前言

我们在配置Spring Xml配置文件的时候，可以在文件路径字符串中加入 ${} 占位符，Spring会自动帮我们解析占位符，这么神奇的操作Spring是怎么帮我们完成的呢？这篇文章我们就来一步步揭秘。

1.示例

ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext();applicationContext.setConfigLocation("${java.version}.xml");applicationContext.refresh();String[] beanNames = applicationContext.getBeanDefinitionNames();for (String beanName : beanNames) { System.out.println(beanName);}

这段代码在我工程里是会报错的，如下：

Caused by: java.io.FileNotFoundException: class path resource [1.8.0_144.xml] cannot be opened because it does not exist at org.springframework.core.io.ClassPathResource.getInputStream(ClassPathResource.java:190) at org.springframework.beans.factory.xml.XmlBeanDefinitionReader.loadBeanDefinitions(XmlBeanDefinitionReader.java:336) ... 11 more

可以看到报错里面的文件路径变成了1.8.0_144.xml，也就是说Spring帮我们把${java.version}解析成了实际值。

2.原理

AbstractRefreshableConfigApplicationContext
我们在之前的文章里提到过这个类的resolve方法，我们再来瞧一眼：

/** * Resolve the given path, replacing placeholders with corresponding * environment property values if necessary. Applied to config locations. * @param path the original file path * @return the resolved file path * @see org.springframework.core.env.Environment#resolveRequiredPlaceholders(String) */ protected String resolvePath(String path) { //通过当前环境去 解析 必要的占位符 return getEnvironment().resolveRequiredPlaceholders(path); }

获取当前环境，这个环境在示例代码中就是 StandardEnvironment ,并且根据当前环境去解析占位符，这个占位符解析不到还会报错。

resolveRequiredPlaceHolders由StandardEnvironment的父类AbstractEnvironment实现。

AbstractEnvironment

//把propertySources放入 Resolver中private final ConfigurablePropertyResolver propertyResolver = new PropertySourcesPropertyResolver(this.propertySources);@Overridepublic String resolveRequiredPlaceholders(String text) throws IllegalArgumentException { return this.propertyResolver.resolveRequiredPlaceholders(text);}

这里的propertySources很重要了，从命名也可以看出我们解析占位符的来源就是从这个集合中来的。这个集合是在我们StandardEnvironment实例化的时候去自定义的。

StandardEnvironment

/** * Create a new {@code Environment} instance, calling back to * {@link #customizePropertySources(MutablePropertySources)} during construction to * allow subclasses to contribute or manipulate(操作) {@link PropertySource} instances as * appropriate. * @see #customizePropertySources(MutablePropertySources) */ //StandardEnvironment 实例化调用 public AbstractEnvironment() { customizePropertySources(this.propertySources); }@Overrideprotected void customizePropertySources(MutablePropertySources propertySources) { //todo Java提供了System类的静态方法getenv()和getProperty()用于返回系统相关的变量与属性， //todo getenv方法返回的变量大多于系统相关， //todo getProperty方法返回的变量大多与java程序有关。 //https:///Baronboy/p/6030443.html propertySources.addLast(new MapPropertySource(SYSTEM_PROPERTIES_PROPERTY_SOURCE_NAME, getSystemProperties())); //SystemEnvironmentPropertySource 是System.getenv() propertySources.addLast(new SystemEnvironmentPropertySource(SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME, getSystemEnvironment()));}

最重要的肯定是我们的 propertyResolver.resolveRequiredPlaceholders 方法了，propertyResolver.resolveRequiredPlaceholders其实是PropertySourcesPropertyResolver的父类AbstractPropertyResolver来实现。

AbstractPropertyResolver

//创建一个占位符的helper去解析@Overridepublic String resolveRequiredPlaceholders(String text) throws IllegalArgumentException { if (this.strictHelper == null) { //不忽略 this.strictHelper = createPlaceholderHelper(false); } return doResolvePlaceholders(text, this.strictHelper);} //私有方法 //是否忽略 无法解决的占位符private PropertyPlaceholderHelper createPlaceholderHelper(boolean ignoreUnresolvablePlaceholders) { //默认使用${ placeholderPrefix return new PropertyPlaceholderHelper(this.placeholderPrefix, this.placeholderSuffix, this.valueSeparator, ignoreUnresolvablePlaceholders);}private String doResolvePlaceholders(String text, PropertyPlaceholderHelper helper) { //PlaceholderResolver function interface //todo important 重要的是这个getPropertyAsRawString return helper.replacePlaceholders(text, this::getPropertyAsRawString); }

这里的 this::getPropertyAsRawString 很重要，利用了java8的函数式接口来实现。它的定义在AbstractPropertyResolver里

/** * Retrieve the specified property as a raw String, * i.e. without resolution of nested placeholders. * @param key the property name to resolve * @return the property value or {@code null} if none found */ @Nullable protected abstract String getPropertyAsRawString(String key);

