Java字節碼（class文件）解析

Java編譯後生成的.class字節碼文件裏面的內容究竟是什麼呢？一直比較困擾，現在終於看到了廬山真面目，比如對於test.class使用javap -p -verbose test可以查看生成的字節碼裏面的內容。用一個簡單的test類來分析字節碼裏面的內容。

test.java:

/**

* @author :zhengrf1

* @date 創建時間：2017年7月19日上午10:53:08

* @version 1.0

* @parameter

* @since

* @return

public class test {

private int a = 99;

public Long b = 88l;

String d = "hello";

Object e = new Object();

static Object c = new Object();

static String f ="world";

{

a=999;

}

static{

f ="world2";

}

test(int param){

a = param;

}

public static void fun(Stringstr){

System.out.println(str);

}

/**

* @author : zhengrf1

* @date 創建時間：2017年7月19日上午10:53:08

public static void main(String[]args) {

// TODOAuto-generated method stub

test t= newtest(999);

test.fun(t.d);

}

源碼很簡單，主要內容就是1.定義了幾個非靜態成員變量和靜態成員變量，2.並在非靜態語句塊{}和靜態語句塊static{}中重新賦值，3.重載了構造方法，4.創建了一個靜態成員方法fun,5.在main方法中創建了一個test對象並調用了靜態方法fun

執行了javap -verbose test命令後，生成test.class的詳細內容信息：

Classfile/D:/javadevelop/eclipse/user/workspace/RedisDao/bin/test.class

Last modified 2017-7-19; size 1116 bytes

MD5 checksum a211a3dc33eaac639dc28689a541fdf4

Compiled from "test.java"

publicclass test

minor version: 0

major version: 52

flags: ACC_PUBLIC, ACC_SUPER

Constantpool:

#1 = Class #2 // test

#2 = Utf8 test

#3 = Class #4 // java/lang/Object

#4 = Utf8 java/lang/Object

#5 = Utf8 a

#6 = Utf8 I

#7 = Utf8 b

#8 = Utf8 Ljava/lang/Long;

#9 = Utf8 d

#10 = Utf8 Ljava/lang/String;

#11 = Utf8 e

#12 = Utf8 Ljava/lang/Object;

#13 = Utf8 c

#14 = Utf8 f

#15 = Utf8 <clinit>

#16 = Utf8 ()V

#17 = Utf8 Code

#18 = Methodref #3.#19 //java/lang/Object."<init>":()V

#19 = NameAndType #20:#16 // "<init>":()V

#20 = Utf8 <init>

#21 = Fieldref #1.#22 // test.c:Ljava/lang/Object;

#22 = NameAndType #13:#12 // c:Ljava/lang/Object;

#23 = String #24 // world

#24 = Utf8 world

#25 = Fieldref #1.#26 // test.f:Ljava/lang/String;

#26 = NameAndType #14:#10 // f:Ljava/lang/String;

#27 = String #28 // world2

#28 = Utf8 world2

#29 = Utf8 LineNumberTable

#30 = Utf8 LocalVariableTable

#31 = Utf8 (I)V

#32 = Fieldref #1.#33 // test.a:I

#33 = NameAndType #5:#6 // a:I

#34 = Long 88l

#36 = Methodref #37.#39 // java/lang/Long.valueOf:(J)Ljava/lan

g/Long;

#37 = Class #38 // java/lang/Long

#38 = Utf8 java/lang/Long

#39 = NameAndType #40:#41 // valueOf:(J)Ljava/lang/Long;

#40 = Utf8 valueOf

#41 = Utf8 (J)Ljava/lang/Long;

#42 = Fieldref #1.#43 // test.b:Ljava/lang/Long;

#43 = NameAndType #7:#8 // b:Ljava/lang/Long;

#44 = String #45 // hello

#45 = Utf8 hello

#46 = Fieldref #1.#47 // test.d:Ljava/lang/String;

#47 = NameAndType #9:#10 // d:Ljava/lang/String;

#48 = Fieldref #1.#49 // test.e:Ljava/lang/Object;

#49 = NameAndType #11:#12 // e:Ljava/lang/Object;

#50 = Utf8 this

#51 = Utf8 Ltest;

