[ 成为架构师系列 ] 1. 第一个 Java Web 程序
- 2019 年 11 月 27 日
- 筆記
[ 成为架构师系列 ] 1. 第一个 Java Web 程序
1.新建 maven webapp 工程
打开 idea, New Project, 选择 Maven, 从 maven-archetype 创建, 找到 maven-archetype-webapp:
创建 maven 项目
-DgroupId=com.light.sword -DartifactId=simple-javaweb -Dversion=1.0-SNAPSHOT -DarchetypeGroupId=org.apache.maven.archetypes -DarchetypeArtifactId=maven-archetype-webapp -DarchetypeVersion=RELEASE org.apache.maven.plugins:maven-archetype-plugin:RELEASE:generate
得到项目文件目录如下
$ tree . ├── pom.xml ├── simplejavaweb.iml └── src └── main └── webapp ├── WEB-INF │ └── web.xml └── index.jsp 4 directories, 4 files
2. maven-archetype-webapp 是什么?
Maven Webapp Archetype: http://maven.apache.org/archetypes/maven-archetype-webapp/
Maven Webapp Archetype
maven-archetype-webapp is an archetype which generates a sample Maven webapp project:
project |-- pom.xml `-- src `-- main `-- webapp |-- WEB-INF | `-- web.xml `-- index.jsp
Usage
To generate a new project from this archetype, type:
mvn archetype:generate -DarchetypeGroupId=org.apache.maven.archetypes -DarchetypeArtifactId=maven-archetype-webapp -DarchetypeVersion=1.4
3. Servlet 是什么?
Servlet 本质上就是一个java类:
- A servlet is a small Java program that runs within a Web server.
- Servlets receive and respond to requests from Web clients, usually across HTTP, the HyperText Transfer Protocol.
/** * Defines methods that all servlets must implement. * * <p> * A servlet is a small Java program that runs within a Web server. Servlets * receive and respond to requests from Web clients, usually across HTTP, the * HyperText Transfer Protocol. * * <p> * To implement this interface, you can write a generic servlet that extends * <code>javax.servlet.GenericServlet</code> or an HTTP servlet that extends * <code>javax.servlet.http.HttpServlet</code>. * * <p> * This interface defines methods to initialize a servlet, to service requests, * and to remove a servlet from the server. These are known as life-cycle * methods and are called in the following sequence: * <ol> * <li>The servlet is constructed, then initialized with the <code>init</code> * method. * <li>Any calls from clients to the <code>service</code> method are handled. * <li>The servlet is taken out of service, then destroyed with the * <code>destroy</code> method, then garbage collected and finalized. * </ol> * * <p> * In addition to the life-cycle methods, this interface provides the * <code>getServletConfig</code> method, which the servlet can use to get any * startup information, and the <code>getServletInfo</code> method, which allows * the servlet to return basic information about itself, such as author, * version, and copyright. * * @see GenericServlet * @see javax.servlet.http.HttpServlet */ public interface Servlet { /** * Called by the servlet container to indicate to a servlet that the servlet * is being placed into service. * * <p> * The servlet container calls the <code>init</code> method exactly once * after instantiating the servlet. The <code>init</code> method must * complete successfully before the servlet can receive any requests. * * <p> * The servlet container cannot place the servlet into service if the * <code>init</code> method * <ol> * <li>Throws a <code>ServletException</code> * <li>Does not return within a time period defined by the Web server * </ol> * * * @param config * a <code>ServletConfig</code> object containing the servlet's * configuration and initialization parameters * * @exception ServletException * if an exception has occurred that interferes with the * servlet's normal operation * * @see UnavailableException * @see #getServletConfig */ public void init(ServletConfig config) throws ServletException; /** * * Returns a {@link ServletConfig} object, which contains initialization and * startup parameters for this servlet. The <code>ServletConfig</code> * object returned is the one passed to the <code>init</code> method. * * <p> * Implementations of this interface are responsible for storing the * <code>ServletConfig</code> object so that this method can return it. The * {@link GenericServlet} class, which implements this interface, already * does this. * * @return the <code>ServletConfig</code> object that initializes this * servlet * * @see #init */ public ServletConfig getServletConfig(); /** * Called by the servlet container to allow the servlet to respond to a * request. * * <p> * This method is only called after the servlet's <code>init()</code> method * has completed successfully. * * <p> * The status code of the response always should be set for a servlet that * throws or sends an error. * * * <p> * Servlets typically run inside multithreaded servlet containers that can * handle multiple requests concurrently. Developers must be aware to * synchronize access to any shared resources such as files, network * connections, and as well as the servlet's class and instance variables. * More information on multithreaded programming in Java is available in <a * href * ="http://java.sun.com/Series/Tutorial/java/threads/multithreaded.html"> * the Java tutorial on multi-threaded programming</a>. * * * @param req * the <code>ServletRequest</code> object that contains the * client's request * * @param res * the <code>ServletResponse</code> object that contains the * servlet's response * * @exception ServletException * if an exception occurs that interferes with the servlet's * normal operation * * @exception IOException * if an input or output exception occurs */ public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException; /** * Returns information about the servlet, such as author, version, and * copyright. * * <p> * The string that this method returns should be plain text and not markup * of any kind (such as HTML, XML, etc.). * * @return a <code>String</code> containing servlet information */ public String getServletInfo(); /** * Called by the servlet container to indicate to a servlet that the servlet * is being taken out of service. This method is only called once all * threads within the servlet's <code>service</code> method have exited or * after a timeout period has passed. After the servlet container calls this * method, it will not call the <code>service</code> method again on this * servlet. * * <p> * This method gives the servlet an opportunity to clean up any resources * that are being held (for example, memory, file handles, threads) and make * sure that any persistent state is synchronized with the servlet's current * state in memory. */ public void destroy(); }
Tomcat与Servlet的关系
Servlet运行在Tomcat中
Servlet与普通的Java程序的区别
Servlet本质上就是一个Java类 Servlet类必须实现接口javax.servlet.Servlet接口 运行在Web容器中，tomcat就是一个Web容器。 能够接收浏览器发送的请求，并且做出响应给浏览器。
4. 编写自己的 Servlet
pom.xml中添加 servlet-api 依赖
<dependency> <groupId>org.apache.tomcat</groupId> <artifactId>servlet-api</artifactId> <version>6.0.53</version> </dependency>
写一个类继承 HttpServlet
HttpServlet是个抽象类它已经实现了Servlet接口。
我们写一个 HelloServlet 类继承 HttpServlet:
package com.light.sword; import javax.servlet.http.HttpServlet; /** * @author: Jack * 2019-11-23 22:50 */ public class HelloServlet extends HttpServlet { }
重写 doGet 或 doPost 方法，分别处理表单的get或post请求。
package com.light.sword; 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.io.PrintWriter; import java.util.Date; /** * @author: Jack * 2019-11-23 22:50 */ public class HelloServlet extends HttpServlet { @Override public void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { resp.setContentType("text/html"); resp.setCharacterEncoding("UTF8"); PrintWriter writer = resp.getWriter(); writer.println(new Date() + ", Hello World, by Servlet."); } }
如果直接在浏览器输入地址访问，使用的是get方法。
Note: Get 方法是幂等的,无状态,无副作用的. The GET method should be safe, that is, without any side effects
5. 编写web.xml配置文件
