View Javadoc

    1   /*
    2    * Licensed to the Apache Software Foundation (ASF) under one or more
    3    * contributor license agreements.  See the NOTICE file distributed with
    4    * this work for additional information regarding copyright ownership.
    5    * The ASF licenses this file to You under the Apache License, Version 2.0
    6    * (the "License"); you may not use this file except in compliance with
    7    * the License.  You may obtain a copy of the License at
    8    *
    9    *     http://www.apache.org/licenses/LICENSE-2.0
    10   *
    11   * Unless required by applicable law or agreed to in writing, software
    12   * distributed under the License is distributed on an "AS IS" BASIS,
    13   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    14   * See the License for the specific language governing permissions and
    15   * limitations under the License.
    16   */
    17  package org.apache.commons.configuration.interpol;
    18
    19  import java.util.HashMap;
    20  import java.util.Map;
    21  import java.util.Set;
    22
    23  import org.apache.commons.lang.text.StrLookup;
    24
    25  /***
    26   * <p>
    27   * A class that handles interpolation (variable substitution) for configuration
    28   * objects.
    29   * </p>
    30   * <p>
    31   * Each instance of <code>AbstractConfiguration</code> is associated with an
    32   * object of this class. All interpolation tasks are delegated to this object.
    33   * </p>
    34   * <p>
    35   * <code>ConfigurationInterpolator</code> works together with the
    36   * <code>StrSubstitutor</code> class from <a
    37   * href="http://commons.apache.org/lang">Commons Lang</a>. By extending
    38   * <code>StrLookup</code> it is able to provide values for variables that
    39   * appear in expressions.
    40   * </p>
    41   * <p>
    42   * The basic idea of this class is that it can maintain a set of primitive
    43   * <code>StrLookup</code> objects, each of which is identified by a special
    44   * prefix. The variables to be processed have the form
    45   * <code>${prefix:name}</code>. <code>ConfigurationInterpolator</code> will
    46   * extract the prefix and determine, which primitive lookup object is registered
    47   * for it. Then the name of the variable is passed to this object to obtain the
    48   * actual value. It is also possible to define a default lookup object, which
    49   * will be used for variables that do not have a prefix or that cannot be
    50   * resolved by their associated lookup object.
    51   * </p>
    52   * <p>
    53   * When a new instance of this class is created it is initialized with a default
    54   * set of primitive lookup objects. This set can be customized using the static
    55   * methods <code>registerGlobalLookup()</code> and
    56   * <code>deregisterGlobalLookup()</code>. Per default it contains the
    57   * following standard lookup objects:
    58   * </p>
    59   * <p>
    60   * <table border="1">
    61   * <tr>
    62   * <th>Prefix</th>
    63   * <th>Lookup object</th>
    64   * </tr>
    65   * <tr>
    66   * <td valign="top">sys</td>
    67   * <td>With this prefix a lookup object is associated that is able to resolve
    68   * system properties.</td>
    69   * </tr>
    70   * <tr>
    71   * <td valign="top">const</td>
    72   * <td>The <code>const</code> prefix indicates that a variable is to be
    73   * interpreted as a constant member field of a class (i.e. a field with the
    74   * <b>static final</b> modifiers). The name of the variable must be of the form
    75   * <code>&lt;full qualified class name&gt;.&lt;field name&gt;</code>, e.g.
    76   * <code>org.apache.commons.configuration.interpol.ConfigurationInterpolator.PREFIX_CONSTANTS
    77   * </code>.</td>
    78   * </tr>
    79   * </table>
    80   * </p>
    81   * <p>
    82   * After an instance has been created the current set of lookup objects can be
    83   * modified using the <code>registerLookup()</code> and
    84   * <code>deregisterLookup()</code> methods. The default lookup object (that is
    85   * invoked for variables without a prefix) can be set with the
    86   * <code>setDefaultLookup()</code> method. (If a
    87   * <code>ConfigurationInterpolator</code> instance is created by a
    88   * configuration object, this lookup points to the configuration itself, so that
    89   * variables are resolved using the configuration's properties. This ensures
    90   * backward compatibility to earlier version of Commons Configuration.)