#52 = Utf8 param

#53 = Utf8 fun

#54 = Utf8 (Ljava/lang/String;)V

#55 = Fieldref #56.#58 // java/lang/System.out:Ljava/io/Print

Stream;

#56 = Class #57 // java/lang/System

#57 = Utf8 java/lang/System

#58 = NameAndType #59:#60 // out:Ljava/io/PrintStream;

#59 = Utf8 out

#60 = Utf8 Ljava/io/PrintStream;

#61 = Methodref #62.#64 // java/io/PrintStream.println:(Ljava/

lang/String;)V

#62 = Class #63 // java/io/PrintStream

#63 = Utf8 java/io/PrintStream

#64 = NameAndType #65:#54 // println:(Ljava/lang/String;)V

#65 = Utf8 println

#66 = Utf8 str

#67 = Utf8 main

#68 = Utf8 ([Ljava/lang/String;)V

#69 = Methodref #1.#70 // test."<init>":(I)V

#70 = NameAndType #20:#31 // "<init>":(I)V

#71 = Methodref #1.#72 // test.fun:(Ljava/lang/String;)V

#72 = NameAndType #53:#54 // fun:(Ljava/lang/String;)V

#73 = Utf8 args

#74 = Utf8 [Ljava/lang/String;

#75 = Utf8 t

#76 = Utf8 SourceFile

#77 = Utf8 test.java

{

private int a;

descriptor: I

flags: ACC_PRIVATE

publicjava.lang.Long b;

descriptor: Ljava/lang/Long;

flags: ACC_PUBLIC

java.lang.String d;

descriptor: Ljava/lang/String;

flags:

java.lang.Object e;

descriptor: Ljava/lang/Object;

flags:

static java.lang.Object c;

descriptor: Ljava/lang/Object;

flags: ACC_STATIC

static java.lang.String f;

descriptor: Ljava/lang/String;

flags: ACC_STATIC

static {};

descriptor: ()V

flags: ACC_STATIC

Code:

stack=2, locals=0, args_size=0

0: new #3 // class java/lang/Object

3: dup

4: invokespecial #18 // Method java/lang/Object."<init>

":()V

7: putstatic #21 // Field c:Ljava/lang/Object;

10: ldc #23 // String world

12: putstatic #25 // Field f:Ljava/lang/String;

15: ldc #27 // String world2

17: putstatic #25 // Field f:Ljava/lang/String;

20: return

LineNumberTable:

line 15: 0

line 16: 10

line 21: 15

line 22: 20

LocalVariableTable:

Start Length Slot Name Signature

test(int);

descriptor: (I)V

flags:

Code:

stack=3, locals=2, args_size=2

0: aload_0

1: invokespecial #18 // Methodjava/lang/Object."<init>

":()V

4: aload_0

5: bipush 99

7: putfield #32 // Field a:I

10: aload_0

11: ldc2_w #34 // long 88l

14: invokestatic #36 // Methodjava/lang/Long.valueOf:(

J)Ljava/lang/Long;

17: putfield #42 // Field b:Ljava/lang/Long;

20: aload_0

21: ldc #44 // String hello

23: putfield #46 // Field d:Ljava/lang/String;

26: aload_0

27: new #3 // class java/lang/Object

30: dup

31: invokespecial #18 // Method java/lang/Object."<init>

":()V

34: putfield #48 // Field e:Ljava/lang/Object;

37: aload_0

38: sipush 999

41: putfield #32 // Field a:I

44: aload_0

45: iload_1

46: putfield #32 // Field a:I

49: return

LineNumberTable:

line 24: 0

line 11: 4

line 12: 10

line 13: 20

line 14: 26

line 18: 37

line 25: 44

line 26: 49

LocalVariableTable:

Start Length Slot Name Signature

0 50 0 this Ltest;

0 50 1 param I

public static void fun(java.lang.String);

descriptor: (Ljava/lang/String;)V

flags: ACC_PUBLIC, ACC_STATIC

Code:

stack=2, locals=1, args_size=1

0: getstatic #55 // Field java/lang/System.out:Ljav

a/io/PrintStream;

3: aload_0

4: invokevirtual #61 // Methodjava/io/PrintStream.prin

tln:(Ljava/lang/String;)V

7: return

LineNumberTable:

line 29: 0

line 30: 7

LocalVariableTable:

