职责链模式（Chain of Responsibility Pattern）

所有一切链式的、分步完成的、"传递性"的都可以想到用职责链模式。其实现方式的关键类似于链表的结构。职责链由一些职责结点组成，每个结点能够将任务传递给下一个结点。最后每个结点都遍历且执行了该结点规定执行的任务。举个例子：
类图：
关键是RequestHandler的实现。



代码：
/**
*
* RequestHandler
*
*/
public abstract class RequestHandler {

private static final Logger LOGGER = LoggerFactory.getLogger(RequestHandler.class);

private RequestHandler next;

public RequestHandler(RequestHandler next) {
this.next = next;
}

/**
* Request handler
*/
public void handleRequest(Request req) {
if (next != null) {
next.handleRequest(req);
}
}

protected void printHandling(Request req) {
LOGGER.info("{} handling request \"{}\"", this, req);
}

@Override
public abstract String toString();
}

/**
*
* OrcOfficer
*
*/
public class OrcOfficer extends RequestHandler {

public OrcOfficer(RequestHandler handler) {
super(handler);
}

@Override
public void handleRequest(Request req) {
if (req.getRequestType().equals(RequestType.TORTURE_PRISONER)) {
printHandling(req);
req.markHandled();
} else {
super.handleRequest(req);
}
}

@Override
public String toString() {
return "Orc officer";
}

}

/**
*
* OrcCommander
*
*/
public class OrcCommander extends RequestHandler {

public OrcCommander(RequestHandler handler) {
super(handler);
}

@Override
public void handleRequest(Request req) {
if (req.getRequestType().equals(RequestType.DEFEND_CASTLE)) {
printHandling(req);
req.markHandled();
} else {
super.handleRequest(req);
}
}

@Override
public String toString() {
return "Orc commander";
}
}

/**
*
* OrcSoldier
*
*/
public class OrcSoldier extends RequestHandler {

public OrcSoldier(RequestHandler handler) {
super(handler);
}

@Override
public void handleRequest(Request req) {
if (req.getRequestType().equals(RequestType.COLLECT_TAX)) {
printHandling(req);
req.markHandled();
} else {
super.handleRequest(req);
}
}

@Override
public String toString() {
return "Orc soldier";
}
}

/**
* Request
*/
public class Request {

/**
* The type of this request, used by each item in the chain to see if they should or can handle
* this particular request
*/
private final RequestType requestType;

/**
* A description of the request
*/
private final String requestDescription;

/**
* Indicates if the request is handled or not. A request can only switch state from unhandled to
* handled, there's no way to 'unhandle' a request
*/
private boolean handled;

/**
* Create a new request of the given type and accompanied description.
*
* @param requestType        The type of request
* @param requestDescription The description of the request
*/
public Request(final RequestType requestType, final String requestDescription) {
this.requestType = Objects.requireNonNull(requestType);
this.requestDescription = Objects.requireNonNull(requestDescription);
}

/**
* Get a description of the request
*
* @return A human readable description of the request
*/
public String getRequestDescription() {
return requestDescription;
}

/**
* Get the type of this request, used by each person in the chain of command to see if they should
* or can handle this particular request
*
* @return The request type
*/
public RequestType getRequestType() {
return requestType;
}

/**
* Mark the request as handled
*/
public void markHandled() {
this.handled = true;
}

/**
* Indicates if this request is handled or not
*
* @return <tt>true</tt> when the request is handled, <tt>false</tt> if not
*/
public boolean isHandled() {
return this.handled;
}

@Override
public String toString() {
return getRequestDescription();
}

}

/**
*
* RequestType enumeration
*
*/
public enum RequestType {

DEFEND_CASTLE, TORTURE_PRISONER, COLLECT_TAX

}

/**
*
* OrcKing makes requests that are handled by the chain.
*
*/
public class OrcKing {

RequestHandler chain;

public OrcKing() {
buildChain();
}

private void buildChain() {
chain = new OrcCommander(new OrcOfficer(new OrcSoldier(null)));
}

public void makeRequest(Request req) {
chain.handleRequest(req);
}

}

/**
*
* The Chain of Responsibility pattern is a design pattern consisting of command objects and a
* series of processing objects. Each processing object contains logic that defines the types of
* command objects that it can handle; the rest are passed to the next processing object in the
* chain. A mechanism also exists for adding new processing objects to the end of this chain.
* <p>
* In this example we organize the request handlers ({@link RequestHandler}) into a chain where each
* handler has a chance to act on the request on its turn. Here the king ({@link OrcKing}) makes
* requests and the military orcs ({@link OrcCommander}, {@link OrcOfficer}, {@link OrcSoldier})
* form the handler chain.
*
*/
public class App {

/**
* Program entry point
*
* @param args command line args
*/
public static void main(String[] args) {

OrcKing king = new OrcKing();
king.makeRequest(new Request(RequestType.DEFEND_CASTLE, "defend castle"));
king.makeRequest(new Request(RequestType.TORTURE_PRISONER, "torture prisoner"));
king.makeRequest(new Request(RequestType.COLLECT_TAX, "collect tax"));

}
}