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What Java Bytecode Instructions Actually Look Like and Do
- Java bytecode is what the Java Virtual Machine reads from .class files rather than running the Java file directly.
- The .class file contains a structured layout, starting with a magic number, version, and a constant pool storing method names, string values, and structural info.
- Following the constant pool, the class file describes the class itself with details like extensions, interfaces, fields, and methods.
- Methods in bytecode consist of instructions such as iload, imul, bipush, iadd, istore, etc., to handle data manipulation and calculations.
- Exception handling is also represented in bytecode through an exception table that specifies how to manage exceptions.
- Bytecode includes attributes for source file tracking, line numbers, and a LocalVariableTable for debugging and profiling purposes.
- Bytecode instructions operate in a stack-based manner, moving values between the stack and local variables for data manipulation.
- Arithmetic and logical instructions are used for operations on the stack, with one instruction per operation.
- Object-oriented instructions in bytecode handle object references, field access, method calls, and instance variable manipulations.
- Jump instructions in bytecode control flow, including conditionals, loops, switch statements, and method returns to exit execution.
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