JSP：带有 java.lang.Byte[] 的 jstl 核心条件

Question

我的 Spring MVC 应用程序的 JSP 中有这段代码，部署在 WebLogic Server 版本：12.1.2.0.0 中。我正在使用 jstl-1.2

2-> [${item.companyLogo}]
3-> [${empty item.companyLogo}]
4-> [${item.companyLogo eq null}]
<c:choose>
<c:when test="${empty item.companyLogo} ">
IS EMPTY
<img src="${contextPath}/images/devices-logo.gif"  />
</c:when>
<c:otherwise>
NOT EMPTY
</c:otherwise>
</c:choose>

也试过

<c:when test="${empty item.companyLogo == true} ">

但在屏幕上我看到了这些：

2-> [] 
3-> [true] 
4-> [true] 

NOT EMPTY  

如果我尝试使用 ${item.companyLogo.length} 检查长度 我收到了这个错误：

]] Root cause of ServletException.
java.lang.NumberFormatException: For input string: "length"
        at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
        at java.lang.Integer.parseInt(Integer.java:492)
        at java.lang.Integer.parseInt(Integer.java:527)
        at javax.el.ArrayELResolver.toInteger(ArrayELResolver.java:378)
        at javax.el.ArrayELResolver.getValue(ArrayELResolver.java:198)
        at javax.el.CompositeELResolver.getValue(CompositeELResolver.java:188)
        at com.sun.el.parser.AstValue.getValue(AstValue.java:138)
        at com.sun.el.parser.AstValue.getValue(AstValue.java:183)
        at com.sun.el.ValueExpressionImpl.getValue(ValueExpressionImpl.java:224)
        at weblogic.servlet.jsp.ELHelper.evaluate(ELHelper.java:32)
        at jsp_servlet._web_45_inf._jsp._result.__productcategoryresult._jspService(__productcategoryresult.java:330)
        at weblogic.servlet.jsp.JspBase.service(JspBase.java:34)
        at weblogic.servlet.internal.StubSecurityHelper$ServletServiceAction.run(StubSecurityHelper.java:280)
        at weblogic.servlet.internal.StubSecurityHelper$ServletServiceAction.run(StubSecurityHelper.java:254)
        at weblogic.servlet.internal.StubSecurityHelper.invokeServlet(StubSecurityHelper.java:136)
        at weblogic.servlet.internal.ServletStubImpl.execute(ServletStubImpl.java:341)
        at weblogic.servlet.internal.ServletStubImpl.onAddToMapException(ServletStubImpl.java:478)
        at weblogic.servlet.internal.ServletStubImpl.execute(ServletStubImpl.java:367)
        at weblogic.servlet.internal.ServletStubImpl.execute(ServletStubImpl.java:238)
        at weblogic.servlet.internal.RequestDispatcherImpl.invokeServlet(RequestDispatcherImpl.java:573)
        at weblogic.servlet.internal.RequestDispatcherImpl.include(RequestDispatcherImpl.java:480)
        at org.apache.taglibs.standard.tag.common.core.ImportSupport.acquireString(ImportSupport.java:347)
        at org.apache.taglibs.standard.tag.common.core.ImportSupport.doEndTag(ImportSupport.java:204)
        at jsp_servlet._web_45_inf._jsp._template.__template._jsp__tag10(__template.java:659)

这是companyLogo属性的类型

Byte[] companyLogo;

Answer 1

改用JSTL函数length：

• 在页面的开头也导入函数 taglib：

  <%@ taglib uri="http://java.sun.com/jsp/jstl/functions" prefix="fn" %>
  

• 然后得到长度：

  ${fn:length(item.companyLogo)}