Start Length Slot Name Signature

0 8 0 str Ljava/lang/String;

public static void main(java.lang.String[]);

descriptor: ([Ljava/lang/String;)V

flags: ACC_PUBLIC, ACC_STATIC

Code:

stack=3, locals=2, args_size=1

0: new #1 // class test

3: dup

4: sipush 999

7: invokespecial #69 // Method"<init>":(I)V

10: astore_1

11: aload_1

12: getfield #46 // Field d:Ljava/lang/String;

15: invokestatic #71 // Methodfun:(Ljava/lang/String;)

18: return

LineNumberTable:

line 38: 0

line 39: 11

line 40: 18

LocalVariableTable:

Start Length Slot Name Signature

0 19 0 args [Ljava/lang/String;

11 8 1 t Ltest;

}

SourceFile: "test.java"

裏面的內容看似雜亂，其實稍微分析下就會很清楚了

第一部分深紅色模塊：

Classfile/D:/javadevelop/eclipse/user/workspace/RedisDao/bin/test.class

Last modified 2017-7-19; size 1116 bytes

MD5 checksum a211a3dc33eaac639dc28689a541fdf4

Compiled from "test.java"

--就是關於.calss文件的文件本身的信息介紹，有最後修改時間，文件大小，MD5校驗碼，來自哪個.java文件

第二部分紅色模塊：

publicclass test

minor version: 0

major version: 52

flags: ACC_PUBLIC, ACC_SUPER

--就是關於test這個類的信息，包括編譯版本，和flags:修飾符訪問權限,可見test是ACC_PUBLIC（public）

第三部分藍色模塊：

Constantpool:

#1 = Class #2 // test

#2 = Utf8 test

#3 = Class #4 // java/lang/Object

#4 = Utf8 java/lang/Object

#5 = Utf8 a

#6 = Utf8 I

#7 = Utf8 b

#8 = Utf8 Ljava/lang/Long;

#9 = Utf8 d

#10 = Utf8 Ljava/lang/String;

#11 = Utf8 e

#12 = Utf8 Ljava/lang/Object;

#13 = Utf8 c

#14 = Utf8 f

#15 = Utf8 <clinit>

#16 = Utf8 ()V

#17 = Utf8 Code

#18 = Methodref #3.#19 //java/lang/Object."<init>":()V

#19 = NameAndType #20:#16 // "<init>":()V

#20 = Utf8 <init>

#21 = Fieldref #1.#22 // test.c:Ljava/lang/Object;

#22 = NameAndType #13:#12 // c:Ljava/lang/Object;

#23 = String #24 // world

#24 = Utf8 world

#25 = Fieldref #1.#26 // test.f:Ljava/lang/String;

#26 = NameAndType #14:#10 // f:Ljava/lang/String;

#27 = String #28 // world2

#28 = Utf8 world2

#29 = Utf8 LineNumberTable

#30 = Utf8 LocalVariableTable

#31 = Utf8 (I)V

#32 = Fieldref #1.#33 // test.a:I

#33 = NameAndType #5:#6 // a:I

#34 = Long 88l

#36 = Methodref #37.#39 // java/lang/Long.valueOf:(J)Ljava/lan

g/Long;

#37 = Class #38 // java/lang/Long

#38 = Utf8 java/lang/Long

#39 = NameAndType #40:#41 // valueOf:(J)Ljava/lang/Long;

#40 = Utf8 valueOf

#41 = Utf8 (J)Ljava/lang/Long;

#42 = Fieldref #1.#43 // test.b:Ljava/lang/Long;

#43 = NameAndType #7:#8 // b:Ljava/lang/Long;

#44 = String #45 // hello

#45 = Utf8 hello

#46 = Fieldref #1.#47 // test.d:Ljava/lang/String;

#47 = NameAndType #9:#10 // d:Ljava/lang/String;

#48 = Fieldref #1.#49 // test.e:Ljava/lang/Object;

#49 = NameAndType #11:#12 // e:Ljava/lang/Object;

#50 = Utf8 this

#51 = Utf8 Ltest;

#52 = Utf8 param

#53 = Utf8 fun

#54 = Utf8 (Ljava/lang/String;)V

#55 = Fieldref #56.#58 // java/lang/System.out:Ljava/io/Print

Stream;

