Inside the Hello World Application via Java Class Viewer

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Introduction

The Hello World Application is the first application for everybody that opens the program world. Here, we describe the secrets inside the Hello World application. And this will help you to understand the Java programming in depth.

• See the article for the Java Class Viewer tool
• See the Source Code of the tool

The Hello World Application

Here is an example of Hello World application that comes from The Java Tutorials:

/**
 * The "Hello World!" Application.
 * <p>
 * This application comes from: 
 * http://docs.oracle.com/javase/tutorial/getStarted/cupojava/index.html .
 * </p>
 */
class HelloWorldApp {
    public static void main(String[] args) {
        System.out.println("Hello World!");
    }
}

Here is the analysis result of the corresponding class file HelloWorldApp.class:

Inside the Hello World Application

The Constant Pool

The constant pool is a table of structures representing various components in the class file, like string constants, class and interface names, field names, etc. It includes:

• Class
• Fieldref
• Methodref
• InterfaceMethodref
• String
• Integer
• Float
• Long
• Double
• NameAndType
• Utf8
• MethodHandle
• MethodType
• InvokeDynamic

Although the 'Hello World' application is so simple to contain only 1 line of code, the constant pool of this class contains 34 items, as the following screen shot shows:

The following screen shows the extracted data (human readable) of the constant pool:

1. The first part shows the 'tag', it is the type of the constant pool object
2. The rest shows the value of the constant pool object

Where is the Text of 'Hello World!'?

The 'Hello World!' string is contained in constant pool item 3 (tag/type: String), and item 3 is redirected to item 23 (tag/type: Utf8):

The Methods

Although there is only one main method that is defined in the HelloWorldApp.java source code, 2 methods exist in the class file:

1. void <init> ()
2. public static void main (java.lang.String[])

When compiled, the javac tool generated one method named <init> .

The Method: <init>

This method is used to initialize the object. If no constructor is defined in the class source code, the method <init> will also be generated. By default, the method will call the java.lang.Object.<init> method directly.

Here is the source code of the method in binary format, it has 5 bytes:

2A  B7  00  01  B1

Here is the extracted source code (technically readable) of the <init> method. The code is in opcode format, and the Java Class Viewer adds the description of the binary code:

There are 3 commands in the opcode:

1. aload_0

   Load the 0^th variable local variable table, it is a reference to a Java Object Instance. In our case, it loads the this object from the local variable.

   There are similar opcode instructions to aload_n:

   aaload	Load reference from array
   aload	Load reference from local variable
   aload_n	Load reference from local variable, The n must be an index into the local variable array of the current frame
   baload	Load byte or boolean from array
   caload	Load char from array
   daload	Load double from array
   dload	Load double from local variable
   dload_n	Load double from local variable
   faload	Load float from array
   fload	Load float from local variable
   fload_n	Load float from local variable
   iaload	Load int from array
   iload	Load int from local variable
   iload_n	Load int from local variable
   laload	Load long from array
   lload	Load long from local variable
   lload_n	Load long from local variable
   saload	Load short from array

2. invokespecial

   Call the instance method <init> of super class java.lang.Object.

3. return

   Return void from current method.

The Method: main

This is the main() method compiled from the source code.

Here is the source code of the method in binary format, it has 9 bytes:

B2  00  02  12  03  B6  00  04  B1

Here is the extracted source code (technically readable) of the main() method. The code is in opcode format, and the Java Class Viewer adds the description of the binary code:

There are 4 commands in the opcode:

1. getstatic

   Get the static field out from class java.lang.System.

2. ldc

   Push item 3 from run-time constant pool; the tag (type) of item 3 is CONSTANT_String_info, and it refers to item 23, whose type is CONSTANT_Utf8_info.

   As described above, the value of item 23 is 'Hello World!'.

3. invokevirtual

   Invoke the instance method java.io.PrintStream.println.

4. return

   Return void from current method.

The Attribute of the Class

Attributes are used in the Class, Field, Method, and Code objects of the class file in JVM.

In our case, there is a SourceFile attribute of the HelloWorldApp class.

This attribute refers to item 19 of constant pool, and its value is 'HelloWorldApp.java', specifying the source code file name of current class.

History

• 2014-04-22 - Created this article
• 2014-04-24 - First version of this article