How to access environment variables from JSP page

问题:

How can I access environment variables from a JSP page? Does one of the implicit objects give access to them? I couldn't find an example addressing this specific issue. Ideally I'm looking for something like:

<c:set var="where" value="${myEnvironment.machineName}">

回答1:

You can read the properties file at the server start-up using ServletContextListener and store it as application scoped attribute to access it from anywhere in the application.

Steps to follow:

.properties:

machineName=xyz

web.xml:

<listener>
    <listener-class>com.x.y.z.AppServletContextListener</listener-class>
</listener>

AppServletContextListener.java:

public class AppServletContextListener implements ServletContextListener {

    private static Properties properties = new Properties();

    static {
        // load properties file
        try {
            // absolute path on server outside the war 
            // where properties files are stored

            String absolutePath = ..; 
            File file = new File(absolutePath);
            properties.load(new FileInputStream(file));
        } catch (Exception e) {
            e.printStackTrace();
        }

    }

    @Override
    public void contextDestroyed(ServletContextEvent servletContextEvent) {

    }

    @Override
    public void contextInitialized(ServletContextEvent servletContextEvent) {
        servletContextEvent.getServletContext().
                                    setAttribute("myEnvironment", properties);
    }
}

JSP:

Then you can just treat it as Map in EL.

${myEnvironment['machineName']}

or

${myEnvironment.machineName}

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Read more about JSTL Core c:set Tag

The <c:set> tag is JSTL-friendly version of the setProperty action. The tag is helpful because it evaluates an expression and uses the results to set a value of a JavaBean or a java.util.Map object.

The <c:set> tag has following attributes:

If target is specified, property must also be specified.

Read more about it HERE

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If you are looking for sample code then find it here. Please find it at below posts. It might help you.

• Acces value between two jsp with jstl

• JSP - Standard Tag Library (JSTL) Tutorial

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More samples on other scopes.

    <%-- Set scoped variables --%>
    <c:set var="para" value="${41+1}" scope="page" />
    <c:set var="para" value="${41+1}" scope="request" />
    <c:set var="para" value="${41+1}" scope="session" />
    <c:set var="para" value="${41+1}" scope="application" />

    <%-- Print the values --%>
    <c:out value="${pageScope.para}" />
    <c:out value="${requestScope.para}" />
    <c:out value="${sessionScope.para}" />
    <c:out value="${applicationScope.para}" />

In your case you have set an attribute where in default page scope.