對于constantvalue_index意義如下:
long CONSTANT_Long
float CONSTANT_Float
double CONSTANT_Double
int, short, char, byte, boolean CONSTANT_Integer
String CONSTANT_String
ConstantValue用于field_info 中,用于描述一個static常量,
且此時field_info的access_flags應為ACC_STATIC
3. Code : attribute_info被替代為:
Code_attribute {
u2 attribute_name_index;
u4 attribute_length;
u2 max_stack; //執行此函數時可用的棧的最大深度
u2 max_locals; //執行此函數可用到的最大本地變量數目,包括參數。
// 注意:一個long/double相當于2個變量數目.
u4 code_length; //本函數用到的代碼長度。
u1 code[code_length]; //實現本函數的真正字節碼
u2 exception_table_length;
{ u2 start_pc;
u2 end_pc; //捕獲違例時執行代碼數組中的[start_pc, end_pc)部分
u2 handler_pc; //現在還不大明白他是干嘛的!!
u2 catch_type; //指向constant_pool的索引,對應CONSTANT_Class_info
}exception_table[exception_table_length];
u2 attributes_count;
attribute_info attributes[attributes_count];
}
CONSTANT_Class_info {
u1 tag; //必須為CONSTANT_Class (7)
u2 name_index; //不用我再說了吧?
}
Code屬性用于method_info結構中。
4. Exceptions : attribute_info被替代為:
Exceptions_attribute {
u2 attribute_name_index;
u4 attribute_length;
u2 number_of_exceptions;
u2 exception_index_table[number_of_exceptions];
}
5. InnerClasses : attribute_info被替代為:
InnerClasses_attribute {
u2 attribute_name_index;
u4 attribute_length;
u2 number_of_classes;
{ u2 inner_class_info_index;
u2 outer_class_info_index;
u2 inner_name_index;
u2 inner_class_access_flags;
} classes[number_of_classes];
}
6. Synthetic : attribute_info被替代為:
Synthetic_attribute {
u2 attribute_name_index; //不用廢話了吧?
u4 attribute_length; //必須為0
}
Synthetic用在 field_info、 method_info 中,
一個沒有出現在源程序中的變量必須使用Synthetic標記。
7. LineNumberTable : attribute_info被替代為:
LineNumberTable_attribute {
u2 attribute_name_index;
u4 attribute_length;
u2 line_number_table_length;
{ u2 start_pc; //代碼數組中的開始處
u2 line_number; //源文件中的行號(對于每一非空行都有這么一項)
} line_number_table[line_number_table_length];
}
LineNumberTable用于Code屬性中,通常用于調試。
8. LocalVariableTable : attribute_info被替代為:
LocalVariableTable_attribute {
u2 attribute_name_index;
u4 attribute_length;
u2 local_variable_table_length;
{ u2 start_pc;
u2 length; //當解釋到代碼數組的[start_pc,start_pc+length]
//時變量必須被賦值??
u2 name_index;
u2 descriptor_index;
u2 index; //到本地變量數組的一個索引
} local_variable_table[local_variable_table_length];
}
9. Deprecated : attribute_info被替代為:
Deprecated_attribute {
u2 attribute_name_index;
u4 attribute_length; //必須為0
}
當然你也可以定義自己的屬性,但要你自己的編譯器和虛擬機實現。JVM將忽略自己不認可的屬性。
來實踐一下吧!
編寫一個最簡單的程序:
class Test
{
public static void main(String[] args)
{
System.out.println("Hello World!");
}
}
c:\work>javac Test.java
c:\work>filedump Test.class
File Dump V0.3 Beta by cloud (safesuite@363.net).
01:00 ca fe ba be 00 03 00 2d 00 1d 0a 00 06 00 0f 09 .......-........
01:01 00 10 00 11 08 00 12 0a 00 13 00 14 07 00 15 07 ................
01:02 00 16 01 00 06 3c 69 6e 69 74 3e 01 00 03 28 29 .....<init>...()
01:03 56 01 00 04 43 6f 64 65 01 00 0f 4c 69 6e 65 4e V...Code...LineN
01:04 75 6d 62 65 72 54 61 62 6c 65 01 00 04 6d 61 69 umberTable...mai
01:05 6e 01 00 16 28 5b 4c 6a 61 76 61 2f 6c 61 6e 67 n...([Ljava/lang
01:06 2f 53 74 72 69 6e 67 3b 29 56 01 00 0a 53 6f 75 /String;)V...Sou
01:07 72 63 65 46 69 6c 65 01 00 09 54 65 73 74 2e 6a rceFile...Test.j
>d
02:00 61 76 61 0c 00 07 00 08 07 00 17 0c 00 18 00 19 ava.............
