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); }}