将会话属性传递到自定义JSP标记中

Question

Is it possible to pass object from session into JSP tag attribute?

Ie I have a class storing information about menu nodes. I store its object in session under "app.menu" key.

I've created menu tag handler:

public class MenuTagHandler extends TagSupport {

    private static final long serialVersionUID = 7116890687703774365L;

    private Menu menu;
    private String id;
    private String cssClass;
    private Logger logger = Logger.getLogger(this.getClass());

    @Override
    public int doStartTag() throws JspException {
        logger.info("tagSupport doStartTag method entered");

        return SKIP_BODY;
    }

    public Menu getMenu() {
        return menu;
    }

    public void setMenu(Menu menu) {
        this.menu = menu;
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getCssClass() {
        return cssClass;
    }

    public void setCssClass(String cssClass) {
        this.cssClass = cssClass;
    }

}

TLD file:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE taglib
  PUBLIC "-//Sun Microsystems, Inc.//DTD JSP Tag Library 1.2//EN"
  "http://java.sun.com/dtd/web-jsptaglibrary_1_2.dtd">

<taglib>
    <tlib-version>1.0</tlib-version>
    <jsp-version>2.0</jsp-version>
    <short-name>menu</short-name>
    <description>Vertical menu tag</description>
    <tag>
        <name>cloudUMLMenu</name>
        <tag-class>clouduml.menu.MenuTagHandler</tag-class>
        <description>Menu tag handler</description>
        <attribute>
            <name>menu</name>
            <required>true</required>
            <rtexprvalue>true</rtexprvalue>
            <type>clouduml.menu.Menu</type>
        </attribute>
        <attribute>
            <name>id</name>
            <required>false</required>
        </attribute>
        <attribute>
            <name>cssClass</name>
            <required>false</required>
        </attribute>
    </tag>
</taglib> 

Invocation in JSP file:

<menu:cloudUMLMenu menu="#session['app.menu']"/>

Unfortunatelly I always get Exception:

org.apache.jasper.JasperException: Unable to convert string "#session['clouduml.menu']" to class clouduml.menu.Menu

Ofc I can pass key "app.menu" into menu attribute and inside MenuTagHandler read menu data from session, but I wonder if it is possible using OGNL.

Answer 1

If you want to access a Menu object in session, you don't need to set it as an attribute to the tag class. Try the following, which involves removing the Menu member of the class:

public class MenuTagHandler extends TagSupport {

    private String id;
    private String cssClass;
    private Logger logger = Logger.getLogger(this.getClass());

    @Override
    public int doStartTag() throws JspException {
        logger.info("tagSupport doStartTag method entered");

        HttpServletRequest request = (HttpServletRequest)pageContext.getRequest();
        HttpSession session = request.getSession();

        Menu menu = (Menu)session.getAttribute("app.menu");

        // do whatever you need to do with the menu data

        return SKIP_BODY;
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getCssClass() {
        return cssClass;
    }

    public void setCssClass(String cssClass) {
        this.cssClass = cssClass;
    }
}

Answer 2

Unlike EL, which is parsed by the JSP engine, OGNL expressions in JSP are just strings. Only when a tag parses that string as an OGNL expression does it create or yield an object from the ValueStack. You would need to treat the value (in your case, menu) as a String and then parse it as an OGNL expression. This is how the Struts2 tags work, as well. Note that you can also access the session using EL and pass the value to your custom tag.