02:01 01 00 0c 48 65 6c 6c 6f 20 57 6f 72 6c 64 21 07 ...Hello World!.
02:02 00 1a 0c 00 1b 00 1c 01 00 04 54 65 73 74 01 00 ..........Test..
02:03 10 6a 61 76 61 2f 6c 61 6e 67 2f 4f 62 6a 65 63 .java/lang/Objec
02:04 74 01 00 10 6a 61 76 61 2f 6c 61 6e 67 2f 53 79 t...java/lang/Sy
02:05 73 74 65 6d 01 00 03 6f 75 74 01 00 15 4c 6a 61 stem...out...Lja
02:06 76 61 2f 69 6f 2f 50 72 69 6e 74 53 74 72 65 61 va/io/PrintStrea
02:07 6d 3b 01 00 13 6a 61 76 61 2f 69 6f 2f 50 72 69 m;...java/io/Pri
>d
03:00 6e 74 53 74 72 65 61 6d 01 00 07 70 72 69 6e 74 ntStream...print
03:01 6c 6e 01 00 15 28 4c 6a 61 76 61 2f 6c 61 6e 67 ln...(Ljava/lang
03:02 2f 53 74 72 69 6e 67 3b 29 56 00 20 00 05 00 06 /String;)V. ....
03:03 00 00 00 00 00 02 00 00 00 07 00 08 00 01 00 09 ................
03:04 00 00 00 1d 00 01 00 01 00 00 00 05 2a b7 00 01 ............*...
03:05 b1 00 00 00 01 00 0a 00 00 00 06 00 01 00 00 00 ................
03:06 01 00 09 00 0b 00 0c 00 01 00 09 00 00 00 25 00 ..............%.
03:07 02 00 01 00 00 00 09 b2 00 02 12 03 b6 00 04 b1 ................
>d
04:00 00 00 00 01 00 0a 00 00 00 0a 00 02 00 00 00 05 ................
04:01 00 08 00 06 00 01 00 0d 00 00 00 02 00 0e ..............
>
解讀一下:
ca fe ba be magic
00 03 00 2d 次主版本號,換算一下: 45.3 (注意,不是高字節在前,別犯職業!)
00 1d constant_pool元素個數加一 : 29 那么constant_pool就是[1-28]
constant_pool: 1-28
1. 0a 00 06 00 0f
0x0a :CONSTANT_InterfaceMethodref 0x06 :class index 0x0f :name-type-index
2. 09 00 10 00 11
0x09 : CONSTANT_Fieldref 0x10: . . . 0x11 : . . . .
3. 08 00 12 0x08 : CONSTANT_String 0x12 : string_index
4. 0a 00 13 00 14 0x0a同于1.
5. 07 00 15 0x07 : CONSTANT_Class 0x15 : name_index
6. 07 00 16
7. 01 00 06 3c 69 6e 69 74 3e 01 ...<init>
0x01 CONSTANT_Utf8 0x06 : string length "<init>" : 構造函數
8. 01 00 03 28 29 56 ...()V 函數,無參數
0x01 . . . . . . 0x03 : . . . . "()V" : . . .
9. 01 00 04 43 6f 64 65 ...Code
10. 01 00 0f 4c 69 6e 65 4e 75 6d 62 65 72 54 61 62 6c 65 ...LineNumberTable
11. 01 00 04 6d 61 69 6e ...main
12. 01 00 16 28 5b 4c 6a 61 76 61 2f 6c 61 6e 67 2f 53 74 72 69 6e 67 3b 29 56
...([Ljava/lang/String;)
V 函數,參數為String[]類型
13. 01 00 0a 53 6f 75 72 63 65 46 69 6c 65 ...SourceFile
14. 01 00 09 54 65 73 74 2e 6a 61 76 61 ...Test.java
15. 0c 00 07 00 08 0x0c:CONSTANT_NameAndType 07 : name-index 08:name-type-index
16. 07 00 17
17. 0c 00 18 00 19
18. 01 00 0c 48 65 6c 6c 6f 20 57 6f 72 6c 64 21 ...Hello World!
