/*

  * Licensed to the Apache Software Foundation (ASF) under one or more

  * contributor license agreements.  See the NOTICE file distributed with

  * this work for additional information regarding copyright ownership.

  * The ASF licenses this file to You under the Apache License, Version 2.0

  * (the "License"); you may not use this file except in compliance with

  * the License.  You may obtain a copy of the License at

  *

  *     http://www.apache.org/licenses/LICENSE-2.0

  *

  * Unless required by applicable law or agreed to in writing, software

  * distributed under the License is distributed on an "AS IS" BASIS,

  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

  * See the License for the specific language governing permissions and

  * limitations under the License.

  */

 package org.apache.commons.configuration.interpol;

 import java.util.HashMap;

 import java.util.Map;

 import java.util.Set;

 import org.apache.commons.lang.text.StrLookup;

 /**

  * <p>

  * A class that handles interpolation (variable substitution) for configuration

  * objects.

  * </p>

  * <p>

  * Each instance of <code>AbstractConfiguration</code> is associated with an

  * object of this class. All interpolation tasks are delegated to this object.

  * </p>

  * <p>

  * <code>ConfigurationInterpolator</code> works together with the

  * <code>StrSubstitutor</code> class from <a

  * href="http://commons.apache.org/lang">Commons Lang</a>. By extending

  * <code>StrLookup</code> it is able to provide values for variables that

  * appear in expressions.

  * </p>

  * <p>

  * The basic idea of this class is that it can maintain a set of primitive

  * <code>StrLookup</code> objects, each of which is identified by a special

  * prefix. The variables to be processed have the form

  * <code>${prefix:name}</code>. <code>ConfigurationInterpolator</code> will

  * extract the prefix and determine, which primitive lookup object is registered

  * for it. Then the name of the variable is passed to this object to obtain the

  * actual value. It is also possible to define a default lookup object, which

  * will be used for variables that do not have a prefix or that cannot be

  * resolved by their associated lookup object.

  * </p>

  * <p>

  * When a new instance of this class is created it is initialized with a default

  * set of primitive lookup objects. This set can be customized using the static

  * methods <code>registerGlobalLookup()</code> and

  * <code>deregisterGlobalLookup()</code>. Per default it contains the

  * following standard lookup objects:

  * </p>

  * <p>

  * <table border="1">

  * <tr>

  * <th>Prefix</th>

  * <th>Lookup object</th>

  * </tr>

  * <tr>

  * <td valign="top">sys</td>

  * <td>With this prefix a lookup object is associated that is able to resolve

  * system properties.</td>

  * </tr>

  * <tr>

  * <td valign="top">const</td>

  * <td>The <code>const</code> prefix indicates that a variable is to be

  * interpreted as a constant member field of a class (i.e. a field with the

  * <b>static final</b> modifiers). The name of the variable must be of the form

  * <code>&lt;full qualified class name&gt;.&lt;field name&gt;</code>, e.g.

  * <code>org.apache.commons.configuration.interpol.ConfigurationInterpolator.PREFIX_CONSTANTS

  * </code>.</td>

  * </tr>

  * </table>

  * </p>

  * <p>

  * After an instance has been created the current set of lookup objects can be

  * modified using the <code>registerLookup()</code> and

  * <code>deregisterLookup()</code> methods. The default lookup object (that is

  * invoked for variables without a prefix) can be set with the

  * <code>setDefaultLookup()</code> method. (If a

  * <code>ConfigurationInterpolator</code> instance is created by a

  * configuration object, this lookup points to the configuration itself, so that

  * variables are resolved using the configuration's properties. This ensures

  * backward compatibility to earlier version of Commons Configuration.)

  * </p>

  * <p>

  * Implementation node: Instances of this class are not thread-safe related to

  * modifications of their current set of registered lookup objects. It is

  * intended that each instance is associated with a single

  * <code>Configuration</code> object and used for its interpolation tasks.

  * </p>

  *

  * @version $Id: ConfigurationInterpolator.java 561230 2007-07-31 04:17:09Z rahul $

  * @since 1.4

  * @author <a

  * href="http://commons.apache.org/configuration/team-list.html">Commons

  * Configuration team</a>

  */

 public class ConfigurationInterpolator extends StrLookup

 {

     /**

      * Constant for the prefix of the standard lookup object for resolving

      * system properties.

      */

     public static final String PREFIX_SYSPROPERTIES = "sys";

     /**

      * Constant for the prefix of the standard lookup object for resolving

      * constant values.

      */

     public static final String PREFIX_CONSTANTS = "const";

     /** Constant for the prefix separator. */

     private static final char PREFIX_SEPARATOR = ':';

     /** A map with the globally registered lookup objects. */

     private static Map globalLookups;

     /** A map with the locally registered lookup objects. */

     private Map localLookups;

     /** Stores the default lookup object. */

     private StrLookup defaultLookup;

     /**

      * Creates a new instance of <code>ConfigurationInterpolator</code>.

      */

     public ConfigurationInterpolator()

     {

         synchronized (globalLookups)

         {

             localLookups = new HashMap(globalLookups);

         }

     }

     /**

      * Registers the given lookup object for the specified prefix globally. This

      * means that all instances that are created later will use this lookup

      * object for this prefix. If for this prefix a lookup object is already

      * registered, the new lookup object will replace the old one. Note that the

      * lookup objects registered here will be shared between multiple clients.

