责任链模式
模板模式、策略模式和责任链模式，这三种模式具有相同的作用：复用和扩展，在实际的项目开发中比较常用，特别是框架开发中，我们可以利用它们来提供框架的扩展点，能够让框架的使用者在不修改框架源码的情况下，基于扩展点定制化框架的功能。
这篇文章主要讲解责任链模式的原理和实现。除此之外还会贴合实战，通过剖析Servlet Filter、Spring Interceptor来看，如利用责任链模式实现框架中常用的过滤器、拦截器。
原理
职责链模式的英文翻译是Chain Of Responsibility Design Pattern。在GoF的《设计模式》中，它是这么定义的：
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.
翻译成中文就是：将请求的发送和接收解耦，让多个接收对象都有机会处理这个请求。将这些接收对象串成一条链，并沿着这条链传递这个请求，直到链上的某个接收对象能够处理它为止。
这么说比较抽象，我用更加容易理解的话来进一步解读一下。
在职责链模式中，多个处理器（也就是刚刚定义中说的“接收对象”）依次处理同一个请求。一个请求先经过A处理器处理，然后再把请求传递给B处理器，B处理器处理完后再传递给C处理器，以此类推，形成一个链条。链条上的每个处理器各自承担各自的处理职责，所以叫作职责链模式。
实现
关于职责链模式，我们先来看看它的代码实现。结合代码实现，你会更容易理解它的定义。职责链模式有多种实现方式，我们这里介绍两种比较常用的。
第一种实现方式如下所示。其中，Handler是所有处理器类的抽象父类，handle()是抽象方法。每个具体的处理器类（HandlerA、HandlerB）的handle()函数的代码结构类似，如果它能处理该请求，就不继续往下传递；如果不能处理，则交由后面的处理器来处理（也就是调用successor.handle()）。HandlerChain是处理器链，从数据结构的角度来看，它就是一个记录了链头、链尾的链表。其中，记录链尾是为了方便添加处理器。
public abstract class Handler {
protected Handler successor = null;
public void setSuccessor(Handler successor) {
this.successor = successor;
}
public abstract void handle();
}
public class Handler extends Handler {
@Override
public void handle() {
boolean handled = false;
//...
if (!handled && successor != null) {
successor.handle();
}
}
}
public class HandlerB extends Handler {
@Override
public void handle() {
boolean handled = false;
//...
if (!handled && successor != null) {
successor.handle();
}
}
}
public class HandlerChain {
private Handler head = null;
private Handler tail = null;
public void addHandler(Handler handler) {
handler.setSuccessor(null);
if (head == null) {
head = handler;
tail = handler;
return;
}
tail.setSuccessor(handler);
tail = handler;
}
public void handle() {
if (head != null) {
head.handle();
}
}
}
// 使用举例
public class Application {
public static void main(String[] args) {
HandlerChain chain = new HandlerChain();
chain.addHandler(new HandlerA());
chain.addHandler(new HandlerB());
chain.handle();
}
}
实际上，上面的代码实现不够优雅。处理器类的handle()函数，不仅包含自己的业务逻辑，还包含对下一个处理器的调用，也就是代码中的successor.handle()。一个不熟悉这种代码结构的程序员，在添加新的处理器类的时候，很有可能忘记在handle()函数中调用successor.handle()，这就会导致代码出现bug。
针对这个问题，我们对代码进行重构，利用模板模式，将调用successor.handle()的逻辑从具体的处理器类中剥离出来，放到抽象父类中。这样具体的处理器类只需要实现自己的业务逻辑就可以了。重构之后的代码如下所示：
public abstract class Handler {
protected Handler successor = null;
public void setSuccessor(Handler successor) {
this.successor = successor;
}
public final void handle() {
boolean handled = doHandle();
if (successor != null && !handled) {
successor.handle();
}
}
protected abstract boolean doHandle();
}
public class HandlerA extends Handler {
@Override
protected boolean doHandle() {
boolean handled = false;
//...
return handled;
}
}
public class HandlerB extends Handler {
@Override
protected boolean doHandle() {
boolean handled = false;
//...
return handled;
}
}
// HandlerChain和Application代码不变
框架中的责任链模式
职责链模式常用在框架的开发中，为框架提供扩展点，让框架的使用者在不修改框架源码的情况下，基于扩展点添加新的功能。实际上，更具体点来说，职责链模式最常用来开发框架的过滤器和拦截器。今天，我们就通过Servlet Filter、Spring Interceptor这两个Java开发中常用的组件，来具体讲讲它在框架开发中的应用。
Servlet责任链：
/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.catalina.core;
import java.io.IOException;
import java.security.Principal;
import java.security.PrivilegedActionException;
import java.util.Set;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.Servlet;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.apache.catalina.Globals;
import org.apache.catalina.security.SecurityUtil;
import org.apache.tomcat.util.ExceptionUtils;
import org.apache.tomcat.util.res.StringManager;
/**
* Implementation of <code>javax.servlet.FilterChain</code> used to manage the execution of a set of filters for a
* particular request. When the set of defined filters has all been executed, the next call to <code>doFilter()</code>
* will execute the servlet's <code>service()</code> method itself.