#56 = Class #57 // java/lang/System

#57 = Utf8 java/lang/System

#58 = NameAndType #59:#60 // out:Ljava/io/PrintStream;

#59 = Utf8 out

#60 = Utf8 Ljava/io/PrintStream;

#61 = Methodref #62.#64 // java/io/PrintStream.println:(Ljava/

lang/String;)V

#62 = Class #63 // java/io/PrintStream

#63 = Utf8 java/io/PrintStream

#64 = NameAndType #65:#54 // println:(Ljava/lang/String;)V

#65 = Utf8 println

#66 = Utf8 str

#67 = Utf8 main

#68 = Utf8 ([Ljava/lang/String;)V

#69 = Methodref #1.#70 // test."<init>":(I)V

#70 = NameAndType #20:#31 // "<init>":(I)V

#71 = Methodref #1.#72 // test.fun:(Ljava/lang/String;)V

#72 = NameAndType #53:#54 // fun:(Ljava/lang/String;)V

#73 = Utf8 args

#74 = Utf8 [Ljava/lang/String;

#75 = Utf8 t

#76 = Utf8 SourceFile

#77 = Utf8 test.java

---這就是大名鼎鼎的常量池了，裏面包含了很多類型的常量，比如

#1 =Class #2 // test

#2 =Utf8 test

--這個意思是在代碼區用到的#1這個索引買就是用到了一個Class對象，這個Class對象就是test,簡單來說就是#1就是Class，對一個類或接口的符號引用，而這個Class的名就是#2，而#2就是test

再比如後面的代碼內容中有一行是：

4: invokespecial #18 // Methodjava/lang/Object."<init>":()V

--來我們分析下是什麼回事，首先看命令invokespecial（調用需要特殊處理的實例方法：invokespecial），說明這行命令是調用一個實例方法，調用哪個方法呢？調用常量池中的#18，再去看看#18的內容：

#18 = Methodref #3.#19 //java/lang/Object."<init>":()V

--#18是一個Methodref類型常量，意思這個常量是對一個類中方法的符號引用，而#18由#3.#19組成，那看看#3和#19

#3 =Class #4 // java/lang/Object

#4 =Utf8 java/lang/Object

#19 =NameAndType #20:#16 // "<init>":()V

#20 =Utf8 <init>

#16 =Utf8 ()V

--#3是Class，對一個類或接口的符號引用，那個類名由#4確定，#4對應java/lang/Object，說明#3全名就是java/lang/Object這個類型，而#19是NameAndType, 對一個字段或方法的部分符號引用,而引用的方法由

#20:#16組成，而#20和#16是Utf8,utf-8編碼的字符串。分別是<init>和()V，其中<init>是構造方法的方法名，而()V是方法的簽名，表示方法是個沒有參數返回類型爲void。所以#18的最後的全名是java/lang/Object."<init>":()V，表示這是個Object類的默認構造方法。這是查找的過程，其實javap生成的結果中#18常量池這行已經在//後面幫我們拼湊出了最後的全名

--說到這裏，所謂的神祕的常量池就被我們扒的差不多了，但是常量池的作用是什麼呢？就以上面4: invokespecial #18例子看，當我們加載Class文件到JVM中，常量池的內容會存放在方法區的常量池區域，當我們執行代碼到invokespecial #18這行時，JVM就知道要去找Object類的構造方法init，如果Object類還沒加載，那就加載Object.class進jvm，加載Object.class後，很明顯JVM將在方法區中存儲Object類的所有方法，包括構造方法init，這樣這個方法的物理內存地址就確定了，假設這個地址是ff0809,那麼這個ff0809這個值就會回寫到常量池中，假設是#18 = Methodref #3.#19 ff0809，那麼後續只要其他代碼執行到invokespecial #18，就直接調用ff0809這個方法入口就行，不需要再次去解析。這個就是常量池的符號引用解析成內存直接引用的一步。

