Java 无法实例化 Log4j appender
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Log4j appender could not be instantiated
提问by Chaos
I'm following a tutorial on Log4j logging in web applications.
我正在学习有关 Log4j 登录 Web 应用程序的教程。
This is my log4j.propertiesfile:
这是我的log4j.properties文件：
#log to console
log4j.rootLogger=INFO, , demoappender
log4j.appender.demoappender=org.apache.log4j.ConsoleAppender
log4j.appender.demoappender=System.out
log4j.appender.demoappender.layout=org.apache.log4j.PatternLayout
log4j.appender.demoappender.layout.ConversionPattern=%d{ABSOLUTE} %5p %c:%L - %m%n
I am instantiating the logger using an init() method:
我正在使用 init() 方法实例化记录器：
private Logger logger = null;
public void init(ServletConfig config) throws ServletException {
logger = Logger.getRootLogger();
}
This is where I try to log some sample messages:
这是我尝试记录一些示例消息的地方：
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
logger.info("logging INFO message");
logger.error("logging ERROR message");
logger.warn("loggging WARNING message");
logger.fatal("logging FATAL message");
response.setContentType("text/html");
When I run the servlet on Tomcat, I get this error on the console:
当我在 Tomcat 上运行 servlet 时，我在控制台上收到此错误：
log4j:ERROR Could not find value for key log4j.appender.
log4j:ERROR Could not instantiate appender named "".
log4j:ERROR Could not instantiate class [System.out].
java.lang.ClassNotFoundException: System.out
at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1676)
at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1521)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:190)
at org.apache.log4j.helpers.Loader.loadClass(Loader.java:198)
at org.apache.log4j.helpers.OptionConverter.instantiateByClassName(OptionConverter.java:327)
at org.apache.log4j.helpers.OptionConverter.instantiateByKey(OptionConverter.java:124)
at org.apache.log4j.PropertyConfigurator.parseAppender(PropertyConfigurator.java:785)
at org.apache.log4j.PropertyConfigurator.parseCategory(PropertyConfigurator.java:768)
at org.apache.log4j.PropertyConfigurator.configureRootCategory(PropertyConfigurator.java:648)
at org.apache.log4j.PropertyConfigurator.doConfigure(PropertyConfigurator.java:514)
at org.apache.log4j.PropertyConfigurator.doConfigure(PropertyConfigurator.java:580)
at org.apache.log4j.helpers.OptionConverter.selectAndConfigure(OptionConverter.java:526)
at org.apache.log4j.LogManager.<clinit>(LogManager.java:127)
at org.apache.log4j.Logger.getRootLogger(Logger.java:135)
at Log4jDemoServlet.init(Log4jDemoServlet.java:22)
at org.apache.catalina.core.StandardWrapper.initServlet(StandardWrapper.java:1189)
at org.apache.catalina.core.StandardWrapper.loadServlet(StandardWrapper.java:1103)
at org.apache.catalina.core.StandardWrapper.allocate(StandardWrapper.java:813)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:135)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:164)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:462)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:164)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:562)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:395)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:250)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:188)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:302)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:724)
log4j:ERROR Could not instantiate appender named "demoappender".
log4j:WARN No appenders could be found for logger (root).
log4j:WARN Please initialize the log4j system properly.
Not sure how to resolve this, and why these errors are coming up, since I followed the tutorial to a tee.
不知道如何解决这个问题，以及为什么会出现这些错误，因为我按照教程进行了发球。
采纳答案by Paul Vargas
There are two errors in your file.
您的文件中有两个错误。
There is an empty appenderin
log4j.rootLogger=INFO, , demoappender. The value of this line is splitby,so there is an empty element. The first element is the level.The class of the appender must implement the interface
org.apache.log4j.AppenderandSystem.outdoes not implement that interface.
有一个空的appender在
log4j.rootLogger=INFO, , demoappender。此行的值被分割，,因此有一个空元素。第一个元素是级别。appender 的类必须实现接口
org.apache.log4j.Appender，System.out不实现该接口。
You need to make several changes to your file.
您需要对文件进行几处更改。
# Set root logger level to INFO and its only appender to demoappender
log4j.rootLogger=INFO, demoappender
# demoappender is set to be a ConsoleAppender.
log4j.appender.demoappender=org.apache.log4j.ConsoleAppender
# log4j.appender.demoappender=System.out <-- Remove this line
# demoappender uses PatternLayout.
log4j.appender.demoappender.layout=org.apache.log4j.PatternLayout
log4j.appender.demoappender.layout.ConversionPattern=%d{ABSOLUTE} %5p %c:%L - %m%n
See more in Apache log4j 1.2 - Short introduction to log4j.
在Apache log4j 1.2 - log4j 的简短介绍中查看更多信息。
