9.2. Bytecode Verification

When a class loader presents the bytecodes of a newly loaded Java platform class to the virtual machine, these bytecodes are first inspected by a verifier. The verifier checks that the instructions cannot perform actions that are obviously damaging. All classes except for system classes are verified.

Here are some of the checks that the verifier carries out:

• Variables are initialized before they are used.

• Method calls match the types of object references.

• Rules for accessing private data and methods are not violated.

• Local variable accesses fall within the runtime stack.

• The runtime stack does not overflow.

If any of these checks fails, the class is considered corrupted and will not be loaded.

Note

If you ...

Get Core Java® Volume II—Advanced Features, Ninth Edition now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.