    91   * </p>
    92   * <p>
    93   * Implementation node: Instances of this class are not thread-safe related to
    94   * modifications of their current set of registered lookup objects. It is
    95   * intended that each instance is associated with a single
    96   * <code>Configuration</code> object and used for its interpolation tasks.
    97   * </p>
    98   *
    99   * @version $Id: ConfigurationInterpolator.java 561230 2007-07-31 04:17:09Z rahul $
    100  * @since 1.4
    101  * @author <a
    102  * href="http://commons.apache.org/configuration/team-list.html">Commons
    103  * Configuration team</a>
    104  */
    105 public class ConfigurationInterpolator extends StrLookup
    106 {
    107     /***
    108      * Constant for the prefix of the standard lookup object for resolving
    109      * system properties.
    110      */
    111     public static final String PREFIX_SYSPROPERTIES = "sys";
    112
    113     /***
    114      * Constant for the prefix of the standard lookup object for resolving
    115      * constant values.
    116      */
    117     public static final String PREFIX_CONSTANTS = "const";
    118
    119     /*** Constant for the prefix separator. */
    120     private static final char PREFIX_SEPARATOR = ':';
    121
    122     /*** A map with the globally registered lookup objects. */
    123     private static Map globalLookups;
    124
    125     /*** A map with the locally registered lookup objects. */
    126     private Map localLookups;
    127
    128     /*** Stores the default lookup object. */
    129     private StrLookup defaultLookup;
    130
    131     /***
    132      * Creates a new instance of <code>ConfigurationInterpolator</code>.
    133      */
    134     public ConfigurationInterpolator()
    135     {
    136         synchronized (globalLookups)
    137         {
    138             localLookups = new HashMap(globalLookups);
    139         }
    140     }
    141
    142     /***
    143      * Registers the given lookup object for the specified prefix globally. This
    144      * means that all instances that are created later will use this lookup
    145      * object for this prefix. If for this prefix a lookup object is already
    146      * registered, the new lookup object will replace the old one. Note that the
    147      * lookup objects registered here will be shared between multiple clients.
    148      * So they should be thread-safe.
    149      *
    150      * @param prefix the variable prefix (must not be <b>null</b>)
    151      * @param lookup the lookup object to be used for this prefix (must not be
    152      * <b>null</b>)
    153      */
    154     public static void registerGlobalLookup(String prefix, StrLookup lookup)
    155     {
    156         if (prefix == null)
    157         {
    158             throw new IllegalArgumentException(
    159                     "Prefix for lookup object must not be null!");
    160         }
    161         if (lookup == null)
    162         {
    163             throw new IllegalArgumentException(
    164                     "Lookup object must not be null!");
    165         }
    166         synchronized (globalLookups)
    167         {
    168             globalLookups.put(prefix, lookup);
    169         }
    170     }
    171
    172     /***
    173      * Deregisters the global lookup object for the specified prefix. This means
    174      * that this lookup object won't be available for later created instances
    175      * any more. For already existing instances this operation does not have any
    176      * impact.
    177      *
    178      * @param prefix the variable prefix
    179      * @return a flag whether for this prefix a lookup object had been
    180      * registered
    181      */
    182     public static boolean deregisterGlobalLookup(String prefix)
    183     {
    184         synchronized (globalLookups)
    185         {
    186             return globalLookups.remove(prefix) != null;
    187         }
    188     }
    189
    190     /***
    191      * Registers the given lookup object for the specified prefix at this
    192      * instance. From now on this lookup object will be used for variables that
    193      * have the specified prefix.
    194      *
    195      * @param prefix the variable prefix (must not be <b>null</b>)
    196      * @param lookup the lookup object to be used for this prefix (must not be
    197      * <b>null</b>)
    198      */
    199     public void registerLookup(String prefix, StrLookup lookup)
    200     {
    201         if (prefix == null)
    202         {
    203             throw new IllegalArgumentException(
    204                     "Prefix for lookup object must not be null!");
    205         }
    206         if (lookup == null)
    207         {
    208             throw new IllegalArgumentException(
    209                     "Lookup object must not be null!");
    210         }
    211         localLookups.put(prefix, lookup);
    212     }
    213
    214     /***
    215      * Deregisters the lookup object for the specified prefix at this instance.
