Java Web学习总结（3）Servlet（二）

一，Servlet访问URL映射配置

由于客户端是通过URL地址访问web服务器中的资源，所以Servlet程序若想被外界访问，必须把servlet程序映射到一个URL地址上，这个工作在web.xml文件中使用<servlet>元素和<servlet-mapping>元素完成。

Servlet：该元素用于注册Servlet，它包含有两个主要的子元素：<servlet-name>和<servlet-class>，分别用于设置Servlet的注册名称和Servlet的完整类名。

servlet-mapping：该元素用于映射一个已注册的Servlet的一个对外访问路径，它包含有两个子元素：<servlet-name>和<url-pattern>，分别用于指定Servlet的注册名称和Servlet的对外访问路径。

<servlet>
    <servlet-name>HelloServlet</servlet-name><!-- servlet名称 -->
    <servlet-class>com.zender.servlet.HelloServlet</servlet-class><!-- servlet关联的类 -->
</servlet>
<servlet-mapping>
    <servlet-name>HelloServlet</servlet-name>
    <url-pattern>/hello.html</url-pattern><!-- servlet访问路径 -->
</servlet-mapping>

同一个Servlet可以被映射到多个URL上，即多个<servlet-mapping>元素的<servlet-name>子元素的设置值可以是同一个Servlet的注册名

<servlet>
    <servlet-name>HelloServlet</servlet-name><!-- servlet名称 -->
    <servlet-class>com.zender.servlet.HelloServlet</servlet-class><!-- servlet关联的类 -->
</servlet>
<servlet-mapping>
    <servlet-name>HelloServlet</servlet-name>
    <url-pattern>/hello.html</url-pattern><!-- servlet访问路径 -->
</servlet-mapping>
<servlet-mapping>
    <servlet-name>HelloServlet</servlet-name>
    <url-pattern>/hello1.html</url-pattern><!-- servlet访问路径 -->
</servlet-mapping>
<servlet-mapping>
    <servlet-name>HelloServlet</servlet-name>
    <url-pattern>/hello2.html</url-pattern><!-- servlet访问路径 -->
</servlet-mapping>

二，ServletURL通配符：*

在Servlet映射到的URL中也可以使用*通配符，但是只能有两种固定的格式：

格式一：*.扩展名"

<servlet>
    <servlet-name>HelloServlet</servlet-name><!-- servlet名称 -->
    <servlet-class>com.zender.servlet.HelloServlet</servlet-class><!-- servlet关联的类 -->
</servlet>
<servlet-mapping>
    <servlet-name>HelloServlet</servlet-name>
    <url-pattern>*.html</url-pattern><!-- servlet访问路径 -->
</servlet-mapping>

格式二：以正斜杠（/）开头并以"/*"结尾

<servlet>
    <servlet-name>HelloServlet</servlet-name><!-- servlet名称 -->
    <servlet-class>com.zender.servlet.HelloServlet</servlet-class><!-- servlet关联的类 -->
</servlet>
<servlet-mapping>
    <servlet-name>HelloServlet</servlet-name>
    <url-pattern>/hello/*</url-pattern><!-- servlet访问路径 -->
</servlet-mapping>

三，缺省Servlet

Servlet的映射路径仅仅为一个正斜杠（/），那么这个Servlet就成为当前Web应用程序的缺省Servlet。 凡是在web.xml文件中找不到匹配的<servlet-mapping>元素的URL，它们的访问请求都将交给缺省Servlet处理。

<servlet>
    <servlet-name>HelloServlet</servlet-name><!-- servlet名称 -->
    <servlet-class>com.zender.servlet.HelloServlet</servlet-class><!-- servlet关联的类 -->
</servlet>
<servlet-mapping>
    <servlet-name>HelloServlet</servlet-name>
    <url-pattern>/</url-pattern><!-- servlet访问路径 -->
</servlet-mapping>

四，ServletConfig

ServletConfig代表当前Servlet在web.xml中的配置信息。

1，Servlet初始化参数

在Servlet的配置文件中，可以使用一个或多个<init-param>标签为servlet配置一些初始化参数。

例如：

<servlet>
    <servlet-name>HelloServlet</servlet-name><!-- servlet名称 -->
    <servlet-class>com.zender.servlet.HelloServlet</servlet-class><!-- servlet关联的类 -->
    <init-param>
        <param-name>name</param-name>
        <param-value>Zender</param-value>
    </init-param>
    <init-param>
        <param-name>encode</param-name>
        <param-value>UTF-8</param-value>
    </init-param>
</servlet>
<servlet-mapping>
    <servlet-name>HelloServlet</servlet-name>
    <url-pattern>/hello.html</url-pattern><!-- servlet访问路径 -->
</servlet-mapping>