19. 07 00 1a
20. 0c 00 1b 00 1c
21. 01 00 04 54 65 73 74 ...Test
22. 01 00 10 6a 61 76 61 2f 6c 61 6e 67 2f 4f 62 6a 65 63 74 ...java/lang/Object
23. 01 00 10 6a 61 76 61 2f 6c 61 6e 67 2f 53 79 73 74 65 6d ...java/lang/System
24. 01 00 03 6f 75 74 ...out
25. 01 00 15 4c 6a 61 76 61 2f 69 6f 2f 50 72 69 6e 74 53 74 72 65 61 6d 3b
...Ljava/io/PrintStream;
26. 01 00 13 6a 61 76 61 2f 69 6f 2f 50 72 69 6e 74 53 74 72 65 61 6d...java/io/PrintStream
27. 01 00 07 70 72 69 6e 74 6c 6e ...println
28. 01 00 15 28 4c 6a 61 76 61 2f 6c 61 6e 67 2f 53 74 72 69 6e 67 3b 29 56
...(Ljava/lang/String;)V
00 20 access_flags
00 05 this_class
00 06 super_class
00 00 interfaces_count
00 00 fields_count
00 02 methods_count
methods[2]:
method_info {
u2 access_flags;
u2 name_index;
u2 descriptor_index;
u2 attributes_count;
attribute_info attributes[attributes_count];
}
0. 00 00 access_flags;
00 07 name_index; 看看constant_pool的第7項: <init> 表明當前描述構造函數
00 08 descriptor_index;
00 01 attributes_count;
00 09 attribute_name_index 0x09 看看constant_pool的第9項,簡單明了Code !
00 00 00 1d attribute_length = 29
00 01 max_stack
00 01 max_locals
00 00 00 05 code_length
2a b7 00 01 b1 JVM定義的操作碼代碼段數組
00 00 exception_table_length
00 01 attributes_count 一個,接下來是attribute_info結構
00 0a attribute_name_index 看看constant_pool的第10項: LineNumberTable(顯然調試用)
00 00 00 06 attribute_length
00 01 line_number_table_length
00 00 start_pc
00 01 line_number
1. 00 09 access_flags PUBLIC & STATIC
00 0b name_index; 表明當前描述main函數
00 0c descriptor_index; ...([Ljava/lang/String;)V
00 01 attributes_count;
00 09 attribute_name_index Code
00 00 00 25 attribute_length = 37
00 02 max_stack
00 01 max_locals
00 00 00 09 code_length
b2 00 02 12 03 b6 00 04 b1 代碼數組 codeArray1[0-8]
00 00 exception_table_length
00 01 attributes_count 接下來是attribute_info結構
00 0a attribute_name_index LineNumberTable
00 00 00 0a attribute_length
00 02 line_number_table_length
00 00 start_pc
00 05 line_number
00 08 start_pc : codeArray1[8] = 0xb1 --> return
00 06 line_number 第6行源程序為 }
00 01 attributes_count
00 0d attribute_name_index 屬性為SourceFile
00 00 00 02 attribute_length
00 0e sourcefile_index constant_pool[0x0e] --- > "Test.java"
接下來我們看看main()函數對應的代碼:
b2 00 02 12 03 b6 00 04 b1
0xb2 0x00 0x02 : getstatic #2
看看constant_pool[0x02] :09 00 10 00 11
0x09 : CONSTANT_Fieldref 0x10: class index 0x11 :name-type-index
constant_pool[0x10]: --> constant_pool[0x17] : java/lang/System
constant_pool[0x11]: 0x18 : class index 0x19 :name-type-index
constant_pool[0x18] : out
constant_pool[0x19] : Ljava/io/PrintStream
即 System.out 為 java.io.PrintStream 類型
0x12 0x03 : ldc #3
看看 constant_pool[3] : 08 00 12 0x08 : CONSTANT_String 0x12 : string_index
指向一個字符串 constant_pool[0x12]: "Hello World!"
故該指令加載了字符串索引到本地棧
0xb6 0x00 0x04: invokevirtual #4
------->到constant_pool查查 0x13 :class 0x14 :name-type
看看constant_pool[0x13]:java/io/PrintStream
constant_pool[20] :--> 00 1b 00 1c
constant_pool[0x1b]:println
. . . . :(Ljava/lang/String;)V
故該指令調用了 java.io.PrintStream.println(java.lang.String)
而參數則為 ldc #3 加載的 "Hello World!"
0xb1 : return
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