在閱讀flume源碼的時(shí)候發(fā)下如下函數(shù)：

  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()這兩個(gè)函數(shù)不是很理解，google一下發(fā)下兩個(gè)函數(shù)的區(qū)別：
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).