A brief analysis of java bytecode framework ASM operating bytecode method

We have introduced ASM in detail before, and those who need it can click here: an in-depth study of java bytecode framework ASM

JVM's type signature comparison table

Type Signature	Java Type
Z	boolean
B	byte
C	char
S	short
I	int
J	long
F	float
D	double
L	fully-qualified-class ;fully-qualified-class
[ type	type[]
( arg-types ) ret-type	method type

For example, the java method is

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

The corresponding type signature is

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

Another example is the java method

private void hi(double a, List<String> b);

The corresponding type signature is

hi (DLjava/util/List;)V

Next, ASM can be used to verify whether the above two types of signatures are correct:

public class Test {

 public static void main(String[] args) throws Exception {
 ClassPrinter printer = new ClassPrinter();
 // Read the static inner class Bazhang
 ClassReader cr = new ClassReader("Test$Bazhang");
 cr.accept(printer, 0);

 }

 // Static inner class 
 static class Bazhang {

 public Bazhang(int a) {
 }

 private long f (int n, String s, int[] arr){
  return 0;
 }

 private void hi(double a, List<String> 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);
  // Print out the parent class name And this kind of name
  System.out.println(superName + " " + name);
 }

 @Override
 public MethodVisitor visitMethod(int access, String name, String desc, String signature, String[] exceptions) {
  // Print out the method name and type signature 
  System.out.println(name + " " + desc);
  return super.visitMethod(access, name, desc, signature, exceptions);
 }
 }

}

The last thing you print out:

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

Verify the previous correctness, where you can see that the default constructor is also printed.

So, to do something interesting, let's add a and method to the Bazhang class, which is:

public void newFunc(String str){
 
}

This time you need to use ClassWriter, used to concatenate bytecode, ClassReader, ClassVisitor, ClassWriter articles can see this article: ASM source learning ClassReader, ClassVisitor and ClassWriter details

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();

 //  Get generated class File corresponding 2 Base flow 
 byte[] code = cw.toByteArray();
 
 // will 2 The base stream writes out/ Under the 
 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<String> b) {
 }

 public void newFunc(String var1) {
 }
}

Before finishing JVM instruction set, using bytecode ASM direct operation is no problem, at the end of the enclosed ASM source download address: http: / / forge ow2. org/projects asm /

conclusion

The above is the whole content of this article, I hope the content of this article to your study or work can bring 1 definite help, if you have questions you can leave a message to communicate.