先看经典职责链代码,
(代码来自维基)
- package pattern;
- import java.io.*;
- abstract class PurchasePower {
- protected final double base = 500;
- protected PurchasePower successor;
- public void setSuccessor(PurchasePower successor) {
- this.successor = successor;
- }
- abstract public void processRequest(PurchaseRequest request);
- }
- class ManagerPPower extends PurchasePower {
- private final double ALLOWABLE = 10 * base;
- public void processRequest(PurchaseRequest request) {
- if (request.getAmount() < ALLOWABLE)
- System.out.println("Manager will approve $" + request.getAmount());
- else if (successor != null)
- successor.processRequest(request);
- }
- }
- class DirectorPPower extends PurchasePower {
- private final double ALLOWABLE = 20 * base;
- public void processRequest(PurchaseRequest request) {
- if (request.getAmount() < ALLOWABLE)
- System.out.println("Director will approve $" + request.getAmount());
- else if (successor != null)
- successor.processRequest(request);
- }
- }
- class VicePresidentPPower extends PurchasePower {
- private final double ALLOWABLE = 40 * base;
- public void processRequest(PurchaseRequest request) {
- if (request.getAmount() < ALLOWABLE)
- System.out.println("Vice President will approve $"
- + request.getAmount());
- else if (successor != null)
- successor.processRequest(request);
- }
- }
- class PresidentPPower extends PurchasePower {
- private final double ALLOWABLE = 60 * base;
- public void processRequest(PurchaseRequest request) {
- if (request.getAmount() < ALLOWABLE)
- System.out
- .println("President will approve $" + request.getAmount());
- else
- System.out.println("Your request for $" + request.getAmount()
- + " needs a board meeting!");
- }
- }
- class PurchaseRequest {
- private int number;
- private double amount;
- private String purpose;
- public PurchaseRequest(int number, double amount, String purpose) {
- this.number = number;
- this.amount = amount;
- this.purpose = purpose;
- }
- public double getAmount() {
- return amount;
- }
- public void setAmount(double amt) {
- amount = amt;
- }
- public String getPurpose() {
- return purpose;
- }
- public void setPurpose(String reason) {
- purpose = reason;
- }
- public int getNumber() {
- return number;
- }
- public void setNumber(int num) {
- number = num;
- }
- }
- public class CheckAuthority {
- public static void main(String[] args) {
- ManagerPPower manager = new ManagerPPower();
- DirectorPPower director = new DirectorPPower();
- VicePresidentPPower vp = new VicePresidentPPower();
- PresidentPPower president = new PresidentPPower();
- manager.setSuccessor(director);
- director.setSuccessor(vp);
- vp.setSuccessor(president);
- // enter ctrl+c to kill.
- try {
- while (true) {
- System.out
- .println("Enter the amount to check who should approve your expenditure.");
- System.out.print(">");
- double d = Double.parseDouble(new BufferedReader(
- new InputStreamReader(System.in)).readLine());
- manager.processRequest(new PurchaseRequest(0, d, "General"));
- }
- } catch (Exception e) {
- System.exit(1);
- }
- }
- }
这段代码实现了不同审批限额由不同级别的人去审批的功能。
该模式有如下特点:
1. 几个对象组成链表结构,用来向后传递请求;
2. 请求被处理,则返回;否则继续向后传递;(有的实现则是不论请求是否被处理,都向后传递)。
3. 对象都继承自同一个父类或实现了共同的用来处理请求的接口。
优点:
1. 发起者(调用者)与接收者分离。
2. 可动态插入职责链。
缺点:
1. 遍历链表有性能损耗。
职责链模式的经典应用是Servlet中的的Filter技术。略过Filter的很简单的使用
方法,我们来看看它在tomcat中的实现。为了说明方便,这里只是它的实现的一
个简化演示代码。
用户自定义的filter都需实现了如下的Filter接口,
doFilter(ServletRequest request,ServletResponse response,FilterChain chain)throws java.io.IOException,ServletException
用户在这个接口里写的代码可以截获请求。对截获的请求处理后,可以把处理权
传递给下一个filter。传递给下一个filter的方法是调用如下一行代码,
chain.doFilter(request,response);
这行代码是Servlet里的Filter技术的核心,所以接下来我们主要就是分析、模拟
这行代码做了什么,它是如何把请求传递给下一个filter的,下一个filter是何
时被创建的。
下面是对整个过程的简单模拟。很简单,就是读配置文件,生成多个
ApplicationFilterConfig对象(称呼它为配置对象),该配置对象负责生成
filter对象。
- final class ApplicationFilterChain implements FilterChain {
- private List filterConfigs = new ArrayList();
- private Iterator iterator = null;
- public void doFilter(ServletRequest request, ServletResponse response) {
- if (this.iterator == null)
- this.iterator = filters.iterator();
- if (this.iterator.hasNext()) {
- ApplicationFilterConfig filterConfig = (ApplicationFilterConfig) iterator
- .next();// 在这里取得下一个filter的配置对象
- Filter filter = filterConfig.getFilter();// 注意:在这里取得了下一个filter,这个filter就是你实现了filter接口的filter.
- filter.doFilter(request, response, this);// 把请求传递个下一个filter处理
- }
- }
- public void startFilter() {
- ApplicationFilterConfig filterConfig = (ApplicationFilterConfig) iterator
- .next();
- Filter filter = filterConfig.getFilter();
- filter.doFilter(request, response, this);
- }
- public void addFilterConfig(ApplicationFilterConfig filterConfig) {
- filterConfigs.add(filterConfig);
- }
- }
- final class ApplicationFilterConfig implements FilterConfig {
- private Filter filter = null;
- private String filterClass;
- public Filter getFilter() {
- if (this.filter != null)
- return (this.filter);
- Class clazz = classLoader.loadClass(filterClass);
- this.filter = (Filter) clazz.newInstance();
- return this.filter;
- }
- }
- public class MainTest {
- public FilterChain create() {
- ApplicationFilterChain a = new ApplicationFilterChain();
- List filters = readConfigFile();// 读取配置文件,获取filter信息,实现过程略
- for (int i = 0; i < filters.size(); i++) {
- ApplicationFilterConfig afc = (ApplicationFilterConfig) filters
- .get(i);
- a.addFilterConfig(afc);
- }
- return a;
- }
- public static void main(String args[]) {
- (new MainTest()).create().startFilter();
- }
- }
tomcat的chain实现有3个要点,
1. 不是一下子全部初始化所有的filter对象,而是只初始化所有的配置对象。配
置对象的生成比filter对象的生成节省资源,生成配置对象只是存储了一些配置
信息。filter对象的生成需要反射,相对消耗资源。
2. 在第一次调用filter的时候,由配置对象生成filter对象。下一次再次调用则
直接使用已经生成的filter对象。缓存思路。
3. 由FilterChain维护一个链表,链表中存放着配置对象的链条,每次用户调用
一次chain.doFilter(req, res),链表就去取下一个配置对象,再通过配置对象
得到下一个filter,然后调用该filter,接着你在filter里写的逻辑就被执行了。