Resin : Tutorials : Application configuration files using a custom JNDI object

Application configuration files using a custom JNDI object

Resin 3.1

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JNDI appconfig

Periodic Task

IoC

JMS

Applications often need to read, and possibly write, configuration files. An excellent way to accomplish this is to implement a custom JNDI object, which is easily configured and easily obtained from anywhere in the application.

This implementation of the concept allows you to configure a base directory for configuration files. An object of type AppConfig is obtained with a jndi lookup. It is used to open files relative to the base directory.

Demo

Files in this tutorial

WEB-INF/web.xml	Configure the AppConfig object as a resource
WEB-INF/classes/example/AppConfig.java	The AppConfig object provides input and output streams to configuration files
WEB-INF/classes/example/TestServlet.java	A simple example usage of AppConfig that reads and writes a file
index.jsp	The starting page for the tutorial

The java code for a custom JNDI object

A custom JNDI object is implemented similar to a java-bean (see Bean-style initialization). Setter methods like setFoo(String foo) are used to set values that are specified in the configuration.

In this case, a single setter is provided that matches the configuration parameter "config-files-location". The init() method is called by Resin after all of the setters have been called.

AppConfig.java

public class AppConfig {
  ConfigFilesLocation _cfl = null;

  /**
   * Set the base for subsequent call's to openConfigFileRead()
   * and openConfigFileWrite()
   *
   * @param location a file path or url
   */
  public void setConfigFilesLocation(String location)
    throws Exception
  {
    _cfl = new ConfigFilesLocation();
    _cfl.setLocation(location);
  }

  public void init()
    throws Exception
  {
    if (_cfl == null)
      throw new Exception("'config-files-location' must be set");
  }

  ...

Configuring the custom JNDI object

Configuration of the JNDI object is done with the resource tag.

The example here configures the location of the configuration files as WEB-INF/config (which means you need to make sure the directory exists for the example to work). It is good to hide the files somewhere under WEB-INF, because a browser will not be able to read the files, just the application.

The EL configuration variable app.docDir is used.

Configuring the AppConfig JNDI object

  <resource name='config/Application'>
    <type>example.AppConfig</type>
    <init>
      <config-files-location>${'${'}app.docDir}/WEB-INF/config</config-files-location>
    </init>
  </resource>

Obtaining and using the object

An instance of the object is retrieved in the application using JNDI. Since the configuration will not change, it is best to store the results of the lookup.

In this example servlet, an instance of the object is retrieved in the init() method and stored in _appConfig.

Obtaining the AppConfig JNDI object

  final static String JNDI_NAME = "java:comp/env/config/Application";

  public void init()
    throws ServletException
  {
    try {
      _appConfig = (AppConfig) new InitialContext().lookup(JNDI_NAME);

      if (_appConfig == null)
        throw new ServletException("`" + JNDI_NAME + "' is an unknown JNDI resource");
    } catch (NamingException e) {
      throw new ServletException(e);
    }

_appConfig is used to open the configuration files for reading and writing.

Using the AppConfig JNDI object

...

    InputStream is = _appConfig.openConfigFileRead(inputFile);

...

    OutputStream os = _appConfig.openConfigFileWrite(outputFile);

...

Variation - Hiding the configuration file with getters

The example in this tutorial is easily modified to allow the hiding of the configuration file behind get methods of the bean. Implementing getters on the confugration bean abstracts the configuration information, protecting code which uses the configuration information from implementation details of how the configuration information is read and stored.

Hiding the configuration file with getters

package example;

import java.util.*;
import java.io.*;
import javax.naming.*;

public class AppConfig {
  public final static String APPCONFIG_JNDINAME = "java:comp/env/config/Application";
  private final static String DEFAULT_PROPERTIES = "example/AppConfig.properties";

  private String _configFile;
  private Properties _properties;


