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老師這個在類里面是如何寫的 logger

如何實現(xiàn)的在MessageDao類里面

丶子非魚

2017-06-19

源自：通過自動回復機器人學Mybatis---基礎版 4-3

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丶子非魚提問者
2017-06-19

我的可以實現(xiàn)但是提示logger這個對象空指針 ? ?這個Logger對象如何實例化

2017-06-19?10:50:13,034[http-bio-8080-exec-3]DEBUG?[Message.selectbycommand]-==>??Preparing:?SELECT?ID,COMMAND,DESCRIPTION,CONTENT?FROM?message?where?1=1?
2017-06-19?10:50:13,036[http-bio-8080-exec-3]DEBUG?[Message.selectbycommand]-==>?Parameters:?
2017-06-19?10:50:13,043[http-bio-8080-exec-3]DEBUG?[Message.selectbycommand]-<==??????Total:?6
六月?19,?2017?10:50:13?上午?org.apache.catalina.core.StandardWrapperValve?invoke
嚴重:?Servlet.service()?for?servlet?[ListServlet]?in?context?with?path?[/Mybatis]?threw?exception
java.lang.NullPointerException
	at?Com.DAO.Service.selectbycommand(Service.java:40)
	at?Com.Servlet.ListServlet.doGet(ListServlet.java:34)
	at?Com.Servlet.ListServlet.doPost(ListServlet.java:49)
	at?javax.servlet.http.HttpServlet.service(HttpServlet.java:647)
	at?javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
	at?org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
	at?org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
	at?org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222)
	at?org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123)
	at?org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
	at?org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171)
	at?org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:99)
	at?org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:953)
	at?org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
	at?org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
	at?org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1023)
	at?org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:589)
	at?org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:312)
	at?java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
	at?java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
	at?java.lang.Thread.run(Thread.java:745)