      * So they should be thread-safe.

      *

      * @param prefix the variable prefix (must not be <b>null</b>)

      * @param lookup the lookup object to be used for this prefix (must not be

      * <b>null</b>)

      */

     public static void registerGlobalLookup(String prefix, StrLookup lookup)

     {

         if (prefix == null)

         {

             throw new IllegalArgumentException(

                     "Prefix for lookup object must not be null!");

         }

         if (lookup == null)

         {

             throw new IllegalArgumentException(

                     "Lookup object must not be null!");

         }

         synchronized (globalLookups)

         {

             globalLookups.put(prefix, lookup);

         }

     }

     /**

      * Deregisters the global lookup object for the specified prefix. This means

      * that this lookup object won't be available for later created instances

      * any more. For already existing instances this operation does not have any

      * impact.

      *

      * @param prefix the variable prefix

      * @return a flag whether for this prefix a lookup object had been

      * registered

      */

     public static boolean deregisterGlobalLookup(String prefix)

     {

         synchronized (globalLookups)

         {

             return globalLookups.remove(prefix) != null;

         }

     }

     /**

      * Registers the given lookup object for the specified prefix at this

      * instance. From now on this lookup object will be used for variables that

      * have the specified prefix.

      *

      * @param prefix the variable prefix (must not be <b>null</b>)

      * @param lookup the lookup object to be used for this prefix (must not be

      * <b>null</b>)

      */

     public void registerLookup(String prefix, StrLookup lookup)

     {

         if (prefix == null)

         {

             throw new IllegalArgumentException(

                     "Prefix for lookup object must not be null!");

         }

         if (lookup == null)

         {

             throw new IllegalArgumentException(

                     "Lookup object must not be null!");

         }

         localLookups.put(prefix, lookup);

     }

     /**

      * Deregisters the lookup object for the specified prefix at this instance.

      * It will be removed from this instance.

      *

      * @param prefix the variable prefix

      * @return a flag whether for this prefix a lookup object had been

      * registered

      */

     public boolean deregisterLookup(String prefix)

     {

         return localLookups.remove(prefix) != null;

     }

     /**

      * Returns a set with the prefixes, for which lookup objects are registered

      * at this instance. This means that variables with these prefixes can be

      * processed.

      *

      * @return a set with the registered variable prefixes

      */

     public Set prefixSet()

     {

         return localLookups.keySet();

     }

     /**

      * Returns the default lookup object.

      *

      * @return the default lookup object

      */

     public StrLookup getDefaultLookup()

     {

         return defaultLookup;

     }

     /**

      * Sets the default lookup object. This lookup object will be used for all

      * variables without a special prefix. If it is set to <b>null</b>, such

      * variables won't be processed.

      *

      * @param defaultLookup the new default lookup object

      */

     public void setDefaultLookup(StrLookup defaultLookup)

     {

         this.defaultLookup = defaultLookup;

     }

     /**

      * Resolves the specified variable. This implementation will try to extract

      * a variable prefix from the given variable name (the first colon (':') is

      * used as prefix separator). It then passes the name of the variable with

      * the prefix stripped to the lookup object registered for this prefix. If

      * no prefix can be found or if the associated lookup object cannot resolve

      * this variable, the default lookup object will be used.

      *

      * @param var the name of the variable whose value is to be looked up

      * @return the value of this variable or <b>null</b> if it cannot be

      * resolved

      */

     public String lookup(String var)

     {

         if (var == null)

         {

             return null;

         }

         int prefixPos = var.indexOf(PREFIX_SEPARATOR);

         if (prefixPos >= 0)

         {

             String prefix = var.substring(0, prefixPos);

             String name = var.substring(prefixPos + 1);

             String value = fetchLookupForPrefix(prefix).lookup(name);

             if (value != null)

             {

                 return value;

             }

         }

         return fetchNoPrefixLookup().lookup(var);

     }

     /**

      * Returns the lookup object to be used for variables without a prefix. This

      * implementation will check whether a default lookup object was set. If

      * this is the case, it will be returned. Otherwise a <b>null</b> lookup

      * object will be returned.

      *

      * @return the lookup object to be used for variables without a prefix

      */

     protected StrLookup fetchNoPrefixLookup()

     {

         return (getDefaultLookup() != null) ? getDefaultLookup() : StrLookup.noneLookup();

     }

     /**

      * Obtains the lookup object for the specified prefix. This method is called

      * by the <code>lookup()</code> method. This implementation will check

      * whether a lookup object is registered for the given prefix. If not, a

      * <b>null</b> lookup object will be returned.

      *

      * @param prefix the prefix

      * @return the lookup object to be used for this prefix

      */

     protected StrLookup fetchLookupForPrefix(String prefix)

     {

         StrLookup lookup = (StrLookup) localLookups.get(prefix);

         if (lookup == null)

         {

             lookup = StrLookup.noneLookup();

         }

         return lookup;

     }

     // static initializer, sets up the map with the standard lookups

     static

     {

         globalLookups = new HashMap();

         globalLookups.put(PREFIX_SYSPROPERTIES, StrLookup.systemPropertiesLookup());

         globalLookups.put(PREFIX_CONSTANTS, new ConstantLookup());

     }

 }