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Lecture 19: Java Byte Codes

This lecture discusses Java Byte Codes, including the Java Virtual Machine, stack-based computation, and various instructions for arithmetic, control flow, and method calls.

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Lecture 19: Java Byte Codes

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  1. Lecture 19: 0xCAFEBABE (Java Byte Codes) David Evans http://www.cs.virginia.edu/evans CS201j: Engineering Software University of Virginia Computer Science

  2. Menu • Running Programs • Crash Course in Architecture (CS333) • Crash Course in Compilers (CS571) • Java Virtual Machine • Byte Codes CS 201J Fall 2002

  3. Computer Architecture Processor does computation Memory stores bits Input Devices (mouse, keyboard) get input from user Output Devices (display, speakers) present output to user CS 201J Fall 2002

  4. Central Processing Unit (CPU) CS 201J Fall 2002

  5. Intel 4004 • First general purpose microprocessor, 1971 • 4-bit data • 46 instructions • 8-bit instructions! CS 201J Fall 2002

  6. PC Motherboard Memory CPU From http://www.cyberiapc.com/hardwarebeg.htm CS 201J Fall 2002

  7. Inside the CPU • Registers • Loads and decodes instructions from memory • ALU: Arithmetic Logic Unit • Does arithmetic • Can only operate on values in registers • Must load values from memory into registers before computing with them CS 201J Fall 2002

  8. Compiler • Translates a program in a high-level language into machine instructions • Calling convention • How are parameters passed to functions • How is the stack managed to return • Register allocation • Figure out how to use registers efficiently CS 201J Fall 2002

  9. 6: int max (int a, int b) { 00401010 push ebp 00401011 mov ebp,esp 00401013 sub esp,40h 00401016 push ebx 00401017 push esi 00401018 push edi 00401019 lea edi,[ebp-40h] 0040101C mov ecx,10h 00401021 mov eax,0CCCCCCCCh 00401026 rep stos dword ptr [edi] 7: if (a > b) { 00401028 mov eax,dword ptr [ebp+8] 0040102B cmp eax,dword ptr [ebp+0Ch] 0040102E jle max+25h (00401035) 8: return b; 00401030 mov eax,dword ptr [ebp+0Ch] 00401033 jmp max+28h (00401038) 9: } else { 10: return a; 00401035 mov eax,dword ptr [ebp+8] 00401038 pop edi 00401039 pop esi 0040103A pop ebx 0040103B mov esp,ebp 0040103D pop ebp 0040103E ret push instruction is 1 byte mov instruction is 2 bytes Dealing with function call: updating stack, moving arguments In Visual C++, see assembly code by running Debug, then Window | Disassembly int max (int a, int b) { if (a > b) { return b; } else { return a; } } CS 201J Fall 2002

  10. Java Virtual Machine CS 201J Fall 2002

  11. Java Ring (1998) CS 201J Fall 2002

  12. Java Card CS 201J Fall 2002

  13. Java Virtual Machine • Small and simple to implement • All VMs will run all programs the same way • Secure CS 201J Fall 2002

  14. Java Byte Codes • Stack-based virtual machine • Small instruction set: 202 instructions • Intel x86: ~280 instructions (1 to 17 bytes long!) • Memory is typed • Every Java class file begins with magic number 3405691582 = 0xCAFEBABE in base 16 CS 201J Fall 2002

  15. Stack-Based Computation • push – put something on the top of the stack • pop – get and remove the top of the stack Stack push 2 5 2 push 3 3 add Does 2 pops, pushes sum CS 201J Fall 2002

  16. Some Java Instructions CS 201J Fall 2002

  17. Some Java Instructions Constant may be an int, float or String ldc “Hello” ldc 201 The String is really a reference to an entry in the string constant table! CS 201J Fall 2002

  18. Arithmetic iconst_2 iconst_3 iadd CS 201J Fall 2002

  19. Arithmetic CS 201J Fall 2002

  20. Java Byte Code Instructions • 0: nop • 1-20: putting constants on the stack • 96-119: arithmetic on ints, longs, floats, doubles • What other kinds of instructions do we need? CS 201J Fall 2002

