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ConstantsTag


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Useless Tag Library?

Inspired by this post, I wrote a tag library to expose the fields of my Contants.java class to my JSPs. It allows a user to specify a single variable using:

<appfuse:constants var="USER_KEY"/>

Or all variables:

<appfuse:constants/>

By default, it exposes the values from the Contants class imported into the tag library, but also allows a className variable to specify a different class. The main reason I wrote this was to prove it was possible. The second reason was to get around importing my Contants class (and using <%=Contants.VARNAME%>). With this tag, both of the following work:

Scriptlet: <%=VARNAME%>, JSTL: <c:out value="${VARNAME}"/>

The only thing it gives you over the import/scriptlet way is to expose the attribute to different scopes - i.e. page for JSTL. After writing it, I think it's a fairly useless tag library - I don't know if I'll ever use it, but maybe someone else will find it useful.

BTW, you need to use it on the JSP you plan to use it in a scriptlet and you can't wrap it with oscache's <cache> tags.


package org.appfuse.webapp.taglib;

import java.lang.reflect.AccessibleObject;
import java.lang.reflect.Field;

import javax.servlet.jsp.JspException;
import javax.servlet.jsp.PageContext;
import javax.servlet.jsp.tagext.TagSupport;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.apache.struts.taglib.TagUtils;
import org.appfuse.common.Constants;


/**
 <p>This class is designed to put all the public variables in a class to a
 * specified scope - designed for exposing a Constants class to Tag
 * Libraries.</p>
 *
 <p>It is designed to be used as follows:
 <pre>&lt;tag:constants /&gt;</pre>
 </p>
 *
 <p>Optional values are "className" (fully qualified) and "scope".</p>
 *
 <p>
 * <a href="BaseAction.java.html"><i>View Source</i></a>
 </p>
 *
 @author <a href="mailto:[email protected]">Matt Raible</a>
 @version $Revision: 1.1 $ $Date: 2003/12/31 13:59:32 $
 *
 * @jsp.tag name="constants" bodycontent="empty"
 *  tei-class="us.co.adams.apptracker.webapp.taglib.ConstantsTei"
 */
public class ConstantsTag extends TagSupport {
    private Log log = LogFactory.getLog(ConstantsTag.class);

    /**
     * The class to expose the variables from.
     */
    public String clazz = Constants.class.getName();

    /**
     * The scope to be put the variable in.
     */
    protected String scope = null;

    /**
     * The single variable to expose.
     */
    protected String var = null;

    public int doStartTag() throws JspException {
        // Using reflection, get the available field names in the class
        Class c = null;
        int toScope = PageContext.PAGE_SCOPE;

        if (scope != null) {
            // default to pageScope
            toScope = TagUtils.getInstance().getScope(scope);
        }

        try {
            c = Class.forName(clazz);
        catch (ClassNotFoundException cnf) {
            log.error("ClassNotFound - maybe a typo?");
            throw new JspException(cnf.getMessage());
        }

        try {
            // if var is null, expose all variables
            if (var == null) {
                Field[] fields = c.getDeclaredFields();

                AccessibleObject.setAccessible(fields, true);

                for (int i = 0; i < fields.length; i++) {
                    /*if (log.isDebugEnabled()) {
                       log.debug("putting '" + fields[i].getName() + "=" +
                                 fields[i].get(this) + "' into " + scope +
                                 " scope");
                       }*/
                    pageContext.setAttribute(fields[i].getName(),
                                             fields[i].get(this), toScope);
                }
            else {
                try {
                    String value = (Stringc.getField(var).get(this);
                    pageContext.setAttribute(c.getField(var).getName(), value,
                                             toScope);
                catch (NoSuchFieldException nsf) {
                    log.error(nsf.getMessage());
                    throw new JspException(nsf);
                }
            }
        catch (IllegalAccessException iae) {
            log.error("Illegal Access Exception - maybe a classloader issue?");
            throw new JspException(iae);
        }

        // Continue processing this page
        return (SKIP_BODY);
    }

    /**
     * @jsp.attribute
     */
    public void setClassName(String clazz) {
        this.clazz = clazz;
    }

    public String getClassName() {
        return this.clazz;
    }

    /**
     * @jsp.attribute
     */
    public void setScope(String scope) {
        this.scope = scope;
    }

    public String getScope() {
        return (this.scope);
    }

    /**
     * @jsp.attribute
     */
    public void setVar(String var) {
        this.var = var;
    }

    public String getVar() {
        return (this.var);
    }

    /**
     * Release all allocated resources.
     */
    public void release() {
        super.release();
        clazz = null;
        scope = Constants.class.getName();
    }
}


package org.appfuse.webapp.taglib;

import java.lang.reflect.AccessibleObject;
import java.lang.reflect.Field;

import java.util.ArrayList;
import java.util.List;

import javax.servlet.jsp.tagext.TagData;
import javax.servlet.jsp.tagext.TagExtraInfo;
import javax.servlet.jsp.tagext.VariableInfo;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.appfuse.common.Constants;


/**
 * Implementation of <code>TagExtraInfo</code> for the <b>constants</b>
 * tag, identifying the scripting object(s) to be made visible.
 *
 @author Matt Raible
 @version $Revision: 1.1 $ $Date: 2003/12/31 13:59:32 $
 */
public class ConstantsTei extends TagExtraInfo {
    private Log log = LogFactory.getLog(ConstantsTei.class);

    /**
     * Return information about the scripting variables to be created.
     */
    public VariableInfo[] getVariableInfo(TagData data) {
        // loop through and expose all attributes
        List vars = new ArrayList();

        try {
            String clazz = data.getAttributeString("className");

            if (clazz == null) {
                clazz = Constants.class.getName();
            }

            Class c = Class.forName(clazz);

            // if no var specified, get all
            if (data.getAttributeString("var"== null) {
                Field[] fields = c.getDeclaredFields();

                AccessibleObject.setAccessible(fields, true);

                for (int i = 0; i < fields.length; i++) {
                    vars.add(new VariableInfo(fields[i].getName(),
                                              "java.lang.String", true,
                                              VariableInfo.AT_END));
                }
            else {
                String var = data.getAttributeString("var");
                vars.add(new VariableInfo(c.getField(var).getName(),
                                          "java.lang.String", true,
                                          VariableInfo.AT_END));
            }
        catch (Exception cnf) {
            log.error(cnf.getMessage());
            cnf.printStackTrace();
        }

        return (VariableInfo[]) vars.toArray(new VariableInfo[] {  });
    }
}



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This particular version was published on 06-Nov-2006 13:52:28 MST by MattRaible.