关于java字节码框架ASM的学习

来源:转载

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

 1 package com.asm3; 2 3 import java.io.File; 4 import java.io.FileNotFoundException; 5 import java.io.FileOutputStream; 6 import java.io.IOException; 7 8 import org.objectweb.asm.ClassWriter; 9 import org.objectweb.asm.Opcodes;10 11 /**12 * 通过asm生成类的字节码13 * @author Administrator14 *15 */16 public class GeneratorClass {17 18 public static void main(String[] args) throws IOException {19 //生成一个类只需要ClassWriter组件即可20 ClassWriter cw = new ClassWriter(0);21 //通过visit方法确定类的头部信息22 cw.visit(Opcodes.V1_5, Opcodes.ACC_PUBLIC+Opcodes.ACC_ABSTRACT+Opcodes.ACC_INTERFACE,23 "com/asm3/Comparable", null, "java/lang/Object", new String[]{"com/asm3/Mesurable"});24 //定义类的属性25 cw.visitField(Opcodes.ACC_PUBLIC+Opcodes.ACC_FINAL+Opcodes.ACC_STATIC,26 "LESS", "I", null, new Integer(-1)).visitEnd();27 cw.visitField(Opcodes.ACC_PUBLIC+Opcodes.ACC_FINAL+Opcodes.ACC_STATIC,28 "EQUAL", "I", null, new Integer(0)).visitEnd();29 cw.visitField(Opcodes.ACC_PUBLIC+Opcodes.ACC_FINAL+Opcodes.ACC_STATIC,30 "GREATER", "I", null, new Integer(1)).visitEnd();31 //定义类的方法32 cw.visitMethod(Opcodes.ACC_PUBLIC+Opcodes.ACC_ABSTRACT, "compareTo",33 "(Ljava/lang/Object;)I", null, null).visitEnd();34 cw.visitEnd(); //使cw类已经完成35 //将cw转换成字节数组写到文件里面去36 byte[] data = cw.toByteArray();37 File file = new File("D://Comparable.class");38 FileOutputStream fout = new FileOutputStream(file);39 fout.write(data);40 fout.close();41 }42 }

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:

1 public interface com.asm3.Comparable extends com.asm3.Mesurable {2 public static final int LESS;3 4 public static final int EQUAL;5 6 public static final int GREATER;7 8 public abstract int compareTo(java.lang.Object);9 }

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

C.java

1 package com.asm5;2 3 public class C {4 public void m() throws InterruptedException{5 Thread.sleep(100); 6 }7 }

将C.java类的内容改为如下:

 1 package com.asm5; 2 3 public class C { 4 public static long timer; 5 public void m() throws InterruptedException{ 6 timer -= System.currentTimeMillis(); 7 Thread.sleep(100); 8 timer += System.currentTimeMillis(); 9 }10 }

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

LADDPUTSTATIC 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

 1 package com.asm5; 2 3 import org.objectweb.asm.ClassAdapter; 4 import org.objectweb.asm.ClassVisitor; 5 import org.objectweb.asm.FieldVisitor; 6 import org.objectweb.asm.MethodAdapter; 7 import org.objectweb.asm.MethodVisitor; 8 import org.objectweb.asm.Opcodes; 9 10 public class AddTimeClassAdapter extends ClassAdapter {11 private String owner;12 private boolean isInterface;13 public AddTimeClassAdapter(ClassVisitor cv) {14 super(cv);15 }16 @Override17 public void visit(int version, int access, String name, String signature,18 String superName, String[] interfaces) {19 cv.visit(version, access, name, signature, superName, interfaces);20 owner = name;21 isInterface = (access & Opcodes.ACC_INTERFACE) != 0;22 }23 @Override24 public MethodVisitor visitMethod(int access, String name, String desc,25 String signature, String[] exceptions) {26 MethodVisitor mv = cv.visitMethod(access, name, desc, signature, exceptions);27 if(!name.equals("<init>") && !isInterface && mv!=null){28 //为方法添加计时功能29 mv = new AddTimeMethodAdapter(mv);30 }31 return mv;32 }33 @Override34 public void visitEnd() {35 //添加字段36 if(!isInterface){37 FieldVisitor fv = cv.visitField(Opcodes.ACC_PUBLIC+Opcodes.ACC_STATIC, "timer", "J", null, null);38 if(fv!=null){39 fv.visitEnd();40 }41 }42 cv.visitEnd();43 }44 45 class AddTimeMethodAdapter extends MethodAdapter{46 public AddTimeMethodAdapter(MethodVisitor mv) {47 super(mv);48 }49 @Override50 public void visitCode() {51 mv.visitCode();52 mv.visitFieldInsn(Opcodes.GETSTATIC, owner, "timer", "J");53 mv.visitMethodInsn(Opcodes.INVOKESTATIC, "java/lang/System", "currentTimeMillis", "()J");54 mv.visitInsn(Opcodes.LSUB);55 mv.visitFieldInsn(Opcodes.PUTSTATIC, owner, "timer", "J");56 }57 @Override58 public void visitInsn(int opcode) {59 if((opcode>=Opcodes.IRETURN && opcode<=Opcodes.RETURN) || opcode==Opcodes.ATHROW){60 mv.visitFieldInsn(Opcodes.GETSTATIC, owner, "timer", "J");61 mv.visitMethodInsn(Opcodes.INVOKESTATIC, "java/lang/System", "currentTimeMillis", "()J");62 mv.visitInsn(Opcodes.LADD);63 mv.visitFieldInsn(Opcodes.PUTSTATIC, owner, "timer", "J");64 }65 mv.visitInsn(opcode);66 }67 @Override68 public void visitMaxs(int maxStack, int maxLocal) {69 mv.visitMaxs(maxStack+4, maxLocal);70 }71 }72 73 }

Generator.java

 1 package com.asm5; 2 3 import java.io.File; 4 import java.io.FileNotFoundException; 5 import java.io.FileOutputStream; 6 import java.io.IOException; 7 8 import org.objectweb.asm.ClassAdapter; 9 import org.objectweb.asm.ClassReader;10 import org.objectweb.asm.ClassWriter;11 12 13 14 public class Generator {15 16 public static void main(String[] args){17 try {18 ClassReader cr = new ClassReader("com/asm5/C");19 ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_MAXS);20 ClassAdapter classAdapter = new AddTimeClassAdapter(cw);21 //使给定的访问者访问Java类的ClassReader22 cr.accept(classAdapter, ClassReader.SKIP_DEBUG);23 byte[] data = cw.toByteArray();24 File file = new File(System.getProperty("user.dir") + "//WebRoot//WEB-INF//classes//com//asm5//C.class");25 FileOutputStream fout = new FileOutputStream(file);26 fout.write(data);27 fout.close();28 System.out.println("success!");29 } catch (FileNotFoundException e) {30 e.printStackTrace();31 } catch (IOException e) {32 e.printStackTrace();33 }34 }35 36 }

下面是一个测试类:

 1 package com.asm5; 2 3 public class Test { 4 public static void main(String[] args) throws InterruptedException, NoSuchFieldException, SecurityException, IllegalArgumentException, IllegalAccessException { 5 C c = new C(); 6 c.m(); 7 Class cc = c.getClass(); 8 System.out.println(cc.getField("timer").get(c)); 9 }10 }

输出结果为:100

未完待续.......


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