日志组件不同时，会告警，出现如下信息。

log4j:WARN No appenders could be found for logger (com.alibaba.druid.pool.DruidDataSource).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.

建议将无logType时的加载顺序中slf4j放到第一顺序。

LogFactory的类中定义如下：

定义的logType

String logType= System.getProperty("druid.logType");
if(logType != null){
if(logType.equalsIgnoreCase("slf4j")){
tryImplementation("org.slf4j.Logger", "com.alibaba.druid.support.logging.SLF4JImpl");
}else if(logType.equalsIgnoreCase("log4j")){
tryImplementation("org.apache.log4j.Logger", "com.alibaba.druid.support.logging.Log4jImpl");
}else if(logType.equalsIgnoreCase("log4j2")){
tryImplementation("org.apache.logging.log4j.Logger", "com.alibaba.druid.support.logging.Log4j2Impl");
}else if(logType.equalsIgnoreCase("commonsLog")){
tryImplementation("org.apache.commons.logging.LogFactory",
"com.alibaba.druid.support.logging.JakartaCommonsLoggingImpl");
}else if(logType.equalsIgnoreCase("jdkLog")){
tryImplementation("java.util.logging.Logger", "com.alibaba.druid.support.logging.Jdk14LoggingImpl");
}
}

如果没定义logType
// 优先选择log4j,而非Apache Common Logging. 因为后者无法设置真实Log调用者的信息
tryImplementation("org.apache.log4j.Logger", "com.alibaba.druid.support.logging.Log4jImpl");
tryImplementation("org.apache.logging.log4j.Logger", "com.alibaba.druid.support.logging.Log4j2Impl");
tryImplementation("org.slf4j.Logger", "com.alibaba.druid.support.logging.SLF4JImpl");
tryImplementation("org.apache.commons.logging.LogFactory",
"com.alibaba.druid.support.logging.JakartaCommonsLoggingImpl");
tryImplementation("java.util.logging.Logger", "com.alibaba.druid.support.logging.Jdk14LoggingImpl");