元数据提供了被包含数据的信息，它被数据对象显式引用。
Information about included data is provided by metadata, explicitly referenced by the data object.
然而，在sql语句中，可以始终显式引用一个隐式隐藏的列。
However, an implicitly hidden column can always be referenced explicitly in SQL statements.
当原生代码需要在JNI调用之间保存VM对象的句柄，需要显式引用管理。
Requiring explicit reference management when the native code wants to keep handles to VM objects between JNI calls.
应用推荐