java injection (candi)

application configuration files using a webbeans object

Applications often need to read, and possibly write, configuration files. An excellent way to accomplish this is to implement a custom singleton, 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 javax.enterprise.inject.Current injection. It is used to open files relative to the base directory.

Demo

Files in this tutorial

FILE	DESCRIPTION
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 Singleton

A custom singleton is a standard java-bean (see Resin-IoC). 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 @PostConstruct annotation tells Resin to call the init() method after all of the setters have been called.

AppConfig.java

import javax.annotations.PostConstruct;

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);
  }

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

  ...

Configuring the custom singleton

Configuration of the singleton is done with the <example:AppConfig> 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 webApp.root is used.

Configuring the AppConfig singleton in resin-web.xml

<web-app xmlns="http://caucho.com/ns/resin">

  <example:AppConfig xmlns:example="urn:java:example">
    <config-files-location>${webApp.root}/WEB-INF/config</config-files-location>
  </example:AppConfig>

</web-app>

Obtaining and using the object

An instance of the object is retrieved in the application using dependency injection. In this example servlet, we'll use field-based injection, marked by the @javax.enterprise.inject.Current annotation. We could also use method injection.

Resin will look in the WebBeans registry for the AppConfig object that we've configured in the resin.conf, and inject it into the servlet. Resin will report any errors in looking up the AppConfig object, e.g. if it's not configured in the resin.conf or if you've configured multiple AppConfig instances.

Obtaining the AppConfig object

import javax.enterprise.inject.Current;

public class TestServlet extends GenericServlet {
  @Current AppConfig _appConfig;
}

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

Using the AppConfig 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 configuration 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.*;

public class AppConfig {
  private final static String DEFAULT_PROPERTIES = "example/AppConfig.properties";

  private String _configFile;
  private Properties _properties;


  /**
   * 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;
  }

  @PostConstruct
  public void init()
    throws Exception
  {
    InputStream is = null;

    if (_configFile != null) {
      // the properties in _configFile override the defaults
      is = new FileInputStream(_configFile);

      _properties = new Properties(defaults);
      _properties.load(is);
    }
    else {
      // try to find a default configuration file in the classpath
      ClassLoader loader = Thread.currentThread().getContextClassLoader();
      is = loader.getResourceAsStream(DEFAULT_PROPERTIES);

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

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

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


<web-app xmlns="http://caucho.com/ns/resin">

 <example:AppConfig xmlns:example="urn:java:example"/>

</web-app>

or

<web-app xmlns="http://caucho.com/ns/resin">

 <example:AppConfig xmlns:example="urn:java:example">
    <config-file>${webApp.root}/WEB-INF/AppConfig-override.properties</config-file>
 </example:AppConfig>

</web-app>

package example;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.enterprise.inject.Current;

import java.io.*;
import java.util.*;

public class TestServlet extends HttpServlet {
  @Current AppConfig _appConfig;

  ...

  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 <example:AppConfig> 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-INF/resin-web.xml local to web-app

<web-app xmlns="http://caucho.com/ns/resin">

 <example:AppConfig xmlns:example="urn:java:example"/>

</web-app>

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

shared host configuration in resin.conf

<resin xmlns="http://caucho.com/ns/resin">
<cluster id="">

  <host id="">

    <example:AppConfig xmlns:example="urn:java:example"/>

    ...

  </host>

</cluster>
</resin>

If the <example:AppConfig> is placed as a child of <cluster>, that instance of AppConfig is available to all web-apps within all hosts within that cluster.

shared cluster configuration in resin.conf

<resin xmlns="http://caucho.com/ns/resin">
<cluster id="">

   <example:AppConfig xmlns:example="urn:java:example"/>

   <host id="">
      ...
   </host id="">
  ...
</cluster>
</resin>

In the case of <cluster> 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> tag.

Demo