Advice
1. Advice is the action and decision part of the crosscutting puzzle. It helps you define “What to do”.
2. An advice can be broken into three parts: the advice declaration, the pointcut specification, and the advice body.
一个advice分为三个部分：advice描述、pointcut声明和advice主体。
Example:
before(Connection connection) ： 1) advice declaration
connectionOperation(connection) 2) pointcut specification
{
System.out.println(“Performing operation on ”+connction); 3) advice body
}
The advice declaration also specifies the context information available to the advice body, such as the execution object and arguments. It also specifies any checked exception throw by the advice.
advice declaration也会指明可用于advice body的上下文信息，例如execution对象和参数。它也指明了advice需要抛出的任何异常。
3. before advice
The before advice is typically used for performing pre-operation tasks, such as policy enforcement, logging, and authentication.
4. after advice
1) The after advice executes after the execution of a join point.
2) after() It will be executed after a call to a method, regardless of how it
returns-normally or by throwing an exception.
无论返回是正常还是抛出异常都在调用一个函数之后执行。
after() returning only executed after a successful completion of captured join point.
仅仅当成功完成捕获join point后执行
after() throwing executed only when the advised join point throws an exception.
当join point抛出异常时执行
5. around advice
1）It has ability to bypass the execution of the captured join point completely, or to execute the join point with the same or different arguments.
它有能力完全绕过join point处的execution（注：即完全不执行原来的方法体），或者以相同或不同的参数执行（即改变原方法的参数，很强吧。。。）
2）除非显示调用proceed()，否则join point出处的操作将被忽略掉（注：即完全不执行原来的方法体），不予执行，只执行around() body中的代码
3) proceed()中的arguments可以和join point的一样或完全不同，最重要的是arguments的数量和类型必须和advice收集的context一致。
4）如果在around中使用了proceed()，则around就是简单地返回proceed返回的值，例如return proceed();
如果是proceed(); return ….; 则返回的是return后面的值。
如果没有调用proceed()，则需要返回一个适当的值。
6. Passing context from a join point to advice
1）Aspectj provides the this(), target(), and args() pointcuts to collect the context.
Aspectj提供了this(), target(), and args()用于收集上下文信息。
我的理解：如果用Type作为以上三者的参数，如this(Account)，那么其仅仅起到判断的作用，而无法收集context；如果用OjbectIdentifier作为参数则可以，如this(account)。
