BCELでHello, World!!

Hello, World。すぐ自動生成コードのための自動生成コードが欲しくなると思う。.NETもそうだったし。

public class HelloWorld {

    public static void main(String[] args) {
        System.out.println("Hello, World!!");
    }

}

生成用コード。

import org.apache.bcel.Constants;
import org.apache.bcel.generic.ArrayType;
import org.apache.bcel.generic.ClassGen;
import org.apache.bcel.generic.ConstantPoolGen;
import org.apache.bcel.generic.InstructionConstants;
import org.apache.bcel.generic.InstructionFactory;
import org.apache.bcel.generic.InstructionList;
import org.apache.bcel.generic.MethodGen;
import org.apache.bcel.generic.ObjectType;
import org.apache.bcel.generic.PUSH;
import org.apache.bcel.generic.Type;


public class Main {

    public static void main(String[] args) throws Exception {

        ClassGen helloWorldClassGen = 
            new ClassGen(
                    "HelloWorld", 
                    "java.lang.Object", 
                    "<generated>", 
                    Constants.ACC_PUBLIC | Constants.ACC_SUPER, 
                    null
            );
        
        ConstantPoolGen helloWorldClassConstantPoolGen = 
            helloWorldClassGen.getConstantPool();
        InstructionList instructionList = new InstructionList();
        
        MethodGen mainMethodGen = 
            new MethodGen(
                    Constants.ACC_STATIC | Constants.ACC_PUBLIC, 
                    Type.VOID, 
                    new Type[] { new ArrayType(Type.STRING, 1) },
                    new String[] { "args" },
                    "main", 
                    "HelloWorld",
                    instructionList,
                    helloWorldClassConstantPoolGen
            );
        InstructionFactory instructionFactory = 
            new InstructionFactory(helloWorldClassGen); 
        
        ObjectType printStreamObjectType = new ObjectType("java.io.PrintStream");
        instructionList.append(
                instructionFactory.createFieldAccess(
                    "java.lang.System", 
                    "out", 
                    printStreamObjectType,
                    Constants.GETSTATIC
                )
        );
        instructionList.append(new PUSH(helloWorldClassConstantPoolGen, "Hello, World!!"));
        instructionList.append(
                instructionFactory.createInvoke(
                        "java.io.PrintStream", 
                        "println", 
                        Type.VOID, 
                        new Type[] { Type.STRING }, 
                        Constants.INVOKEVIRTUAL
                )
        );
        instructionList.append(InstructionConstants.RETURN);
        
        mainMethodGen.setMaxStack();
        helloWorldClassGen.addMethod(mainMethodGen.getMethod());
        instructionList.dispose();
        helloWorldClassGen.addEmptyConstructor(Constants.ACC_PUBLIC);
        
        helloWorldClassGen.getJavaClass().dump("HelloWorld.class");
        
    }

}