
builder.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
}

总结一下，其实上面这么多代码调来调去，只有一个目的，就是向spring容器中注入一个BeanDefinition，这个BeanDefinition有两个最重要的属性：

BeanClass为PropertySourcesPlaceholderConfigurer。
有一个属性为location，对应properties文件的位置。

PropertySourcesPlaceholderConfigurer的调用

上面向spring容器中注入一个PropertySourcesPlaceholderConfigurer类型BeanDefinition，先来看下这个类的继承关系：

从上图的继承关系可以看出PropertySourcesPlaceholderConfigurer实现了BeanFactoryPostProcessor，所以这个类的核心方法为postProcessBeanFactory()。

public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
if (this.propertySources == null) { // null
this.propertySources = new MutablePropertySources();
if (this.environment != null) {
// environment中存储的是系统属性和环境变量
this.propertySources.addLast(
new PropertySource<Environment>(ENVIRONMENT_PROPERTIES_PROPERTY_SOURCE_NAME, this.environment) {
@Override
@Nullable
public String getProperty(String key) {
return this.source.getProperty(key);
}
}
);
}
try {
// 加载application.properties为Properties，包装为PropertySource
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);
}
}
// 处理占位符
processProperties(beanFactory, new PropertySourcesPropertyResolver(this.propertySources));
this.appliedPropertySources = this.propertySources;
}

上面的方法的前面一大截的主要作用为将系统属性、环境变量以及properties文件中的属性整合到MutablePropertySources中，这样就可以直接调用MutablePropertySources.getProperties()方法根据属性名拿到对应的属性值了。MutablePropertySources里面其实是一个Map的链表，这样就可以先遍历链表，然后再根据属性名从Map中找到对应的属性值。

protected void processProperties(ConfigurableListableBeanFactory beanFactoryToProcess,
final ConfigurablePropertyResolver propertyResolver) throws BeansException {
propertyResolver.setPlaceholderPrefix(this.placeholderPrefix); // ${
propertyResolver.setPlaceholderSuffix(this.placeholderSuffix); // }
propertyResolver.setValueSeparator(this.valueSeparator); // :
// 下面的doProcessProperties会回调这个lambda表达式
// 真正的解析逻辑在resolveRequiredPlaceholders
/**
* @see AbstractPropertyResolver#resolveRequiredPlaceholders(java.lang.String)
*/
StringValueResolver valueResolver = strVal -> {
String resolved = (this.ignoreUnresolvablePlaceholders ?
propertyResolver.resolvePlaceholders(strVal) :
propertyResolver.resolveRequiredPlaceholders(strVal));
if (this.trimValues) {
resolved = resolved.trim();
}
return (resolved.equals(this.nullValue) ? null : resolved);
};
// 这里会遍历所有的BD，挨个处理占位符
doProcessProperties(beanFactoryToProcess, valueResolver);
}

org.springframework.beans.factory.config.PlaceholderConfigurerSupport#doProcessProperties

protected void doProcessProperties(ConfigurableListableBeanFactory beanFactoryToProcess,
StringValueResolver valueResolver) {
BeanDefinitionVisitor visitor = new BeanDefinitionVisitor(valueResolver);
String[] beanNames = beanFactoryToProcess.getBeanDefinitionNames();
for (String curName : beanNames) {
// Check that we're not parsing our own bean definition,
// to avoid failing on unresolvable placeholders in properties file locations.
if (!(curName.equals(this.beanName) && beanFactoryToProcess.equals(this.beanFactory))) {
BeanDefinition bd = beanFactoryToProcess.getBeanDefinition(curName);
try {
// 遍历BD
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);
}