Servlet---注解开发

1.引入

在Servlet3.0以后的版本提供了Servlet注解配置，大大简化了代码编写。它可以替代xml文件的配置

2.代码实现

<1>注解开发代码如下：

@WebServlet(value = "/servlet1", initParams = {@WebInitParam(name = "charset", value = "utf-8")})
//【注：如果是只需要映射路径则可以直接在双引号中写】
//@WebServlet("/servlet1")
public class ServletDemo extends HttpServlet {
}

<2>它等同于xml配置文件

<?xml version="1.0" encoding="UTF-8"?>
<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"><servlet><servlet-name>ServletDemo </servlet-name><servlet-class>com.itheima.servlet.ServletDemo</servlet-class><init-param><param-name>charset</param-name><param-value>utf-8</param-value></init-param></servlet><servlet-mapping><servlet-name>ServletDemo </servlet-name><url-pattern>/servlet1</url-pattern></servlet-mapping>
</web-app>

<3>ServletDemo获取配置信息代码

@WebServlet(value = "/servlet1", initParams = {@WebInitParam(name = "charset", value = "utf-8")})
//@WebServlet("/servlet1")
public class ServletDemo extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {ServletConfig servletConfig = this.getServletConfig();Enumeration<String> parameterNames = servletConfig.getInitParameterNames();while (parameterNames.hasMoreElements()) {String name = parameterNames.nextElement();String value = servletConfig.getInitParameter(name);System.out.println(name + "..." + value);}}@Overrideprotected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {doGet(req, resp);}
}打印结果：
------------------------------------------------------
charset...utf-8

3.自动注解开发的注解详解

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface WebServlet {
"指定Servlet的名称"
"--等效于web.xml文件<servlet>标签下的<servlet-name>"String name() default "";
"--用于映射servlet"
"等效于<url-pattern>"
//两个属性只能写一个String[] value() default {};String[] urlPatterns() default {};
"/指定servlet的加载时机"
"等效于<load-on-startup>"
//-1表示访问时创建int loadOnStartup() default -1;
"指定servlet的初始化参数"
"等效于<init-param>"WebInitParam[] initParams() default {};
"指定servlet是否支持异步"boolean asyncSupported() default false;
"指定servlet的小图标"String smallIcon() default "";
"指定servlet的大图标"String largeIcon() default "";
"指定servlet的描述信息"String description() default "";
"指定servlet的显示名称"String displayName() default "";
}