完成 Servlet 的编写, 还要配置一下 web.xml 文件，对Servlet进行配置，才能通过浏览器来访问。
配置 web.xml
<!DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN" "http://java.sun.com/dtd/web-app_2_3.dtd" > <web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd" version="4.0"> <display-name>Hello World, Archetype Created Web Application</display-name> <description> This is a simple web application with a source code organization based on the recommendations of the Application Developer's Guide. </description> <servlet> <servlet-name>HelloServlet</servlet-name> <servlet-class>com.light.sword.HelloServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>HelloServlet</servlet-name> <url-pattern>/hello</url-pattern> </servlet-mapping> </web-app>
6. war 打包配置
http://maven.apache.org/plugins/maven-war-plugin/examples/adding-filtering-webresources.html
maven 打 war包配置:
<build> <finalName>simple-javaweb</finalName> <sourceDirectory>${project.basedir}/src/main/java</sourceDirectory> <plugins> <plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-compiler-plugin</artifactId> <version>3.3</version> <configuration> <!-- 指定JDK版本 --> <source>1.8</source> <target>1.8</target> </configuration> </plugin> <plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-war-plugin</artifactId> <version>2.4</version> </plugin> </plugins> </build>
工程目录规范:
/simplejavaweb$ tree . ├── README.md ├── package.sh ├── pom.xml ├── simplejavaweb.iml └── src └── main ├── java │ └── com │ └── light │ └── sword │ └── HelloServlet.java └── webapp ├── WEB-INF │ └── web.xml ├── hello.jsp └── index.html 8 directories, 8 files
Including and Excluding Files From the WAR: http://maven.apache.org/plugins/maven-war-plugin/examples/including-excluding-files-from-war.html
7. Tomcat 启动部署
apache-tomcat-7.0.82$ ./bin/startup.sh Using CATALINA_BASE: /Users/jack/soft/apache-tomcat-7.0.82 Using CATALINA_HOME: /Users/jack/soft/apache-tomcat-7.0.82 Using CATALINA_TMPDIR: /Users/jack/soft/apache-tomcat-7.0.82/temp Using JRE_HOME: /Library/Java/JavaVirtualMachines/jdk1.8.0_40/Contents/Home Using CLASSPATH: /Users/jack/soft/apache-tomcat-7.0.82/bin/bootstrap.jar:/Users/jack/soft/apache-tomcat-7.0.82/bin/tomcat-juli.jar Tomcat started.
上传 war 包, 然后 deploy .
8. 运行测试
访问 http://localhost:8080/simple-javaweb/
点击 To a JSP page. http://localhost:8080/simple-javaweb/hello.jsp
<%@ page import="java.util.Date" %> <%@ page session="false" pageEncoding="UTF-8" contentType="text/html; charset=UTF-8" %> <!DOCTYPE html> <html> <body> <h2> <%= new Date() + ", Hello World, by JSP." %> </h2> </body> </html>
点击 To a Servlet. http://localhost:8080/simple-javaweb/hello
package com.light.sword; 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.io.PrintWriter; import java.util.Date; /** * @author: Jack * 2019-11-23 22:50 */ public class HelloServlet extends HttpServlet { @Override public void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { resp.setContentType("text/html"); resp.setCharacterEncoding("UTF8"); PrintWriter writer = resp.getWriter(); writer.println(new Date() + ", Hello World, by Servlet."); } }
9. FAQ 问题
HTTP Status 500 – Error instantiating servlet class
用maven打了war包之后部署到tomcat下居然无法执行,看了一下原来没有任何编译的.class文件. 这个问题通常是打 war 包,没有把编译好的类文件打进去.
配置 build 标签下面的sourceDirectory:
<build> <finalName>simple-javaweb</finalName> <sourceDirectory>${project.basedir}/src/main/java</sourceDirectory> <plugins> <plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-compiler-plugin</artifactId> <version>3.3</version> <configuration> <!-- 指定JDK版本 --> <source>1.8</source> <target>1.8</target> </configuration> </plugin> <plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-war-plugin</artifactId> <version>2.4</version> </plugin> </plugins> </build>
#!/usr/bin/env bash mvn clean compile package
.gitignore
*.iml .idea
工程源码
https://github.com/to-be-architect/simple-javaweb
参考资料
https://blog.csdn.net/RookiexiaoMu_a/article/details/89254719
