使用cglib动态为Java类添加方法

CGLib（Code Generation Library）是一个强大的，高性能，高质量的字节码操作类库，它可以在运行期扩展Java类与实现Java接口，Spring、Hibernate等很多著名的框架都使用了它。
https://github.com/cglib/cglib/

其他的库还有：
Javassist (Java Programming Assistant)、ASM、Apache Commons BCEL（Byte Code Engineering Library）

public class CGLibExample {

    @SuppressWarnings("unchecked")
    public static void main(String[] args) {
    	
    	// 定义一个参数是字符串类型的setCreatedAt方法
        InterfaceMaker im = new InterfaceMaker();
        im.add(new Signature("setCreatedAt", Type.VOID_TYPE, 
        		new Type[] { Type.getType(String.class) }), null);

        Class myInterface = im.create();

        Enhancer enhancer = new Enhancer();
        enhancer.setSuperclass(ExampleBean.class);
        enhancer.setInterfaces(new Class[] { myInterface });
        enhancer.setCallback(new MethodInterceptor() {
            public Object intercept(Object obj, Method method, Object[] args,
                    MethodProxy proxy) throws Throwable {
                
                ExampleBean bean = (ExampleBean) obj;
                
                // 调用字符串类型的setCreatedAt方法时，转换成Date型后调用Setter
                if (method.getName().startsWith("setCreatedAt")
                        && args[0] != null && args[0] instanceof String) {

                    SimpleDateFormat sdf = new SimpleDateFormat("yyyyMMdd");
                    Date date = null;
                    try {
                        date = sdf.parse((String) args[0]);
                    } catch (final Exception e) { /* nop */ }
                    bean.setCreatedAt(date);
                    return null;

                }
                return proxy.invokeSuper(obj, args);
            }
        });

        // 生成一个Bean
        ExampleBean bean = (ExampleBean) enhancer.create();
        bean.setId(999);

        try {
            Method method = bean.getClass().getMethod("setCreatedAt", new Class[] {String.class});
            method.invoke(bean, new Object[]{"20100531"});
        } catch (final Exception e) {
            e.printStackTrace();
        }
        
        System.out.printf("id : [%d] createdAt : [%s]\n", bean.getId(), bean.getCreatedAt());
    }
}

class ExampleBean implements Serializable {
    private static final long serialVersionUID = -8121418052209958014L;
    
    private int id;
    private Date createdAt;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public Date getCreatedAt() {
        return createdAt;
    }

    public void setCreatedAt(Date createdAt) {
        this.createdAt = createdAt;
    }
}

运行结果：

引用

id : [999] createdAt : [Mon May 31 00:00:00 CST 2010]