EnvVarResolverProperties.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.flume.node;

import com.google.common.base.Preconditions;

import java.util.Properties;
import java.util.regex.Matcher;
import java.util.regex.Pattern;


/**
 * A class that extends the Java built-in Properties overriding
 * {@link java.util.Properties#getProperty(String)} to allow ${ENV_VAR_NAME}-style environment
 * variable inclusions
 */
public class EnvVarResolverProperties extends Properties {
  /**
   * @param input The input string with ${ENV_VAR_NAME}-style environment variable names
   * @return The output string with ${ENV_VAR_NAME} replaced with their environment variable values
   */
  protected static String resolveEnvVars(String input) {
    Preconditions.checkNotNull(input);
    // match ${ENV_VAR_NAME}
    Pattern p = Pattern.compile("\\$\\{(\\w+)\\}");
    Matcher m = p.matcher(input);
    StringBuffer sb = new StringBuffer();
    while (m.find()) {
      String envVarName = m.group(1);
      String envVarValue = System.getenv(envVarName);
      m.appendReplacement(sb, null == envVarValue ? "" : envVarValue);
    }
    m.appendTail(sb);
    return sb.toString();
  }

  /**
   * @param key the property key
   * @return the value of the property key with ${ENV_VAR_NAME}-style environment variables replaced
   */
  @Override
  public String getProperty(String key) {
    return resolveEnvVars(super.getProperty(key));
  }
}