设计模式-行为型模式-模板方法模式(Template Method)

Posted 2021-03-05 sxrtb

行为型-模板方法模式

　　个人理解，通俗的说，就是我们将实现某功能的模板写好，其中实现这个功能的具体步骤可以抽离出来，由子类来写具体的实现。

　　其实抽象类就体现了模板方法这个思想，抽象类将子类需要实现的方法抽象出来，形成抽象方法；也可以理解为抽象类作为了子类的通用模板。设计模板方法时，我们的父类一般都是抽象类。

 

　　类图:

 

 

 

　　1.定义AbstractClass

public abstract class TemplateMethod {

    protected abstract void doOperation1();

    public void doOperation2(){
        System.out.println("TemplateMethod doOperation2");
    }

    /**
     * 模板方法
     */
    public void templateMethod(){
        doOperation1();
        doOperation2();
    }
}

　　2.实现具体步骤的子类

public class FirstSubTemplateMethod extends TemplateMethod {
    @Override
    protected void doOperation1() {
        System.out.println("FirstSubTemplateMethod doOperation1");
    }

    @Override
    public void doOperation2() {
        System.out.println("FirstSubTemplateMethod doOperation2");
    }
}

public class SecodeSubTemplateMethod extends TemplateMethod{
    @Override
    protected void doOperation1() {
        System.out.println("SecodeSubTemplateMethod doOperation1");
    }
}

　　3.测试模板方法

public class TemplateMethodTest {
    public static void main(String[] args) {
        TemplateMethod templateMethod1 = new FirstSubTemplateMethod();
        TemplateMethod templateMethod2 = new SecodeSubTemplateMethod();

        templateMethod1.templateMethod();
        templateMethod2.templateMethod();
    }
}

　　4.输出

FirstSubTemplateMethod doOperation1
FirstSubTemplateMethod doOperation2
SecodeSubTemplateMethod doOperation1
TemplateMethod doOperation2

 

　　其实这样，模板方法模式就结束了。

 

　　那么我们的父类能不能是普通的类，或者是接口呢，这个问题讲完模板方法后再将。

　　个人理解，其实模板方法体现在，有一个模板方法，这个方法中调用了中另外一个或这几个方法。而被调用的方法需要子类实现(或者提供了默认实现，子类可以重写)，则它的修饰符可以使public，protected，甚至可以省略(一般不使用这种方式)。

　　这里我们再说父类能不能是普通的类和接口。先说接口，JDK1.8之前，接口中的方法只能是抽象方法，JDK1.8之后，接口中可以有类方法和默认方法，而设计模式这种思想早就被人提出，所以我们没有看到使用接口来写模板模式的(如果那一天看到别人用默认将默认的方法写出一种模板方法，我个人还是觉得可以接受的，java语言使我们手中的工具，我们可以用java编写能带给人们便利的软件)。

　　再说父类能不能是普通的类。如果父类是普通的类，而父类中的方法就都不能用abstract，那么这样，。。。感觉也未尝不可。

　　再说模板方法，感觉在编程中，某个方法调用一个到几个其他方法的方法感觉到处都是。若父类是普通的类，那么，无处不是模板方法(那么父类到底可不可以是普通类呢，实在要这个编程，那也没错)。

　　模板方法是人们对常用编程方式的一种提炼，很好的体现了开闭原则和里氏替换原则。通常上，人们将父类定义为抽象类。

　　抽象类中不一定有抽象方法，模板方法中也不是一定要有抽象方法。

　　设计模式，或者我们现在讨论的模板方法设计模式，体现的一种编程思想。

　　

　　使用模板方法的例子：

　　JAVA WEB程序离不开Servlet（SpringMVC底层就封装了Servlet，虽然有可能我们平时开发中没有直接使用到Servlet）。编写一个Servlet，可以参考https://blog.csdn.net/a376298333/article/details/79121548。

public class FirstServlet extends javax.servlet.http.HttpServlet {
    protected void doPost(javax.servlet.http.HttpServletRequest request, javax.servlet.http.HttpServletResponse response) throws javax.servlet.ServletException, IOException {