2，通过ServletConfig获取Servlet的初始化参数

当servlet配置了初始化参数后，web容器在创建servlet实例对象时，会自动将这些初始化参数封装到ServletConfig对象中，并在调用servlet的init方法时，将ServletConfig对象传递给servlet。进而，通过ServletConfig对象就可以得到当前servlet的初始化参数信息。

public class HelloServlet extends HttpServlet{
 
    private static final long serialVersionUID = 3903946972744326948L;

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        this.doPost(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取ServletConfig
        ServletConfig config = this.getServletConfig();
        //获取初始化参数
        String name = config.getInitParameter("name");
        String encode = config.getInitParameter("encode");
        //设置编码格式，否则中文会出现乱码
        resp.setCharacterEncoding("GBK");
        PrintWriter out = resp.getWriter();
        out.println("name:" + name);
        out.println("encode:" + encode);
    }
}

五，ServletContext

WEB容器在启动时，它会为每个WEB应用程序都创建一个对应的ServletContext对象，它代表当前web应用。

ServletConfig对象中维护了ServletContext对象的引用，开发人员在编写servlet时，可以通过ServletConfig.getServletContext()方法获得ServletContext对象。

由于一个WEB应用中的所有Servlet共享同一个ServletContext对象，因此Servlet对象之间可以通过ServletContext对象来实现通讯。ServletContext对象通常也被称之为context域对象。

1，获取WEB应用的初始化参数

在web.xml文件中使用<context-param>标签配置WEB应用的初始化参数：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns="http://java.sun.com/xml/ns/javaee"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
    id="WebApp_ID" version="3.0">
    <display-name>ServletDemo</display-name>
    <welcome-file-list>
        <welcome-file>index.jsp</welcome-file>
    </welcome-file-list>
    <!-- 配置WEB应用的初始化参数 -->
    <context-param>
        <param-name>name</param-name>
        <param-value>Zender</param-value>
    </context-param>
    <context-param>
        <param-name>encode</param-name>
        <param-value>UTF-8</param-value>
    </context-param>
 
    <servlet>
        <servlet-name>HelloServlet</servlet-name><!-- servlet名称 -->
        <servlet-class>com.zender.servlet.HelloServlet</servlet-class><!-- servlet关联的类 -->
    </servlet>
    <servlet-mapping>
        <servlet-name>HelloServlet</servlet-name>
        <url-pattern>/hello.html</url-pattern><!-- servlet访问路径 -->
    </servlet-mapping>
</web-app>

获取Web应用的初始化参数：

public class HelloServlet extends HttpServlet{

    private static final long serialVersionUID = 3903946972744326948L;

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        this.doPost(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取ServletContext
        ServletContext context = this.getServletContext();
        //获取初始化参数
        String name = context.getInitParameter("name");
        String encode = context.getInitParameter("encode");
        //设置编码格式，否则中文会出现乱码
        resp.setCharacterEncoding("GBK");
        PrintWriter out = resp.getWriter();
        out.println("name:" + name);
        out.println("encode:" + encode);
    }
}

2，多个Servlet通过ServletContext对象实现数据共享

例如 ：ServletTwo获得ServletOne所共享的数据

ServletOne：

public class ServletOne extends HttpServlet{
 
    private static final long serialVersionUID = 3903946972744326948L;

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        this.doPost(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //共享的数据
        String name = "zender";
        //获取ServletContext
        ServletContext context = this.getServletContext();
        //将name存储到ServletContext对象中
        context.setAttribute("name", name);
    }
}

ServletTwo：

public class ServletTwo extends HttpServlet{
 
    private static final long serialVersionUID = 3903946972744326948L;

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        this.doPost(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取ServletContext
        ServletContext context = this.getServletContext();
        //获取初始化参数
        String name = (String) context.getAttribute("name");
        PrintWriter out = resp.getWriter();
        out.println("name:" + name);
    }
}

