初识ASM,简单的HelloWord

今天看到一个很有趣的ASM+Spring的应用写法.可以根据接口定义自动生成具体的Dao.主要还是ASM不熟悉.所以决定从基础看起来.比如我要定义个类:public class Example
{

public void Hello(String str)
{
System.out.println("Hello world! " + str);
}
}
如果我要定义一个ASM动态生成则:
  public static void main(final String args[]) throws Exception
{

/*
* 如果我要生成一个如下的java代码:
*
* public void Hello(String str)
* {
* System.out.println("Hello world! " + str);
* }
*/

// 定义ClassWriter 准备写类
ClassWriter cw = new ClassWriter(0);
cw.visit(V1_6, ACC_PUBLIC, "Example", null, "java/lang/Object", null);

// 创建默认构造函数
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();

//创建Hello方法 ,形参是 String
MethodVisitor mv = cw.visitMethod(ACC_PUBLIC,
"Hello",
"(Ljava/lang/String;)V",
null,
null);
mv.visitCode();
Label l0 = new Label();
mv.visitLabel(l0);
mv.visitLineNumber(12, l0);
mv.visitFieldInsn(GETSTATIC,
"java/lang/System",
"out",
"Ljava/io/PrintStream;");
mv.visitTypeInsn(NEW, "java/lang/StringBuffer");
mv.visitInsn(DUP);
mv.visitLdcInsn("Hello world! ");
mv.visitMethodInsn(INVOKESPECIAL,
"java/lang/StringBuffer",
"<init>",
"(Ljava/lang/String;)V");
mv.visitVarInsn(ALOAD, 1);
mv.visitMethodInsn(INVOKEVIRTUAL,
"java/lang/StringBuffer",
"append",
"(Ljava/lang/String;)Ljava/lang/StringBuffer;");
mv.visitMethodInsn(INVOKEVIRTUAL,
"java/lang/StringBuffer",
"toString",
"()Ljava/lang/String;");
mv.visitMethodInsn(INVOKEVIRTUAL,
"java/io/PrintStream",
"println",
"(Ljava/lang/String;)V");
Label l1 = new Label();
mv.visitLabel(l1);
mv.visitLineNumber(13, l1);
mv.visitInsn(RETURN);
Label l2 = new Label();
mv.visitLabel(l2);
mv.visitLocalVariable("this", "Lcom/test/Example;", null, l0, l2, 0);
mv.visitLocalVariable("str", "Ljava/lang/String;", null, l0, l2, 1);
mv.visitMaxs(4, 2);
mv.visitEnd();

/*
* 写入文件
*/
byte[] code = cw.toByteArray();
FileOutputStream fos = new FileOutputStream("Example.class");
fos.write(code);
fos.close();

/*
* 从文件加载类
*/
HelloWord loader = new HelloWord();
Class exampleClass = loader.defineClass("Example", code, 0, code.length);

/*
* 反射生成类,并传入参数
*/
Object obj = exampleClass.getConstructor(null).newInstance(null);
exampleClass.getMethod("Hello", new Class[] {String.class})
.invoke(obj, new Object[] {"zjy"});

}
 
最后输出就
 Hello world! zjy