一、什么是ASM
ASM是一个java字节码操纵框架,它能被用来动态生成类或者增强既有类的功能。ASM 可以直接产生二进制 class 文件,也可以在类被加载入 Java 虚拟机之前动态改变类行为。Java class 被存储在严格格式定义的 .class文件里,这些类文件拥有足够的元数据来解析类中的所有元素:类名称、方法、属性以及 Java 字节码(指令)。ASM从类文件中读入信息后,能够改变类行为,分析类信息,甚至能够根据用户要求生成新类。
使用ASM框架需要导入asm的jar包,下载链接:asm-3.2.jar。
二、如何使用ASM
ASM框架中的核心类有以下几个:
① ClassReader:该类用来解析编译过的class字节码文件。
② ClassWriter:该类用来重新构建编译后的类,比如说修改类名、属性以及方法,甚至可以生成新的类的字节码文件。
③ ClassAdapter:该类也实现了ClassVisitor接口,它将对它的方法调用委托给另一个ClassVisitor对象。
示例1.通过asm生成类的字节码
package com.asm3;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.Opcodes;
/**
* 通过asm生成类的字节码
* @author Administrator
*
*/
public class GeneratorClass {
public static void main(String[] args) throws IOException {
//生成一个类只需要ClassWriter组件即可
ClassWriter cw = new ClassWriter(0);
//通过visit方法确定类的头部信息
cw.visit(Opcodes.V1_5, Opcodes.ACC_PUBLIC+Opcodes.ACC_ABSTRACT+Opcodes.ACC_INTERFACE,
"com/asm3/Comparable", null, "java/lang/Object", new String[]{"com/asm3/Mesurable"});
//定义类的属性
cw.visitField(Opcodes.ACC_PUBLIC+Opcodes.ACC_FINAL+Opcodes.ACC_STATIC,
"LESS", "I", null, new Integer(-1)).visitEnd();
cw.visitField(Opcodes.ACC_PUBLIC+Opcodes.ACC_FINAL+Opcodes.ACC_STATIC,
"EQUAL", "I", null, new Integer(0)).visitEnd();
cw.visitField(Opcodes.ACC_PUBLIC+Opcodes.ACC_FINAL+Opcodes.ACC_STATIC,
"GREATER", "I", null, new Integer(1)).visitEnd();
//定义类的方法
cw.visitMethod(Opcodes.ACC_PUBLIC+Opcodes.ACC_ABSTRACT, "compareTo",
"(Ljava/lang/Object;)I", null, null).visitEnd();
cw.visitEnd(); //使cw类已经完成
//将cw转换成字节数组写到文件里面去
byte[] data = cw.toByteArray();
File file = new File("D://Comparable.class");
FileOutputStream fout = new FileOutputStream(file);
fout.write(data);
fout.close();
}
}
生成一个类的字节码文件只需要用到ClassWriter类即可,生成Comparable.class后用javap指令对其进行反编译:javap -c Comparable.class >test.txt ,编译后的结果如下:
- public interface com.asm3.Comparable extends com.asm3.Mesurable {
- public static final int LESS;
- public static final int EQUAL;
- public static final int GREATER;
- public abstract int compareTo(java.lang.Object);
- }
注:一个编译后的java类不包含package和import段,因此在class文件中所有的类型都使用的是全路径。
示例2.修改类的字节码文件
- package com.asm5;
- public class C {
- public void m() throws InterruptedException{
- Thread.sleep(100);
- }
- }
- package com.asm5;
- public class C {
- public static long timer;
- public void m() throws InterruptedException{
- timer -= System.currentTimeMillis();
- Thread.sleep(100);
- timer += System.currentTimeMillis();
- }
- }
为了弄清楚ASM是如何实现的,我们先编译这两个类,然后比对它们的TraceClassVisitor的输出,我们可以发现如下的不同(粗体表示)
GETSTATIC C.timer : J
INVOKESTATIC java/lang/System.currentTimilis()J
LSUB
PUTSTATIC C.timer : J
LDC 100
INVOKESTATIC java/lang/Thread.sleep(J)V
GETSTATIC C.timer : J
INVOKESTATIC java/lang/System.currentTimilis()J
LADD
PUTSTATIC C.timer : J
RETURN
MAXSTACK=4
MAXLOCALS=1
通过比对上面的指令,我们可以发现必须在m()方法的最前面增加四条指令,在RETURN指令前也增加四条指令,同时这四条必须位于xRETURN和ATHROW之前,因为这些指令都会结束方法的执行。
具体代码如下:
AddTimeClassAdapter.java
- package com.asm5;
- import org.objectweb.asm.ClassAdapter;
- import org.objectweb.asm.ClassVisitor;
- import org.objectweb.asm.FieldVisitor;
- import org.objectweb.asm.MethodAdapter;
- import org.objectweb.asm.MethodVisitor;
- import org.objectweb.asm.Opcodes;
- public class AddTimeClassAdapter extends ClassAdapter {
- private String owner;
