通常的构造策略是将系统分层，将用户界面逻辑、商业逻辑、系统逻辑和持久性逻辑彼此分开。
A common architectural strategy is to layer your system, separating your user interface logic, business logic, system logic, and persistence logic from each other.
在分层应用程序(J2EE应用程序通常属于此类)中，需要在所有层次执行授权——尤其是在最底层，这一点非常重要。
In layered applications, which J2EE applications frequently are, it is important to perform authorization at all layers — but particularly at the lowest layer.
如图1所示，网络协议通常是分层的。
Network protocols are typically layered, as depicted in Figure 1.
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