    216      * It will be removed from this instance.
    217      *
    218      * @param prefix the variable prefix
    219      * @return a flag whether for this prefix a lookup object had been
    220      * registered
    221      */
    222     public boolean deregisterLookup(String prefix)
    223     {
    224         return localLookups.remove(prefix) != null;
    225     }
    226
    227     /***
    228      * Returns a set with the prefixes, for which lookup objects are registered
    229      * at this instance. This means that variables with these prefixes can be
    230      * processed.
    231      *
    232      * @return a set with the registered variable prefixes
    233      */
    234     public Set prefixSet()
    235     {
    236         return localLookups.keySet();
    237     }
    238
    239     /***
    240      * Returns the default lookup object.
    241      *
    242      * @return the default lookup object
    243      */
    244     public StrLookup getDefaultLookup()
    245     {
    246         return defaultLookup;
    247     }
    248
    249     /***
    250      * Sets the default lookup object. This lookup object will be used for all
    251      * variables without a special prefix. If it is set to <b>null</b>, such
    252      * variables won't be processed.
    253      *
    254      * @param defaultLookup the new default lookup object
    255      */
    256     public void setDefaultLookup(StrLookup defaultLookup)
    257     {
    258         this.defaultLookup = defaultLookup;
    259     }
    260
    261     /***
    262      * Resolves the specified variable. This implementation will try to extract
    263      * a variable prefix from the given variable name (the first colon (':') is
    264      * used as prefix separator). It then passes the name of the variable with
    265      * the prefix stripped to the lookup object registered for this prefix. If
    266      * no prefix can be found or if the associated lookup object cannot resolve
    267      * this variable, the default lookup object will be used.
    268      *
    269      * @param var the name of the variable whose value is to be looked up
    270      * @return the value of this variable or <b>null</b> if it cannot be
    271      * resolved
    272      */
    273     public String lookup(String var)
    274     {
    275         if (var == null)
    276         {
    277             return null;
    278         }
    279
    280         int prefixPos = var.indexOf(PREFIX_SEPARATOR);
    281         if (prefixPos >= 0)
    282         {
    283             String prefix = var.substring(0, prefixPos);
    284             String name = var.substring(prefixPos + 1);
    285             String value = fetchLookupForPrefix(prefix).lookup(name);
    286             if (value != null)
    287             {
    288                 return value;
    289             }
    290         }
    291         return fetchNoPrefixLookup().lookup(var);
    292     }
    293
    294     /***
    295      * Returns the lookup object to be used for variables without a prefix. This
    296      * implementation will check whether a default lookup object was set. If
    297      * this is the case, it will be returned. Otherwise a <b>null</b> lookup
    298      * object will be returned.
    299      *
    300      * @return the lookup object to be used for variables without a prefix
    301      */
    302     protected StrLookup fetchNoPrefixLookup()
    303     {
    304         return (getDefaultLookup() != null) ? getDefaultLookup() : StrLookup.noneLookup();
    305     }
    306
    307     /***
    308      * Obtains the lookup object for the specified prefix. This method is called
    309      * by the <code>lookup()</code> method. This implementation will check
    310      * whether a lookup object is registered for the given prefix. If not, a
    311      * <b>null</b> lookup object will be returned.
    312      *
    313      * @param prefix the prefix
    314      * @return the lookup object to be used for this prefix
    315      */
    316     protected StrLookup fetchLookupForPrefix(String prefix)
    317     {
    318         StrLookup lookup = (StrLookup) localLookups.get(prefix);
    319         if (lookup == null)
    320         {
    321             lookup = StrLookup.noneLookup();
    322         }
    323         return lookup;
    324     }
    325
    326     // static initializer, sets up the map with the standard lookups
    327     static
    328     {
    329         globalLookups = new HashMap();
    330         globalLookups.put(PREFIX_SYSPROPERTIES, StrLookup.systemPropertiesLookup());
    331         globalLookups.put(PREFIX_CONSTANTS, new ConstantLookup());
    332     }
    333 }