- private boolean isInterface;
- public AddTimeClassAdapter(ClassVisitor cv) {
- super(cv);
- }
- @Override
- public void visit(int version, int access, String name, String signature,
- String superName, String[] interfaces) {
- cv.visit(version, access, name, signature, superName, interfaces);
- owner = name;
- isInterface = (access & Opcodes.ACC_INTERFACE) != 0;
- }
- @Override
- public MethodVisitor visitMethod(int access, String name, String desc,
- String signature, String[] exceptions) {
- MethodVisitor mv = cv.visitMethod(access, name, desc, signature, exceptions);
- if(!name.equals("<init>") && !isInterface && mv!=null){
- //为方法添加计时功能
- mv = new AddTimeMethodAdapter(mv);
- }
- return mv;
- }
- @Override
- public void visitEnd() {
- //添加字段
- if(!isInterface){
- FieldVisitor fv = cv.visitField(Opcodes.ACC_PUBLIC+Opcodes.ACC_STATIC, "timer", "J", null, null);
- if(fv!=null){
- fv.visitEnd();
- }
- }
- cv.visitEnd();
- }
- class AddTimeMethodAdapter extends MethodAdapter{
- public AddTimeMethodAdapter(MethodVisitor mv) {
- super(mv);
- }
- @Override
- public void visitCode() {
- mv.visitCode();
- mv.visitFieldInsn(Opcodes.GETSTATIC, owner, "timer", "J");
- mv.visitMethodInsn(Opcodes.INVOKESTATIC, "java/lang/System", "currentTimeMillis", "()J");
- mv.visitInsn(Opcodes.LSUB);
- mv.visitFieldInsn(Opcodes.PUTSTATIC, owner, "timer", "J");
- }
- @Override
- public void visitInsn(int opcode) {
- if((opcode>=Opcodes.IRETURN && opcode<=Opcodes.RETURN) || opcode==Opcodes.ATHROW){
- mv.visitFieldInsn(Opcodes.GETSTATIC, owner, "timer", "J");
- mv.visitMethodInsn(Opcodes.INVOKESTATIC, "java/lang/System", "currentTimeMillis", "()J");
- mv.visitInsn(Opcodes.LADD);
- mv.visitFieldInsn(Opcodes.PUTSTATIC, owner, "timer", "J");
- }
- mv.visitInsn(opcode);
- }
- @Override
- public void visitMaxs(int maxStack, int maxLocal) {
- mv.visitMaxs(maxStack+4, maxLocal);
- }
- }
- }
Generator.java
- package com.asm5;
- import java.io.File;
- import java.io.FileNotFoundException;
- import java.io.FileOutputStream;
- import java.io.IOException;
- import org.objectweb.asm.ClassAdapter;
- import org.objectweb.asm.ClassReader;
- import org.objectweb.asm.ClassWriter;
- public class Generator {
- public static void main(String[] args){
- try {
- ClassReader cr = new ClassReader("com/asm5/C");
- ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_MAXS);
- ClassAdapter classAdapter = new AddTimeClassAdapter(cw);
- //使给定的访问者访问Java类的ClassReader
- cr.accept(classAdapter, ClassReader.SKIP_DEBUG);
- byte[] data = cw.toByteArray();
- File file = new File(System.getProperty("user.dir") + "\\WebRoot\\WEB-INF\\classes\\com\\asm5\\C.class");
- FileOutputStream fout = new FileOutputStream(file);
- fout.write(data);
- fout.close();
- System.out.println("success!");
- } catch (FileNotFoundException e) {
- e.printStackTrace();
- } catch (IOException e) {
- e.printStackTrace();
- }
- }
- }
- package com.asm5;
- public class Test {
- public static void main(String[] args) throws InterruptedException, NoSuchFieldException, SecurityException, IllegalArgumentException, IllegalAccessException {
- C c = new C();
- c.m();
- Class cc = c.getClass();
- System.out.println(cc.getField("timer").get(c));
- }
- }