Java學習路線-48：Servlet學習

課時1 Servlet是什麼

Servlet作用是處理請求

接收請求

處理請求

完成響應

課時2 實作Servlet方式

依賴

<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>javax.servlet-api</artifactId>
    <version>4.0.1</version>
    <scope>provided</scope>
</dependency>

實作方式

// 實作接口：
javax.servlet.Servlet

// 繼承類：
javax.servlet.GenericServlet

// 繼承類：
javax.servlet.HttpServlet

繼承示例

AServlet.java

import javax.servlet.*;
import java.io.IOException;

public class AServlet implements Servlet{

    // 建立時執行
    @Override
    public void init(ServletConfig servletConfig) throws ServletException {

    }

    // 擷取配置資訊
    @Override
    public ServletConfig getServletConfig() {
        return null;
    }

    // 處理請求
    @Override
    public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {

    }

    // 擷取servlet資訊
    @Override
    public String getServletInfo() {
        return null;
    }

    // 銷毀前調用
    @Override
    public void destroy() {

    }
}

課時3 Servlet的生命周期

生命周期

init    執行個體化調用
service 每次處理請求都調用
destroy 銷毀調用

特性

（1）單例，每個類隻有一個對象

（2）線程不安全，效率最高

Servlet類由使用者自定義，對象由伺服器來建立，并有伺服器調用對應的方法

浏覽器通路Servlet

給Servlet配置一個路徑

web.xml

<?xml version="1.0" encoding="utf-8" ?>

<web-app>
    <!-- 注冊 Servlet，幫助web伺服器反射該類 -->
    <servlet>
        <servlet-name>demo</servlet-name>
        <servlet-class>AServlet</servlet-class>
    </servlet>
    
    <!-- 映射 Servlet 資源，用url-pattern元素标示 URL -->
    <servlet-mapping>
        <servlet-name>demo</servlet-name>
        <url-pattern>demo</url-pattern>
    </servlet-mapping>
</web-app>

目錄結構

webapp
    └── WEB-INF
        ├── classes
        │   └── AServlet.class
        └── web.xml

課時4 ServletConfig介紹

interface ServletConfig{
    // 擷取servlet-name中的内容
    String getServletName() 

    // 擷取Servlet上下文對象
    ServletContext getServletContext() 

    // 擷取指定初始化參數值
    String getInitParameter(String name) 

    // 擷取所有初始化參數值
    Enumeration<String> getInitParameterNames() 

}

<servlet> 
    <init-param>
        <param-name>name</param-name>
        <param-value>Tom</param-value>
    </init-param>
</servlet>

擷取參數

@Override
    public void init(ServletConfig servletConfig) throws ServletException {
        System.out.println(servletConfig.getInitParameter("name"));
    }
@Override
    public void init(ServletConfig servletConfig) throws ServletException {
        System.out.println(servletConfig.getInitParameter("name"));
    }

課時5 ServletRequest和Servletresponse對象

interface ServletRequest;

interface ServletResponse;

課時6 GenericServlet介紹

GenericServlet可以隻覆寫service方法，不用全寫

基本原理

import javax.servlet.*;
import java.io.IOException;

public class AServlet implements Servlet {
    private ServletConfig config = null;

    @Override
    public void init(ServletConfig servletConfig) {
        this.config = servletConfig;
        this.init();
    }

    // 實作此方法，初始化後可以調用
    public void init() {

    }

    @Override
    public ServletConfig getServletConfig() {
        return this.config;
    }

    @Override
    public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {

    }

    @Override
    public String getServletInfo() {
        return null;
    }

    @Override
    public void destroy() {

    }
}

使用示例

import javax.servlet.*;
import java.io.IOException;

public class AServlet extends GenericServlet {

    @Override
    public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws IOException {
        servletResponse.getWriter().write("<h2>hello<h2>");
    }
}

課時7 HttpServlet介紹

源碼

package javax.servlet.http;


public abstract class HttpServlet extends GenericServlet {
    // 重寫
    protected void doGet(HttpServletRequest req, HttpServletResponse resp);
    
