Question

本书中有一个例子，它讨论了简单标签来创建动态html选择标签。如需参考，请参阅页码：542。

我收到以下异常。

org.apache.jasper.JasperException: java.lang.NullPointerException

我不知道如何将optionList属性值设置/传递给标记处理程序的setter方法。我认为这就是我得到的原因NullPointerException. tld没有问题。

选择-beer.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ taglib prefix="my" uri="http://example.com/tags/forms" %>
<!DOCTYPE html>
<html>
<head>
<title>Select Beer</title>
</head>
<body>
<form action="select.do" method="POST">
<p>Select Beer Characteristics: </p>
Color:
<my:select optionList="${applicationScope.colorList}" name="color" size="1"/>
<br><br>
<input type="submit" value="Submit"/>
</form>
</body>
</html>

Taghandler代码：

public class SelectTagHandler extends SimpleTagSupport {
private String name;
private String size;
private String[] optionList;
private static final String ATTR_TEMPLATE = "%s='%s'";
private static final String OPTION_TEMPLATE = "<option value='1$s'>%1$s</option>";

public void setName(String name) {
    this.name = name;
}

public void setSize(String size) {
    this.size = size;
}

public void setOptionList(String[] optionList) {
   this.optionList = optionList;
}

@Override
public void doTag() throws JspException, IOException {
JspWriter out = getJspContext().getOut();
out.print("<select");
out.print(String.format(ATTR_TEMPLATE, "name", this.name));
out.print(String.format(ATTR_TEMPLATE, "size", this.size));
out.println('>');

for (Object option : this.optionList) {
String optionTag = String.format(OPTION_TEMPLATE, option.toString());
out.println(optionTag);
}
}
}

堆栈跟踪

18:38:34,039 ERROR [org.apache.catalina.core.ContainerBase.[jboss.web].[default-host].[/jee_war_exploded].[jsp]] (http--127.0.0.1-8000-1) Servlet.service() for servlet jsp threw exception: java.lang.NullPointerException
    at com.jstl.select.SelectTagHandler.doTag(SelectTagHandler.java:40) [classes:]
    at org.apache.jsp.select_002dbeer_jsp._jspx_meth_my_005fselect_005f0(select_002dbeer_jsp.java:101)
    at org.apache.jsp.select_002dbeer_jsp._jspService(select_002dbeer_jsp.java:68)
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70) [jbossweb-7.0.13.Final.jar:]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:847) [jboss-servlet-api_3.0_spec-1.0.0.Final.jar:1.0.0.Final]
    at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:369) [jbossweb-7.0.13.Final.jar:]
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:326) [jbossweb-7.0.13.Final.jar:]
    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:253) [jbossweb-7.0.13.Final.jar:]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:847) [jboss-servlet-api_3.0_spec-1.0.0.Final.jar:1.0.0.Final]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:329) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:248) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:275) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:161) [jbossweb-7.0.13.Final.jar:]
    at org.jboss.as.web.security.SecurityContextAssociationValve.invoke(SecurityContextAssociationValve.java:153) [jboss-as-web-7.1.1.Final.jar:7.1.1.Final]
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:155) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:368) [jbossweb-7.0.13.Final.jar:]
    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:877) [jbossweb-7.0.13.Final.jar:]
    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:671) [jbossweb-7.0.13.Final.jar:]
    at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:930) [jbossweb-7.0.13.Final.jar:]
    at java.lang.Thread.run(Thread.java:724) [rt.jar:1.7.0_25]

Answer 1

您的标记处理程序遍历optionList数组，默认情况下为null。因此，如果不将此数组设置为非空值，则标记将失败。

您正尝试使用

设置此值

<my:select optionList="${applicationScope.colorList}" name="color" size="1"/>

这将从servlet上下文中检索属性colorList并在标记处理程序中设置它。

为此，您需要在servlet上下文中拥有类型为colorList的属性String[]。因此，在执行JSP之前，必须执行以下代码行：

servletContext.setAttribute("colorList", someNonNullStringArray);

应放置代码行的地方取决于您和您的要求。由于它是应用程序的全局，因此将它放在ServletContextListener中是有意义的，在部署应用程序时调用它。但是对于测试，您还可以使用在JSP之前调用的servlet。 Servlet上下文可以从servlet请求获得：

ServletContext servletContext = request.getServletContext();

如何在jsp简单标记中设置/检索applicationScope属性？

选择-beer.jsp

Taghandler代码：

堆栈跟踪

1 个答案: