Example - Calculating Prime Numbers

JSP Tutorials - Herong's Tutorial Examples

∟Example - Calculating Prime Numbers

This section provides a tutorial example to calculate prime numbers using a JSTL Core library c:forEach tag. But example works. But it is not perfect, because it has no break statement.

As my second JSTL core example, JstlPrimeNumbers.jsp, is to calculate if a given number is prime number or not.

<?xml version="1.0"?>
<jsp:root xmlns:jsp="http://java.sun.com/JSP/Page"
   xmlns:c="http://java.sun.com/jsp/jstl/core" version="2.1">
<!-- JstlPrimeNumbers.jspx - It doesn't work, can not change the index
 - Copyright (c) 2006 HerongYang.com. All Rights Reserved.
-->
<jsp:directive.page contentType="text/html"/>
<html><body>
<p>Checking prime numbers:</p>
<c:set var="upperLimit" value="${20}"/>
<c:forEach var="i" begin="${3}" end="${upperLimit}">
 <c:set var="isPrime" value="${true}"/>
 <c:forEach var="j" begin="${2}" end="${i-1}">
  <c:if test="${i%j == 0}">
   <c:set var="isPrime" value="${false}"/>
   <!-- We should break the loop here -->
  </c:if>
 </c:forEach>
 <c:choose>
  <c:when test="${isPrime}">
   <c:out value="${i} is a prime number."/><br/>
  </c:when>
  <c:otherwise>
   <c:out value="${i} is a not prime number."/><br/>
  </c:otherwise>
 </c:choose>
</c:forEach>
</body></html>
</jsp:root>

I am not showing you the output here. But the output is correct. Trust me. As you can see in the source code, I am not breaking the loop on "j" when "i" has already been approved as a non-prime number, because I don't know how.

One of the emails I received after publishing the above notes suggests me to insert a Java break statement through a scriptlet to break the "forEach" tag. To try this idea, I revised my JSP page to JstlPrimeNumbersRevised.jsp:

<?xml version="1.0"?>
<jsp:root xmlns:jsp="http://java.sun.com/JSP/Page"
   xmlns:c="http://java.sun.com/jsp/jstl/core" version="2.1">
<!-- JstlPrimeNumbersRevised.jsp - Compilation error
 - Copyright (c) 2006 HerongYang.com. All Rights Reserved.
-->
<jsp:directive.page contentType="text/html"/>
<html><body>
<p>Checking prime numbers:</p>
<c:set var="upperLimit" value="${20}"/>
<c:forEach var="i" begin="${3}" end="${upperLimit}">
 <c:set var="isPrime" value="${true}"/>
 <c:forEach var="j" begin="${2}" end="${i-1}">
  <c:if test="${i%j == 0}">
   <c:set var="isPrime" value="${false}"/>
   <jsp:scriptlet>break;</jsp:scriptlet>
  </c:if>
 </c:forEach>
 <c:choose>
  <c:when test="${isPrime}">
   <c:out value="${i} is a prime number."/><br/>
  </c:when>
  <c:otherwise>
   <c:out value="${i} is a not prime number."/><br/>
  </c:otherwise>
 </c:choose>
</c:forEach>
</body></html>
</jsp:root>

If the scriptlet statement can merge with the JSTL tags nicely, the logic will work. However if you view this page on Tomcat, you will get a compilation error saying that there is an unreachable statement:

HTTP Status 500 - Unable to compile class for JSP:

type Exception report

message Unable to compile class for JSP:

description The server encountered an internal error
   that prevented it from fulfilling this request.

exception
org.apache.jasper.JasperException: Unable to compile class for JSP:

An error occurred at line: 8 in the jsp file:
/JstlPrimeNumbersRevised.jspx
Unreachable code

5:  - Copyright () 2012, HerongYang.com, All Rights Reserved.
6: -->
7: <jsp:directive.page contentType="text/html"/>
8: <html><body>
9: <p>Checking prime numbers:</p>
10: <c:set var="upperLimit" value="${20}"/>
11: <c:forEach var="i" begin="${3}" end="${upperLimit}">

Stacktrace:
   org.apache.jasper.compiler.DefaultErrorHandler.javacError()
   org.apache.jasper.compiler.ErrorDispatcher.javacError()
   org.apache.jasper.compiler.JDTCompiler.generateClass()
   org.apache.jasper.compiler.Compiler.compile()
   org.apache.jasper.compiler.Compiler.compile()
   org.apache.jasper.compiler.Compiler.compile()
   org.apache.jasper.JspCompilationContext.compile()
   org.apache.jasper.servlet.JspServletWrapper.service()
   org.apache.jasper.servlet.JspServlet.serviceJspFile()
   org.apache.jasper.servlet.JspServlet.service()
   javax.servlet.http.HttpServlet.service()
...

