ASM是什么？
ASM is an all purpose Java bytecode manipulation and analysis framework. It can be used to modify existing classes or dynamically generate classes, directly in binary form. Provided common transformations and analysis algorithms allow to easily assemble custom complex transformations and code analysis tools.
ASM offer similar functionality as other bytecode frameworks, but it is focused on simplicity of use and performance. Because it was designed and implemented to be as small and as fast as possible, it makes it very attractive for using in dynamic systems*.
(*) ASM can of course be used in a static way too.
如何学习ASM？
The best way to learn to use ASM is to write a Java source file that is equivalent to what you want to generate and then use the ASMifier mode of the Bytecode Outline plugin for Eclipse (or the ASMifier tool) to see the equivalent ASM code. If you want to implement a class transformer, write two Java source files (before and after transformation) and use the compare view of the plugin in ASMifier mode to compare the equivalent ASM code.
ASM的核心
在ASM的核心实现中，它主要有以下几个类、接口（在org.objectweb.asm包中）：
ClassReader类：字节码的读取与分析引擎。它采用类似SAX的事件读取机制，每当有事件发生时，调用注册的ClassVisitor、AnnotationVisitor、FieldVisitor、MethodVisitor做相应的处理。
ClassVisitor接口：定义在读取Class字节码时会触发的事件，如类头解析完成、注解解析、字段解析、方法解析等。
A visitor to visit a Java class. The methods of this class must be called in the following order: <tt>visit</tt> [ <tt>visitSource</tt> ] [
<tt>visitOuterClass</tt> ] ( <tt>visitAnnotation</tt> |
<tt>visitTypeAnnotation</tt> | <tt>visitAttribute</tt> )* (
<tt>visitInnerClass</tt> | <tt>visitField</tt> | <tt>visitMethod</tt> )*
<tt>visitEnd</tt>.
AnnotationVisitor接口：定义在解析注解时会触发的事件，如解析到一个基本值类型的注解、enum值类型的注解、Array值类型的注解、注解值类型的注解等。
A visitor to visit a Java annotation. The methods of this class must be
called in the following order: ( <tt>visit</tt> | <tt>visitEnum</tt> |
<tt>visitAnnotation</tt> | <tt>visitArray</tt> )* <tt>visitEnd</tt>.
FieldVisitor接口：定义在解析字段时触发的事件，如解析到字段上的注解、解析到字段相关的属性等。
MethodVisitor接口：定义在解析方法时触发的事件，如方法上的注解、属性、代码等。
ClassWriter类：它实现了ClassVisitor接口，用于拼接字节码。
A {@link ClassVisitor} that generates classes in bytecode form. More
precisely this visitor generates a byte array conforming to the Java class file format. It can be used alone, to generate a Java class "from scratch",or with one or more {@link ClassReader ClassReader} and adapter class visitor to generate a modified class from one or more existing Java classes.
AnnotationWriter类：它实现了AnnotationVisitor接口，用于拼接注解相关字节码。
FieldWriter类：它实现了FieldVisitor接口，用于拼接字段相关字节码。
MethodWriter类：它实现了MethodVisitor接口，用于拼接方法相关字节码。
SignatureReader类：对类定义、字段定义、方法定义、本地变量定义的签名的解析。Signature因范型引入，用于存储范型定义时的元数据（因为这些元数据在运行时会被擦除）。
SignatureVisitor接口：定义在解析Signature时会触发的事件，如正常的Type参数、类或接口的边界等。
SignatureWriter类：它实现了SignatureVisitor接口，用于拼接范型相关字节码。
Attribute类：字节码中属性的类抽象。
ByteVector类：字节码二进制存储的容器。
Opcodes接口：字节码指令的一些常量定义。
Type类：类型相关的常量定义以及一些基于其上的操作。
JVM操作在ASM中的定义
/**
* Defines the JVM opcodes, access flags and array type codes. This interface
* does not define all the JVM opcodes because some opcodes are automatically
* handled. For example, the xLOAD and xSTORE opcodes are automatically replaced
* by xLOAD_n and xSTORE_n opcodes when possible. The xLOAD_n and xSTORE_n
* opcodes are therefore not defined in this interface. Likewise for LDC,
* automatically replaced by LDC_W or LDC2_W when necessary, WIDE, GOTO_W and
* JSR_W.
