PropertyPlaceholderBeanDefinitionParser.java 文件源码

@Override
protected Class<?> getBeanClass(Element element) {
    // As of Spring 3.1, the default value of system-properties-mode has changed from
    // 'FALLBACK' to 'ENVIRONMENT'. This latter value indicates that resolution of
    // placeholders against system properties is a function of the Environment and
    // its current set of PropertySources
    if (element.getAttribute(SYSTEM_PROPERTIES_MODE_ATTRIB).equals(SYSTEM_PROPERTIES_MODE_DEFAULT)) {
        return PropertySourcesPlaceholderConfigurer.class;
    }

    // the user has explicitly specified a value for system-properties-mode. Revert
    // to PropertyPlaceholderConfigurer to ensure backward compatibility.
    return PropertyPlaceholderConfigurer.class;
}