Java ASM系列：（018）Opcodes介绍
本文属于[Java ASM系列一：Core API](https://blog.51cto.com/lsieun/2924583)当中的一篇。
`Opcodes`是一个接口，它定义了许多字段。这些字段主要是在`ClassVisitor.visitXxx()`和`MethodVisitor.visitXxx()`方法中使用。
## 1. ClassVisitor
### 1.1 ASM Version
字段含义：`Opcodes.ASM4`~`Opcodes.ASM9`标识了ASM的版本信息。
应用场景：用于创建具体的`ClassVisitor`实例，例如`ClassVisitor(int api, ClassVisitor classVisitor)`中的`api`参数。
```java
public interface Opcodes {
// ASM API versions.
int ASM4 = 4 << 16 | 0 << 8;
int ASM5 = 5 << 16 | 0 << 8;
int ASM6 = 6 << 16 | 0 << 8;
int ASM7 = 7 << 16 | 0 << 8;
int ASM8 = 8 << 16 | 0 << 8;
int ASM9 = 9 << 16 | 0 << 8;
}
```
### 1.2 Java Version
字段含义：`Opcodes.V1_1`~`Opcodes.V16`标识了`.class`文件的版本信息。
应用场景：用于`ClassVisitor.visit(int version, int access, ...)`的`version`参数。
```java
public interface Opcodes {
// Java ClassFile versions
// (the minor version is stored in the 16 most significant bits, and
// the major version in the 16 least significant bits).
int V1_1 = 3 << 16 | 45;
int V1_2 = 0 << 16 | 46;
int V1_3 = 0 << 16 | 47;
int V1_4 = 0 << 16 | 48;
int V1_5 = 0 << 16 | 49;
int V1_6 = 0 << 16 | 50;
int V1_7 = 0 << 16 | 51;
int V1_8 = 0 << 16 | 52;
int V9 = 0 << 16 | 53;
int V10 = 0 << 16 | 54;
int V11 = 0 << 16 | 55;
int V12 = 0 << 16 | 56;
int V13 = 0 << 16 | 57;
int V14 = 0 << 16 | 58;
int V15 = 0 << 16 | 59;
int V16 = 0 << 16 | 60;
}
```
### 1.3 Access Flags
字段含义：`Opcodes.ACC_PUBLIC`~`Opcodes.ACC_MODULE`标识了Class、Field、Method的访问标识（Access Flag）。
应用场景：
- `ClassVisitor.visit(int version, int access, ...)`的`access`参数。
- `ClassVisitor.visitField(int access, String name, ...)`的`access`参数。
- `ClassVisitor.visitMethod(int access, String name, ...)`的`access`参数。
```java
public interface Opcodes {
int ACC_PUBLIC = 0x0001; // class, field, method
int ACC_PRIVATE = 0x0002; // class, field, method
int ACC_PROTECTED = 0x0004; // class, field, method
int ACC_STATIC = 0x0008; // field, method
int ACC_FINAL = 0x0010; // class, field, method, parameter
int ACC_SUPER = 0x0020; // class
int ACC_SYNCHRONIZED = 0x0020; // method
int ACC_OPEN = 0x0020; // module
int ACC_TRANSITIVE = 0x0020; // module requires
int ACC_VOLATILE = 0x0040; // field
int ACC_BRIDGE = 0x0040; // method
int ACC_STATIC_PHASE = 0x0040; // module requires
int ACC_VARARGS = 0x0080; // method
int ACC_TRANSIENT = 0x0080; // field
int ACC_NATIVE = 0x0100; // method
int ACC_INTERFACE = 0x0200; // class
int ACC_ABSTRACT = 0x0400; // class, method
int ACC_STRICT = 0x0800; // method
int ACC_SYNTHETIC = 0x1000; // class, field, method, parameter, module *
int ACC_ANNOTATION = 0x2000; // class
int ACC_ENUM = 0x4000; // class(?) field inner
int ACC_MANDATED = 0x8000; // field, method, parameter, module, module *
int ACC_MODULE = 0x8000; // class
}
```
## 2. MethodVisitor
### 2.1 frame
字段含义：`Opcodes.F_NEW`~`Opcodes.F_SAME1`标识了frame的状态，`Opcodes.TOP`~`Opcodes.UNINITIALIZED_THIS`标识了frame中某一个数据项的具体类型。
应用场景：
- `Opcodes.F_NEW`~`Opcodes.F_SAME1`用在`MethodVisitor.visitFrame(int type, int numLocal, Object[] local, int numStack, Object[] stack)`方法中的`type`参数。
- `Opcodes.TOP`~`Opcodes.UNINITIALIZED_THIS`用在`MethodVisitor.visitFrame(int type, int numLocal, Object[] local, int numStack, Object[] stack)`方法中的`local`参数和`stack`参数。
```java
public interface Opcodes {
// ASM specific stack map frame types, used in {@link ClassVisitor#visitFrame}.
int F_NEW = -1;
int F_FULL = 0;
int F_APPEND = 1;
int F_CHOP = 2;
int F_SAME = 3;
int F_SAME1 = 4;
// Standard stack map frame element types, used in {@link ClassVisitor#visitFrame}.