但是我们在doResolvePlaceholders里指向的this,所以还得看PropertySourcesPropertyResolver类。

PropertySourcesPropertyResolver

//提供给函数接口 PlaceholderResolver //todo 解析 xml配置文件路径占位符的时候调用的是这个 2020-09-11 @Override @Nullable protected String getPropertyAsRawString(String key) { return getProperty(key, String.class, false); }@Nullable protected <T> T getProperty(String key, Class<T> targetValueType, boolean resolveNestedPlaceholders) { if (this.propertySources != null) { //例如遍历的是MutablePropertySources 的propertySourceList for (PropertySource<?> propertySource : this.propertySources) { if (logger.isTraceEnabled()) { logger.trace("Searching for key '" + key + "' in PropertySource '" + propertySource.getName() + "'"); } Object value = propertySource.getProperty(key); if (value != null) { //todo 解析 profile变量的时候 会去 解析 变量中的占位符 2020-09-11 //TODO 解析xml配置文件路径字符串的时候 如果占位符 变量 的值 包含占位符 在这里 不会去解析 通过Helper 去解析 PropertyPlaceholderHelper if (resolveNestedPlaceholders && value instanceof String) { value = resolveNestedPlaceholders((String) value); } logKeyFound(key, propertySource, value); //跳出for 循环 return convertValueIfNecessary(value, targetValueType); } } } if (logger.isTraceEnabled()) { logger.trace("Could not find key '" + key + "' in any property source"); } return null; }

看到没有，我们是遍历this.propertySources集合，然后根据key调用它的getProperty方法获取value。我们从上面的StandardEnvrionment中看到我们定义的是 MapPropertySource 和 SystemEnvironmentPropertySource .

MapPropertySource

//从source中取得属性@Override@Nullablepublic Object getProperty(String name) { return this.source.get(name);}

这里的source就是getSystemProperties()，也就是 AbstractEnvironment中的方法：

@Override@SuppressWarnings({"unchecked", "rawtypes"})public Map<String, Object> getSystemProperties() { try { //Hashtable return (Map) System.getProperties(); } catch (AccessControlException ex) { return (Map) new ReadOnlySystemAttributesMap() { @Override @Nullable protected String getSystemAttribute(String attributeName) { try { return System.getProperty(attributeName); } catch (AccessControlException ex) { if (logger.isInfoEnabled()) { logger.info("Caught AccessControlException when accessing system property '" + attributeName + "'; its value will be returned [null]. Reason: " + ex.getMessage()); } return null; } } }; }}

我们还忘了很重要的一步，就是PropertyPlaceholderHelper的replacePlaceholders方法。

PropertyPlaceholderHelper

//protected 范围protected String parseStringValue( String value, PlaceholderResolver placeholderResolver, Set<String> visitedPlaceholders) { StringBuilder result = new StringBuilder(value); //如果value中没有占位符前缀 那直接返回result int startIndex = value.indexOf(this.placeholderPrefix); while (startIndex != -1) { //找到占位符的最后一个索引 int endIndex = findPlaceholderEndIndex(result, startIndex); if (endIndex != -1) { String placeholder = result.substring(startIndex + this.placeholderPrefix.length(), endIndex); String originalPlaceholder = placeholder; if (!visitedPlaceholders.add(originalPlaceholder)) { throw new IllegalArgumentException( "Circular placeholder reference '" + originalPlaceholder + "' in property definitions"); } //1. todo 2020-09-01 解析出来占位符，比如java.version //解析内嵌占位符 // Recursive invocation, parsing placeholders contained in the placeholder key. placeholder = parseStringValue(placeholder, placeholderResolver, visitedPlaceholders); // Now obtain the value for the fully resolved key... //2.todo 2020-09-01 获取实际值 String propVal = placeholderResolver.resolvePlaceholder(placeholder); if (propVal == null && this.valueSeparator != null) { int separatorIndex = placeholder.indexOf(this.valueSeparator); if (separatorIndex != -1) { String actualPlaceholder = placeholder.substring(0, separatorIndex); String defaultValue = placeholder.substring(separatorIndex + this.valueSeparator.length()); //这里就是实际获取占位符中值得地方。 propVal = placeholderResolver.resolvePlaceholder(actualPlaceholder); } } if (propVal != null) { //从占位符里获取的值也有可能包含占位符 这里可能会报 Circular placeholder reference propVal = parseStringValue(propVal, placeholderResolver, visitedPlaceholders); //替换占位符 为 实际值 result.replace(startIndex, endIndex + this.placeholderSuffix.length(), propVal); if (logger.isTraceEnabled()) { logger.trace("Resolved placeholder '" + placeholder + "'"); } startIndex = result.indexOf(this.placeholderPrefix, startIndex + propVal.length()); } //省略部分代码 } else { startIndex = -1; } } return result.toString();}

到这里我们就可以看到Spring在处理一个小小的占位符就做了这么多设计。可见这个架构是如此严谨。下篇文章我们就来探讨下Spring是如何加载这个Xml文件的。

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