How does the Java virtual machine perform bytecode verification?

Java bytecode verification is a security mechanism to ensure that Java programs comply with specifications and is performed when the JVM is running. It verifies bytecode through the following steps: 1. Structure verification; 2. Type checking; 3. Control flow verification; 4. Stack verification. If the bytecode does not conform to the specification, such as illegal type conversion, the JVM will throw VerifyError.

Java virtual machine bytecode verification

Java bytecode verification is to ensure that the Java program complies with the Java language when running A key security mechanism for the specification. It occurs while the Java Virtual Machine (JVM) is running and is designed to prevent malicious or corrupted bytecode from compromising the system or data.

Verification process

JVM uses the following steps to verify the bytecode:

1. Structure verification:

• Check that the bytecode conforms to JVM specifications (e.g. correct opcodes, class file structure).
• Make sure all references point to valid classes and methods.

2. Type checking:

• Verify whether the operand is compatible with the expected type.
• Ensure that the operand heap conforms to type rules (for example, you cannot store integers in floating-point variables).

3. Control flow verification:

• Check whether all branch and jump operations point to valid bytecode instructions.
• Ensure that control flow does not jump to the beginning of the class or other unexpected locations.

4. Stack verification:

• Trace the JVM stack at runtime.
• Ensure that method calls and returns have correct stack frame structures.

Practical case

Consider the following Java code:

public class InvalidBytecode {
    public static void main(String[] args) {
        int x = 10;
        float y = x + "abc"; // 非法类型转换
    }
}

After compiling this code, the JVM will throw at runtime VerifyError, indicating that bytecode verification failed. This is because assigning a value of type int to a variable of type float is not allowed.

Conclusion

Bytecode verification is an important security feature of the Java Virtual Machine that helps protect the system and data from malicious or corrupted bytes The dangers of coding. By performing strict checks, the JVM ensures that Java programs adhere to required specifications at runtime.

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