Integer TOP = Frame.ITEM_TOP;
Integer INTEGER = Frame.ITEM_INTEGER;
Integer FLOAT = Frame.ITEM_FLOAT;
Integer DOUBLE = Frame.ITEM_DOUBLE;
Integer LONG = Frame.ITEM_LONG;
Integer NULL = Frame.ITEM_NULL;
Integer UNINITIALIZED_THIS = Frame.ITEM_UNINITIALIZED_THIS;
}
```
### 2.2 opcodes
字段含义：`Opcodes.NOP`~`Opcodes.IFNONNULL`表示opcode的值。
应用场景：在`MethodVisitor.visitXxxInsn(opcode)`方法中的`opcode`参数中使用。
```java
public interface Opcodes {
int NOP = 0; // visitInsn
int ACONST_NULL = 1; // -
int ICONST_M1 = 2; // -
int ICONST_0 = 3; // -
int ICONST_1 = 4; // -
int ICONST_2 = 5; // -
int ICONST_3 = 6; // -
int ICONST_4 = 7; // -
int ICONST_5 = 8; // -
int LCONST_0 = 9; // -
int LCONST_1 = 10; // -
int FCONST_0 = 11; // -
int FCONST_1 = 12; // -
int FCONST_2 = 13; // -
int DCONST_0 = 14; // -
int DCONST_1 = 15; // -
int BIPUSH = 16; // visitIntInsn
int SIPUSH = 17; // -
int LDC = 18; // visitLdcInsn
int ILOAD = 21; // visitVarInsn
int LLOAD = 22; // -
int FLOAD = 23; // -
int DLOAD = 24; // -
int ALOAD = 25; // -
int IALOAD = 46; // visitInsn
int LALOAD = 47; // -
int FALOAD = 48; // -
int DALOAD = 49; // -
int AALOAD = 50; // -
int BALOAD = 51; // -
int CALOAD = 52; // -
int SALOAD = 53; // -
int ISTORE = 54; // visitVarInsn
int LSTORE = 55; // -
int FSTORE = 56; // -
int DSTORE = 57; // -
int ASTORE = 58; // -
int IASTORE = 79; // visitInsn
int LASTORE = 80; // -
int FASTORE = 81; // -
int DASTORE = 82; // -
int AASTORE = 83; // -
int BASTORE = 84; // -
int CASTORE = 85; // -
int SASTORE = 86; // -
int POP = 87; // -
int POP2 = 88; // -
int DUP = 89; // -
int DUP_X1 = 90; // -
int DUP_X2 = 91; // -
int DUP2 = 92; // -
int DUP2_X1 = 93; // -
int DUP2_X2 = 94; // -
int SWAP = 95; // -
int IADD = 96; // -
int LADD = 97; // -
int FADD = 98; // -
int DADD = 99; // -
int ISUB = 100; // -
int LSUB = 101; // -
int FSUB = 102; // -
int DSUB = 103; // -
int IMUL = 104; // -
int LMUL = 105; // -
int FMUL = 106; // -
int DMUL = 107; // -
int IDIV = 108; // -
int LDIV = 109; // -
int FDIV = 110; // -
int DDIV = 111; // -
int IREM = 112; // -
int LREM = 113; // -
int FREM = 114; // -
int DREM = 115; // -
int INEG = 116; // -
int LNEG = 117; // -
int FNEG = 118; // -
int DNEG = 119; // -
int ISHL = 120; // -
int LSHL = 121; // -
int ISHR = 122; // -
int LSHR = 123; // -
int IUSHR = 124; // -
int LUSHR = 125; // -
int IAND = 126; // -
int LAND = 127; // -
int IOR = 128; // -
int LOR = 129; // -
int IXOR = 130; // -
int LXOR = 131; // -
int IINC = 132; // visitIincInsn
int I2L = 133; // visitInsn
int I2F = 134; // -
int I2D = 135; // -
int L2I = 136; // -
int L2F = 137; // -
int L2D = 138; // -
int F2I = 139; // -
int F2L = 140; // -
int F2D = 141; // -
int D2I = 142; // -
int D2L = 143; // -
int D2F = 144; // -
int I2B = 145; // -
int I2C = 146; // -
int I2S = 147; // -
int LCMP = 148; // -
int FCMPL = 149; // -
int FCMPG = 150; // -
