How can I access System Properties to determine a param-value for a context-param in Web.xml?

[George Andrews]

I am currently trying to use System Properties to determine which property file to load for a Generic Servlet as described below. I would like to access the System Property "platform.type" in particular, though if this is possible, I will also use others such as "serverName".

<?xml version="1.0" encoding="UTF-8"?>
<web-app id="WebApp_ID" version="2.4" 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 http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">

	<display-name>Generic Application</display-name>

	<context-param>
		<param-name>GenericProperties</param-name>
		<param-value>
			classpath:${platform.type}.generic.properties
		</param-value>
		<description>The appropriate Generic Properties file should be loaded for the Generic Servlet below 
                        based on the System Property - platform.type.</description>
	</context-param>

	<servlet>
		<servlet-name>GenericServlet</servlet-name>
		<servlet-class>
			com.common.GenericServlet
		</servlet-class>
		<init-param>
			<param-name>keepgenerated</param-name>
			<param-value>false</param-value>
		</init-param>
		<init-param>
			<param-name>debug</param-name>
			<param-value>9</param-value>
		</init-param>
		<init-param>
			<param-name>sendErrToClient</param-name>
			<param-value>true</param-value>
		</init-param>
		<load-on-startup>1</load-on-startup>
	</servlet>

	<servlet-mapping>
		<servlet-name>GenericServlet</servlet-name>
		<url-pattern>/GenericServlet</url-pattern>
	</servlet-mapping>

</web-app>

I have already been through multiple posts regarding the following: ContextLoaderListener and ContextLoaderServlet. They will not work for this case because I need those properties accessible when the Generic Servlet accesses the above listed Context Parameter.

I have even created a Wrapper Servlet, let's call it GenericInitializationServlet, in which I attempt to replace the properties by: using a ServletConfig Proxy to access the Context Properties using the getContextParameter method and then substituting the property value with the matching System Property.

Example:

public class PropertySubstituter {

	public static String substitute(final String value) {
		return substitute(value, System.getProperties());
	}

	public static String substitute(String value, final Properties props) {
		if (value == null) {
			return null;
		}
		String initialValue = null;
		final Pattern p = Pattern.compile("\\$\\{([\\p{Alnum}|\\.|\\-]*)\\}");
		while (!value.equals(initialValue)) {
			initialValue = value;
			final Matcher m = p.matcher(value);
			while (m.find()) {
				final String propName = m.group(1);
				final String propValue = props.getProperty(propName, null);
				if (propValue != null) {
					value = value.replaceAll("\\$\\{" + propName + "\\}",
							Matcher.quoteReplacement(propValue));
				}
			}
		}
		return value;
	}
}

Does anyone know of a way to accomplish this? To reiterate, I need to perform property replacement on a Context Parameter that will be used by a Servlet.

Thanks!

[Tomasz Szymanski]

I'm pretty sure you can't manipulate the init parameters in a container-independent way. There is simply no standard API for this.

One of the things I can think of is using your own ServletContext implementation, that would wrap the original one and implement the getInitParameter method appropriately. Is that what you meant by "proxy"? Have you tried it?

[George Andrews]

Below you can find the code I was able to get to work:

web.xml:

<?xml version="1.0" encoding="UTF-8"?>  
<web-app id="WebApp_ID" version="2.4" 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 http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">  
 
	<display-name>Generic Application</display-name>

	<context-param>
		<param-name>GenericProperties</param-name>
		<param-value>/resources/${platform.type}-env.properties</param-value>
	</context-param>
 
	<servlet>
		<servlet-name>WrapperServlet</servlet-name>
		<servlet-class>com.application.WrapperServlet</servlet-class>
		<init-param>
			<param-name>keepgenerated</param-name>
			<param-value>false</param-value>
		</init-param>
		<init-param>
			<param-name>debug</param-name>
			<param-value>9</param-value>
		</init-param>
		<init-param>
			<param-name>sendErrToClient</param-name>
			<param-value>true</param-value>
		</init-param>
		<load-on-startup>1</load-on-startup>
	</servlet>

	<servlet-mapping>
		<servlet-name>WrapperServlet</servlet-name>
		<url-pattern>/WrapperServlet</url-pattern>
	</servlet-mapping>

</web-app>  

WrapperServlet.java:

package com.application;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

import javax.servlet.ServletConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;

import com.application.ParameterSubstituter;

import com.application.GenericServlet;

/**
 * Wrapper servlet used for property replacement in the web.xml.
 * @author andrewg
 * @version $Author: andrewg $Date: Feb 24, 2010 1:01:50 PM $Revision: 1.0
 */
public class WrapperServlet extends GenericServlet {

	private static final long serialVersionUID = -9166749147614441775L;
	private ServletConfig servletConfigProxy;
	private ServletContext servletContextProxy;

