When and Where is the String initialised/stored in

问题:

This is the source code I have:

public class Koray {
    public static void main(String [] args) {
            System.out.println("This is a sample program.");
    } 
}

And when I compile this, I get the bytecode. When I look at the bytecode with a Hexadecimal viewer I see part:

19 54 68 69 73 20 69 73 20 61 20 73 61 6D 70 6C 65 20 70 72 6F 67 72 61 6D 2E

which can be read as

This is a sample program. 

if the bytes are interpreted as characters.

And when I do

javap -c Koray.class

do disassemble this class I see:

Compiled from "Koray.java"
public class Koray {
  public Koray();
    Code:
       0: aload_0       
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return        

  public static void main(java.lang.String[]);
    Code:
       0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
       3: ldc           #3                  // String This is a sample program.
       5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
       8: bipush        10
      10: istore_1      
      11: return        
}

My question is, where is this String seen in the disassembled text? I only see it in a comment.

回答1:

See that ldc instruction? It loads a constant from the runtime constant pool. That's where the your string is stored. To print the constant pool as well, add -verbose option to javap call.

回答2:

ldc #3; //String This is a sample program.

This line uses the opcode ldc, which loads a constant onto the operand stack. At this point, we're going to load whatever constant is in index #3 of our constant pool table.

The constant pool table is where most of the literal constant values are stored. (ref) The command javap -c -verbose <classfile> provides the output for constant pool table.

Example output:

const #22 = String      #23;    // This is a sample program 
const #23 = Asciz       This is a sample program;

More info from VM Spec ( http://java.sun.com/docs/books/jvms/second_edition/html/ClassFile.doc.html ):

The constant_pool is a table of structures (§4.4) representing various string constants, class and interface names, field names, and other constants that are referred to within the ClassFile structure and its substructures. The format of each constant_pool table entry is indicated by its first "tag" byte. The constant_pool table is indexed from 1 to constant_pool_count-1.

回答3:

ldc #3 loads the string constant stored in the constant pool, which is a separate table that stores constants such as String literals and class names.

See http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.4 from the JVM specification:

The constant_pool is a table of structures (§4.4) representing various string constants, class and interface names, field names, and other constants that are referred to within the ClassFile structure and its substructures. The format of each constant_pool table entry is indicated by its first "tag" byte.

You can view the string table using javap -verbose ClassName.

A sample output would be something like:

Compiled from "Test.java"
public class tests.Test extends java.lang.Object
  SourceFile: "Test.java"
  minor version: 0
  major version: 50
  Constant pool:
const #1 = class        #2;     //  tests/Test
const #2 = Asciz        tests/Test;
const #3 = String        #4;     //  This is a sample program.
...