    }

    protected void doGet(javax.servlet.http.HttpServletRequest request, javax.servlet.http.HttpServletResponse response) throws javax.servlet.ServletException, IOException {

    }
}

　　而javax.servlet.http.HttpServlet的源码

public abstract class HttpServlet extends GenericServlet {
    private static final long serialVersionUID = 1L;
    private static final String METHOD_DELETE = "DELETE";
    private static final String METHOD_HEAD = "HEAD";
    private static final String METHOD_GET = "GET";
    private static final String METHOD_OPTIONS = "OPTIONS";
    private static final String METHOD_POST = "POST";
    private static final String METHOD_PUT = "PUT";
    private static final String METHOD_TRACE = "TRACE";
    private static final String HEADER_IFMODSINCE = "If-Modified-Since";
    private static final String HEADER_LASTMOD = "Last-Modified";
    private static final String LSTRING_FILE = "javax.servlet.http.LocalStrings";
    private static final ResourceBundle lStrings = ResourceBundle.getBundle("javax.servlet.http.LocalStrings");

    public HttpServlet() {
    }

    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(405, msg);
        } else {
            resp.sendError(400, msg);
        }

    }

    protected long getLastModified(HttpServletRequest req) {
        return -1L;
    }

    protected void doHead(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        if (DispatcherType.INCLUDE.equals(req.getDispatcherType())) {
            this.doGet(req, resp);
        } else {
            NoBodyResponse response = new NoBodyResponse(resp);
            this.doGet(req, response);
            response.setContentLength();
        }

    }

    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(405, msg);
        } else {
            resp.sendError(400, msg);
        }

    }

    protected void doPut(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String protocol = req.getProtocol();
        String msg = lStrings.getString("http.method_put_not_supported");
        if (protocol.endsWith("1.1")) {
            resp.sendError(405, msg);
        } else {
            resp.sendError(400, msg);
        }

    }

    protected void doDelete(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String protocol = req.getProtocol();
        String msg = lStrings.getString("http.method_delete_not_supported");
        if (protocol.endsWith("1.1")) {
            resp.sendError(405, msg);
        } else {
            resp.sendError(400, msg);
        }

    }

    private static Method[] getAllDeclaredMethods(Class<?> c) {
        if (c.equals(HttpServlet.class)) {
            return null;
        } else {
            Method[] parentMethods = getAllDeclaredMethods(c.getSuperclass());
            Method[] thisMethods = c.getDeclaredMethods();
            if (parentMethods != null && parentMethods.length > 0) {
                Method[] allMethods = new Method[parentMethods.length + thisMethods.length];
                System.arraycopy(parentMethods, 0, allMethods, 0, parentMethods.length);
                System.arraycopy(thisMethods, 0, allMethods, parentMethods.length, thisMethods.length);
                thisMethods = allMethods;
            }

            return thisMethods;
        }
    }

    protected void doOptions(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        Method[] methods = getAllDeclaredMethods(this.getClass());
        boolean ALLOW_GET = false;
        boolean ALLOW_HEAD = false;
        boolean ALLOW_POST = false;
        boolean ALLOW_PUT = false;
        boolean ALLOW_DELETE = false;
        boolean ALLOW_TRACE = true;
        boolean ALLOW_OPTIONS = true;
        Class clazz = null;

        try {
            clazz = Class.forName("org.apache.catalina.connector.RequestFacade");
            Method getAllowTrace = clazz.getMethod("getAllowTrace", (Class[])null);
            ALLOW_TRACE = (Boolean)getAllowTrace.invoke(req, (Object[])null);
        } catch (NoSuchMethodException | SecurityException | IllegalAccessException | IllegalArgumentException | InvocationTargetException | ClassNotFoundException var14) {
            ;
        }

        for(int i = 0; i < methods.length; ++i) {
            Method m = methods[i];
            if (m.getName().equals("doGet")) {
                ALLOW_GET = true;
                ALLOW_HEAD = true;
            }

            if (m.getName().equals("doPost")) {
                ALLOW_POST = true;
            }

            if (m.getName().equals("doPut")) {
                ALLOW_PUT = true;
            }

            if (m.getName().equals("doDelete")) {
                ALLOW_DELETE = true;
            }
        }

