A brief analysis of the method of operating bytecode in java bytecode framework ASM

We have introduced ASM in detail before. Friends who need it can click here: In-depth study of java bytecode framework ASM

JVM type signature comparison table

For example, the java method is

long f (int n, String s, int[] arr);

and the corresponding type signature is

f (ILjava/lang/String;[I)J

Another example, The java method is

private void hi(double a, List b);

, and the corresponding type signature is

hi (DLjava/util/List;)V

. Next, you can use ASM to verify whether the above two type signatures are correct:

public class Test {
 
 public static void main(String[] args) throws Exception {
 ClassPrinter printer = new ClassPrinter();
 //读取静态内部类Bazhang
 ClassReader cr = new ClassReader("Test$Bazhang");
 cr.accept(printer, 0);
 
 }
 
 //静态内部类
 static class Bazhang {
 
 public Bazhang(int a) {
 }
 
 private long f (int n, String s, int[] arr){
  return 0;
 }
 
 private void hi(double a, List b){
 
 }
 }
 
 static class ClassPrinter extends ClassVisitor {
 
 public ClassPrinter() {
  super(Opcodes.ASM5);
 }
 
 @Override
 public void visit(int version, int access, String name, String signature, String superName, String[] interfaces) {
  super.visit(version, access, name, signature, superName, interfaces);
  //打印出父类name和本类name
  System.out.println(superName + " " + name);
 }
 
 @Override
 public MethodVisitor visitMethod(int access, String name, String desc, String signature, String[] exceptions) {
  //打印出方法名和类型签名
  System.out.println(name + " " + desc);
  return super.visitMethod(access, name, desc, signature, exceptions);
 }
 }
 
}

The last printed content:

java/lang/Object Test$Bazhang
 ()V
f (ILjava/lang/String;[I)J
hi (DLjava/util/List;)V

Verified the previous correctness, where you can see the default The constructor is also printed.

Then let’s do something interesting next. Let’s add a new method to the Bazhang class and set it as:

public void newFunc(String str){
  
}

At this time, you need to use ClassWriter , used to splice bytecode. For specific articles about ClassReader, ClassVisitor, and ClassWriter, you can view this article: Detailed explanation of ClassReader, ClassVisitor, and ClassWriter for ASM source code learning

public static void main(String[] args) throws Exception {
 ClassReader cr = new ClassReader(Bazhang.class.getName());
 ClassWriter cw = new ClassWriter(cr, ClassWriter.COMPUTE_MAXS);
 
 cr.accept(cw, Opcodes.ASM5);
 
 MethodVisitor mv = cw.visitMethod(ACC_PUBLIC, "newFunc", "(Ljava/lang/String;)V", null, null);
 
 mv.visitInsn(Opcodes.RETURN);
 mv.visitEnd();
 
 // 获取生成的class文件对应的二进制流
 byte[] code = cw.toByteArray();
  
 //将二进制流写到out/下
 FileOutputStream fos = new FileOutputStream("out/Bazhang222.class");
 fos.write(code);
 fos.close();
 
}

This will generate Bazhang222.class in the out/folder:

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//
 
import java.util.List;
 
class Test$Bazhang {
 Test$Bazhang() {
 }
 
 private long f(int n, String s, int[] arr) {
 return 0L;
 }
 
 private void hi(double a, List b) {
 }
 
 public void newFunc(String var1) {
 }
}

Combined with the previously organized JVM instruction set, use ASM to directly operate the bytecode It’s no problem. At the end, the ASM source code download address is attached: http://forge.ow2.org/projects/asm/

Summary

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