设计模式-责任链模式

理解

责任链的执行序列是一环连着一环,后一环的执行依赖前一环的执行,层层递进.

流程图

组成部分

1. 链条:负责承前启后;
2. 链条上的环:执行业务逻辑,决定是否继续执行下一个环（有的情况由链条决定）;
3. 执行流:
   
4. 前期:由外层到里层;
   
5. 后期:由里层到外层

特点

1. 链条上相邻的环之间会影响,影响入参和出参;
2. 每个环负责相对独立的任务,比如有的负责字符编码设置,有的负责流的编码和解码(比如gzip)，有的负责记录日志等

实例

一个简单的 demo

项目代码

链条类

public class DaoFilterChain { private List<IFilter> filterList = new ArrayList<>(); private int index = 0; private boolean hasAddDefaultFilter = false; public DaoFilterChain addFilter(IFilter filter) { if (hasAddDefaultFilter) { throw new RuntimeException("自定义过滤器必须在默认过滤器之前添加"); } this.filterList.add(filter); return this; } /*** * 专门添加最基础的DaoFilter * @param filter * @return */ public DaoFilterChain addDefaultFilter(IFilter filter) { this.filterList.add(filter); hasAddDefaultFilter = true; return this; } public void reset() { this.index = 0; } private IFilter next() { if (index == filterList.size()) { return null; } IFilter filter = filterList.get(index); index++; return filter; } public void doFilter(DaoRequest request, DaoResponse response) { IFilter filter = next(); if (null == filter) { System.out.println("结束 index :" + index); return; } filter.doFilter(request, response, this); } }

过滤器接口

public interface IFilter { /*** * 最后一个过滤器一定是DefaultDaoFilter * @param request * @param response * @param filterChain */ void doFilter(DaoRequest request, DaoResponse response, DaoFilterChain filterChain); }

javax.servlet.Filter

过滤器核心方法 :

public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException;

链条:org.apache.catalina.core.ApplicationFilterChain
源码见:http://www.docjar.com/html/api/org/apache/catalina/core/ApplicationFilterChain.java.html



doFilter 核心代码 (去掉了其他代码):

ApplicationFilterConfig filterConfig = filters[pos++]; Filter filter = null; filter = filterConfig.getFilter(); filter.doFilter(request, response, this);

过滤器实例 :支持跨域





其他过滤器实例

租户拦截器

接口：

public interface IRequestFilter { void doFilter(HttpFormContentRequestWrapper request, HttpServletResponse response, RequestFilterChain filterChain) throws IOException, ServletException; }

链条类 RequestFilterChain

 public class RequestFilterChain { private List<IRequestFilter> filterList = new ArrayList<>(); private final static Logger logger = LoggerFactory.getLogger(RequestFilterChain.class); private ThreadLocal<Integer> indexLocal = new ThreadLocal<Integer>() { @Override protected Integer initialValue() { return 0; } }; private ThreadLocal<Boolean> hasAddDefaultFilter = new ThreadLocal<Boolean>() { @Override protected Boolean initialValue() { return Boolean.FALSE; } }; public RequestFilterChain addFilter(IRequestFilter filter) { if (hasAddDefaultFilter.get()) { throw new RuntimeException("自定义过滤器必须在默认过滤器之前添加"); } this.filterList.add(filter); return this; } public RequestFilterChain removeFilter(IRequestFilter filter) { this.filterList.remove(filter); return this; } public RequestFilterChain addDefaultFilter(IRequestFilter filter) { this.filterList.add(filter); hasAddDefaultFilter.set(true); // DefaultFormRequestWrapperFilter defaultDaoFilter = (DefaultFormRequestWrapperFilter) filter; return this; } public void reset() { indexLocal.set(0); } private IRequestFilter next() { if (indexLocal.get() == filterList.size()) { return null; } IRequestFilter filter = filterList.get(indexLocal.get()); // index++; indexLocal.set(indexLocal.get() + 1); return filter; } public void doFilter(HttpFormContentRequestWrapper request, HttpServletResponse response) throws IOException, ServletException { IRequestFilter filter = next(); if (null == filter) { return; } try { filter.doFilter(request, response, this); } catch (Exception e) { e.printStackTrace(); throw e; } } }

MultipleTenantsFilter 做了什么

适用场景

1. 同时有请求和应答的场景,例如HTTP请求,数据库查询;
2. 映射层转化
3. 需要决定是否中断流程的执行;
4. 业务的执行分为多个中间环节，中间环节的个数可以定制；

和aop的区别

aop体现的是代理模式,实际是动态代理或 cglib

感想

1. 责任链模式，依然符合开闭原则,比如由于业务的发展,需要添加新的逻辑,那么就可以增加一个过滤器，而不用修改原来的逻辑处理;
2. 对于逻辑比较复杂，且容易变动的逻辑，可以充分使用责任链模式，把相对聚合的逻辑抽取出来，作为一个个环，后面增加新的逻辑只需要增加新的环即可，也可以根据实际需求修改链条上环的顺序。