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Introduction to JVM
Based on material produced by Bill Venners
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The Java programming environment
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The Java platform
- byte code generated by the Java front-end is an
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Java compilation model
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Java bytecode format
- void spin () { int i; for (i = 0; i < 100; i++) {;} }
0 iconst_0 // push int constant 0 1 istore_1 // store into var 1 "i=0" 2 goto 8 // first time: don't incr 5 iinc 1 1 // incr var 1 by 1 "i++" 8 iload_1 // push local var 1 "i" 9 bipush 100 // push byte const "100" 11 if_icmplt 5 // loop if less "i < 100" 14 return // return void when done
- branch instructions use relative displacement, i.e.,
add/subtract from PC => easy combination
- disassembled code uses absolute pseudo-labels
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Java bytecode basics
- no run-time tag checking (but objects have metadata)
- "untyped" local variables, reused for different types
- type tags are carried along the instructions
- the verification phase of class loading ensures
validity: types are OK, no operand stack overflow..
- bytecode categories
– arithmetic: add, sub, mul, rem, div (typed versions) – operand stack management: load/store, dup, swap – control transfer: goto <offset>, if_icmpeq, ifeq, .. – type conversions: i2l, i2f, l2f, f2i, d2i, int2byte, etc. – method invocation and return: invokevirtual, invokestatic, ireturn, lreturn, return, etc. – throwing exceptions, monitors, etc..
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Back-end transformation and execution
(1) simple JVM – byte code interpretation, including resolution of symbolic references:
- finding the entity identified by a text symbol