𝑰’𝒎 𝒉𝒉𝒈, 𝑰 𝒂𝒎 𝒂 𝒈𝒓𝒂𝒅𝒖𝒂𝒕𝒆 𝒔𝒕𝒖𝒅𝒆𝒏𝒕 𝒇𝒓𝒐𝒎 𝑵𝒂𝒏𝒋𝒊𝒏𝒈, 𝑪𝒉𝒊𝒏𝒂.

• 🏫 𝑺𝒉𝒄𝒐𝒐𝒍: 𝑯𝒐𝒉𝒂𝒊 𝑼𝒏𝒊𝒗𝒆𝒓𝒔𝒊𝒕𝒚
• 🌱 𝑳𝒆𝒂𝒓𝒏𝒊𝒏𝒈: 𝑰’𝒎 𝒄𝒖𝒓𝒓𝒆𝒏𝒕𝒍𝒚 𝒍𝒆𝒂𝒓𝒏𝒊𝒏𝒈 𝒅𝒆𝒔𝒊𝒈𝒏 𝒑𝒂𝒕𝒕𝒆𝒓𝒏, 𝑳𝒆𝒆𝒕𝒄𝒐𝒅𝒆, 𝒅𝒊𝒔𝒕𝒓𝒊𝒃𝒖𝒕𝒆𝒅 𝒔𝒚𝒔𝒕𝒆𝒎, 𝒎𝒊𝒅𝒅𝒍𝒆𝒘𝒂𝒓𝒆 𝒂𝒏𝒅 𝒔𝒐 𝒐𝒏.
• 💓 𝑯𝒐𝒘 𝒕𝒐 𝒓𝒆𝒂𝒄𝒉 𝒎𝒆：𝑽𝑿
• 📚 𝑴𝒚 𝒃𝒍𝒐𝒈: 𝒉𝒕𝒕𝒑𝒔://𝒉𝒉𝒈𝒚𝒚𝒅𝒔.𝒃𝒍𝒐𝒈.𝒄𝒔𝒅𝒏.𝒏𝒆𝒕/
• 💼 𝑷𝒓𝒐𝒇𝒆𝒔𝒔𝒊𝒐𝒏𝒂𝒍 𝒔𝒌𝒊𝒍𝒍𝒔：𝒎𝒚 𝒅𝒓𝒆𝒂𝒎
  

1：模版方法

1-1：定义

☘️大白话说，就是父类中有一个方法调用抽象方法step1，抽象方法step2… ，来完成某一个逻辑。这种方法就是模版方法，step1，step2是抽象方法。也就是说，模版方法定义这些抽象方法执行顺序，把你安排起来，并且子类别不允许覆写模板方法。具体方法怎么实现是子类自己实现的。形象的来说，我定义了一共99八十一难，每一难怎么过，你自己看着办。

1-2：适用场景

有一个设计模式，我们就应该想一下为什么要有模版方法模式？

• 把子类公共部分的代码抽取出来。
• 定规矩，流程怎么走，我说了算，你不能改，都得按照我这个流程走。

1-3：简单demo

• GoOnlineHelper

  package com.company.design.template;
  
  public abstract class GoOnlineHelper {
      abstract void login();
  
      abstract void scan();
  
      abstract void logout();
  
      protected final void surfTheInternet() {
          this.login();
          this.scan();
          this.logout();
      }
  }
  

• chromeHelper

  package com.company.design.template;
  
  public class ChromeHelper extends GoOnlineHelper {
      @Override
      void login() {
          System.out.println("chrome help you login");
      }
  
      @Override
      void scan() {
          System.out.println("chrome help you scan");
      }
  
      @Override
      void logout() {
          System.out.println("chrome help you logout");
      }
  }
  

• firefoxHelper

  package com.company.design.template;
  
  public class FireFoxHelper extends GoOnlineHelper {
      @Override
      void login() {
          System.out.println("firefox help you login");
      }
  
      @Override
      void scan() {
          System.out.println("firefox help you scan");
      }
  
      @Override
      void logout() {
          System.out.println("firefox help you logout");
      }
  }
  

☘️GoOnlineHelper 抽象类定义了3个动作以及模版surfTheInternet规定了3者的顺序，从而实现类只需要去实现流程中的细节部分，流程已经被父抽象类定义好了。那么，怎么去理解子类可以重新定义某些特定的步骤呢？比如chrome不想logout。可以加一个标识，来标识是否想要logout。于是乎，有了下面的代码：

• GoOnlineHelper

  package com.company.design.template;
  
  public abstract class GoOnlineHelper {
  
      boolean logoutFlag;
  
