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During phase one, the class-file verifier checks everything that's possible to check in a class file by looking at only the class file itself (without examining any other classes or interfaces). Phase one of the class-file verifier makes sure the imported class file is properly formed, internally consistent, adheres to the constraints of the Java programming language, and contains bytecodes that will be safe for the Java virtual machine to execute. If the class-file verifier finds that any of these are not true, it throws an error, and the class file is never used by the program.
Checking format and internal consistency
Besides verifying the integrity of the bytecodes, the verifier performs many checks for proper class file format and internal
consistency during phase one. For example, every class file must start with the same four bytes, the magic number: 0xCAFEBABE. The purpose of magic numbers is to make it easy for file parsers to recognize a certain type of file. Thus, the first thing
a class-file verifier likely checks is that the imported file does indeed begin with 0xCAFEBABE.
The class-file verifier also checks to make sure the class file is neither truncated nor enhanced with extra trailing bytes. Although different class files can be different lengths, each individual component contained inside a class file indicates its length as well as its type. The verifier can use the component types and lengths to determine the correct total length for each individual class file. In this way, it can verify that the imported file has a length consistent with its internal contents.
The verifier also looks at individual components to make sure they are well-formed instances of their type of component. For example, a method descriptor (the method's return type and the number and types of its parameters) is stored in the class file as a string that must adhere to a certain context-free grammar. One of the checks the verifier performs on individual components is to make sure each method descriptor is a well-formed string of the appropriate grammar.
In addition, the class-file verifier checks that the class itself adheres to certain constraints placed on it by the specification
of the Java programming language. For example, the verifier enforces the rule that all classes, except class Object, must have a superclass. Thus, the class-file verifier checks at runtime some of the Java language rules that should have
been enforced at compile-time. Because the verifier has no way of knowing if the class file was generated by a benevolent,
bug-free compiler, it checks each class file to make sure the rules are followed.
Once the class-file verifier has successfully completed the checks for proper format and internal consistency, it turns its attention to the bytecodes. During this part of phase one, which is commonly called the "bytecode verifier," the Java virtual machine performs a data-flow analysis on the streams of bytecodes that represent the methods of the class. To understand the bytecode verifier, you need to understand a bit about bytecodes and frames.