昨天上課

發表了 HW2 & 3
真夠勁，打算用禮拜六日來寫

講到了Class檔的format
如何把java所有資訊塞到class檔.const data, code segement這些東西
Java在執行過程中擁有完整的資訊，所以可以實現Reflection機制
及一些安全性的東西(validation)
JVM在class還沒開始執行前會檢查檔案是否被修改過
由於register是一個stack，它會檢查這個stack在每個instruction的狀態

這節課讓我對JVM的design有初步認識

把一個class dump出來看看,長這樣
這是一種二進位檔的format, 有點像ELF/PE這類的東西
有標頭檔, Data, code segment, 給JVM吃的
其中Code Segment就沒有解碼了

u4 magic;                           cafebabe
u2 minor_version;                   0000
u2 major_version;                   0032
u2 constant_pool_count;             0039 (57)

constant_pool: 
01: 10: Methodref(class: 15, 29)
02:  9: Field(class: 11, 30)
03:  9: Field(class: 31, 32)
04:  7: Class(33)
05: 10: Methodref(class: 4, 29)
06:  8: String(34)
07: 10: Methodref(class: 4, 35)
08: 10: Methodref(class: 4, 36)
09: 10: Methodref(class: 4, 37)
10: 10: Methodref(class: 38, 39)
11:  7: Class(40)
12: 10: Methodref(class: 11, 41)
13: 10: Methodref(class: 11, 42)
14:  8: String(43)
15:  7: Class(44)
16:  7: Class(45)
17:  1: UTF8(4,"data")
18:  1: UTF8(1,"I")
19:  1: UTF8(6,"")
20:  1: UTF8(4,"(I)V")
21:  1: UTF8(4,"Code")
22:  1: UTF8(15,"LineNumberTable")
23:  1: UTF8(3,"run")
24:  1: UTF8(3,"()V")
25:  1: UTF8(4,"main")
26:  1: UTF8(22,"([Ljava/lang/String;)V")
27:  1: UTF8(10,"SourceFile")
28:  1: UTF8(13,"MyThread.java")
29: 12: NameAndType(19,24)
30: 12: NameAndType(17,18)
31:  7: Class(46)
32: 12: NameAndType(47,48)
33:  1: UTF8(23,"java/lang/String/Builder")
34:  1: UTF8(4,"Run ")
35: 12: NameAndType(49,50)
36: 12: NameAndType(49,51)
37: 12: NameAndType(52,53)
38:  7: Class(54)
39: 12: NameAndType(55,56)
40:  1: UTF8(8,"MyThread")
41: 12: NameAndType(19,20)
42: 12: NameAndType(23,24)
43:  1: UTF8(9,"JVM test!")
44:  1: UTF8(16,"java/lang/Object")
45:  1: UTF8(18,"java/lang/Runnable")
46:  1: UTF8(16,"java/lang/System")
47:  1: UTF8(3,"out")
48:  1: UTF8(21,"Ljava/io/PrintStream;")
49:  1: UTF8(6,"append")
50:  1: UTF8(45,"(Ljava/lang/String;)Ljava/lang/StringBuilder;")
51:  1: UTF8(28,"(I)Ljava/lang/StringBuilder;")
52:  1: UTF8(8,"toString")
53:  1: UTF8(20,"()Ljava/lang/String;")
54:  1: UTF8(19,"java/io/PrintStream")
55:  1: UTF8(7,"println")
56:  1: UTF8(21,"(Ljava/lang/String;)V")

u2 access_flags;                        0020
u2 this_class;                          000b
u2 super_class;                         000f
u2 interfaces_count;                    0001 
u2 interfaces[interfaces_count];        0010
u2 fields_count;   0001
field_info fields[fields_count];

filelds[1] {
  u2 access_flages;                     0000
  u2 name_index;                        0011
  u2 descriptor_index;                  0012
  u2 attributes_count;                  0000
  attribute_info attributes;
}

u2 methods_count;                       0003

Method[1] {
  u2 access_flags;                      0000
  u2 name_index;                        0013
  u2 descriptor_index;                  0014
  u2 attributes_count;                  0001
    u2 attribute_name_index;            0015
    u4 attribute_length;                0000 002a (42)
    u1 info[attribute_length];
          00 02 00 02 00 00 00 0a 2a b7 00 01 2a 1b b5 00 02 b1 00 00 00 01 00 16 00 00 00 0e 00 03 00 00 00 07 00 04 00 08 00 09 00 09
}



Method[2] {
  u2 access_flags;                      0001
  u2 name_index;                        0017
  u2 descriptor_index;                  0018
  u2 attributes_count;                  0001
    u2 attribute_name_index;            0015
    u4 attribute_length;                0000 0039 (57)
    u1 info[attribute_length];
   00 03 00 01 00 00 00 1d b2 00 03 bb 00 04 59 b7 00 05 12 06 b6 00 07 2a b4 00 02 b6 00 08 b6 00 09 b6 00 0a b1 00 00 00 01 00 16 00 00 00 0a 00 02 00 00 00 0b 00 1c 00 0c
}

Methid[3] {
  u2 access_flags;                      0009
  u2 name_index;                        0019
  u2 descriptor_index;                  001a
  u2 attributes_count;                  0001
    u2 attribute_name_index;            0015
    u4 attribute_length;                0000 003b (59)
    u1 info[attribute_length];
        00 03 00 02 00 00 00 17 bb 00 0b 59 10 0a b7 00 0c 4c 2b b6 00 0d b2 00 03 12 0e b6 00 0a b1 00 00 00 01 00 16 00 00 00 12 00 04 00 00 00 0e 00 0a 00 0f 00 0e 00 10 00 16 00 11
}

Attributes count:                       0001
Attributes:
               
    SourceFile_attribute {
        u2 attribute_name_index; 001b
        u4 attribute_length;  0000 0002
        u2 sourcefile_index;  001c
    }