看到canal源碼的時(shí)候發(fā)現(xiàn)了好多這樣的寫法

<bean id="eventParser" class="com.alibaba.otter.canal.parse.inbound.mysql.MysqlEventParser">
        <property name="destination" value="${canal.instance.destination}" />
        <property name="slaveId" value="${canal.instance.mysql.slaveId:1234}" />
        <!-- 心跳配置 -->
        <property name="detectingEnable" value="${canal.instance.detecting.enable:false}" />
        <property name="detectingSQL" value="${canal.instance.detecting.sql}" />
        <property name="detectingIntervalInSeconds" value="${canal.instance.detecting.interval.time:5}" />
        <property name="haController">
            <bean class="com.alibaba.otter.canal.parse.ha.HeartBeatHAController">
                <property name="detectingRetryTimes" value="${canal.instance.detecting.retry.threshold:3}" />
                <property name="switchEnable" value="${canal.instance.detecting.heartbeatHaEnable:false}" />
            </bean>
        </property>

property的屬性${canal.instance.mysql.slaveId:1234} 取配置文件key的時(shí)候帶了:后面跟了一個(gè)值 后來才發(fā)現(xiàn)這是對(duì)spring placeholder 設(shè)置默認(rèn)值的一種擴(kuò)展。

/**
* 擴(kuò)展Spring的
* {@linkplain org.springframework.beans.factory.config.PropertyPlaceholderConfigurer}
* ，增加默認(rèn)值的功能。 例如：${placeholder:defaultValue}，假如placeholder的值不存在，則默認(rèn)取得
* defaultValue。
* @author jianghang 2013-1-24 下午03:37:56
* @version 1.0.0
* */
看一下類圖

PropertyPlaceholderConfigurer類圖

這里只要實(shí)現(xiàn)了InitializingBean的afterPropertiesSet方法 和 ResourceLoaderAware 的setResourceLoader方法將資源的路徑解析出來，然后加載資源 。重寫了 PropertyPlaceholderConfigurer 的resolvePlaceholder 方法，先解析成DefaultablePlaceholder，然后發(fā)現(xiàn)取不到一個(gè)屬性的時(shí)候?qū)⒛J(rèn)值賦給該屬性，其中默認(rèn)值是冒號(hào)后面的值。DefaultablePlaceholder 是新定義的內(nèi)部類。
貼出主要實(shí)現(xiàn)的代碼

@Override
    protected String resolvePlaceholder(String placeholder, Properties props, int systemPropertiesMode) {
        DefaultablePlaceholder dp = new DefaultablePlaceholder(placeholder);
        String value = super.resolvePlaceholder(dp.placeholder, props, systemPropertiesMode);

        if (value == null) {
            value = dp.defaultValue;
        }

        return trimToEmpty(value);
    }
 private static class DefaultablePlaceholder {

        private final String defaultValue;
        private final String placeholder;

        public DefaultablePlaceholder(String placeholder){
            int commaIndex = placeholder.indexOf(":");
            String defaultValue = null;

            if (commaIndex >= 0) {
                defaultValue = trimToEmpty(placeholder.substring(commaIndex + 1));
                placeholder = trimToEmpty(placeholder.substring(0, commaIndex));
            }

            this.placeholder = placeholder;
            this.defaultValue = defaultValue;
        }
    }