    // 重寫
    protected void doPost(HttpServletRequest req, HttpServletResponse resp);

    protected void service(HttpServletRequest req, HttpServletResponse resp);

    public void service(ServletRequest req, ServletResponse res){
        if (req instanceof HttpServletRequest && res instanceof HttpServletResponse) {
            HttpServletRequest request = (HttpServletRequest)req;
            HttpServletResponse response = (HttpServletResponse)res;
            this.service(request, response);
        } else {
            throw new ServletException("non-HTTP request or response");
        }
    }
}

使用示例: 接收GET請求

import javax.servlet.*;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class AServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.getWriter().write("<h2>hello</h2>");
    }
}

課時8 Servlet的細節

1、非線程安全

一個Servlet類隻有一個執行個體對象，不是線程安全的，工作效率高

（1）不要在Servlet中建立成員，建立局部變量即可

（2）可以建立無狀态成員

（3）可以建立有狀态的成員，但是狀态必須為隻讀

2、建立時啟動

預設情況下，伺服器會在第一次通路Servlet時建立執行個體對象

給load-on-startup設定一個非負整數

正數的值越小，啟動該servlet的優先級越高

可以配置建立時啟動

<servlet>
    <servlet-name>servletName</servlet-name>
    <load-on-startup>0</load-on-startup>
</servlet>

3、url-pattern

可以有多個通路路徑

可以使用字首或者字尾通配符

<servlet>
    <servlet-name>servletName</servlet-name>
    <!-- 字尾比對 -->
    <url-pattern>/demo/*</url-pattern>
    <!-- 字首比對 -->
    <url-pattern>*.do</url-pattern>
    <!-- 比對所有url -->
    <url-pattern>/*</url-pattern>
</servlet>

課時9 在conf下的web.xml檔案内容介紹

${CATALINA_HOME}/conf/web.xml

<!-- 優先級最低 -->
<servlet-mapping>
    <servlet-name>default</servlet-name>
    <url-pattern>/</url-pattern>
</servlet-mapping>

<!-- 過期時間30分鐘 -->
<session-config>
    <session-timeout>30</session-timeout>
</session-config>

<!-- 類型映射 -->
<mime-mapping>
    <extension>xml</extension>
    <mime-type>application/xml</mime-type>
</mime-mapping>

課時10 Servlet與反射

Servlet通過反射，擷取配置檔案的類名，進行執行個體化和方法調用

課時11 ServletContext概述

一個項目隻有一個ServletContext對象

可以在多個Servlet中傳遞對象

Tomcat啟動時建立，關閉時銷毀

課時12 擷取ServletContext對象

擷取ServletContext

class ServeletConfig{
    ServletContext getServletContext();
}

class GenericServlet{
    ServletContext getServletContext();
}

課時13 示範ServletContext

域對象: 在多個Servlet中傳遞資料

class ServletContext{
    // 設定 多次調用會覆寫
    void setAttribute(String name, Object vlaue);
    // 擷取
    Object getAttribute(String name);
    // 移除
    void removeAttribute(String name);
    // 擷取全部屬性名稱
    Enumeration getAttributeNames();
}

例如: AServlet的資料可以傳遞到BServlet

package com.pengshiyu;

import javax.servlet.ServletContext;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class AServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
        String name = request.getParameter("name");
        ServletContext context = getServletContext();
        context.setAttribute("name", name);
        response.getWriter().write("<h2>" + name +"</h2>");
    }

}

package com.pengshiyu;

import javax.servlet.ServletContext;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class BServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
        ServletContext context = getServletContext();
        String name = (String)context.getAttribute("name");
        response.getWriter().println(name);
    }
}

課時14 ServletContext擷取公共的初始化參數

Servlet隻能擷取自己的初始化參數

ServletContext擷取公共的初始化參數

<web-app>
    <context-param>
        <param-name>key</param-name>
        <param-value>value</param-value>
    </context-param>
</web-app>

package com.pengshiyu;

import javax.servlet.ServletContext;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class BServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
        ServletContext context = getServletContext();
        String name = context.getInitParameter("key");
        response.getWriter().println(name);
    }
}

