/*
* 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><full qualified class name>.<field name></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());
}
}