Identification of Exception

Question

Hi All,
I am doing few research on the exception handling in JVM.
I want to understand, when a malfunction occurs in the program, how JVM knows that this is exception. What triggers the first exception initiation event at the low level.
Hope I have put the requried in proper words.
thanks in advance,
Andy

Answer 1

mJK wrote:
When a str.toString() method is executed, a null reference is found JVM thows an error. I want to understand at the machine level, how JVM knows this is an exception.
Does JVM has rules which says it is an error or does JVM tries to perform the action and fails so it declares the exception.
I want to understand the exception handling of JVM at the fundamental level.The class
class Mjk {
    public void run() {
        String str = null;
        System.out.println(str.toString()); // NullPointerException because reference variable str is null
    public static void main(String[] arg) {
        new Mjk().run();
}Has the bytecode
C:>javap -c -verbose Mjk
Compiled from "Mjk.java"
class Mjk extends java.lang.Object
  SourceFile: "Mjk.java"
  minor version: 0
  major version: 49
  Constant pool:
const #1 = Method       #8.#18; //  java/lang/Object."<init>":()V
const #2 = Field        #19.#20;        //  java/lang/System.out:Ljava/io/PrintStream;
const #3 = Method       #21.#22;        //  java/lang/String.toString:()Ljava/lang/String;
const #4 = Method       #23.#24;        //  java/io/PrintStream.println:(Ljava/lang/String;)V
const #5 = class        #25;    //  Mjk
const #6 = Method       #5.#18; //  Mjk."<init>":()V
const #7 = Method       #5.#26; //  Mjk.run:()V
const #8 = class        #27;    //  java/lang/Object
const #9 = Asciz        <init>;
const #10 = Asciz       ()V;
const #11 = Asciz       Code;
const #12 = Asciz       LineNumberTable;
const #13 = Asciz       run;
const #14 = Asciz       main;
const #15 = Asciz       ([Ljava/lang/String;)V;
const #16 = Asciz       SourceFile;
const #17 = Asciz       Mjk.java;
const #18 = NameAndType #9:#10;//  "<init>":()V
const #19 = class       #28;    //  java/lang/System
const #20 = NameAndType #29:#30;//  out:Ljava/io/PrintStream;
const #21 = class       #31;    //  java/lang/String
const #22 = NameAndType #32:#33;//  toString:()Ljava/lang/String;
const #23 = class       #34;    //  java/io/PrintStream
const #24 = NameAndType #35:#36;//  println:(Ljava/lang/String;)V
const #25 = Asciz       Mjk;
const #26 = NameAndType #13:#10;//  run:()V
const #27 = Asciz       java/lang/Object;
const #28 = Asciz       java/lang/System;
const #29 = Asciz       out;
const #30 = Asciz       Ljava/io/PrintStream;;
const #31 = Asciz       java/lang/String;
const #32 = Asciz       toString;
const #33 = Asciz       ()Ljava/lang/String;;
const #34 = Asciz       java/io/PrintStream;
const #35 = Asciz       println;
const #36 = Asciz       (Ljava/lang/String;)V;
Mjk();
  Code:
   Stack=1, Locals=1, Args_size=1
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return
  LineNumberTable:
   line 1: 0
public void run();
  Code:
   Stack=2, Locals=2, Args_size=1
   0:   aconst_null
   1:   astore_1
   2:   getstatic       #2; //Field java/lang/System.out:Ljava/io/PrintStream;
   5:   aload_1
   6:   invokevirtual   #3; //Method java/lang/String.toString:()Ljava/lang/String;
   9:   invokevirtual   #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   12:  return
  LineNumberTable:
   line 3: 0
   line 4: 2
   line 5: 12
public static void main(java.lang.String[]);
  Code:
   Stack=2, Locals=1, Args_size=1
   0:   new     #5; //class Mjk
   3:   dup
   4:   invokespecial   #6; //Method "<init>":()V
   7:   invokevirtual   #7; //Method run:()V
   10:  return
  LineNumberTable:
   line 7: 0
   line 8: 10
}The exception occurs at offset 6 in the run() method.
[JVM Spec|http://java.sun.com/docs/books/jvms/second_edition/html/Instructions2.doc6.html#invokevirtual]
Otherwise, if objectref is null, the invokevirtual instruction throws a NullPointerException.