java基础：System.getenv() VS System.getProperty()

在阅读flume源码的时候发下如下函数：

  private static void initSysPropFromEnvVar(String sysPropName, String envVarName,
                                            String description) {
    if (System.getProperty(sysPropName) != null) {
      LOGGER.debug("Global SSL " + description + " has been initialized from system property.");
    } else {
      String envVarValue = System.getenv(envVarName);
      if (envVarValue != null) {
        System.setProperty(sysPropName, envVarValue);
        LOGGER.debug("Global SSL " + description +
            " has been initialized from environment variable.");
      } else {
        LOGGER.debug("No global SSL " + description + " specified.");
      }
    }
  }

其中System.getenv() & System.getProperty()这两个函数不是很理解，google一下发下两个函数的区别：
getenv gets an environment variable. getProperty gets a Java property. Environment variables are specified at the OS level. Java properties are specified by passing the -D option to the JVM (and can be set programmatically).