*
* @author Craig R. McClanahan
*/
public final class ApplicationFilterChain implements FilterChain {
// Used to enforce requirements of SRV.8.2 / SRV.14.2.5.1
private static final ThreadLocal<ServletRequest> lastServicedRequest;
private static final ThreadLocal<ServletResponse> lastServicedResponse;
static {
if (ApplicationDispatcher.WRAP_SAME_OBJECT) {
lastServicedRequest = new ThreadLocal<>();
lastServicedResponse = new ThreadLocal<>();
} else {
lastServicedRequest = null;
lastServicedResponse = null;
}
}
// -------------------------------------------------------------- Constants
public static final int INCREMENT = 10;
// ----------------------------------------------------- Instance Variables
/**
* Filters.
*/
private ApplicationFilterConfig[] filters = new ApplicationFilterConfig[0];
/**
* The int which is used to maintain the current position in the filter chain.
*/
private int pos = 0;
/**
* The int which gives the current number of filters in the chain.
*/
private int n = 0;
/**
* The servlet instance to be executed by this chain.
*/
private Servlet servlet = null;
/**
* Does the associated servlet instance support async processing?
*/
private boolean servletSupportsAsync = false;
/**
* The string manager for our package.
*/
private static final StringManager sm = StringManager.getManager(ApplicationFilterChain.class);
/**
* Static class array used when the SecurityManager is turned on and <code>doFilter</code> is invoked.
*/
private static final Class<?>[] classType =
new Class[] { ServletRequest.class, ServletResponse.class, FilterChain.class };
/**
* Static class array used when the SecurityManager is turned on and <code>service</code> is invoked.
*/
private static final Class<?>[] classTypeUsedInService =
new Class[] { ServletRequest.class, ServletResponse.class };
// ---------------------------------------------------- FilterChain Methods
/**
* Invoke the next filter in this chain, passing the specified request and response. If there are no more filters in
* this chain, invoke the <code>service()</code> method of the servlet itself.
*
* @param request The servlet request we are processing
* @param response The servlet response we are creating
*
* @exception IOException if an input/output error occurs
* @exception ServletException if a servlet exception occurs
*/
@Override
public void doFilter(ServletRequest request, ServletResponse response) throws IOException, ServletException {
if (Globals.IS_SECURITY_ENABLED) {
final ServletRequest req = request;
final ServletResponse res = response;
try {
java.security.AccessController.doPrivileged((java.security.PrivilegedExceptionAction<Void>) () -> {
internalDoFilter(req, res);
return null;
});
} catch (PrivilegedActionException pe) {
Exception e = pe.getException();
if (e instanceof ServletException) {
throw (ServletException) e;
} else if (e instanceof IOException) {
throw (IOException) e;
} else if (e instanceof RuntimeException) {
throw (RuntimeException) e;
} else {
throw new ServletException(e.getMessage(), e);
}
}
} else {
internalDoFilter(request, response);
}
}
private void internalDoFilter(ServletRequest request, ServletResponse response)
throws IOException, ServletException {
// Call the next filter if there is one
if (pos < n) {
ApplicationFilterConfig filterConfig = filters[pos++];
try {
Filter filter = filterConfig.getFilter();
if (request.isAsyncSupported() &&
"false".equalsIgnoreCase(filterConfig.getFilterDef().getAsyncSupported())) {
request.setAttribute(Globals.ASYNC_SUPPORTED_ATTR, Boolean.FALSE);
}
if (Globals.IS_SECURITY_ENABLED) {
final ServletRequest req = request;
final ServletResponse res = response;
Principal principal = ((HttpServletRequest) req).getUserPrincipal();
Object[] args = new Object[] { req, res, this };
SecurityUtil.doAsPrivilege("doFilter", filter, classType, args, principal);
} else {
filter.doFilter(request, response, this);
}
} catch (IOException | ServletException | RuntimeException e) {
throw e;
} catch (Throwable e) {
e = ExceptionUtils.unwrapInvocationTargetException(e);
ExceptionUtils.handleThrowable(e);
throw new ServletException(sm.getString("filterChain.filter"), e);
}
return;
}
// We fell off the end of the chain -- call the servlet instance
try {
if (ApplicationDispatcher.WRAP_SAME_OBJECT) {
lastServicedRequest.set(request);
lastServicedResponse.set(response);
}
if (request.isAsyncSupported() && !servletSupportsAsync) {
request.setAttribute(Globals.ASYNC_SUPPORTED_ATTR, Boolean.FALSE);
}
// Use potentially wrapped request from this point
if ((request instanceof HttpServletRequest) && (response instanceof HttpServletResponse) &&
Globals.IS_SECURITY_ENABLED) {
final ServletRequest req = request;
final ServletResponse res = response;
Principal principal = ((HttpServletRequest) req).getUserPrincipal();
Object[] args = new Object[] { req, res };
SecurityUtil.doAsPrivilege("service", servlet, classTypeUsedInService, args, principal);
} else {
servlet.service(request, response);
}
} catch (IOException | ServletException | RuntimeException e) {
throw e;
} catch (Throwable e) {
e = ExceptionUtils.unwrapInvocationTargetException(e);
ExceptionUtils.handleThrowable(e);
throw new ServletException(sm.getString("filterChain.servlet"), e);
} finally {
if (ApplicationDispatcher.WRAP_SAME_OBJECT) {
lastServicedRequest.set(null);
lastServicedResponse.set(null);
}
}
}
/**
* The last request passed to a servlet for servicing from the current thread.
