JVM Bytecodes
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This document discusses JVM bytecodes. It begins by explaining that a virtual machine is a high-level abstraction that emulates a physical machine. The JVM is a stack-based process virtual machine that enables platform independence. It then discusses stack-based vs register-based virtual machines and how bytecodes are unaffected. The rest of the document dives into details of JVM bytecodes - what they are, the instruction set, examples of bytecode for a simple Java program, and tools like javap for viewing bytecodes. It emphasizes that understanding bytecodes provides insight into how the JVM works and enables modifying or developing new languages that target the JVM.
![Simple Program
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, world");
}
}
> javac HelloWorld.java
> javap -c HelloWorld.class
> javap -verbose HelloWorld.class](https://image.slidesharecdn.com/jvmbytecodes-170422070235/85/JVM-Bytecodes-16-320.jpg)
JVM Bytecodes
- 1. JVM Bytecodes ~ Anvay Patil
- 2. We spend 90% of our time in developing/maintaining 10% of code.
- 3. From the book: Screenshot from Chapter 5
- 4. JVM=J+VM Let’s get little curious here
- 5. What is VM? A Virtual Machine is a high level abstraction on top of native operating system that emulates a physical machine. Two types of Virtual Machines: 1. Process Virtual Machine. 2. System Virtual Machine.
- 6. JVM -> Process Virtual Machine A Process Virtual machine enables the same platform/environment for execution on multiple operating systems and different hardware architectures. They could be of two type: 1. Stack Based Process Virtual machine (HotSpot Virtual machine/ Primary JVM) 2. Register Based Process Virtual machine (Android’s Dalvik Virtual machine)
- 7. Stack Based vs Register based Virtual Machine Types
- 8. Now few questions? 1. Stack based vs register based Virtual machine, does it affect bytecodes which is generated by Java Compiler ? 2. What makes Java achieve platform independency ? 3. Is platform independency, for only Java language or it is true for other JVM languages too ?
- 9. Bytecodes So what is the Magic behind the bytecodes? What exactly are bytecodes? Why should i be aware about it?
- 10. Bytecode? JVM ➔ one byte instruction set ➔ 256 possible opcodes (unsigned bytecode) ➔ 200 in use on current Instruction sets designed for efficient execution by software interpreter which can do suitable further compilation into machine code may be at runtime. Microsoft CLR ➔ 2 bytes instruction sets ➔ Stack based VM
- 11. Why know JVM Bytecodes? 1. Can give deep understanding about the platform 2. Can read the byte code 3. Can modify the bytecode 4. Can build own language? 5. Use libraries which can generate bytecodes.
- 13. Jython VM CPython x=7,y=8 print x+y Jython Compiler CPython Compiler Cpython Interpreter Jython Interpreter15 CPython VM Java VM CPython Bytecode Java Bytecode Python source code
- 14. JVM Languages: < OMG :-O > Jython Mirah JRuby MicroFocus Visual COBOL JScheme Javascal/Pascal4J Javascal JTclJawk JLua Erjang Jaskell JPerl JPerl Groovy Clojure Xtend Ceylon Kotlin More than 800 implementations of JVM and more than 250 JVM languages Scala Fhatom Gosu Processing Drools Frink Open-XION Redline SmallTalk Ioke Nashorn JS JudoScript LolCode NestVM JPC PizzaNoop jvm.go
- 15. JVM=Polyglot-VM
- 16. Simple Program public class HelloWorld { public static void main(String[] args) { System.out.println("Hello, world"); } } > javac HelloWorld.java > javap -c HelloWorld.class > javap -verbose HelloWorld.class
- 17. javap ● Java Class File disassembler ● Basic operations and class structure ○ javap HelloWorld.class ● -c flag include bytecodes ○ javap -c HelloWorld.class ● -public -private -protected option ○ javap -private HelloWorld.class ○ javap -public HelloWorld.class ○ javap -protected HelloWorld.class ● -verbose for stack size, locals, args etc. ○ javap -verbose HelloWorld.class
- 18. Instruction set: ● Stack instruction, since stack based machine ● Basic operations ● Flow control ● Class structures ● Exception Handling
- 20. Typed Opcodes: b byte s short c char i int l long f float d double a reference <type> <operation> Constant Values Local vars (load, store) Array Operations (aload, astore) Math ops (add, sub, mul, div) Boolean bitwise Comparisons Conversions Type Operations There is no instruction for boolean, and can play with 0 or 1
- 21. Constant Values:
- 22. Local variables:
- 23. Arrays:
- 24. Math Operations:
- 25. Boolean and Bitwise Operations:
- 26. Conversions:
- 28. Flow Control:
- 32. Let’s execute HelloWorld stack=2, locals=1, args_size=1 0: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream; 3: ldc #3 // String Hello Java User Group 5: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 8: return
- 33. Q & A :)
- 34. References ● Java Virtual Machine Specification ● Inside JVM ● JVM Programming Languages
- 35. Thank you :)