javax.el.E​​LException：无法将 java.util.ArrayList 类型的 [light, amber, brown, dark] 转换为 [Ljava.lang.String;

【问题标题】：javax.el.ELException: Cannot convert [light, amber, brown, dark] of type class java.util.ArrayList to class [Ljava.lang.String;javax.el.E​​LException：无法将 java.util.ArrayList 类型的 [light, amber, brown, dark] 转换为 [Ljava.lang.String;
【发布时间】：2014-02-28 15:28:51
【问题描述】：

我正在练习 Head First Servlets 和 JSP 书中的示例。

我正在尝试创建一个类似于html select 标记的简单标记。我根据书中给出的示例创建了 tld、标记处理程序和其余的 servlet 以及 JSP。如需参考，请参阅第 542 页。

但是，我得到了上述异常。任何帮助将不胜感激。

顶级域名：

<?xml version="1.0" encoding="ISO-8859-1" ?>
<!DOCTYPE taglib
        PUBLIC "-//Sun Microsystems, Inc.//DTD JSP Tag Library 1.2//EN"
        "http://java.sun.com/j2ee/dtds/web-jsptaglibrary_1_2.dtd">
<taglib>
    <tlib-version>1.2</tlib-version>
    <jsp-version>1.2</jsp-version>
    <short-name>Forms Taglib</short-name>
    <uri>http://example.com/tags/forms</uri>
    <description>
        An example tab library of replacements for the HTML form tags.
    </description>
    <tag>
        <name>select</name>
        <tag-class>com.jstl.select.SelectTagHandler</tag-class>
        <body-content>empty</body-content>
        <attribute>
            <name>optionList</name>
            <required>true</required>
            <rtexprvalue>true</rtexprvalue>
            <type>java.util.List</type>
        </attribute>
        <attribute>
            <name>name</name>
            <required>true</required>
        </attribute>
        <attribute>
            <name>size</name>
            <required>true</required>
        </attribute>
    </tag>
</taglib>

ColorServlet-首先调用：

public class ColorServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        List colorList = new ArrayList();

        colorList.add("light");
        colorList.add("amber");
        colorList.add("brown");
        colorList.add("dark");

        getServletContext().setAttribute("colorList", colorList);
        RequestDispatcher view = request.getRequestDispatcher("select-beer.jsp");
        view.forward(request, response);

    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request, response);
    }
}

jsp(select-beer.jsp);

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ taglib prefix="my" uri="http://example.com/tags/forms" %>

<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>

TagHandler 类：

public class SelectTagHandler extends SimpleTagSupport {
    private String name;
    private String size;
    private List 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(List 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

20:41:53,383 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: javax.el.ELException: Cannot convert [light, amber, brown, dark] of type class java.util.ArrayList to class [Ljava.lang.String;
    at org.apache.el.lang.ELSupport.coerceToType(ELSupport.java:420) [jbossweb-7.0.13.Final.jar:]
    at org.apache.el.ValueExpressionImpl.getValue(ValueExpressionImpl.java:191) [jbossweb-7.0.13.Final.jar:]
    at org.apache.jasper.runtime.PageContextImpl.proprietaryEvaluate(PageContextImpl.java:935) [jbossweb-7.0.13.Final.jar:]
    at org.apache.jsp.select_002dbeer_jsp._jspx_meth_my_005fselect_005f0(select_002dbeer_jsp.java:96)
    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.ApplicationDispatcher.invoke(ApplicationDispatcher.java:840) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:622) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:560) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:488) [jbossweb-7.0.13.Final.jar:]
    at com.jstl.select.ColorServlet.doPost(ColorServlet.java:26) [classes:]
    at com.jstl.select.ColorServlet.doGet(ColorServlet.java:31) [classes:]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:734) [jboss-servlet-api_3.0_spec-1.0.0.Final.jar:1.0.0.Final]
    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]

20:41:53,391 ERROR [org.apache.catalina.core.ContainerBase.[jboss.web].[default-host].[/jee_war_exploded].[ColorServlet]] (http--127.0.0.1-8000-1) Servlet.service() for servlet ColorServlet threw exception: javax.el.ELException: Cannot convert [light, amber, brown, dark] of type class java.util.ArrayList to class [Ljava.lang.String;
    at org.apache.el.lang.ELSupport.coerceToType(ELSupport.java:420) [jbossweb-7.0.13.Final.jar:]
    at org.apache.el.ValueExpressionImpl.getValue(ValueExpressionImpl.java:191) [jbossweb-7.0.13.Final.jar:]
    at org.apache.jasper.runtime.PageContextImpl.proprietaryEvaluate(PageContextImpl.java:935) [jbossweb-7.0.13.Final.jar:]
    at org.apache.jsp.select_002dbeer_jsp._jspx_meth_my_005fselect_005f0(select_002dbeer_jsp.java:96)
    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.ApplicationDispatcher.invoke(ApplicationDispatcher.java:840) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:622) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:560) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:488) [jbossweb-7.0.13.Final.jar:]
    at com.jstl.select.ColorServlet.doPost(ColorServlet.java:26) [classes:]
    at com.jstl.select.ColorServlet.doGet(ColorServlet.java:31) [classes:]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:734) [jboss-servlet-api_3.0_spec-1.0.0.Final.jar:1.0.0.Final]
    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]

【问题讨论】：

    标签： jsp servlets el


    【解决方案1】：

    问题在于您尝试将ArrayList 转换为“字符串数组”（JVM 使用符号[Ljava.lang.String 作为“字符串数组”的类名）。

    javax.el.ELException: Cannot convert [light, amber, brown, dark] of type class java.util.ArrayList to class [Ljava.lang.String;

    在评估语句时尝试使用 # 而不是 $。类型也可以更改，它应该是String[] 而不是ArrayList。 

    String[] color = {"light", "amber", "brown", "dark"};

    如果你必须使用 ArrayLists 试试这个：

    getServletContext().setAttribute("colorList", colorList.toArray(new String[colorList.size()]));

    【讨论】：

    • 用#代替$是什么意思？是为了EL表达式吧？
    • 我希望将其设置为 ArrayList。为什么只有 String[]？
    • 使用 # 表示 EL 表达式
    • 对不起。我没听懂你。
    • 使用 #{} 而不是 ${}。虽然我必须承认我无法回答有什么区别，但对我来说它们只是“旧式 EL”和“新式 EL”，功能应该相同。
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