Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.
使多个对象都有机会处理请求，从而避免了请求的发送者和接受者之间的耦合关系。将这些对象连成一条链，并沿着这条链传递该请求，直到有对象处理它为止。

The classes and objects participating in this pattern are:

• defines an interface for handling the requests
• (optional) implements the successor link

• handles requests it is responsible for
• can access its successor
• if the ConcreteHandler can handle the request, it does so; otherwise it forwards the request to its successor

• initiates the request to a ConcreteHandler object on the chain