The Java Virtual Machine. Monday, November 14, 16

Transcription

1 The Java Virtual Machine

2 Java model is erpretation Java VMs assume.class files that contain byte code Byte code is an artificial machine code Java byte code assumes a stack machine Operands are pushed onto a stack on operated on by operations It is smaller than machine code Byte code is more portable

3 Why more portable A C iler must have iler back ends, libraries, linkers, loaders, etc., ported to a new machine architecture Since every machine runs Java byte code with the same execution model, only the JVM needs to be ported to a new machine An inefficient JVM port can be pretty easy

4 outer: for ( i = 2; i < 1000; i++) { for ( j = 2; j < i; j++) { if (i % j == 0) continue outer; } System.out.prln (i); } 0: iconst_2 1: istore_1 2: iload_1 3: sipush : if_icmpge Java 44Byte Code 9: iconst_2 10: istore_2 11: iload_2 12: iload_1 13: if_icmpge 31 16: iload_1 17: iload_2 18: irem 19: ifne 25 22: goto 38 25: iinc 2, 1 28: goto 11 31: getstatic #84; // Field java/lang/system.out:ljava/io/ PrStream; 34: iload_1 35: invokevirtual #85; // Method java/io/prstream.prln:(i)v 38: iinc 1, 1 41: goto 2 44: return

5 VM startup load named class execute main load classes as needed (self modifying code!) ile code as needed Method binary (native) code x.class y.class x.class Java stack Native method stack class loader Heap (VM objects, such as classes, user program object) class object VFT, byte code, static vars, etc. Execution Engine Native libraries Native erface

6 Execution Engine class object VFT, byte code, static vars, etc. Byte-code erpreter Garbage collector Java stack Hotspot detector Method binary (native) code Dynamic (JIT) iler Native method stack

7 Class loading What happens when we have something like A.var (a static variable reference) or new Foo( )? If A is loaded, the reference is done or the call is made If A is not loaded, load it Find A.class Check if it is valid if so, load class info (including VFT) o heap as the class This is basically the loader operations patch up code to reference loaded class This is basically the linker operations

8 Invalid classes and java ilation model Malformed classes are invalid Java allows me to ile my Your.java against a local Your.java and then run against Your.class that you have on your machine What if Main.main( ) calls your Your.foo( ), and foo( ) doesn t exist in your Your, but it does in my Your? What if My.main( ) doesn t call Your.bar( ), my Your doesn t have bar( ), but your Your does?

9 Actual Your loaded must have all of the functionality of the Your iled against It must implement the Your specification This ability allows older Java classes to run against upgraded Java classes on another machine, without reiling the older Java class

10 Consider the following A runtime stack illustration m1 calls m2 m2 calls m1 (not what happens in the VM) m1 (erpreted)... somewhere along the way m2 detected as hot, get iled m2 (erpreted) m1 (erpreted) m2 (iled) to to m1 (erpreted)

11 Consider the following m1 calls m2 m2 calls m1 iled and A runtime stack illustration (not what happens in the VM) m1 (erpreted)... erpreted code for m2 executing somewhere at the same along the way m2 time detected as hot, get iled to m2 (erpreted) m1 (erpreted) m2 (iled) m1 (erpreted) to

12 One strategy erpreted to iled: erpreted code needs to push args onto iled code stack. On return, return values need to be pushed onto erpreted code stack iled to erpreted -- basically the opposite In both cases need to run out of common memory, i.e., heap Addressing made simpler by all data being in runtime stack or head iled code lilly pad or trampoline erpreted code args does stack manipulations and other fixup iled code Indirection saves the day, another reason to inline if possible