How to use regex function on user agent string in jsp?

Question

I'm pretty new to the Java, JSP and Taglib world and one simple problem bothers me. I just want to check if the User's browser agent string contains "Opera Mini" or an old Android Version to check if it's compatible with my app.

I tested my function in Eclipse this is what it looks like:

package test.Regexp;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class UserAgentRegexTest {
    public static void main(String args[]){
        System.out.println(isCompatible());
    }

    static boolean isCompatible(){
        //String content = "Mozilla/5.0 (Linux; U; Android 4.1; en-us; GT-N7100 Build/JRO03C) AppleWebKit/534.30 (KHTML, like Gecko) Version/4.0 Mobile Safari/534.30";
        //String content = "Mozilla/5.0 (Linux; Android 6.0; Nexus 5 Build/MRA58N) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/48.0.2564.23 Mobile Safari/537.36";
        //String content = "Opera/9.80 (J2ME/MIDP; Opera Mini/5.0.16823Mod.by.Handler/22.387; U; en) Presto/2.5.25 Version/10.54";
        String content = "Mozilla/5.0 (Linux; Android 4.4.2; Nexus 4 Build/KOT49H) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/48.0.2564.23 Mobile Safari/537.36";

        String patternString = "Android (\\d.\\d)|Opera Mini";
        Pattern pattern = Pattern.compile(patternString, Pattern.CASE_INSENSITIVE);
        Matcher matcher = pattern.matcher(content);
        if(matcher.find()) {
            if(matcher.group(1) != null){
                double version = Double.parseDouble(matcher.group(1).toString());
                if(version > 4.4){
                    return true;
                }
            }   
            return false;
        }
        return true;
    }
}

Now I want to use this function embedded as a jar taglib and use it in my jsp-File. I found a lot of examples for custom tag libraries but I didn't found an example for a taglib that simply has a function that returns a boolean. Is it even possible to use a taglib jar like that? I tried to create a jar taglib and I couldn't make it work.

According to this example at the bottom of this page: https://docs.oracle.com/javaee/1.4/tutorial/doc/JSPIntro7.html#wp77078 it seems to be possible. But I guess when you create a jar file for the lib the -Tag is required. I couldn't recreate the example. These is part of my taglib decriptor within the jar:

...
<uri>http://www.imaginary.url/my_taglib_name</uri>    
<function>
      <name>hello</name>
      <function-class>my.package.Functions</function-class>
      <function-signature>java.lang.String hello(java.lang.String)</function-signature>
</function> 
...

This is the class:

package my.package;

public class Functions {
   public static String hello(String name) {
     return "Hiya, " + name + ".";
   }
}

I tried it on my local xampp tomcat. I referenced the taglib in the web.xml and and created a tld-file in the tags-directory. In the jsp I referenced it with:

<%@ taglib prefix="test" uri="http://www.imaginary.url/my_taglib_name" %>
<test:hello("bla") />

This throws the following exception: No tag "hello(" defined in tag library imported with prefix "test".

What did I miss or what did I do wrong. I think by being able to run the example I'd be able to emebd my user-agent-if-statement easily. Or is there even a cleaner/easier way?

Answer 1

I found a solution: JSP:

<%@ taglib prefix="comp" uri="http://www.myurl.com/taglibs/comp" %>
<c:out value="${header['User-Agent']}" />
    <c:set var="andver">
        <comp:check UA="${header['User-Agent']}" />   
    </c:set>
<c:out value="${andver}" />

comp.tld in my tags folder:

<?xml version="1.0" encoding="ISO-8859-1" ?>
    <taglib version="2.1" 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">
    <tlib-version>1.0</tlib-version>
    <display-name>Check my app compatibility</display-name>
    <short-name>comp</short-name>
    <uri>http://www.myurl.com/taglibs/comp</uri>   
</taglib>

comp.tld (taglib descriptor within jar):

<?xml version="1.0" encoding="UTF-8"?>
<taglib version="2.0" xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee web-jsptaglibrary_2_0.xsd">
  <description>Check compatibility</description>
  <tlib-version>1.0</tlib-version>
  <short-name>comp</short-name>
  <uri>http://www.myurl.com/taglibs/comp</uri>
  <tag>
    <description>
      Check if browser is compatible with my app
    </description>
    <name>check</name>
    <tag-class>my.package.comp.check</tag-class>
    <body-content>scriptless</body-content>
    <attribute>
        <name>UA</name>
        <required>true</required>       
        <type>java.lang.String</type>
        <rtexprvalue>true</rtexprvalue>
    </attribute>    
  </tag>
</taglib>

my java file:

package my.package.comp;

import javax.servlet.http.*;
import javax.servlet.jsp.*;
import javax.servlet.jsp.tagext.*;
import java.util.*;
import java.util.regex.*; 

public class check extends TagSupport
{
    private String content;
    public void setUA(String UA) {
        this.content = UA;
    }

    public int doStartTag() throws JspException
    {
        try
        {
            JspWriter out = pageContext.getOut();
            HttpServletResponse response = (HttpServletResponse)pageContext.getResponse();
            out.print(isCompatible());  
            //out.write(UA);
        }
        catch(Exception e)
        {   
            throw new JspException(e.getMessage());
        }
        return EVAL_PAGE;

    }

    boolean isCompatible(){     
        String patternString = "Android (\\d.\\d)|Opera Mini";
        Pattern pattern = Pattern.compile(patternString, Pattern.CASE_INSENSITIVE);
        Matcher matcher = pattern.matcher(content);
        if(matcher.find()) {
            if(matcher.group(1) != null){
                double version = Double.parseDouble(matcher.group(1).toString());
                if(version > 4.4){
                    return true;
                }
            }   
            return false;
        }
        return true;
    }

}

web.xml reference:

<taglib> 
    <taglib-uri>/tags/comp.tld</taglib-uri> 
    <taglib-location>http://www.myurl.com/taglibs/comp</taglib-location> 
</taglib>

Well - maybe it's not the perfect solution but at least something that works for the start. :D

Answer 2

You don't need taglibs for this if you don't want to. The below is perhaps a bit ugly but it's the fastest way to do what you want, and elegant IMHO:

<% if (test.Regexp.UserAgentRegexTest.isCompatible(request)) { %>
  ... rest of your JSP in case of true ...
<% } else { %>
  ... rest of your JSP in case of false ...
<% } %>

Notice you need to pass request to your isCompatible method so inside your method you can fetch the actual User-Agent header. You'll also need to make the method public. For instance:

public static boolean isCompatible(HttpServletRequest request) {
        String content = request.getHeader("User-Agent");
        ...