Where are dependencies stored in a compiled Java class?

Question

In which section of a compiled class are dependencies to other classes (the imports) stored and can they be viewed (with an editor or something not programatically) ?

Thanks,

Answer 1

This is stored in the "constant pool" of the class (along with everything else, classes and method names, etc).

You would use javap to look at the bytecode in a readable format.

See Anton Arhipov's excellent blog on bytecode fundamentals

An example taken from there for class:

public class Foo {
    private String bar;

    public String getBar(){ 
      return bar; 
    }
    ...

... yields bytecode:

public java.lang.String getBar();
  Code:
   0:   aload_0
   1:   getfield        #2; //Field bar:Ljava/lang/String;
   4:   areturn

And has constant pool:

Compiled from "Foo.java"
public class Foo extends java.lang.Object
  SourceFile: "Foo.java"
  minor version: 0
  major version: 50
  Constant pool:
const #1 = Method       #4.#17; //  java/lang/Object."<init>":()V
const #2 = Field        #3.#18; //  Foo.bar:Ljava/lang/String;
const #3 = class        #19;    //  Foo
const #4 = class        #20;    //  java/lang/Object
const #5 = Asciz        bar;
const #6 = Asciz        Ljava/lang/String;;
const #7 = Asciz        <init>
const #8 = Asciz        ()V;
const #9 = Asciz        Code;
const #10 = Asciz       LineNumberTable;
const #11 = Asciz       getBar;
const #12 = Asciz       ()Ljava/lang/String;;
const #13 = Asciz       setBar;
const #14 = Asciz       (Ljava/lang/String;)V;
const #15 = Asciz       SourceFile;
const #16 = Asciz       Foo.java;
const #17 = NameAndType #7:#8;//  "<init>":()V
const #18 = NameAndType #5:#6;//  bar:Ljava/lang/String;
const #19 = Asciz       Foo;
const #20 = Asciz       java/lang/Object;

You can trace the reference to String from Foo#getBar through the constant pool as:

• getfield #2
• where #2 is a Field, referencing #3.#18
  • where #3 is referencing Class #19
    • which is class Foo
  • and #18 is a NameAndType referencing #5 and #6
    • and #5 is bar (field)
    • and #6 is the reference to java.lang.String

I strongly advise reading Anton's blog. It's really well done!

I'd also read James D. Bloom's JVM Internals article as it covers a lot of JVM internal architecture. Be aware of dates and versions referenced though as things change (such as the use of the java heap in java 8 and String inlining.