uimaj-core/src/main/java/org/apache/uima/util/Settings.java

/*
 * 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.uima.util;

import java.io.IOException;
import java.io.InputStream;
import java.util.Set;

import org.apache.uima.resource.ResourceConfigurationException;

//@formatter:off
/**
 * A <code>Settings</code> object holds the properties used for external parameter overrides.
 * 
 * Similar to java.util.Properties but: 
 *  - supports UTF-8 (so \\uXXXX escapes are not needed or supported)
 *  - keys must be valid Java identifiers (actually must not contain '=' ':' '}' or white-space)
 *  - reverses priority in that duplicate entries are ignored, i.e. once set values cannot be changed
 *  - multiple files can be loaded
 *  - values can contain references to other values, e.g. name = .... ${key} ....
 *  - arrays are represented as strings, e.g. '[elem1,elem2]', and can span multiple lines
 *  - '\' can be used in values to escape '$' '{' '[' ',' ']' 
 *   
 * @author burn
 */
//@formatter:on
public interface Settings {

  /**
   * Load properties from an input stream. Existing properties are not changed and a warning is
   * logged if the new value is different. May be called multiple times, so effective search is in
   * load order. Arrays are enclosed in [] and the elements may be separated by <code>,</code> or
   * new-line, so can span multiple lines without using a final \
   * 
   * @param in
   *          - Stream holding properties
   * @throws IOException
   *           if name characters illegal
   */
  void load(InputStream in) throws IOException;

  /**
   * Load properties from the comma-separated list of files specified in the system property
   * UimaExternalOverrides Files are loaded in order --- so in descending priority.
   * 
   * @throws ResourceConfigurationException
   *           wraps IOException
   */
  void loadSystemDefaults() throws ResourceConfigurationException;

  /**
   * Look up the value for a property. Perform one substitution pass on ${key} substrings replacing
   * them with the value for key. Recursively evaluate the value to be substituted. NOTE: infinite
   * loops not detected! If the key variable has not been defined, an exception is thrown. To avoid
   * evaluation and get ${key} in the output escape the $ or { Arrays are returned as a
   * comma-separated string, e.g. "[elem1,elem2]" 
   * 
   * Note: escape characters are not removed as they may affect array separators.
   * 
   * @param name
   *          - name to look up
   * @return - value of property
   * @throws ResourceConfigurationException
   *           if the value references an undefined property
   */
  String lookUp(String name) throws ResourceConfigurationException;

  /**
   * Return a set of keys of all properties loaded
   * 
   * @return - set of strings
   */
  Set<String> getKeys();

  /**
   * Get the value of an external override setting.
   * 
   * @param name
   *          - the name of the parameter
   * @return - the value found in the settings file(s), or null if missing.
   * @throws ResourceConfigurationException
   *           if the value references an undefined property, or the value is an array
   */
  String getSetting(String name) throws ResourceConfigurationException;

  /**
   * Get the array of values for an external override setting.
   * 
   * @param name
   *          - the name of the parameter
   * @return - an array of values found in the settings file(s), or null if missing.
   * @throws ResourceConfigurationException
   *           if the value references an undefined property, or the value is not an array
   */
  String[] getSettingArray(String name) throws ResourceConfigurationException;

}