1.jar包：

2.原java文件：

package club.jiajia.test3;
public class Examp3 {
	public int method(int a){
		switch (a) {
			case 123:
			    a=1230;
				break;
			case 128:
				a=1280;
				break;
			case 12:
				a=120;
				break;
		}
		return a;
	}
}

3.javap -c命令反编译此java的字节码文件

Compiled from "Examp3.java"
public class club.jiajia.test3.Examp3 {
  public club.jiajia.test3.Examp3();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public int method(int);
    Code:
       0: iload_1
       1: lookupswitch  { // 3

                    12: 50

                   123: 36

                   128: 43
               default: 53
          }
      36: sipush        1230
      39: istore_1
      40: goto          53
      43: sipush        1280
      46: istore_1
      47: goto          53
      50: bipush        120
      52: istore_1
      53: iload_1
      54: ireturn
}

    我们只看method方法

4.按照上面的字节码文件编写java代码如下：

package club.jiajia.test3;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.Label;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.Opcodes;
public class TestAsm3 implements Opcodes{
	public static void main(String[] args) throws InstantiationException, IllegalAccessException{
		ClassWriter cw=new ClassWriter(0);
		String className="Example3";
		cw.visit(0x31,ACC_PUBLIC,className,null,"java/lang/Object",new String[]{"java/lang/Cloneable",ITest.class.getName().replace('.','/')});
		String method="method";
		MethodVisitor mv;
		
		/***********************************************************/
		Label end = new Label();
		mv=cw.visitMethod(ACC_PUBLIC,method,"(I)I",null,null);
		mv.visitCode();
		mv.visitVarInsn(ILOAD, 1);
		Label def=new Label();
		int val[]=new int[]{12,123,128};
		Label labs[]=new Label[3];
		labs[0]=new Label();
		labs[1]=new Label();
		labs[2]=new Label();
		mv.visitLookupSwitchInsn(def,val,labs);
		
		mv.visitLabel(labs[0]);
		mv.visitFrame(F_SAME, 0, null, 0, null);
		mv.visitIntInsn(SIPUSH,1230);
		mv.visitVarInsn(ISTORE,1);
		mv.visitJumpInsn(GOTO, end);
		
		mv.visitLabel(labs[1]);
		mv.visitFrame(F_SAME, 0, null, 0, null);
		mv.visitIntInsn(SIPUSH,1280);
		mv.visitVarInsn(ISTORE,1);
		mv.visitJumpInsn(GOTO, end);
		
		mv.visitLabel(labs[2]);
		mv.visitFrame(F_SAME, 0, null, 0, null);
		mv.visitIntInsn(SIPUSH,120);
		mv.visitVarInsn(ISTORE,1);
		mv.visitJumpInsn(GOTO, end);
		
		mv.visitLabel(def);
		mv.visitFrame(F_SAME, 0, null, 0, null);
		
		mv.visitLabel(end);
		mv.visitFrame(F_SAME, 0, null, 0, null);
		
		mv.visitVarInsn(ILOAD, 1);
		mv.visitInsn(IRETURN);
		mv.visitMaxs(2, 2);
		mv.visitEnd();
		
		/***********************************************************/
		
		//下面产生构造方法
		mv=cw.visitMethod(ACC_PUBLIC,"<init>","()V",null,null);
		mv.visitCode();
		mv.visitVarInsn(ALOAD,0);
		mv.visitMethodInsn(INVOKESPECIAL,"java/lang/Object","<init>","()V");
		mv.visitInsn(RETURN);
		mv.visitMaxs(1,1);
		mv.visitEnd();
		
		cw.visitEnd();

		byte[] bs=cw.toByteArray();
		File file = new File("E://jiajiajia/Example3.class");
		try {
			FileOutputStream fout = new FileOutputStream(file);
			fout.write(bs);
			fout.close();
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		}
		
		MyCLassLoader loader=new MyCLassLoader();
		Class<?> c=loader.defineClass(bs);
		
		ITest ins=(ITest)c.newInstance();
		System.out.println(ins.method(12003));
	}
	public interface ITest{
		public int method(int a);
	}
	public static class MyCLassLoader extends ClassLoader{
		public Class<?> defineClass(byte[] data){
			return super.defineClass(null,data,0,data.length,null);
		}
	}
}

5.反编译生成的class文件如下：