If you open the converted Java class file of the JSP page, JstlPrimeNumbersRevised_jspx.java, you will see:

/*
 * Generated by the Jasper component of Apache Tomcat
 * Version: Apache Tomcat/9.0.12
   ...
 */
...
//  c:forEach
org.apache.taglibs.standard.tag.rt.core.ForEachTag _jspx_th_c_005ff...
_jspx_th_c_005fforEach_005f0.setPageContext(_jspx_page_context);
_jspx_th_c_005fforEach_005f0.setParent(null);
// /JstlPrimeNumbersRevised.jspx(11,53) name = end type = int reqTi...
_jspx_th_c_005fforEach_005f0.setEnd(((java.lang.Integer) org.apache...
// /JstlPrimeNumbersRevised.jspx(11,53) name = begin type = int req...
_jspx_th_c_005fforEach_005f0.setBegin(((java.lang.Integer) org.apac...
// /JstlPrimeNumbersRevised.jspx(11,53) name = var type = java.lang...
_jspx_th_c_005fforEach_005f0.setVar("i");
int[] _jspx_push_body_count_c_005fforEach_005f0 = new int[] { 0 };
try {
  int _jspx_eval_c_005fforEach_005f0 = _jspx_th_c_005fforEach_005f0...
  if (_jspx_eval_c_005fforEach_005f0 != javax.servlet.jsp.tagext.Ta...
    do {
      if (_jspx_meth_c_005fset_005f1(_jspx_th_c_005fforEach_005f0, ...
        return;
      //  c:forEach
      org.apache.taglibs.standard.tag.rt.core.ForEachTag _jspx_th_c...
      _jspx_th_c_005fforEach_005f1.setPageContext(_jspx_page_contex...
      _jspx_th_c_005fforEach_005f1.setParent((javax.servlet.jsp.tag...
      // /JstlPrimeNumbersRevised.jspx(13,47) name = end type = int...
      _jspx_th_c_005fforEach_005f1.setEnd(((java.lang.Integer) org....
      // /JstlPrimeNumbersRevised.jspx(13,47) name = begin type = i...
      _jspx_th_c_005fforEach_005f1.setBegin(((java.lang.Integer) or...
      // /JstlPrimeNumbersRevised.jspx(13,47) name = var type = jav...
      _jspx_th_c_005fforEach_005f1.setVar("j");
      int[] _jspx_push_body_count_c_005fforEach_005f1 = new int[] {...
      try {
        int _jspx_eval_c_005fforEach_005f1 = _jspx_th_c_005fforEach...
        if (_jspx_eval_c_005fforEach_005f1 != javax.servlet.jsp.tag...
          do {
            //  c:if
            org.apache.taglibs.standard.tag.rt.core.IfTag _jspx_th_...
            _jspx_th_c_005fif_005f0.setPageContext(_jspx_page_conte...
            _jspx_th_c_005fif_005f0.setParent((javax.servlet.jsp.ta...
            // /JstlPrimeNumbersRevised.jspx(14,28) name = test typ...
            _jspx_th_c_005fif_005f0.setTest(((java.lang.Boolean) or...
            int _jspx_eval_c_005fif_005f0 = _jspx_th_c_005fif_005f0...
            if (_jspx_eval_c_005fif_005f0 != javax.servlet.jsp.tage...
              do {
                if (_jspx_meth_c_005fset_005f2(_jspx_th_c_005fif_00...
                  return;
break;
                int evalDoAfterBody = _jspx_th_c_005fif_005f0.doAft...
                if (evalDoAfterBody != javax.servlet.jsp.tagext.Bod...
                  break;
              } while (true);
            }
            if (_jspx_th_c_005fif_005f0.doEndTag() == javax.servlet...
              _005fjspx_005ftagPool_005fc_005fif_0026_005ftest.reus...
              return;
            }
            _005fjspx_005ftagPool_005fc_005fif_0026_005ftest.reuse(...
            int evalDoAfterBody = _jspx_th_c_005fforEach_005f1.doAf...
            if (evalDoAfterBody != javax.servlet.jsp.tagext.BodyTag...
              break;
          } while (true);
        }
        if (_jspx_th_c_005fforEach_005f1.doEndTag() == javax.servle...
          return;
        }
      } catch (java.lang.Throwable _jspx_exception) {
        while (_jspx_push_body_count_c_005fforEach_005f1[0]-- > 0)
          out = _jspx_page_context.popBody();
        _jspx_th_c_005fforEach_005f1.doCatch(_jspx_exception);
      } finally {
        _jspx_th_c_005fforEach_005f1.doFinally();
        _005fjspx_005ftagPool_005fc_005fforEach_0026_005fvar_005fen...
      }
      if (_jspx_meth_c_005fchoose_005f0(_jspx_th_c_005fforEach_005f...
        return;
      int evalDoAfterBody = _jspx_th_c_005fforEach_005f0.doAfterBod...
      if (evalDoAfterBody != javax.servlet.jsp.tagext.BodyTag.EVAL_...
        break;
    } while (true);
  }
...

The reason for the compilation error is clear. The c:forEach tag has been converted to a Java "while" loop - no surprise. However the c:if tag is also converted to a Java "while" loop - big surprise. The break statement provided by the scriptlet is placed in the middle of a while loop without if condition, and it causes the "code not reachable" compilation error.