j2ee 学习笔记

1. Servlet

1.1 基本流程

 (*)-> "login.html"
-right-> "form表单处理给 '/login' "
-down-> "web.xml映射,给LoginServlet处理"
-right-> "LoginServlet处理"
-right-> "实例化Servelt"
-up-> "重写 doGet,doPost等方法,获取参数，设置响应"
-up-> "返回html"

flowchart TD
    A[login.html] -->|form提交到 /login| B(/login)
    B -->|web.xml 映射| C[LoginServlet]
    C --> D[实例化 Servlet]
    D --> E[重写 doGet/doPost]
    E -->|获取参数 & 设置响应| F[返回 HTML]

web.xml

<!--注册servlet name-->
<servlet>
    <servlet-name>HelloServlet</servlet-name> <!--类的位置-->
    <servlet-class>HelloServlet</servlet-class>
</servlet>

<!--设置/hello 映射给HelloServlet-->
<servlet-mapping>
    <servlet-name>HelloServlet</servlet-name>
    <url-pattern>/hello</url-pattern>
</servlet-mapping>

• 主要实现了处理映射(servlet-mapping标签) 和注册Servlet(servlet标签)

loginServlet

@Override
   protected void service(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException {

       System.out.println("3. ========== service ============= 函数");

       /**
        * 中文乱码，设置编码
        */
       request.setCharacterEncoding("UTF-8"); //request(请求) 编码
       response.setContentType("text/html;charset=UTF-8"); //response(响应) 编码

       String name = request.getParameter("name");
       String password = request.getParameter("password");

       if (name.equals("admin") && password.equals("admin")) { //密码正确
           /**
            * 服务器内跳转
            */
           request.getRequestDispatcher("success.html").forward(request, response);
       } else { //密码错误
           /**
            * 客户端中跳转
            */
           response.sendRedirect("fail.html");
       }
   }

• service将请求类型分发给doPost, doGet 等方法，用户只用重写这些方法

/**
*Servlet 的 service 源码
*/
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;
            try {
                ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
            } catch (IllegalArgumentException iae) {
                // Invalid date header - proceed as if none was set
                ifModifiedSince = -1;
            }
            if (ifModifiedSince < (lastModified / 1000 * 1000)) {
                // 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);
    }
}

servlet 跳转

1. 服务端跳转

request.getRequestDispatcher("success.html").forward(request, response);

• 流程如下

@startuml
浏览器 -> Tomcat : <color:#0867ff>         /login</color>
Tomcat -> 浏览器 : <color:#0867ff>把success.html的内容发送给浏览器
@enduml

2. 客户端跳转

response.sendRedirect("fail.html");

• 流程如下

@startuml
浏览器 -> Tomcat : <color: #0867ff>    /login</color>
Tomcat -> 浏览器 : <color: #0867ff>服务器发送一条特殊信息给浏览器让浏览器访问fail.html</color>
浏览器 -> Tomcat : <color: #0867ff> 访问Tomcat</color>
Tomcat -> 浏览器: <color: #0867ff> 服务端获取fail.html的内容，发送给浏览器</color>
@enduml

1.2 生命周期

a. 实例化

/**
 * 构造方法
 * Servlet 是单实例的
 * 无论被网页引用多少次，都只被调用一次
 */
public LoginServlet() {
    System.out.println("1. ============ 构造 =========  函数");
    System.out.println("LoginServlet 构造方法 被调用");
}

b. 初始化

/**
 * 初始化 init
 *
 * @param config
 */
@Override
public void init(ServletConfig config) {
    System.out.println("2. ============ init =========  函数");
    System.out.println("初始化函数");
    System.out.println("init(ServletConfig)");
}

c. 提供服务

• service -> doGet or service -> doPost

d. 销毁

/**
 * 销毁
 */
@Override
public void destroy() {
    System.out.println("4. ============= destroy ============ 函数");
    System.out.println("destroy 执行");
}

e. 被回收

1.3 自启动

• 如果tomcat启动就需要执行一些初始化代码，就需要自启动，如校验数据库完整性
• web.xml 需要配置自启动

<servlet>
    <servlet-name>HelloServlet</servlet-name>
    <servlet-class>HelloServlet</servlet-class>
    <!-- 自启动 数字越[小]优先级越[高], 取值范围1-99-->
    <load-on-startup>10</load-on-startup>
</servlet>

• 且将业务逻辑写在servlet 中的init()部分

/**
* 在此书写逻辑
*/
@Override
public void init(){
    System.out.println("init of Hello Servlet");
}``java

1.4 request常用方法

System.out.println("3. ========== service ============= 函数");

System.out.println("浏览器发出请求时的完整URL，包括协议 主机名 端口（如果有j）:"
                + request.getRequestURL());         //request.getRequestURL();

System.out.println("浏览器发出请求时的资源名部分，去掉了协议和主机名: "
                + request.getRequestURI());         //request.getRequestURI();

System.out.println("请求行中的参数部分: "
                + request.getQueryString());        //request.getQueryString();

System.out.println("浏览器所处于的客户机的IP地址: "
                + request.getRemoteAddr());         //request.getRemoteAddr();

System.out.println("浏览器所处于的客户机的主机名: "
                + request.getRemoteHost());         //request.getRemoteHost();

System.out.println("浏览器所处的客户机使用的网络端口"
                + request.getRemotePort());         //request.getRemotePort();

System.out.println("服务器的ip地址"
                + request.getLocalAddr());          //request.getLocalAddr()

System.out.println("服务器的主机名"
                + request.getLocalName());          //request.getLocalName();

System.out.println("客户机的请求方式"
                + request.getMethod());             //request.getMethod()

• 运行结果 例子