        String allow = null;
        if (ALLOW_GET) {
            allow = "GET";
        }

        if (ALLOW_HEAD) {
            if (allow == null) {
                allow = "HEAD";
            } else {
                allow = allow + ", HEAD";
            }
        }

        if (ALLOW_POST) {
            if (allow == null) {
                allow = "POST";
            } else {
                allow = allow + ", POST";
            }
        }

        if (ALLOW_PUT) {
            if (allow == null) {
                allow = "PUT";
            } else {
                allow = allow + ", PUT";
            }
        }

        if (ALLOW_DELETE) {
            if (allow == null) {
                allow = "DELETE";
            } else {
                allow = allow + ", DELETE";
            }
        }

        if (ALLOW_TRACE) {
            if (allow == null) {
                allow = "TRACE";
            } else {
                allow = allow + ", TRACE";
            }
        }

        if (ALLOW_OPTIONS) {
            if (allow == null) {
                allow = "OPTIONS";
            } else {
                allow = allow + ", OPTIONS";
            }
        }

        resp.setHeader("Allow", allow);
    }

    protected void doTrace(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String CRLF = "
";
        StringBuilder buffer = (new StringBuilder("TRACE ")).append(req.getRequestURI()).append(" ").append(req.getProtocol());
        Enumeration reqHeaderEnum = req.getHeaderNames();

        while(reqHeaderEnum.hasMoreElements()) {
            String headerName = (String)reqHeaderEnum.nextElement();
            buffer.append(CRLF).append(headerName).append(": ").append(req.getHeader(headerName));
        }

        buffer.append(CRLF);
        int responseLength = buffer.length();
        resp.setContentType("message/http");
        resp.setContentLength(responseLength);
        ServletOutputStream out = resp.getOutputStream();
        out.print(buffer.toString());
        out.close();
    }

    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String method = req.getMethod();
        long lastModified;
        if (method.equals("GET")) {
            lastModified = this.getLastModified(req);
            if (lastModified == -1L) {
                this.doGet(req, resp);
            } else {
                long ifModifiedSince;
                try {
                    ifModifiedSince = req.getDateHeader("If-Modified-Since");
                } catch (IllegalArgumentException var9) {
                    ifModifiedSince = -1L;
                }

                if (ifModifiedSince < lastModified / 1000L * 1000L) {
                    this.maybeSetLastModified(resp, lastModified);
                    this.doGet(req, resp);
                } else {
                    resp.setStatus(304);
                }
            }
        } else if (method.equals("HEAD")) {
            lastModified = this.getLastModified(req);
            this.maybeSetLastModified(resp, lastModified);
            this.doHead(req, resp);
        } else if (method.equals("POST")) {
            this.doPost(req, resp);
        } else if (method.equals("PUT")) {
            this.doPut(req, resp);
        } else if (method.equals("DELETE")) {
            this.doDelete(req, resp);
        } else if (method.equals("OPTIONS")) {
            this.doOptions(req, resp);
        } else if (method.equals("TRACE")) {
            this.doTrace(req, resp);
        } else {
            String errMsg = lStrings.getString("http.method_not_implemented");
            Object[] errArgs = new Object[]{method};
            errMsg = MessageFormat.format(errMsg, errArgs);
            resp.sendError(501, errMsg);
        }

    }

    private void maybeSetLastModified(HttpServletResponse resp, long lastModified) {
        if (!resp.containsHeader("Last-Modified")) {
            if (lastModified >= 0L) {
                resp.setDateHeader("Last-Modified", lastModified);
            }

        }
    }

    public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
        HttpServletRequest request;
        HttpServletResponse response;
        try {
            request = (HttpServletRequest)req;
            response = (HttpServletResponse)res;
        } catch (ClassCastException var6) {
            throw new ServletException("non-HTTP request or response");
        }

        this.service(request, response);
    }
}

　　这里就使用到了模板方法设计模式。

　　类图：

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