  /** 
   * A convenience method that emulates the singleton pattern.
   * It is not a good idea to use static member variables to implement the
   * singleton pattern in an app server because of ClassLoader problems.
   * In this case, JNDI is used to store the object in a safe manner.
   *
   * The AppConfig object has already been instantiated by Resin, the JNDI
   * lookup is used to get a reference to an object that already exists.  
   */
  static public AppConfig getInstance()
    throws NamingException
  {
    AppConfig r = (AppConfig) new InitialContext().lookup(APPCONFIG_JNDINAME);
    if (r == null) 
      throw new NamingException("no object found with jndi name `" + APPCONFIG_JNDINAME + "'");
    return r;
  }

  /**
   * Optionally set the name of a file that provides properties that override
   * the defaults.  The defaults are obtained from a file in the classpath 
   * named 'example/AppConfig.properties'
   *
   * For example, the file containing default properties might be in 
   * WEB-INF/classes/example/AppConfig.properties,
   * or if AppConfig.class is in a jar, the AppConfig.properties 
   * could be in the jar file alongside the AppConfig.class file.
   *
   * AppConfig.properties contains values placed there by the developer.
   * The <config-file> is used to indicate a file that specifies properties
   * that override the defaults, perhaps properties that change depending 
   * on the deployment environment.
   */
  public void setConfigFile(String configFile)
    throws Exception
  {
    _configFile = configFile;
  }

  public void init()
    throws Exception
  {
    InputStream is = null;
    Properties defaults;

    // try to find a default configuration file in the classpath
    ClassLoader loader = Thread.currentThread().getContextClassLoader();
    is = loader.getResourceAsStream(DEFAULT_PROPERTIES);

    if (is != null)
      defaults = new Properties();
      defaults.load(is);
    }
    else {
      // throw an exception here to make the defaults required
      throw new FileNotFoundException(DEFAULT_PROPERTIES);
    }

    if (_configFile == null) {
      // just use the defaults
      _properties = defaults;
    } 
    else {
      // the properties in _configFile override the defaults
      is = new FileInputStream(_configFile);
      _properties = new Properties(defaults);
      _properties.load(is); 
    }
  }

  public String getFoo()
  { 
    return _properties.getProperty("foo");
  }

  public String getBar()
  { 
    return _properties.getProperty("bar");
  }
}

<web-app>
 <resource name='config/Application'>
    <type>example.AppConfig</type>
 </resource>
</web-app>

or

<web-app>
 <resource name='config/Application'>
    <type>example.AppConfig</type>
    <init>
      <config-file>${'${'}app.docDir}/WEB-INF/AppConfig-override.properties</config-file>
    </init>
 </resource>
</web-app>

package example;

import javax.servlet.*;
import javax.servlet.http.*;

import java.io.*;
import java.util.*;
import javax.naming.*;

public class TestServlet extends HttpServlet {
  /**
   * _appConfig is stored locally, for efficiency.
   */
  AppConfig _appConfig;

  public void init()
    throws ServletException
  {
    try {
      _appConfig = AppConfig.getInstance();
    } catch (NamingException e) {
      throw new ServletException(e);
    }
  }

  ...

  String foo = _appConfig.getFoo();
  String bar = _appConfig.getBar();

  ...

}

Availability of AppConfig from different web-apps

The availability of AppConfig to different web-apps depends upon the context that the <resource ...> configuration is placed within.

If the configuration is placed as a child of <web-app>, then that instance of AppConfig is available only to that web-app.

<web-app>
 <resource name='config/Application'>
    <type>example.AppConfig</type>
 </resource>
</web-app>

If it is placed as a child of <host>, that instance of AppConfig is available to all web-app's within the host.

<host>
  ...
 <resource name='config/Application'>
    <type>example.AppConfig</type>
 </resource>
  ...
</host>

If it is placed as a child of <server>, that instance of AppConfig is available to all web-app's within all host's within that server.

<server>
  ...
 <resource name='config/Application'>
    <type>example.AppConfig</type>
 </resource>
  ...
</server>

In the case of <server> or <host>, the example.AppConfig class needs to be available in the classpath. The easiest way to accomplish that is to place a jar with that class in $RESIN_HOME/lib, or you can use an explicit class-loader.

Compatibility

Although the resource tag is Resin specific the pattern shown in this tutorial is good for portability. If your application needs to work in another app-server, you can use a startup facility such as a load-on-startup servlet to manually configure the bean, call the init() method, and stuff the instance into JNDI.