jsp中javabeen中 ArrayList al = new ArrayList();出现警告
jsp页面使用时出现500
500是服务器内部错误，你有没详细报错提示
严重: Servlet.service() for servlet jsp threw exception
java.lang.NullPointerException
at shiwen.Select.selinfo(Select.java:133)
at org.apache.jsp.searchlogin_jsp._jspService(searchlogin_jsp.java:74)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:393)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:320)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:266)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:230)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:104)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:261)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:581)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Thread.java:619)
哎呀，小白飘过，看不大懂啊
单单你写的那行没有问题。
问题可能是你类型转换出错，类型不符，或者有变量为null。
是空指针的问题，跟list没关系，debug 一下就清楚
ava.lang.NullPointerException空指针异常,你看看这个空指针是你代码 什么地方产生的找出原因修改就可以了
空指针异常debug调试一下
List al=new ArrayList()
;
空指针异常
ArrayList al = new ArrayList();
是不是下边没有给赋值，然后就是用了ArrayList中的属性或者方法
代码本身无错，检查你的JSP页面头部是否 引入了 <%@ page import="java.util.ArrayList" %>
空指针异常，最常见的了，检查下值传进去没有