  21. Other Instruction Classes • Control Flow (~20 instructions) • if, goto, return • Method Calls (4 instructions) • Loading and Storing Variables (65 instructions) • Creating objects (1 instruction) • Using object fields (4 instructions) • Arrays (3 instructions) CS 201J Fall 2002

  22. Control Flow • ifeq <label> Pop an int off the stack. If it is zero, jump to the label. Otherwise, continue normally. • if_icmple <label> Pop two ints off the stack. If the second one is <= the first one, jump to the label. Otherwise, continue normally. CS 201J Fall 2002

  23. Method Calls • invokevirtual <method> • Invokes the method <method> on the parameters and object on the top of the stack. • Finds the appropriate method at run-time based on the actual type of the this object. invokevirtual <Method void println(java.lang.String)> CS 201J Fall 2002

  24. Method Calls • invokestatic <method> • Invokes a static (class) method <method> on the parameters on the top of the stack. • Finds the appropriate method at run-time based on the actual type of the this object. CS 201J Fall 2002

  25. Example public class Sample1 { static public void main (String args[]) { System.err.println ("Hello!"); System.exit (1); } } CS 201J Fall 2002

  26. public class Sample1 { static public void main (String args[]) { System.err.println ("Hello!"); System.exit (1); } } > javap -c Sample1 Compiled from Sample1.java public class Sample1 extends java.lang.Object { public Sample1(); public static void main(java.lang.String[]); } Method Sample1() 0 aload_0 1 invokespecial #1 <Method java.lang.Object()> 4 return Method void main(java.lang.String[]) 0 getstatic #2 <Field java.io.PrintStream err> 3 ldc #3 <String "Hello!"> 5 invokevirtual #4 <Method void println(java.lang.String)> 8 iconst_1 9 invokestatic #5 <Method void exit(int)> 12 return CS 201J Fall 2002

  27. Referencing Memory • iload <varnum> • Pushes the int in local variable <varnum> (1 bytes) on the stack • istore <varnum> • Pops the int on the top of the stack and stores it in local variable <varnum> CS 201J Fall 2002

  28. Referencing Example Method void main(java.lang.String[]) 0 iconst_2 1 istore_1 2 iconst_3 3 istore_2 4 iload_1 5 iload_2 6 iadd 7 istore_3 8 getstatic #2 <Field java.io.PrintStream err> 11 new #3 <Class java.lang.StringBuffer> 14 dup 15 invokespecial #4 <Method java.lang.StringBuffer()> 18 ldc #5 <String "c: "> 20 invokevirtual #6 <Method java.lang.StringBuffer append(java.lang.String)> 23 iload_3 24 invokevirtual #7 <Method java.lang.StringBuffer append(int)> 27 invokevirtual #8 <Method java.lang.String toString()> 30 invokevirtual #9 <Method void println(java.lang.String)> 33 return public class Locals1 { static public void main (String args[]) { int a = 2; int b = 3; int c = a + b; System.err.println ("c: " + c); } } CS 201J Fall 2002

