JavaWeb学习心得（一）

标题JavaWeb学习心得（一）

迷人的500错误

Type ：异常报告

消息 ：Error instantiating servlet class [chapter4.SessionCreate]

描述 ：服务器遇到一个意外的情况，阻止它完成请求。

Exception：
javax.servlet.ServletException: Error instantiating servlet class [chapter4.SessionCreate]
org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:490)
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)
org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:678)
org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)
org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:408)
org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:834) org.apache.tomcat.util.net.NioEndpoint$ConnectionHandler.process(AbstractProtocol.java:834)org.apache.tomcat.util.net.NioEndpointSocketProcessor.doRun(NioEndpoint.java:1415)
org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) org.apache.tomcat.util.threads.TaskThread$Worker.run(ThreadPoolExecutor.java:624)org.apache.tomcat.util.threads.TaskThreadWrappingRunnable.run(TaskThread.java:61)
java.lang.Thread.run(Thread.java:748)
Root Cause

java.lang.NoSuchMethodException: chapter4.SessionCreate.()
java.lang.Class.getConstructor0(Class.java:3082)
java.lang.Class.getConstructor(Class.java:1825)
org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:490)
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)
org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:678)
org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)
org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:408)
org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:834) org.apache.tomcat.util.net.NioEndpoint$ConnectionHandler.process(AbstractProtocol.java:834)org.apache.tomcat.util.net.NioEndpointSocketProcessor.doRun(NioEndpoint.java:1415)
org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) org.apache.tomcat.util.threads.TaskThread$Worker.run(ThreadPoolExecutor.java:624)org.apache.tomcat.util.threads.TaskThreadWrappingRunnable.run(TaskThread.java:61)
java.lang.Thread.run(Thread.java:748)
Note 主要问题的全部 stack 信息可以在 server logs 里查看

今天在创建session，并验证session如何保持会话时，出现了500错误，很奇怪…

**解决方案：**代码中继承HttpServlet的类要加public 修饰符。