課時15 ServletContext擷取資源相關方法

以webapp文根目錄

ServletContext context = getServletContext();

// 擷取絕對路徑
String path = context.getRealPath("/index.html");

// 擷取檔案後轉為輸入流
InputStream in = context.getResourceAsStream("/index.html");

// 擷取目錄下的檔案
Set<String> set = context.getResourcePaths("/");
// [/hello.html, /WEB-INF/]

課時16 網站通路量統計小案例

使用ServletContext對象共享統計資料

package com.pengshiyu;

import javax.servlet.ServletContext;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class BServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
        ServletContext context = getServletContext();
        Integer count = (Integer) context.getAttribute("count");

        if (count == null) {
            count = 1; // 第一次通路
        } else {
            count++;
        }

        response.setContentType("text/html; charset=UTF-8");
        response.getWriter().println("<h2>第"+count+"次通路</h2>");
        context.setAttribute("count", count);
    }
}

課時17 擷取類路徑下的資源

<dependency>
    <groupId>commons-io</groupId>
    <artifactId>commons-io</artifactId>
    <version>2.6</version>
</dependency>

代碼執行個體

package com.pengshiyu;

import org.apache.commons.io.IOUtils;

import java.io.IOException;
import java.io.InputStream;
import java.nio.charset.StandardCharsets;

public class Demo {

    public static void main(String[] args) throws IOException {

        Class<Demo> clazz = Demo.class;

        // 1、相對于.class所在目錄
        InputStream input1 = clazz.getResourceAsStream("1.txt");
        System.out.println(IOUtils.toString(input1, StandardCharsets.UTF_8));

        // 2、相對于/classes
        InputStream input2 = clazz.getResourceAsStream("/2.txt");
        System.out.println(IOUtils.toString(input2, StandardCharsets.UTF_8));

        ClassLoader loader = clazz.getClassLoader();

        // 3、相對于/classes
        InputStream input3 = loader.getResourceAsStream("3.txt");
        System.out.println(IOUtils.toString(input3, StandardCharsets.UTF_8));

    }
}

課時18 BaseServlet

一個Servlet中可以有多個請求處理方法

BaseServlet.java

package com.pengshiyu;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.lang.reflect.Method;

public class BaseServlet extends HttpServlet {
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        String methodName = req.getParameter("method");

        if(methodName == null || methodName.trim().isEmpty()){
            throw new RuntimeException("method is null or empty");
        }
        System.out.println(methodName);

        // 通過參數擷取方法
        Class clazz = this.getClass();

        Method method = null;
        try {
            method = clazz.getMethod(methodName, HttpServletRequest.class, HttpServletResponse.class);
        } catch (NoSuchMethodException e) {
            throw new RuntimeException("方法擷取失敗");
        }

        // 調用方法
        try {
            method.invoke(this, req, resp);
        } catch (Exception e) {
            throw new RuntimeException("調用失敗");
        }

    }
}

package com.pengshiyu;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class AServlet extends BaseServlet {

    public void getAge(HttpServletRequest request, HttpServletResponse response) throws IOException {
        response.getWriter().print("getAge");
    }

    public void getName(HttpServletRequest request, HttpServletResponse response) throws IOException {
        response.getWriter().print("getName");
    }

}

調用時傳入查詢參數

http://localhost:8080/demo/hello?method=getAge

Java學習路線-48：Servlet學習

課時1 Servlet是什麼

課時2 實作Servlet方式

課時3 Servlet的生命周期

課時4 ServletConfig介紹

課時5 ServletRequest和Servletresponse對象

課時6 GenericServlet介紹

課時7 HttpServlet介紹

課時8 Servlet的細節

課時9 在conf下的web.xml檔案内容介紹

課時10 Servlet與反射

課時11 ServletContext概述

課時12 擷取ServletContext對象

課時13 示範ServletContext

課時14 ServletContext擷取公共的初始化參數

課時15 ServletContext擷取資源相關方法

課時16 網站通路量統計小案例

課時17 擷取類路徑下的資源

課時18 BaseServlet

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