在項目中常常看到這樣的${}標識方法,如:java
<!-- dataSource 配置 --> <bean id="dataSource" class="com.alibaba.druid.pool.DruidDataSource" init-method="init" lazy-init="true" destroy-method="close"> <!-- 基本屬性 url、user、password --> <property name="url" value="${jdbc.url}"/> <property name="username" value="${jdbc.username}"/> <property name="password" value="${jdbc.password}"/> <!--數據庫加密--> <property name="passwordCallback" ref="dbPasswordCallback"/> <property name="connectionProperties" value="password=${jdbc.password}" /> <!-- 配置初始化大小、最小、最大 --> <property name="initialSize" value="${ds.initialSize}"/> <property name="minIdle" value="${ds.minIdle}"/> <property name="maxActive" value="${ds.maxActive}"/> <!-- 配置獲取鏈接等待超時的時間 --> <property name="maxWait" value="${ds.maxWait}"/> <!-- 配置間隔多久才進行一次檢測,檢測須要關閉的空閒鏈接,單位是毫秒 --> <property name="timeBetweenEvictionRunsMillis" value="${ds.timeBetweenEvictionRunsMillis}"/> <!-- 配置一個鏈接在池中最小生存的時間,單位是毫秒 --> <property name="minEvictableIdleTimeMillis" value="${ds.minEvictableIdleTimeMillis}"/> </bean>
看到了${}這樣的表達式,腦海裏面第一印象咱們jsp中的EL表達式,spring
不過,這個並非EL表達式,Spring提供了本身的EL表達式,但是它的格式是 #{},數據庫
${key}這樣的表達式在XML中,表明引入的properties文件中key對應的一個變量值,徹底是靠第三方jar本身解析的。如:app
在ibatis中${key}的解析(代碼參考:com.ibatis.common.xml.NodeletUtils.java)以下:jsp
public static String parsePropertyTokens(String string, Properties variables) { final String OPEN = "${"; final String CLOSE = "}"; String newString = string; if (newString != null && variables != null) { int start = newString.indexOf(OPEN); int end = newString.indexOf(CLOSE); while (start > -1 && end > start) { String prepend = newString.substring(0, start); String append = newString.substring(end + CLOSE.length()); String propName = newString.substring(start + OPEN.length(), end); String propValue = variables.getProperty(propName); if (propValue == null) { newString = prepend + propName + append; } else { newString = prepend + propValue + append; } start = newString.indexOf(OPEN); end = newString.indexOf(CLOSE); } } return newString; }
在spring中${key}的解析以下:ui
/** * Resolve placeholder values in the given array of patterns. * @return a new array with updated patterns */ protected String[] resolveEmbeddedValuesInPatterns(String[] patterns) { if (this.embeddedValueResolver == null) { return patterns; } else { String[] resolvedPatterns = new String[patterns.length]; for (int i=0; i < patterns.length; i++) { resolvedPatterns[i] = this.embeddedValueResolver.resolveStringValue(patterns[i]); } return resolvedPatterns; } }
Spring的代碼有點很差理解,有興趣的話能夠研究PropertyPlaceholderConfigurer(並且若是它未找到${xxx}中定義的xxx鍵,它還會去JVM系統屬性(System.getProperty())和環境變量(System.getenv())中尋找)this