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This is a Java class file manipulation library that, unlike other bytecode libraries, directly deals with the constructs in the class file without any abstraction. It is still unfinished.

A generic example can be foundhere.

This will create a basic class namedTheName extendingsome.super.Class compiled with Java version 11. Note that java versions less than 8 are not supported and using one will throw an exception.

JavaClassaClass =newJavaClass("TheName","some/super/Class",11);aClass.setAccessFlags(ClassAccessFlags.PUBLIC);

Assuming we have the code from above, adding a constant pool entry is simple. For example, here we add aUTF8_info constant pool entry.

UTF8Entryentry =newUTF8Entry(aClass.getConstantPool(),"Write whatever you want in here");

The entry will automatically add itself to the constant pool on creation.

Here comes the juicy part: methods. Adding a method is slightly more complex.

Methodm =newMethod(aClass.getConstantPool(),"yourMethodName","(Lsome/Type;)V");m.setAccessFlags(MethodAccessFlags.PUBLIC);aClass.getMethods().add(m);

Finally, bytecode! In Java, bytecode goes into a thing called acode attribute, which then goes into the method. Here's a sample:

CodeAttributecode =newCodeAttribute(newUTF8Entry(aClass.getConstantPool(),"Code"),5,2,JvmInstructions.ALOAD.create(0),JvmInstructions.DUP.create());m.getAttributes().add(code);

The code attribute has 4 arguments: aUTF8Entry pointing to the valueCode (this is very important!), the max stack size, the max local variable size, and a list of bytecode instructions. In this case the instructions areaload 0, dup

What's the point of having a Java class if you can't write it? TheJavaClass has a methodwrite(OutputStream out) that writes the class in byte form. It can be used with aFileOutputStream,ByteArrayOutputStream, and all the other variants.

• Add the rest of the bytecode instructions
• Add more attributes
• Add interface and field capabilities to classes
• Add exception tables to the code attribute