*
* @return The last request to be serviced.
*/
public static ServletRequest getLastServicedRequest() {
return lastServicedRequest.get();
}
/**
* The last response passed to a servlet for servicing from the current thread.
*
* @return The last response to be serviced.
*/
public static ServletResponse getLastServicedResponse() {
return lastServicedResponse.get();
}
// -------------------------------------------------------- Package Methods
/**
* Add a filter to the set of filters that will be executed in this chain.
*
* @param filterConfig The FilterConfig for the servlet to be executed
*/
void addFilter(ApplicationFilterConfig filterConfig) {
// Prevent the same filter being added multiple times
for (ApplicationFilterConfig filter : filters) {
if (filter == filterConfig) {
return;
}
}
if (n == filters.length) {
ApplicationFilterConfig[] newFilters = new ApplicationFilterConfig[n + INCREMENT];
System.arraycopy(filters, 0, newFilters, 0, n);
filters = newFilters;
}
filters[n++] = filterConfig;
}
/**
* Release references to the filters and wrapper executed by this chain.
*/
void release() {
for (int i = 0; i < n; i++) {
filters[i] = null;
}
n = 0;
pos = 0;
servlet = null;
servletSupportsAsync = false;
}
/**
* Prepare for reuse of the filters and wrapper executed by this chain.
*/
void reuse() {
pos = 0;
}
/**
* Set the servlet that will be executed at the end of this chain.
*
* @param servlet The Wrapper for the servlet to be executed
*/
void setServlet(Servlet servlet) {
this.servlet = servlet;
}
void setServletSupportsAsync(boolean servletSupportsAsync) {
this.servletSupportsAsync = servletSupportsAsync;
}
/**
* Identifies the Filters, if any, in this FilterChain that do not support async.
*
* @param result The Set to which the fully qualified class names of each Filter in this FilterChain that does not
* support async will be added
*/
public void findNonAsyncFilters(Set<String> result) {
for (int i = 0; i < n; i++) {
ApplicationFilterConfig filter = filters[i];
if ("false".equalsIgnoreCase(filter.getFilterDef().getAsyncSupported())) {
result.add(filter.getFilterClass());
}
}
}
}
Spring拦截器：
/*
* Copyright 2002-2020 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* https://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.springframework.web.servlet;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.springframework.lang.Nullable;
import org.springframework.util.CollectionUtils;
/**
* Handler execution chain, consisting of handler object and any handler interceptors.
* Returned by HandlerMapping's {@link HandlerMapping#getHandler} method.
*
* @author Juergen Hoeller
* @since 20.06.2003
* @see HandlerInterceptor
*/
public class HandlerExecutionChain {
private static final Log logger = LogFactory.getLog(HandlerExecutionChain.class);
private final Object handler;
private final List<HandlerInterceptor> interceptorList = new ArrayList<>();
private int interceptorIndex = -1;
/**
* Create a new HandlerExecutionChain.
* @param handler the handler object to execute
*/
public HandlerExecutionChain(Object handler) {
this(handler, (HandlerInterceptor[]) null);
}
/**
* Create a new HandlerExecutionChain.
* @param handler the handler object to execute
* @param interceptors the array of interceptors to apply
* (in the given order) before the handler itself executes
*/
public HandlerExecutionChain(Object handler, @Nullable HandlerInterceptor... interceptors) {
this(handler, (interceptors != null ? Arrays.asList(interceptors) : Collections.emptyList()));
}
/**
* Create a new HandlerExecutionChain.
