How to convert an arbitrary object to String with

2019-04-21 04:48发布

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问题:

Is there any way to call toString() on an object with the EL and JSTL? (I need the String representation of an enum as index in a map in a JSP EL expression.) I hoped something like ${''+object} would work like in java, but EL isn't that nice, and there does not seem to be any function that does it.

Clarification: I have a variable somemap that maps Strings to Strings, and I have a variable someenum that is an enumeration. I'd like to do something like ${somemap[someenum.toString()]}. (Of course .toString() does not work, but what does?)

回答1:

You just do it like this:

${object}

And it'll toString it for you.

edit: Your nested expression can be resolved like this:

<c:set var="myValue">${someenum}</c:set>
${somemap[myValue]}

The first line stringifies (using toString()) the ${someenum} expression and stores it in the myValue variable. The second line uses myValue to index the map.



回答2:

Couple things you can do.

One, you can use c:set -

<c:set var="nowAString">${yourVar}</c:set>

Another thing you can do is create your own EL function, call it toString, and then call that in JSTL. EL functions are basically static methods hooked up with a taglib file. Straightforward to do.

Edit:

Really? Did you actually, you know, try it?

<%@page contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
   "http://www.w3.org/TR/html4/loose.dtd">

<%@taglib uri="http://java.sun.com/jstl/core" prefix="c" %>
<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
        <title>JSP Page</title>
    </head>
    <body>
        <h1>Hello World!</h1>
        <%
        pageContext.setAttribute("testDate", new java.util.Date());
        %>

        <c:set var="myVar">${testDate}</c:set>
        testDate = ${testDate}<br/>
        myVar = ${myVar}<br/>
        testDate Class = ${testDate.class}<br/>
        myVar Class = ${myVar.class}<br/>
    </body>
</html>

And JSP 2.0 tagfile and JSTL functions are trivial.



回答3:

I think in new versions of JSP api you can call methods, even with parameters!

I just tried ${statusColorMap[jobExecution.exitStatus.toString()]} and it works fine!



回答4:

The answer of Will Hartung should work. Here's a copy'n'paste'n'runnable SSCCE:

<%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<!doctype html>

<%!
    enum MyEnum {
        FOO, BAR
    }
%>
<%
    request.setAttribute("myEnum", MyEnum.FOO);
    java.util.Map<String, String> map = new java.util.HashMap<String, String>();
    map.put("FOO", "value of key FOO");
    map.put("BAR", "value of key BAR");
    request.setAttribute("map", map);
%>

<html lang="en">
    <head>
        <title>Test</title>
    </head>
    <body>
        <p>Map: ${map}
        <p>Enum: ${myEnum}
        <c:set var="myEnumAsString">${myEnum}</c:set>
        <p>Map value: ${map[myEnumAsString]}        
    </body>
</html>

This yields:

Map: {BAR=value of key BAR, FOO=value of key FOO}

Enum: FOO

Map value: value of key FOO

(scriptlets are just for quick prototyping, don't use them in real!)



回答5:

//In java

public class Foo {
    // Define properties and get/set methods
    private int prop1;
    private String prop2;



    public String toString() {
        String jsonString = ...; /// Convert this object to JSON string
        return jsonString;
    }
}

As skaffman said, EL syntax ${obj} will call toString().

So, if a object foo in JSTL is an instance of Foo. Then, 

// test.jsp

<script>
    var a = ${foo};  // ${foo} will be {"prop1": ooo, "prop2": "xxx"}
    console.log(a.prop1);
    console.log(a.prop2);
</script>

Example

If toString() will output JSON format string, for example, Foo's toString() outputs JSON format string. then:

// .java codes
Foo a = ...// a Foo object. => { 'prop1': ooo }
List<Foo> b = ... //< Array. => [ {'prop1': ooo}, {prop1: xxx} ]

// Pass object to JSTL by HttpServletRequest or ..
request.setAttribute('a', a);
request.setAttribute('b', b);

// .jsp codes
<span>${a.prop1}</span>

<script>
    var aa = ${a}; // ${a} => { 'prop1': ooo }
    var bb = ${b}; // ${b} => [ {'prop1': ooo}, {prop1: xxx} ]

    console.log(aa.prop1);
    console.log(bb[0].prop1);
</script>


回答6:

ugly but simple enough

<someTag someValue="${yourObject}${''}" ... />

for example, someValue only accept strings (but declare as java.lang.Object), this way enforce it with string concatenation



标签： java jsp jstl el
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