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Could someone list major tasks that the bytecode verifier has to perform to guarantee correctness of the program? Is there a standard, minimal set of responsibilities defined in JVM specification? I was also wondering whether verifications spans across other phases such as loading and initializing.
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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.
After the class loader in the JVM loads the byte code of . class file to the machine the Bytecode is first checked for validity by the verifier and this process is called as verification.
The bytecode verifier 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.
The JVM is what takes the bytecode and translates it into machine code. The point of bytecode is that you get better performance than a strictly interpreted language (like PHP for example) because the bytecode is already partially compiled and optimized.
Once the byte code is loaded by the JVM, (with the help of the .class file), the bytecode is checked to see the validity with the help of the verifier. The verifier checks the linking so as to perform operations efficiently.
After the class loader in the JVM loads the byte code of .class file to the machine the Bytecode is first checked for validity by the verifier and this process is called as verification. The verifier performs as much checking as possible at the Linking so that expensive operation performed by the interpreter at the run time can be eliminated.
Once the byte code is loaded by the JVM, (with the help of the .class file), the bytecode is checked to see the validity with the help of the verifier. The verifier checks the linking so as to perform operations efficiently. This way, the interpreter performs much efficiently. This process is known as verification.
Whenever you run a Java program, a runtime instance of JVM is created and loaded to the memory. Such one runtime instance is responsible for running only one Java Program. So now let's have a look at how JVM cater its responsibilities that I mentioned above and what components it uses for each of these tasks.
This is specified in the JVM Specification: Chapter 4.10. Verification of class Files .
The bulk of the page describes the various aspects of type safety. To check that the program is type-safe the verifier needs to figure out what types of operands reside in the operand stack at each program point, and make sure that they match the type expected by the respective instruction.
Other things it verifies include, but is not limited to the following:
Branches must be within the bounds of the code array for the method.
The targets of all control-flow instructions are each the start of an instruction. In the case of a wide instruction, the wide opcode is considered the start of the instruction, and the opcode giving the operation modified by that wide instruction is not considered to start an instruction. Branches into the middle of an instruction are disallowed.
No instruction can access or modify a local variable at an index greater than or equal to the number of local variables that its method indicates it allocates.
All references to the constant pool must be to an entry of the appropriate type. (For example, the instruction getfield must reference a field.)
The code does not end in the middle of an instruction.
Execution cannot fall off the end of the code.
For each exception handler, the starting and ending point of code protected by the handler must be at the beginning of an instruction or, in the case of the ending point, immediately past the end of the code. The starting point must be before the ending point. The exception handler code must start at a valid instruction, and it must not start at an opcode being modified by the wide instruction.
As a final step the verifier also performs a data-flow analysis, which makes sure that no instruction reference any uninitialized local variables.
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