* @param handler the handler object to execute
* @param interceptorList the list of interceptors to apply
* (in the given order) before the handler itself executes
* @since 5.3
*/
public HandlerExecutionChain(Object handler, List<HandlerInterceptor> interceptorList) {
if (handler instanceof HandlerExecutionChain) {
HandlerExecutionChain originalChain = (HandlerExecutionChain) handler;
this.handler = originalChain.getHandler();
this.interceptorList.addAll(originalChain.interceptorList);
}
else {
this.handler = handler;
}
this.interceptorList.addAll(interceptorList);
}
/**
* Return the handler object to execute.
*/
public Object getHandler() {
return this.handler;
}
/**
* Add the given interceptor to the end of this chain.
*/
public void addInterceptor(HandlerInterceptor interceptor) {
this.interceptorList.add(interceptor);
}
/**
* Add the given interceptor at the specified index of this chain.
* @since 5.2
*/
public void addInterceptor(int index, HandlerInterceptor interceptor) {
this.interceptorList.add(index, interceptor);
}
/**
* Add the given interceptors to the end of this chain.
*/
public void addInterceptors(HandlerInterceptor... interceptors) {
CollectionUtils.mergeArrayIntoCollection(interceptors, this.interceptorList);
}
/**
* Return the array of interceptors to apply (in the given order).
* @return the array of HandlerInterceptors instances (may be {@code null})
*/
@Nullable
public HandlerInterceptor[] getInterceptors() {
return (!this.interceptorList.isEmpty() ? this.interceptorList.toArray(new HandlerInterceptor[0]) : null);
}
/**
* Return the list of interceptors to apply (in the given order).
* @return the list of HandlerInterceptors instances (potentially empty)
* @since 5.3
*/
public List<HandlerInterceptor> getInterceptorList() {
return (!this.interceptorList.isEmpty() ? Collections.unmodifiableList(this.interceptorList) :
Collections.emptyList());
}
/**
* Apply preHandle methods of registered interceptors.
* @return {@code true} if the execution chain should proceed with the
* next interceptor or the handler itself. Else, DispatcherServlet assumes
* that this interceptor has already dealt with the response itself.
*/
boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
for (int i = 0; i < this.interceptorList.size(); i++) {
HandlerInterceptor interceptor = this.interceptorList.get(i);
if (!interceptor.preHandle(request, response, this.handler)) {
triggerAfterCompletion(request, response, null);
return false;
}
this.interceptorIndex = i;
}
return true;
}
/**
* Apply postHandle methods of registered interceptors.
*/
void applyPostHandle(HttpServletRequest request, HttpServletResponse response, @Nullable ModelAndView mv)
throws Exception {
for (int i = this.interceptorList.size() - 1; i >= 0; i--) {
HandlerInterceptor interceptor = this.interceptorList.get(i);
interceptor.postHandle(request, response, this.handler, mv);
}
}
/**
* Trigger afterCompletion callbacks on the mapped HandlerInterceptors.
* Will just invoke afterCompletion for all interceptors whose preHandle invocation
* has successfully completed and returned true.
*/
void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, @Nullable Exception ex) {
for (int i = this.interceptorIndex; i >= 0; i--) {
HandlerInterceptor interceptor = this.interceptorList.get(i);
try {
interceptor.afterCompletion(request, response, this.handler, ex);
}
catch (Throwable ex2) {
logger.error("HandlerInterceptor.afterCompletion threw exception", ex2);
}
}
}
/**
* Apply afterConcurrentHandlerStarted callback on mapped AsyncHandlerInterceptors.
*/
void applyAfterConcurrentHandlingStarted(HttpServletRequest request, HttpServletResponse response) {
for (int i = this.interceptorList.size() - 1; i >= 0; i--) {
HandlerInterceptor interceptor = this.interceptorList.get(i);
if (interceptor instanceof AsyncHandlerInterceptor) {
try {
AsyncHandlerInterceptor asyncInterceptor = (AsyncHandlerInterceptor) interceptor;
asyncInterceptor.afterConcurrentHandlingStarted(request, response, this.handler);
}
catch (Throwable ex) {
if (logger.isErrorEnabled()) {
logger.error("Interceptor [" + interceptor + "] failed in afterConcurrentHandlingStarted", ex);
}
}
}
}
}
/**
* Delegates to the handler's {@code toString()} implementation.
*/
@Override
public String toString() {
return "HandlerExecutionChain with [" + getHandler() + "] and " + this.interceptorList.size() + " interceptors";
}
}