int DCMPL = 151; // -
int DCMPG = 152; // -
int IFEQ = 153; // visitJumpInsn
int IFNE = 154; // -
int IFLT = 155; // -
int IFGE = 156; // -
int IFGT = 157; // -
int IFLE = 158; // -
int IF_ICMPEQ = 159; // -
int IF_ICMPNE = 160; // -
int IF_ICMPLT = 161; // -
int IF_ICMPGE = 162; // -
int IF_ICMPGT = 163; // -
int IF_ICMPLE = 164; // -
int IF_ACMPEQ = 165; // -
int IF_ACMPNE = 166; // -
int GOTO = 167; // -
int JSR = 168; // -
int RET = 169; // visitVarInsn
int TABLESWITCH = 170; // visiTableSwitchInsn
int LOOKUPSWITCH = 171; // visitLookupSwitch
int IRETURN = 172; // visitInsn
int LRETURN = 173; // -
int FRETURN = 174; // -
int DRETURN = 175; // -
int ARETURN = 176; // -
int RETURN = 177; // -
int GETSTATIC = 178; // visitFieldInsn
int PUTSTATIC = 179; // -
int GETFIELD = 180; // -
int PUTFIELD = 181; // -
int INVOKEVIRTUAL = 182; // visitMethodInsn
int INVOKESPECIAL = 183; // -
int INVOKESTATIC = 184; // -
int INVOKEINTERFACE = 185; // -
int INVOKEDYNAMIC = 186; // visitInvokeDynamicInsn
int NEW = 187; // visitTypeInsn
int NEWARRAY = 188; // visitIntInsn
int ANEWARRAY = 189; // visitTypeInsn
int ARRAYLENGTH = 190; // visitInsn
int ATHROW = 191; // -
int CHECKCAST = 192; // visitTypeInsn
int INSTANCEOF = 193; // -
int MONITORENTER = 194; // visitInsn
int MONITOREXIT = 195; // -
int MULTIANEWARRAY = 197; // visitMultiANewArrayInsn
int IFNULL = 198; // visitJumpInsn
int IFNONNULL = 199; // -
}
```
### 2.3 opcode: newarray
字段含义：`Opcodes.T_BOOLEAN`~`Opcodes.T_LONG`表示数组的类型。
应用场景：对于`MethodVisitor.visitIntInsn(opcode, operand)`方法，当`opcode`为`NEWARRAY`时，`operand`参数中使用。
```java
public interface Opcodes {
// Possible values for the type operand of the NEWARRAY instruction.
int T_BOOLEAN = 4;
int T_CHAR = 5;
int T_FLOAT = 6;
int T_DOUBLE = 7;
int T_BYTE = 8;
int T_SHORT = 9;
int T_INT = 10;
int T_LONG = 11;
}
```
```java
public class HelloWorld {
public void test() {
byte[] bytes = new byte[10];
}
}
```
### 2.4 opcode: invokedynamic
字段含义：`Opcodes.H_GETFIELD`~`Opcodes.H_INVOKEINTERFACE`表示MethodHandle的类型。
应用场景：在创建`Handle(int tag, String owner, String name, String descriptor, boolean isInterface)`时，`tag`参数中使用；而该`Handle`实例会在`MethodVisitor.visitInvokeDynamicInsn()`方法使用到。
```java
public interface Opcodes {
// Possible values for the reference_kind field of CONSTANT_MethodHandle_info structures.
int H_GETFIELD = 1;
int H_GETSTATIC = 2;
int H_PUTFIELD = 3;
int H_PUTSTATIC = 4;
int H_INVOKEVIRTUAL = 5;
int H_INVOKESTATIC = 6;
int H_INVOKESPECIAL = 7;
int H_NEWINVOKESPECIAL = 8;
int H_INVOKEINTERFACE = 9;
}
```
```java
import java.util.function.BiFunction;
public class HelloWorld {
public void test() {
BiFunction<Integer, Integer, Integer> func = Math::max;
}
}
```
## 3. 总结
本文主要对`Opcodes`接口里定义的字段进行介绍，内容总结如下：
- 第一点，在`Opcodes`类定义的字段，主要应用于`ClassVisitor`和`MethodVisitor`类的`visitXxx()`方法。
- 第二点，记忆方法。由于`Opcodes`类定义的字段很多，我们可以分成不同的批次和类别来进行理解，慢慢去掌握。