asm基础学习1

本文给出一个最简单的通过asm生成一个java类的例子：

生成这样一个类：

 public class TestDzl {
  private String name;
  public void sayHello(String s){
   System.out.print("welcome, ");
   System.out.println(s);
   name="zili.dengzl";
  }
  
  public void printName(){
   System.out.println(name);
  }
 }   

生成代码如下：

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.lang.reflect.Constructor;
import java.lang.reflect.Method;

import com.alibaba.ITestDzl;

public class AsmLearn1 implements Opcodes {

 /**
  * @param args
  */
 public static void main(String[] args) {
  byte[] clazzBytes = generateClass();
  printClass(clazzBytes);
  Class<?> clazz = loadClass(clazzBytes);

  callByInterface(clazz);
  callByReflect(clazz); 
 }

 private static void callByReflect(Class<?> clazz) {
  try {
   Constructor<?> cns = clazz.getConstructor();
   Object o = cns.newInstance();
   
   System.out.printf("%s","sayHello:");
   Method method = clazz.getMethod("sayHello", String.class);
   method.invoke(o, "callByReflect");
   
   System.out.printf("%s","printName:");
   Method method2 = clazz.getMethod("printName");
   method2.invoke(o);
  } catch (Exception e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  }
 }

 private static void callByInterface(Class<?> clazz) {
  try {
   Object o = clazz.newInstance();
   System.out.println(o);
   ITestDzl t = (ITestDzl) o;
   t.sayHello("callByInterface");
  
  } catch (InstantiationException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  } catch (IllegalAccessException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  }
 }

 private static Class<?> loadClass(byte[] clazzBytes) {
  Class<?> clazz = new ClassLoader(AsmLearn1.class.getClassLoader()) {
   public Class<?> loadClassFromBytes(String name, byte[] b) {
    return defineClass(name, b, 0, b.length);
   }
  }.loadClassFromBytes("TestDzl", clazzBytes);
  return clazz;
 }

 private static byte[] generateClass() {
  ClassWriter cw = new ClassWriter();
  cw.visit(V1_5, ACC_PUBLIC + ACC_SUPER, "TestDzl", "java/lang/Object",
    new String[]{"com/alibaba/ITestDzl"});

  FieldVisitor fv = cw.visitField(ACC_PRIVATE, "name", "Ljava/lang/String;");
  fv.visitEnd();
  
  
  // 初始化方法
  MethodVisitor mw = cw.visitMethod(ACC_PUBLIC, "<init>", "()V", null,
    null);
  mw.visitVarInsn(ALOAD, 0);
  mw.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "<init>", "()V");
  mw.visitInsn(RETURN);
  mw.visitMaxs(1, 1);
  mw.visitEnd();

  // sayHello方法
  MethodVisitor mw2 = cw.visitMethod(ACC_PUBLIC, "sayHello",
    "(Ljava/lang/String;)V", null, null);

  mw2.visitFieldInsn(GETSTATIC, "java/lang/System", "out",
    "Ljava/io/PrintStream;");
  mw2.visitLdcInsn("welcome, ");
  mw2.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "print",
    "(Ljava/lang/String;)V");

  mw2.visitFieldInsn(GETSTATIC, "java/lang/System", "out",
    "Ljava/io/PrintStream;");
  mw2.visitVarInsn(ALOAD, 1);
  mw2.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println",
    "(Ljava/lang/String;)V");
  
  mw2.visitVarInsn(ALOAD, 0);
  mw2.visitLdcInsn("zili.dengzl");
  mw2.visitFieldInsn(PUTFIELD, "TestDzl", "name", "Ljava/lang/String;");
  
  mw2.visitMaxs(2, 2);
  mw2.visitInsn(RETURN);
  mw2.visitEnd();
  
  
  //printName()方法
  MethodVisitor mw3 = cw.visitMethod(ACC_PUBLIC, "printName",
    "()V", null, null);

  mw3.visitFieldInsn(GETSTATIC, "java/lang/System", "out",
  "Ljava/io/PrintStream;");
  mw3.visitVarInsn(ALOAD, 0);
  mw3.visitFieldInsn(GETFIELD, "TestDzl", "name", "Ljava/lang/String;");
  mw3.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println",
  "(Ljava/lang/String;)V");
  
  mw3.visitMaxs(2, 2);
  mw3.visitInsn(RETURN);
  mw3.visitEnd();

  byte[] clazzBytes = cw.toByteArray();
  return clazzBytes;
 }

 private static void printClass(byte[] clazzBytes) {
  try {
   FileOutputStream fos = new FileOutputStream("d:/TestDzl.class");
   fos.write(clazzBytes);
  } catch (FileNotFoundException e1) {
   // TODO Auto-generated catch block
   e1.printStackTrace();
  } catch (IOException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  }
  System.out.println(clazzBytes);

 }

}

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