org.apache.commons.text.StrLookup Maven / Gradle / Ivy
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/*
* 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.text;
import java.util.Map;
/**
* Lookup a String key to a String value.
*
* This class represents the simplest form of a string to string map.
* It has a benefit over a map in that it can create the result on
* demand based on the key.
*
* This class comes complete with various factory methods.
* If these do not suffice, you can subclass and implement your own matcher.
*
* For example, it would be possible to implement a lookup that used the
* key as a primary key, and looked up the value on demand from the database
*
* @param the type of the values supported by the lookup
* @since 1.0
*/
public abstract class StrLookup {
/**
* Lookup that always returns null.
*/
private static final StrLookup NONE_LOOKUP = new MapStrLookup<>(null);
/**
* Lookup based on system properties.
*/
private static final StrLookup SYSTEM_PROPERTIES_LOOKUP = new SystemPropertiesStrLookup();
//-----------------------------------------------------------------------
/**
* Returns a lookup which always returns null.
*
* @return a lookup that always returns null, not null
*/
public static StrLookup noneLookup() {
return NONE_LOOKUP;
}
/**
* Returns a new lookup which uses a copy of the current
* {@link System#getProperties() System properties}.
*
* If a security manager blocked access to system properties, then null will
* be returned from every lookup.
*
* If a null key is used, this lookup will throw a NullPointerException.
*
* @return a lookup using system properties, not null
*/
public static StrLookup systemPropertiesLookup() {
return SYSTEM_PROPERTIES_LOOKUP;
}
/**
* Returns a lookup which looks up values using a map.
*
* If the map is null, then null will be returned from every lookup.
* The map result object is converted to a string using toString().
*
* @param the type of the values supported by the lookup
* @param map the map of keys to values, may be null
* @return a lookup using the map, not null
*/
public static StrLookup mapLookup(final Map map) {
return new MapStrLookup<>(map);
}
//-----------------------------------------------------------------------
/**
* Constructor.
*/
protected StrLookup() {
super();
}
/**
* Looks up a String key to a String value.
*
* The internal implementation may use any mechanism to return the value.
* The simplest implementation is to use a Map. However, virtually any
* implementation is possible.
*
* For example, it would be possible to implement a lookup that used the
* key as a primary key, and looked up the value on demand from the database
* Or, a numeric based implementation could be created that treats the key
* as an integer, increments the value and return the result as a string -
* converting 1 to 2, 15 to 16 etc.
*
* The {@link #lookup(String)} method always returns a String, regardless of
* the underlying data, by converting it as necessary. For example:
*
* Map<String, Object> map = new HashMap<String, Object>();
* map.put("number", Integer.valueOf(2));
* assertEquals("2", StrLookup.mapLookup(map).lookup("number"));
*
* @param key the key to be looked up, may be null
* @return the matching value, null if no match
*/
public abstract String lookup(String key);
//-----------------------------------------------------------------------
/**
* Lookup implementation that uses a Map.
*/
static class MapStrLookup extends StrLookup {
/** Map keys are variable names and value. */
private final Map map;
/**
* Creates a new instance backed by a Map.
*
* @param map the map of keys to values, may be null
*/
MapStrLookup(final Map map) {
this.map = map;
}
/**
* Looks up a String key to a String value using the map.
*
* If the map is null, then null is returned.
* The map result object is converted to a string using toString().
*
* @param key the key to be looked up, may be null
* @return the matching value, null if no match
*/
@Override
public String lookup(final String key) {
if (map == null) {
return null;
}
final Object obj = map.get(key);
if (obj == null) {
return null;
}
return obj.toString();
}
}
//-----------------------------------------------------------------------
/**
* Lookup implementation based on system properties.
*/
private static class SystemPropertiesStrLookup extends StrLookup {
/**
* {@inheritDoc} This implementation directly accesses system properties.
*/
@Override
public String lookup(final String key) {
if (key.length() > 0) {
try {
return System.getProperty(key);
} catch (final SecurityException scex) {
// Squelched. All lookup(String) will return null.
}
}
return null;
}
}
}