      GoOnlineHelper(boolean logoutFlag) {
          this.logoutFlag = logoutFlag;
      }
  
      boolean logoutOrNot() {
          return this.logoutFlag;
      }
  
      abstract void login();
  
      abstract void scan();
  
      abstract void logout();
  
      protected final void surfTheInternet() {
          this.login();
          this.scan();
          if (logoutOrNot()) {
              this.logout();
          }
      }
  
      public boolean isLogoutFlag() {
          return logoutFlag;
      }
  
      public void setLogoutFlag(boolean logoutFlag) {
          this.logoutFlag = logoutFlag;
      }
  }
  

• chromeHelper

  package com.company.design.template;
  
  public class ChromeHelper extends GoOnlineHelper {
  
      public ChromeHelper(boolean logoutFlag) {
          super(logoutFlag);
      }
  
      @Override
      void login() {
          System.out.println("chrome help you login");
      }
  
      @Override
      void scan() {
          System.out.println("chrome help you scan");
      }
  
      @Override
      void logout() {
          System.out.println("chrome help you logout");
      }
  }
  

• firefoxHelper

  package com.company.design.template;
  
  public class FireFoxHelper extends GoOnlineHelper {
      public FireFoxHelper(boolean logoutFlag) {
          super(logoutFlag);
      }
  
      @Override
      void login() {
          System.out.println("firefox help you login");
      }
  
      @Override
      void scan() {
          System.out.println("firefox help you scan");
      }
  
      @Override
      void logout() {
          System.out.println("firefox help you logout");
      }
  }
  

• client

  package com.company.design.template;
  
  public class Client {
      public static void main(String[] args) {
          GoOnlineHelper chromeHelper = new ChromeHelper(true);
          GoOnlineHelper firefoxHelper = new FireFoxHelper(false);
          chromeHelper.surfTheInternet();
          firefoxHelper.surfTheInternet();
      }
  }	
  ----output----
  chrome help you login
  chrome help you scan
  chrome help you logout
  firefox help you login
  firefox help you scan
  

☘️可以发现，通过子类传递的标识，就可以修改部分的流程逻辑，这里Firefox 就少了一回登录，跳过了一次流程。

1-4：模版方法的缺陷

• 模版方法模式的调用顺序和流程都是被模版方法限制的，这就很好的体现了“模版”二字，你只能按照这个流程走。
• 违背了里式替换原则，因为父类中帮助了子类做了一个default实现。
• 因为算法流程都在模版方法里面，如果这个算法的流程越来越长，那么维护就会越来越难了。

1-5：源码中模版方法-HttpServlet

• HttpServlet

  protected void service(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException
    {
        String method = req.getMethod();
  
        if (method.equals(METHOD_GET)) {
            long lastModified = getLastModified(req);
            if (lastModified == -1) {
                // servlet doesn't support if-modified-since, no reason
                // to go through further expensive logic
                doGet(req, resp);
            } else {
                long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
                if (ifModifiedSince < lastModified) {
                    // If the servlet mod time is later, call doGet()
                    // Round down to the nearest second for a proper compare
                    // A ifModifiedSince of -1 will always be less
                    maybeSetLastModified(resp, lastModified);
                    doGet(req, resp);
                } else {
                    resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
                }
            }
  
        } else if (method.equals(METHOD_HEAD)) {
            long lastModified = getLastModified(req);
            maybeSetLastModified(resp, lastModified);
            doHead(req, resp);
  
        } else if (method.equals(METHOD_POST)) {
            doPost(req, resp);
            
        } else if (method.equals(METHOD_PUT)) {
            doPut(req, resp);
            
        } else if (method.equals(METHOD_DELETE)) {
            doDelete(req, resp);
            
        } else if (method.equals(METHOD_OPTIONS)) {
            doOptions(req,resp);
            
        } else if (method.equals(METHOD_TRACE)) {
            doTrace(req,resp);
            
        } else {
            //
            // Note that this means NO servlet supports whatever
            // method was requested, anywhere on this server.
            //
  
            String errMsg = lStrings.getString("http.method_not_implemented");
            Object[] errArgs = new Object[1];
            errArgs[0] = method;
            errMsg = MessageFormat.format(errMsg, errArgs);
            
            resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
        }
    }
  

  它这个就是一个模版方法，我们看一眼doGet和doPost

  protected void doGet(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException
    {
        String protocol = req.getProtocol();
        String msg = lStrings.getString("http.method_get_not_supported");
        if (protocol.endsWith("1.1")) {
            resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg);
        } else {
            resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg);
        }
    }
  
    protected void doPost(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException
    {
        String protocol = req.getProtocol();
        String msg = lStrings.getString("http.method_post_not_supported");
        if (protocol.endsWith("1.1")) {
            resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg);
        } else {
            resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg);
        }
    }
  

☘️会发现虽然这里有默认实现，但是都是一样的，也就是说这里是等着你去覆盖，这就是为啥我们之前写servlet的时候，extend HttpServlet之后，要覆盖doGet和doPost方法的原因了，这里是一个模版方法模式，我们覆盖具体的方法，执行流程在HttpServlet中已经帮我们定好了。