自定义一个标签，一直报错，求大神指导

power469593710 2016-11-03 09:55:37

报错如下：
重: Servlet.service() for servlet [jsp] in context with path [/web11] threw exception [An exception occurred processing JSP page /hello2.jsp at line 8

5:
6: </head>
7: <body style="font-size=30px;">
8: <c1:hello msg="pwoer" qty="${2+6 }" />
9:
10:
11:

Stacktrace:] with root cause
java.lang.NullPointerException
at tag.HelloTag.<init>(HelloTag.java:31)
at org.apache.jsp.hello2_jsp._jspx_meth_c1_005fhello_005f0(hello2_jsp.java:128)
at org.apache.jsp.hello2_jsp._jspService(hello2_jsp.java:95)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:439)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:395)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:339)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:218)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:505)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:169)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:956)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:442)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1083)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:640)
at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:2517)
at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2506)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Thread.java:619)

simpletagsupport中的内容如下

package tag;

import java.io.IOException;

import javax.servlet.jsp.JspException;
import javax.servlet.jsp.JspWriter;
import javax.servlet.jsp.PageContext;
import javax.servlet.jsp.tagext.SimpleTagSupport;

public class HelloTag extends SimpleTagSupport {
private String msg2;
private int qty;

public void setMsg(String msg) {
this.msg2 = msg;
}

public void setQty(int qty) {
this.qty = qty;
}

PageContext ctx=(PageContext)getJspContext();
JspWriter out=ctx.getOut();

@Override
public void doTag() throws JspException, IOException {

for(int i=0;i<qty;i++){
out.println(msg2);
}

}

}

描述文件如下：
<?xml version="1.0" encoding="UTF-8" ?>

<taglib xmlns="http://java.sun.com/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-jsptaglibrary_2_1.xsd"
version="2.1">

<tlib-version>1.2</tlib-version>
<short-name>c1</short-name>
<uri>http://192.168.30.200</uri>

<tag>

<name>hello</name>
<tag-class>tag.HelloTag</tag-class>
<body-content>empty</body-content>
<attribute>
<name>msg</name>

<required>true</required>

<rtexprvalue>false</rtexprvalue>
</attribute>
<attribute>
<name>qty</name>

<required>true</required>

<rtexprvalue>true</rtexprvalue>
</attribute>

</tag>

</taglib>

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