*
* @author Eric Bruneton
* @author Eugene Kuleshov
*/
public interface Opcodes {
// ASM API versions
int ASM4 = 4 << 16 | 0 << 8 | 0;
int ASM5 = 5 << 16 | 0 << 8 | 0;
// versions
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;
// access flags
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_VOLATILE = 0x0040; // field
int ACC_BRIDGE = 0x0040; // method
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
int ACC_ANNOTATION = 0x2000; // class
int ACC_ENUM = 0x4000; // class(?) field inner
int ACC_MANDATED = 0x8000; // parameter
// ASM specific pseudo access flags
int ACC_DEPRECATED = 0x20000; // class, field, method
// types for NEWARRAY
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;
// tags for Handle
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;
// stack map frame types
/**
* Represents an expanded frame. See {@link ClassReader#EXPAND_FRAMES}.
*/
int F_NEW = -1;
/**
* Represents a compressed frame with complete frame data.
*/
int F_FULL = 0;
/**
* Represents a compressed frame where locals are the same as the locals in
* the previous frame, except that additional 1-3 locals are defined, and
* with an empty stack.
*/
int F_APPEND = 1;
/**
* Represents a compressed frame where locals are the same as the locals in
* the previous frame, except that the last 1-3 locals are absent and with
* an empty stack.
*/
int F_CHOP = 2;
/**
* Represents a compressed frame with exactly the same locals as the
* previous frame and with an empty stack.
*/
int F_SAME = 3;
/**
* Represents a compressed frame with exactly the same locals as the
* previous frame and with a single value on the stack.
*/
int F_SAME1 = 4;
// Do not try to change the following code to use auto-boxing,
// these values are compared by reference and not by value
// The constructor of Integer was deprecated in 9
// but we are stuck with it by backward compatibility
@SuppressWarnings("deprecation") Integer TOP = new Integer(0);
@SuppressWarnings("deprecation") Integer INTEGER = new Integer(1);
@SuppressWarnings("deprecation") Integer FLOAT = new Integer(2);
@SuppressWarnings("deprecation") Integer DOUBLE = new Integer(3);
@SuppressWarnings("deprecation") Integer LONG = new Integer(4);
@SuppressWarnings("deprecation") Integer NULL = new Integer(5);
@SuppressWarnings("deprecation") Integer UNINITIALIZED_THIS = new Integer(6);
// opcodes // visit method (- = idem)
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 LDC_W = 19; // -
// int LDC2_W = 20; // -
int ILOAD = 21; // visitVarInsn
int LLOAD = 22; // -
int FLOAD = 23; // -
int DLOAD = 24; // -
int ALOAD = 25; // -
// int ILOAD_0 = 26; // -
// int ILOAD_1 = 27; // -
// int ILOAD_2 = 28; // -
// int ILOAD_3 = 29; // -
// int LLOAD_0 = 30; // -
// int LLOAD_1 = 31; // -
// int LLOAD_2 = 32; // -
// int LLOAD_3 = 33; // -
// int FLOAD_0 = 34; // -
// int FLOAD_1 = 35; // -
// int FLOAD_2 = 36; // -
// int FLOAD_3 = 37; // -
// int DLOAD_0 = 38; // -
// int DLOAD_1 = 39; // -
// int DLOAD_2 = 40; // -
// int DLOAD_3 = 41; // -
// int ALOAD_0 = 42; // -
// int ALOAD_1 = 43; // -
// int ALOAD_2 = 44; // -
// int ALOAD_3 = 45; // -
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 ISTORE_0 = 59; // -
// int ISTORE_1 = 60; // -
// int ISTORE_2 = 61; // -
// int ISTORE_3 = 62; // -
// int LSTORE_0 = 63; // -
// int LSTORE_1 = 64; // -
// int LSTORE_2 = 65; // -
// int LSTORE_3 = 66; // -
// int FSTORE_0 = 67; // -
// int FSTORE_1 = 68; // -
// int FSTORE_2 = 69; // -
// int FSTORE_3 = 70; // -
// int DSTORE_0 = 71; // -
// int DSTORE_1 = 72; // -
// int DSTORE_2 = 73; // -
// int DSTORE_3 = 74; // -
// int ASTORE_0 = 75; // -
// int ASTORE_1 = 76; // -
// int ASTORE_2 = 77; // -
// int ASTORE_3 = 78; // -
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 WIDE = 196; // NOT VISITED
int MULTIANEWARRAY = 197; // visitMultiANewArrayInsn
int IFNULL = 198; // visitJumpInsn
int IFNONNULL = 199; // -
// int GOTO_W = 200; // -
// int JSR_W = 201; // -
}
参考文献：
【1】http://asm.ow2.org/
【2】http://www.blogjava.net/DLevin/archive/2014/06/25/414292.html