56 - ASM之ClassFile和Instruction

2022-03-10  本文已影响0人  舍是境界

ClassFile对方法的约束

从ClassFile的角度来说，它对于方法接收的参数数量、方法体的大小做了约束。

方法参数的数量（255）

在一个方法当中，方法接收的参数最多有255个。我们分成三种情况来进行说明：

public class HelloWorld {
    public void test(int a, int b) {
        // do nothing
    }

    public static void main(String[] args) {
        for (int i = 1; i <= 255; i++) {
            String item = String.format("int val%d,", i);
            System.out.println(item);
        }
    }
}

在 Java Virtual Machine Specification的 4.11. Limitations of the Java Virtual Machine部分对方法参数的数量进行了限制：

The number of method parameters is limited to 255 by the definition of a method descriptor, where the limit includes one unit for this in the case of instance or interface method invocations.

方法体的大小（65535）

对于方法来说，方法体并不是想写多少代码就写多少代码，它的大小也有一个限制：方法体内最多包含65535个字节。

当方法体的代码超过65535字节（bytes）时，会出现编译错误：code too large。
在 Java Virtual Machine Specification的 4.7.3. The Code Attribute部分定义了Code属性：

Code_attribute {
    u2 attribute_name_index;
    u4 attribute_length;
    u2 max_stack;
    u2 max_locals;
    u4 code_length;
    u1 code[code_length];
    u2 exception_table_length;
    {   u2 start_pc;
        u2 end_pc;
        u2 handler_pc;
        u2 catch_type;
    } exception_table[exception_table_length];
    u2 attributes_count;
    attribute_info attributes[attributes_count];
}

code_length: The value of the code_length item gives the number of bytes in the code array for this method. The value of code_length must be greater than zero (as the code array must not be empty) and less than 65536.

Instruction VS. Opcode

严格的来说，instruction和opcode这两个概念是有区别的。相对来说，instruction是一个比较大的概念，而opcode是一个比较小的概念：

instruction = opcode + operands

A Java Virtual Machine instruction consists of a one-byte opcode specifying the operation to be performed, followed by zero or more operands supplying arguments or data that are used by the operation. Many instructions have no operands and consist only of an opcode.
参考： 2.11. Instruction Set Summary

Opcodes

opcode mnemonic symbol opcode mnemonic symbol opcode mnemonic symbol opcode mnemonic symbol
0 nop 64 lstore_1 128 ior 192 checkcast
1 aconst_null 65 lstore_2 129 lor 193 instanceof
2 iconst_m1 66 lstore_3 130 ixor 194 monitorenter
3 iconst_0 67 fstore_0 131 lxor 195 monitorexit
4 iconst_1 68 fstore_1 132 iinc 196 wide
5 iconst_2 69 fstore_2 133 i2l 197 multianewarray
6 iconst_3 70 fstore_3 134 i2f 198 ifnull
7 iconst_4 71 dstore_0 135 i2d 199 ifnonnull
8 iconst_5 72 dstore_1 136 l2i 200 goto_w
9 lconst_0 73 dstore_2 137 l2f 201 jsr_w
10 lconst_1 74 dstore_3 138 l2d 202 breakpoint
11 fconst_0 75 astore_0 139 f2i 203
12 fconst_1 76 astore_1 140 f2l 204
13 fconst_2 77 astore_2 141 f2d 205
14 dconst_0 78 astore_3 142 d2i 206
15 dconst_1 79 iastore 143 d2l 207
16 bipush 80 lastore 144 d2f 208
17 sipush 81 fastore 145 i2b 209
18 ldc 82 dastore 146 i2c 210
19 ldc_w 83 aastore 147 i2s 211
20 ldc2_w 84 bastore 148 lcmp 212
21 iload 85 castore 149 fcmpl 213
22 lload 86 sastore 150 fcmpg 214
23 fload 87 pop 151 dcmpl 215
24 dload 88 pop2 152 dcmpg 216
25 aload 89 dup 153 ifeq 217
26 iload_0 90 dup_x1 154 ifne 218
27 iload_1 91 dup_x2 155 iflt 219
28 iload_2 92 dup2 156 ifge 220
29 iload_3 93 dup2_x1 157 ifgt 221
30 lload_0 94 dup2_x2 158 ifle 222
31 lload_1 95 swap 159 if_icmpeq 223
32 lload_2 96 iadd 160 if_icmpne 224
33 lload_3 97 ladd 161 if_icmplt 225
34 fload_0 98 fadd 162 if_icmpge 226
35 fload_1 99 dadd 163 if_icmpgt 227
36 fload_2 100 isub 164 if_icmple 228
37 fload_3 101 lsub 165 if_acmpeq 229
38 dload_0 102 fsub 166 if_acmpne 230
39 dload_1 103 dsub 167 goto 231
40 dload_2 104 imul 168 jsr 232
41 dload_3 105 lmul 169 ret 233
42 aload_0 106 fmul 170 tableswitch 234
43 aload_1 107 dmul 171 lookupswitch 235
44 aload_2 108 idiv 172 ireturn 236
45 aload_3 109 ldiv 173 lreturn 237
46 iaload 110 fdiv 174 freturn 238
47 laload 111 ddiv 175 dreturn 239
48 faload 112 irem 176 areturn 240
49 daload 113 lrem 177 return 241
50 aaload 114 frem 178 getstatic 242
51 baload 115 drem 179 putstatic 243
52 caload 116 ineg 180 getfield 244
53 saload 117 lneg 181 putfield 245
54 istore 118 fneg 182 invokevirtual 246
55 lstore 119 dneg 183 invokespecial 247
56 fstore 120 ishl 184 invokestatic 248
57 dstore 121 lshl 185 invokeinterface 249
58 astore 122 ishr 186 invokedynamic 250
59 istore_0 123 lshr 187 new 251
60 istore_1 124 iushr 188 newarray 252
61 istore_2 125 lushr 189 anewarray 253
62 istore_3 126 iand 190 arraylength 254 impdep1
63 lstore_0 127 land 191 athrow 255 impdep2

参考： Chapter 6. The Java Virtual Machine Instruction Set

mnemonic symbol和类型信息

对于大多数的opcode，它的mnemonic symbol名字当中带有类型信息（type information）。一般情况下，规律如下：

有一些opcode，它的mnemonic symbol名字不带有类型信息（type information），但是可以操作多种类型的数据，例如：

还有一些opcode，它的mnemonic symbol名字不带有类型信息（type information），它也不处理任何类型的数据。例如：

For the majority of typed instructions, the instruction type is represented explicitly in the opcode mnemonic by a letter: i for an int operation, l for long, s for short, b for byte, c for char, f for float, d for double, and a for reference. Some instructions for which the type is unambiguous do not have a type letter in their mnemonic. For instance, arraylength always operates on an object that is an array. Some instructions, such as goto, an unconditional control transfer, do not operate on typed operands.
参考： 2.11.1. Types and the Java Virtual Machine

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