Section Notes: Friday 7 November 2003

Taste-testing Byte Codes

The following code declares an array of integers (foo), creates a new array of integers, and assigns values to array elements 0 through 2, and then prints out the contents of array element 2. Running java FooArray on the command line produces the result 3.

The line numbers indicated to the left of the code correspond with the assembly code below to help you figure out what's going out at each step.

14   public class FooArray {
15       public static void main(String[] args) {
16
17           int[] foo;
18           foo = new int[4];
19
20           foo[0] = 1;
21           foo[1] = 2;
22           foo[2] = 3;
23
24           System.out.println(foo[2]);
25       }
26
27    }

The following assembly code was produced by running the command J:\CS201J\JASMIN~1>D-Java -o jasmin FooArray.class. D-java is a class file disassembler that has an option to output java byte codes in Jasmin format (-o jasmin).

The Jasmin assembly file output for FooArray.class looks like the code below:

;
; Output created by D-Java (mailto:umsilve1@cc.umanitoba.ca)
;

;Classfile version:
;    Major: 45
;    Minor: 3

//Standard Initializer:

.source FooArray.java
.class  public synchronized FooArray
.super  java/lang/Object


; >> METHOD 1 <<
//Sets up the stack for the array:

.method public <init>()V
    .limit stack 1
    .limit locals 1
.line 14
.var 0 is this LFooArray; from Label1 to Label2
    //The variable at location 0 contains the call to FooArray
Label1:
    aload_0
    //Pushes the variable at location 0, call to FooArray, onto the stack
    invokenonvirtual java/lang/Object/<init>()V
    return
Label2:
.end method

; >> METHOD 2 <<
//Creates a new stack frame and sets it up for main:
.method public static main([Ljava/lang/String;)V
    .limit stack 3
    .limit locals 2
.line 18
.var 0 is args [Ljava/lang/String; from Label1 to Label3
    //The variable at location 0 in the main stack frame contains the
    //parameters to String[] in line 15
Label1:
    iconst_4
    //push the constants 0 through 4 onto the stack- represents the
    //indexes in the declared array.
    newarray int
    //allocates a single-dimensional array of integers
    astore_1
    //pops the reference to the array off of the stack and stores it 
    //in variable 1.
.line 20
.var 1 is foo [I from Label2 to Label3
Label2:
    aload_1
    //pushes the variable at location 1 (the array) onto the stack
    iconst_0
    //pushes 0 onto the stack
    iconst_1
    //pushes 1 onto the stack
    iastore
   //takes 1 and stores it as the first index (0) in the foo array 
      //of integers.
.line 21
    aload_1
    //pushes the variable at location 1 (the array) onto the stack
    iconst_1
    //pushes 1 onto the stack
    iconst_2
    //pushes 2 onto the stack
    iastore
    //takes 2 and stores it as the second index (1) in the foo array 
    //of integers.
.line 22
    aload_1
    //pushes the variable at location 1 (the array) onto the stack
    iconst_2
    //pushes 2 onto the stack
    iconst_3
    //pushes 3 onto the stack
    iastore
    //takes 3 and stores it as the third index (2) in the foo array
    //of integers
.line 24
    getstatic java/lang/System/out Ljava/io/PrintStream;
    //"Ljava/io/PrintStream;" is a reference to a PrintStream object;
    getstatic
    // pops the reference from the stack, retrieves the value of
    //java/lang/System/out, and pushes it onto the stack
    aload_1
    //pushes the variable at location 1 (the array) onto the stack
    iconst_2
    //pushes 2 onto the stack
    iaload
    //retrieves the value of index 2 (which is 3), which was just 
    //pushed onto the stack, and places it on the stack
    invokevirtual java/io/PrintStream/println(I)V
    //invokes the java method to print the contents at the top of the
    // stack, the value 3.
.line 25
    return
Label3:
.end method

For further descriptions of java byte code instructions in Jasmin syntax, visit http://mrl.nyu.edu/~meyer/jasmin/.

University of Virginia Department of Computer Science CS 201J: Engineering Software

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