	@Override
	public void init(final ServletConfig sc) throws ServletException {
		
		servletContextProxy = (ServletContext) Proxy.newProxyInstance(
				getClass().getClassLoader(),
				new Class[] { ServletContext.class }, new InvocationHandler() {
					public Object invoke(final Object proxy,
							final Method method, final Object[] args)
					throws Throwable {
						if ("getInitParameter".equals(method.getName())) {
							return [b]ParameterSubstituter
							.substitute[/b]((String) method.invoke(sc.getServletContext(), args));
						} else {
							return method.invoke(sc.getServletContext(), args);
						}
					}
				});
		
		servletConfigProxy = (ServletConfig) Proxy.newProxyInstance(
				getClass().getClassLoader(),
				new Class[] { ServletConfig.class }, new InvocationHandler() {
					public Object invoke(final Object proxy,
							final Method method, final Object[] args)
					throws Throwable {
						if ("getServletContext".equals(method.getName())) {
							return servletContextProxy;
						} else {
							return method.invoke(sc, args);
						}
					}
				});
		
		super.init(servletConfigProxy);
	}

	@Override
	public ServletContext getServletContext() {
		return servletContextProxy;
	}
	
	@Override
	public ServletConfig getServletConfig() {
		return servletConfigProxy;
	}

}

ParameterSubstituter.java:

package com.application;

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

/**
 * Used to do substitution of ${variable} variables in a String, dereferencing the variable from Properties.
 * @author andrewg
 * @version $Author: andrewg $Date: Feb 24, 2010 1:58:08 PM $Revision: 1.0
 */
public class ParameterSubstituter {

	/** 
	 * Same as substitute(String, Properties) but assumes System properties 
	 * @param value on which to do the substitution 
	 * @return value with substitutions 
	 */
	public static String substitute(final String value) {
		return substitute(value, System.getProperties());
	}

	/**
	 * Input a string value with variables in the form of ${var-name} and it will
	 * lookup the var-name in props and replace ${var-name} with it. e.g. given
	 * properties "greeting=hello" and "name=sam" if the input value is "${greeting}
	 * ${name}, how are you?" the the output would be "hello sam, how are you?"
	 * <p>
	 * Parameter names must be alphanumeric but can also have characters "-" and
	 * ".".
	 * <p>
	 * Also: it does multiple passes until all variables are resovled, e.g. <code> 
	 *  System.setProperty("first", "${second}"); 
	 *  System.setProperty("second", "${third}"); 
	 *  System.setProperty("third", "fourth"); 
	 *  System.out.println(dereference("${first} ${second} ${third}")); 
	 * </code>
	 * Prints "fourth fourth fourth"
	 * 
	 * @param value
	 *            an optionally parameterized input string
	 * @param props
	 *            are the Properties to search for replacements
	 * @return value with substitutions made
	 */
	public static String substitute(String value, final Properties props) {
		if (value == null) {
			return null;
		}
		String initialValue = null;
		final Pattern p = Pattern.compile("\\$\\{([\\p{Alnum}|\\.|\\-]*)\\}");
		while (!value.equals(initialValue)) {
			initialValue = value;
			final Matcher m = p.matcher(value);
			while (m.find()) {
				final String propName = m.group(1);
				final String propValue = props.getProperty(propName, null);
				if (propValue != null) {
					value = value.replaceAll("\\$\\{" + propName + "\\}",
							Matcher.quoteReplacement(propValue));
				}
			}
		}
		return value;
	}
}

Props to M Simpson in this post for giving me some direction: https://coderanch.com/t/79094/Websphere/environment-variable-referance-Web-xml

[Tomasz Szymanski]

OK, so you're constructing a real proxy. I was thinking of a simple wrapper, something like ServletRequestWrapper or ServletResponseWrapper. Something like:

class ServletContextWrapper implements ServletContext {
  private ServletContext ctx;
  private Map<String, String> myInitParameters;

  ServletContextWrapper(ServletContext ctx) {
    this.ctx = ctx;
    myInitParameters = new HashMap<String, String>();
    // init the parameters, iterating through those in ctx and processing them with your ParameterSubstituter
  }

  // many methods here, most of them just delegating to the original ctx, eg:
  public void setAttribute(String name, Object value) {
    ctx.setAttribute(name, value);
  }
  public Object getAttribute(String name) {
    return ctx.getAttribute(name);
  }
  // etc., but of course:

  public String getInitParameter(String name) {
    return myInitParameters.get(name);
  }  
}

But actually that would be a much longer piece of code than yours, I guess So... what's the problem anyway? Isn't your solution sufficient?

[George Andrews]

Below you can find the code I was able to get to work:

Hi Tomasz,

Actually, I was able to get this to work and posted the solution I am currently using. Thanks!