第四部分綠色部分：

private int a;

descriptor: I

flags: ACC_PRIVATE

publicjava.lang.Long b;

descriptor: Ljava/lang/Long;

flags: ACC_PUBLIC

java.lang.String d;

descriptor: Ljava/lang/String;

flags:

java.lang.Object e;

descriptor: Ljava/lang/Object;

flags:

static java.lang.Object c;

descriptor: Ljava/lang/Object;

flags: ACC_STATIC

static java.lang.String f;

descriptor: Ljava/lang/String;

flags: ACC_STATIC

--這個就是類的靜態成員和非靜態成員的信息描述，其中以static String f爲例子，可見，生產的字節碼中是下面描述：

staticjava.lang.String f;

descriptor: Ljava/lang/String;

flags: ACC_STATIC

-- descriptor表示成員變量的簽名，而flags: ACC_STATIC表示成員變量的訪問權限和靜態屬性。其中可以看到訪問權限是空，也就是使用了默認的包訪問權限。

第五部分紫色部分：

static{};

descriptor: ()V

flags: ACC_STATIC

Code:

stack=2, locals=0, args_size=0

0: new #3 // class java/lang/Object

3: dup

4: invokespecial #18 // Method java/lang/Object."<init>

":()V

7: putstatic #21 // Field c:Ljava/lang/Object;

10: ldc #23 // String world

12: putstatic #25 // Field f:Ljava/lang/String;

15: ldc #27 // String world2

17: putstatic #25 // Field f:Ljava/lang/String;

20: return

LineNumberTable:

line 15: 0

line 16: 10

line 21: 15

line 22: 20

LocalVariableTable:

Start Length Slot Name Signature

test(int);

descriptor: (I)V

flags:

Code:

stack=3, locals=2, args_size=2

0: aload_0

1: invokespecial #18 // Methodjava/lang/Object."<init>

":()V

4: aload_0

5: bipush 99

7: putfield #32 // Field a:I

10: aload_0

11: ldc2_w #34 // long 88l

14: invokestatic #36 // Methodjava/lang/Long.valueOf:(

J)Ljava/lang/Long;

17: putfield #42 // Field b:Ljava/lang/Long;

20: aload_0

21: ldc #44 // String hello

23: putfield #46 // Field d:Ljava/lang/String;

26: aload_0

27: new #3 // class java/lang/Object

30: dup

31: invokespecial #18 // Method java/lang/Object."<init>

":()V

34: putfield #48 // Field e:Ljava/lang/Object;

37: aload_0

38: sipush 999

41: putfield #32 // Field a:I

44: aload_0

45: iload_1

46: putfield #32 // Field a:I

49: return

LineNumberTable:

line 24: 0

line 11: 4

line 12: 10

line 13: 20

line 14: 26

line 18: 37

line 25: 44

line 26: 49

LocalVariableTable:

Start Length Slot Name Signature

0 50 0 this Ltest;

0 50 1 param I

public static void fun(java.lang.String);

descriptor: (Ljava/lang/String;)V

flags: ACC_PUBLIC, ACC_STATIC

Code:

stack=2, locals=1, args_size=1

0: getstatic #55 // Field java/lang/System.out:Ljav

a/io/PrintStream;

3: aload_0

4: invokevirtual #61 // Methodjava/io/PrintStream.prin

tln:(Ljava/lang/String;)V

7: return

LineNumberTable:

line 29: 0

line 30: 7

LocalVariableTable:

Start Length Slot Name Signature

0 8 0 str Ljava/lang/String;

public static void main(java.lang.String[]);

descriptor: ([Ljava/lang/String;)V

flags: ACC_PUBLIC, ACC_STATIC

Code:

stack=3, locals=2, args_size=1

0: new #1 // class test

3: dup

4: sipush 999

7: invokespecial #69 // Method"<init>":(I)V

10: astore_1

11: aload_1

12: getfield #46 // Field d:Ljava/lang/String;

15: invokestatic #71 // Methodfun:(Ljava/lang/String;)

18: return

LineNumberTable:

line 38: 0

line 39: 11

line 40: 18

LocalVariableTable:

Start Length Slot Name Signature

0 19 0 args [Ljava/lang/String;

11 8 1 t Ltest;

--這就是test類的方法代碼模塊，拿構造方法爲例子：

test(int);

descriptor: (I)V

flags:

