1 The JSP runtime reads a taglib directive at
the top of the JSP, indicating that this page
uses tags and where to find the TLD file for
the tag library or libraries it uses.
2 The JSP runtime next locates the TLD and
reads all the information about the library
from it.
3 The TLD’s information is placed in instances of the helper classes.
4 The runtime returns to reading the JSP. When it encounters a tag, it needs
to validate its usage, meaning: (1) checking if it is indeed a tag defined in
the TLD and (2) whether or not it is used properly (i.e., the tag has valid
attributes and proper body type). The TLD also informs it of the tag handler
class that implements this tag, in order for it to use that class in the
servlet being produced.
5 To validate the tag against the TLD, the runtime can make calls to the inmemory
helper objects, instead of referring to the TLD.
TLD的信息加载在TagLibraryInfo，TagInfo，TagAttributeInfo之中