  29. public class Locals { static public void main (String args[]) { int a1, a2, a3, a4, a5, a6, a7, a8, a9; int a10, a11, a12, a13, a14, a15, a16, a17, a18, a19; int a20, a21, a22, a23, a24, a25, a26, a27, a28, a29; int a30, a31, a32, a33, a34, a35, a36, a37, a38, a39; int a40, a41, a42, a43, a44, a45, a46, a47, a48, a49; int a50, a51, a52, a53, a54, a55, a56, a57, a58, a59; int a60, a61, a62, a63, a64, a65, a66, a67, a68, a69; int a70, a71, a72, a73, a74, a75, a76, a77, a78, a79; int a80, a81, a82, a83, a84, a85, a86, a87, a88, a89; int a90, a91, a92, a93, a94, a95, a96, a97, a98, a99; int a100, a101, a102, a103, a104, a105, a106, a107, a108, a109; int a110, a111, a112, a113, a114, a115, a116, a117, a118, a119; int a120, a121, a122, a123, a124, a125, a126, a127, a128, a129; int a130, a131, a132, a133, a134, a135, a136, a137, a138, a139; int a140, a141, a142, a143, a144, a145, a146, a147, a148, a149; int a150, a151, a152, a153, a154, a155, a156, a157, a158, a159; int a160, a161, a162, a163, a164, a165, a166, a167, a168, a169; int a170, a171, a172, a173, a174, a175, a176, a177, a178, a179; int a180, a181, a182, a183, a184, a185, a186, a187, a188, a189; int a190, a191, a192, a193, a194, a195, a196, a197, a198, a199; int a200, a201, a202, a203, a204, a205, a206, a207, a208, a209; int a210, a211, a212, a213, a214, a215, a216, a217, a218, a219; int a220, a221, a222, a223, a224, a225, a226, a227, a228, a229; int a230, a231, a232, a233, a234, a235, a236, a237, a238, a239; int a240, a241, a242, a243, a244, a245, a246, a247, a248, a249; int a250, a251, a252, a253, a254, a255, a256; a255 = 11; a256 = 12; } } bipush <n> Push a one-byte signed integer Why not just use lcd? Method void main(java.lang.String[]) 0 bipush 11 2 istore 255 4 bipush 12 6 istore_w 256 10 return CS 201J Fall 2002

  30. 6: int max (int a, int b) { 00401010 push ebp 00401011 mov ebp,esp 00401013 sub esp,40h 00401016 push ebx 00401017 push esi 00401018 push edi 00401019 lea edi,[ebp-40h] 0040101C mov ecx,10h 00401021 mov eax,0CCCCCCCCh 00401026 rep stos dword ptr [edi] 7: if (a > b) { 00401028 mov eax,dword ptr [ebp+8] 0040102B cmp eax,dword ptr [ebp+0Ch] 0040102E jle max+25h (00401035) 8: return b; 00401030 mov eax,dword ptr [ebp+0Ch] 00401033 jmp max+28h (00401038) 9: } else { 10: return a; 00401035 mov eax,dword ptr [ebp+8] 00401038 pop edi 00401039 pop esi 0040103A pop ebx 0040103B mov esp,ebp 0040103D pop ebp 0040103E ret Max Why is the compiled C code so much longer and more complicated? public class IfExample { public int max (int a, int b) { if (a > b) { return a; } else { return b; } } } Method int max(int, int) 0 iload_1 1 iload_2 2 if_icmple 7 5 iload_1 6 ireturn 7 iload_2 8 ireturn CS 201J Fall 2002

  31. Creating Objects • new <class> • Pushes an object reference to new object of type <class> on the stack. (Note: doesn’t call a constructor!) CS 201J Fall 2002

  32. Creating Objects public class NewExample { public StringBuffer makeBigBuffer () { StringBuffer res; res = new StringBuffer ("Hello!"); return res; } } Method java.lang.StringBuffer makeBigBuffer() 0 new #2 <Class java.lang.StringBuffer> 3 dup 4 ldc #3 <String "Hello!"> 6 invokespecial #4 <Method java.lang.StringBuffer(java.lang.String)> 9 astore_1 10 aload_1 11 areturn Duplicates the reference, not the object! CS 201J Fall 2002

  33. Using Object Fields • getfield <field-spec> • Pops an object reference from the stack, and pushes the value of the field on the stack • putfield <field-spec> • Pops a value and an object reference from the stack, and stores the value in the in the object’s field CS 201J Fall 2002

  34. public class Species { private String name; private Genome genome; public Species (String n, Genome g) { name = n; genome = g; } 0 aload_0 1 invokespecial #1 <Method java.lang.Object()> 4 aload_0 5 aload_1 6 putfield #2 <Field java.lang.String name> 9 aload_0 10 aload_2 11 putfield #3 <Field Genome genome> 14 return CS 201J Fall 2002

  35. Charge • Use javap –c <classname> to look at what the javac compiler produces for your code • How do we know bad things won’t happen? • If you want a review next class, send me questions by 3pm tomorrow! Method void main(java.lang.String[]) 0 iconst_2 1 iadd 2 return CS 201J Fall 2002

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