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