code: private static ClassLoader getBootstrapClassLoader() {
          if (BOOTSTRAP_CLASSLOADER == null) {
              synchronized(ClassLoaderUtil.class) {
                  if (BOOTSTRAP_CLASSLOADER == null) {
                      ClassLoader cl = null;
                      if (System.getSecurityManager() != null) {
                          cl = AccessController.doPrivileged(new PrivilegedAction<ClassLoader>() {
                              @Override
                              public BootstrapClassLoader run() {
                                  return new BootstrapClassLoader();
                              }
                          });
                      } else {
                          cl = new BootstrapClassLoader();
                      }
                      BOOTSTRAP_CLASSLOADER = cl;
                  }
              }
          }
          return BOOTSTRAP_CLASSLOADER;
      }
docs: Lazily create a singleton BootstrapClassLoader.
  This class loader might be necessary when ICU4J classes are
  initialized by bootstrap class loader.
  
  @return The BootStrapClassLoader singleton instance