Code:

stack=3, locals=2, args_size=2

0: aload_0

1: invokespecial #18 // Methodjava/lang/Object."<init>

":()V

4: aload_0

5: bipush 99

7: putfield #32 // Field a:I

10: aload_0

11: ldc2_w #34 // long 88l

14: invokestatic #36 // Methodjava/lang/Long.valueOf:(

J)Ljava/lang/Long;

17: putfield #42 // Field b:Ljava/lang/Long;

20: aload_0

21: ldc #44 // String hello

23: putfield #46 // Field d:Ljava/lang/String;

26: aload_0

27: new #3 // class java/lang/Object

30: dup

31: invokespecial #18 // Method java/lang/Object."<init>

":()V

34: putfield #48 // Field e:Ljava/lang/Object;

37: aload_0

38: sipush 999

41: putfield #32 // Field a:I

44: aload_0

45: iload_1

46: putfield #32 // Field a:I

49: return

LineNumberTable:

line 24: 0

line 11: 4

line 12: 10

line 13: 20

line 14: 26

line 18: 37

line 25: 44

line 26: 49

LocalVariableTable:

Start Length Slot Name Signature

0 50 0 this Ltest;

0 50 1 param I

--可以看到descriptor: (I)V是方法的簽名，表示有一個int類型的參數，返回類型是void的方法，flags:表示訪問權限和靜態屬性，可見改方法是默認的包訪問權限和非靜態，Code:

stack=3, locals=2, args_size=2

0: aload_0

1: invokespecial #18 // Methodjava/lang/Object."<init>

":()V

4: aload_0

5: bipush 99

7: putfield #32 // Field a:I

10: aload_0

11: ldc2_w #34 // long 88l

14: invokestatic #36 // Methodjava/lang/Long.valueOf:(

J)Ljava/lang/Long;

17: putfield #42 // Field b:Ljava/lang/Long;

20: aload_0

21: ldc #44 // String hello

23: putfield #46 // Field d:Ljava/lang/String;

26: aload_0

27: new #3 // class java/lang/Object

30: dup

31: invokespecial #18 // Method java/lang/Object."<init>

":()V

34: putfield #48 // Field e:Ljava/lang/Object;

37: aload_0

38: sipush 999

41: putfield #32 // Field a:I

44: aload_0

45: iload_1

46: putfield #32 // Field a:I

49: return

--其實就是代碼命令了，stack=3,locals=2, args_size=2，stack應該表示棧深，locals應該表示局部變量個數，args_size表示參數個數，分別是this和param。

0: aload_0

1:invokespecial #18 //Method java/lang/Object."<init>":()V

--這兩行是啥意思呢？其實這是編譯器自己添加的代碼，調用父類的構造方法，而test的父類就是上帝類Object。這說明，初始化之類之前必須初始化父類，如果你不寫，編譯器也會幫你寫。

4:aload_0

5:bipush 99

7:putfield #32 // Field a:I

--這個意思是把99壓入棧中並且賦值給成員變量#32，而#32在常量池中就是a。

10:aload_0

11:ldc2_w #34 // long 88l

14:invokestatic #36 // Methodjava/lang/Long.valueOf:(

J)Ljava/lang/Long;

17:putfield #42 // Field b:Ljava/lang/Long;

--這幾行意思就是把88l作爲參數傳入Long.valueOf（long）方法中，生成一個Long類型對象並且賦值給成員變量b,其實就是Long b = 88l;這行代碼，只不過編譯器使用了語法糖自動裝載幫我們增加了調用Long.valueOf的方法生成了Long類型對象。

20:aload_0

21:ldc #44 // String hello

23:putfield #46 // Field d:Ljava/lang/String;

--這幾行意思就是把#44所表示的“hello”字符串賦值給成員變量d，其實就是d = "hello";

26: aload_0

27:new #3 // class java/lang/Object

30: dup

31:invokespecial #18 //Method java/lang/Object."<init>":()V

34:putfield #48 // Field e:Ljava/lang/Object;

37:aload_0

38:sipush 999

41:putfield #32 // Field a:I

44:aload_0

45: iload_1

46:putfield #32 // Field a:I

49:return

--這幾行就不細說了，跟上面大同小異，分別表示代碼：

Object e = new Object();

a=999;

a = param;

