apollo-client/src/main/java/com/ctrip/framework/apollo/util/ConfigUtil.java/ConfigUtil/getCustomizedCacheRoot
class ConfigUtil:  private String getCustomizedCacheRoot() {
    // 1. Get from System Property
    String cacheRoot = System.getProperty(ApolloClientSystemConsts.APOLLO_CACHE_DIR);
    if (Strings.isNullOrEmpty(cacheRoot)) {
      // 2. Get from OS environment variable
      cacheRoot = System.getenv(ApolloClientSystemConsts.APOLLO_CACHE_DIR_ENVIRONMENT_VARIABLES);
    }
    if (Strings.isNullOrEmpty(cacheRoot)) {
      // 3. Get from server.properties
      cacheRoot = Foundation.server().getProperty(ApolloClientSystemConsts.APOLLO_CACHE_DIR, null);
    }
    if (Strings.isNullOrEmpty(cacheRoot)) {
      // 4. Get from app.properties
      cacheRoot = Foundation.app().getProperty(ApolloClientSystemConsts.APOLLO_CACHE_DIR, null);
    }
    if (Strings.isNullOrEmpty(cacheRoot)) {
      // 5. Get from deprecated config
      cacheRoot = getDeprecatedCustomizedCacheRoot();
    }
    return cacheRoot;
  }