详解JAVA输出Hello World

想必大家对这一段JAVA代码一定不会陌生：

public class Test {
    public static void main(String[] args) {
        System.out.println("Hello World!");
    }
}

输出：Hello World!

今天咱们就从头开始分析一下它是如何从编译到输出的。

1、编译

javac Test.java，生成JAVA的字节码文件：Test.class。使用hexdump -C Test.class查看如下：

00000000  ca fe ba be 00 00 00 32  00 1d 0a 00 06 00 0f 09  |.......2........|
00000010  00 10 00 11 08 00 12 0a  00 13 00 14 07 00 15 07  |................|
00000020  00 16 01 00 06 3c 69 6e  69 74 3e 01 00 03 28 29  |.....<init>...()|
00000030  56 01 00 04 43 6f 64 65  01 00 0f 4c 69 6e 65 4e  |V...Code...LineN|
00000040  75 6d 62 65 72 54 61 62  6c 65 01 00 04 6d 61 69  |umberTable...mai|
00000050  6e 01 00 16 28 5b 4c 6a  61 76 61 2f 6c 61 6e 67  |n...([Ljava/lang|
00000060  2f 53 74 72 69 6e 67 3b  29 56 01 00 0a 53 6f 75  |/String;)V...Sou|
00000070  72 63 65 46 69 6c 65 01  00 09 54 65 73 74 2e 6a  |rceFile...Test.j|
00000080  61 76 61 0c 00 07 00 08  07 00 17 0c 00 18 00 19  |ava.............|
00000090  01 00 0c 48 65 6c 6c 6f  20 57 6f 72 6c 64 21 07  |...Hello World!.|
000000a0  00 1a 0c 00 1b 00 1c 01  00 04 54 65 73 74 01 00  |..........Test..|
000000b0  10 6a 61 76 61 2f 6c 61  6e 67 2f 4f 62 6a 65 63  |.java/lang/Objec|
000000c0  74 01 00 10 6a 61 76 61  2f 6c 61 6e 67 2f 53 79  |t...java/lang/Sy|
000000d0  73 74 65 6d 01 00 03 6f  75 74 01 00 15 4c 6a 61  |stem...out...Lja|
000000e0  76 61 2f 69 6f 2f 50 72  69 6e 74 53 74 72 65 61  |va/io/PrintStrea|
000000f0  6d 3b 01 00 13 6a 61 76  61 2f 69 6f 2f 50 72 69  |m;...java/io/Pri|
00000100  6e 74 53 74 72 65 61 6d  01 00 07 70 72 69 6e 74  |ntStream...print|
00000110  6c 6e 01 00 15 28 4c 6a  61 76 61 2f 6c 61 6e 67  |ln...(Ljava/lang|
00000120  2f 53 74 72 69 6e 67 3b  29 56 00 21 00 05 00 06  |/String;)V.!....|
00000130  00 00 00 00 00 02 00 01  00 07 00 08 00 01 00 09  |................|
00000140  00 00 00 1d 00 01 00 01  00 00 00 05 2a b7 00 01  |............*...|
00000150  b1 00 00 00 01 00 0a 00  00 00 06 00 01 00 00 00  |................|
00000160  01 00 09 00 0b 00 0c 00  01 00 09 00 00 00 25 00  |..............%.|
00000170  02 00 01 00 00 00 09 b2  00 02 12 03 b6 00 04 b1  |................|
00000180  00 00 00 01 00 0a 00 00  00 0a 00 02 00 00 00 03  |................|
00000190  00 08 00 04 00 01 00 0d  00 00 00 02 00 0e        |..............|
0000019e

这是二进制文件，看不明没有关系，java提供了javap可以查看内容是什么。

2、查看CLASS文件

javap -verbose Test。其他关于javap的内容，可以查看：http://www.cnblogs.com/liqiu/p/hexdump.html

Compiled from "Test.java"
public class Test extends java.lang.Object
  SourceFile: "Test.java"
  minor version: 0
  major version: 50
  Constant pool:
const #1 = Method       #6.#15; //  java/lang/Object."<init>":()V
const #2 = Field        #16.#17;        //  java/lang/System.out:Ljava/io/PrintStream;
const #3 = String       #18;    //  Hello World!
const #4 = Method       #19.#20;        //  java/io/PrintStream.println:(Ljava/lang/String;)V
const #5 = class        #21;    //  Test
const #6 = class        #22;    //  java/lang/Object
const #7 = Asciz        <init>;
const #8 = Asciz        ()V;
const #9 = Asciz        Code;
const #10 = Asciz       LineNumberTable;
const #11 = Asciz       main;
const #12 = Asciz       ([Ljava/lang/String;)V;
const #13 = Asciz       SourceFile;
const #14 = Asciz       Test.java;
const #15 = NameAndType #7:#8;//  "<init>":()V
const #16 = class       #23;    //  java/lang/System
const #17 = NameAndType #24:#25;//  out:Ljava/io/PrintStream;
const #18 = Asciz       Hello World!;
const #19 = class       #26;    //  java/io/PrintStream
const #20 = NameAndType #27:#28;//  println:(Ljava/lang/String;)V
const #21 = Asciz       Test;
const #22 = Asciz       java/lang/Object;
const #23 = Asciz       java/lang/System;
const #24 = Asciz       out;
const #25 = Asciz       Ljava/io/PrintStream;;
const #26 = Asciz       java/io/PrintStream;
const #27 = Asciz       println;
const #28 = Asciz       (Ljava/lang/String;)V;

{
public Test();
  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 static void main(java.lang.String[]);
  Code:
   Stack=2, Locals=1, Args_size=1
   0:   getstatic       #2; //Field java/lang/System.out:Ljava/io/PrintStream;
   3:   ldc     #3; //String Hello World!
   5:   invokevirtual   #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   8:   return
  LineNumberTable:
   line 3: 0
   line 4: 8

}

别看写的挺多的，分析一下么有什么：

1. 首先说明一下版本和文件名之类的，至于class文件的格式，可以参考：http://blog.csdn.net/tyrone1979/article/details/964560
2. 然后const的部分是将内容放入内存
3. public Test()是默认的构造函数
4. 有用的就是：public static void main() .............................

如果执行java Test，那么

3、申请内存栈

这部分内容比较复杂，可以参考：http://15838341661-139-com.iteye.com/blog/1287866

4、输出内容

这方面咱们比较关心，查看代码：

3:   ldc     #3; //String Hello World!

这句话的意思是：将字符串Hello World!放入栈顶

invokevirtual   #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V

这句话的意思是输出内容：Hello World!