--從上面的構造方法的解析可以看到，雖然我們在源碼中寫的構造方法是test(intparam){a = param;}

但是實際上編譯器生成的構造方法可不這麼簡單，1.首先是調用父類Object的構造方法。2.把成員變量定義時的初始化操作也添加進了構造方法中。3.把{}語句塊中的賦值操作也添加進構造方法中。4.最後一步纔是源碼中的代碼。知道這個步驟，我們會對初始化有更深的瞭解。

除了舉例子說明的構造方法外，可以看到還有其他方法，其中比較奇怪的是

static{};

descriptor: ()V

flags: ACC_STATIC

Code:

stack=2, locals=0, args_size=0

0: new #3 // class java/lang/Object

3: dup

4: invokespecial #18 // Method java/lang/Object."<init>

":()V

7: putstatic #21 // Field c:Ljava/lang/Object;

10: ldc #23 // String world

12: putstatic #25 // Field f:Ljava/lang/String;

15: ldc #27 // String world2

17: putstatic #25 // Field f:Ljava/lang/String;

20: return

--這個其實就是大名鼎鼎的clinit方法（不過奇怪的是爲什麼方法名不是直接寫clinit），由編譯器生成，作用就是初始化靜態成員變量，如上面代碼翻譯過來就是

static Object c = new Object();

static String f = "world";

f = "world2";

--剩下的其他方法就沒什麼好說了，和源碼一一對應

補充：

1. LineNumberTable:

line 24: 0

line 11: 4

line 12: 10

line 13: 20

line 14: 26

line 18: 37

line 25: 44

line 26: 49

LocalVariableTable:

Start Length Slot Name Signature

0 50 0 this Ltest;

0 50 1 param I

--這些是什麼？其實這些不是很重要，只是附加信息，其中LineNumberTable:是方法裏面的命令行數和.java源碼文件代碼行數的對應關係，比如line 12: 10就是源碼中的第12行代碼和字節碼中構造方法中CODE段的第10行命令。應該是爲了檢查排除bug使用。

2. LocalVariableTable:

Start Length Slot Name Signature

0 50 0 this Ltest;

0 50 1 param I

--這個又是什麼，也是附加信息，描述的是方法內部局部變量（包括參數）的信息，比如上面就是對應構造方法，一共有兩個局部變量，分別是隱藏的this和參數param,並且它們的作用域都是構造方法中命令行的0到50行，其實就是整個構造方法內有效。

3. 如果把static{}註釋掉，只要test內部有static變量，那麼編譯器還是會自動添加clinit方法，如果把所有static變量和語句塊註釋掉，那麼編譯器將不再生成clinit方法。只要存在一個static成員變量，則編譯器都會自動生成clinit方法，該方法的方法簽名和默認構造方法一模一樣，只是flag屬性是ACC_STATIC（靜態）

4. 如果再創建一個子類ChildTest繼承Test類，那麼字節碼有什麼變化嗎？應該說變化不大

public final classChildTest extends test

minor version: 0

major version: 52

flags: ACC_PUBLIC, ACC_FINAL, ACC_SUPER

--只是在對Class描述段，裏面顯示了ChildTest繼承於test，並且是final屬性，flags的內容也做相應修改。還有就是因爲test重載了構造方法，創造了一個帶參數的構造方法，所以父類test已經不存在默認構造方法，這就使子類必須手工聲明和調用super(int)來初始化父類。如果父類存在默認構造方法（也就是無參構造方法），則子類不必手工調用super(),編譯器會自動添加，有一點需注意：子類的static成員初始化在父類初始化之前，簡單來說clinit方法在init方法前執行，並且只在加載時執行一次。

總結：通過對class文件的分析，我們能更深入地去了解java

另外附三個擴展連接：裏面有對命令和名詞的更詳細說明，就是有個不好的地方，沒有總體的概念

http://blog.csdn.net/lisulong1/article/details/53001211

http://baike.baidu.com/link?url=qJIIGWuC_EV-MmgebWE08fxVgYYbh_j-xuRDnrokIaO23zn-ogJ5Q5Kp40vbi6OrmrejF4BwtigOKzOWdtdXmIt1whTsMITcVMO_mP_mIXeoPAk3KiG-QRaScJxf5XJO

http://blog.csdn.net/xieyuooo/article/details/17452383