Web.xml：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns="http://java.sun.com/xml/ns/javaee"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
    id="WebApp_ID" version="3.0">
    <display-name>ServletDemo</display-name>
    <welcome-file-list>
        <welcome-file>index.jsp</welcome-file>
    </welcome-file-list>
 
    <servlet>
        <servlet-name>ServletOne</servlet-name><!-- servlet名称 -->
        <servlet-class>com.zender.servlet.ServletOne</servlet-class><!-- servlet关联的类 -->
    </servlet>
    <servlet-mapping>
        <servlet-name>ServletOne</servlet-name>
        <url-pattern>/ServletOne.html</url-pattern><!-- servlet访问路径 -->
    </servlet-mapping>

    <servlet>
        <servlet-name>ServletTwo</servlet-name><!-- servlet名称 -->
        <servlet-class>com.zender.servlet.ServletTwo</servlet-class><!-- servlet关联的类 -->
    </servlet>
    <servlet-mapping>
        <servlet-name>ServletTwo</servlet-name>
        <url-pattern>/ServletTwo.html</url-pattern><!-- servlet访问路径 -->
    </servlet-mapping>
</web-app>

先访问：http://localhost:8081/ServletDemo/ServletOne.html

再访问：http://localhost:8081/ServletDemo/ServletTwo.html

3，用servletContext实现请求转发

context.getRequestDispatcher()方法实现请求转发

ServletOne：

public class ServletOne extends HttpServlet{
 
    private static final long serialVersionUID = 3903946972744326948L;

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        this.doPost(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //共享的数据
        String name = "zender";
        //获取ServletContext
        ServletContext context = this.getServletContext();
        //将name存储到ServletContext对象中
        context.setAttribute("name", name);
        context.getRequestDispatcher("/ServletTwo.html").forward(req, resp);
    }
}

ServletTwo：

public class ServletTwo extends HttpServlet{
 
    private static final long serialVersionUID = 3903946972744326948L;

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        this.doPost(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取ServletContext
        ServletContext context = this.getServletContext();
        //获取初始化参数
        String name = (String) context.getAttribute("name");
        PrintWriter out = resp.getWriter();
        out.println("name:" + name);
    }
}

Web.xml：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns="http://java.sun.com/xml/ns/javaee"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
    id="WebApp_ID" version="3.0">
    <display-name>ServletDemo</display-name>
    <welcome-file-list>
        <welcome-file>index.jsp</welcome-file>
    </welcome-file-list>
 
    <servlet>
        <servlet-name>ServletOne</servlet-name><!-- servlet名称 -->
        <servlet-class>com.zender.servlet.ServletOne</servlet-class><!-- servlet关联的类 -->
    </servlet>
    <servlet-mapping>
        <servlet-name>ServletOne</servlet-name>
        <url-pattern>/ServletOne.html</url-pattern><!-- servlet访问路径 -->
    </servlet-mapping>

    <servlet>
        <servlet-name>ServletTwo</servlet-name><!-- servlet名称 -->
        <servlet-class>com.zender.servlet.ServletTwo</servlet-class><!-- servlet关联的类 -->
    </servlet>
    <servlet-mapping>
        <servlet-name>ServletTwo</servlet-name>
        <url-pattern>/ServletTwo.html</url-pattern><!-- servlet访问路径 -->
    </servlet-mapping>
</web-app>

访问：http://localhost:8081/ServletDemo/ServletOne.html

访问的是ServletOne.html，浏览器显示的却是ServletTwo.html的内容，这就是使用ServletContext实现了请求转发。

4，利用ServletContext对象读取资源文件

获取上图的config.properties资源文件内容：

GetProperties.Java：

public class GetProperties extends HttpServlet{
 
    private static final long serialVersionUID = 3903946972744326948L;

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        this.doPost(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //设置浏览器编码格式为UTF-8
        resp.setHeader("content-type","text/html;charset=UTF-8");
        resp.getWriter().println("读取src目录下的config.properties配置文件：");
        //读取src目录下的properties配置文件
        readProperties(resp,"/WEB-INF/classes/config.properties");

        resp.getWriter().println("<hr/>读取WebRoot目录下的config.properties配置文件：");
        //读取WebRoot目录下的properties配置文件
        readProperties(resp,"config.properties");
    }

    /**
     *
     * @方法名: readProperties
     * @描述: 获取properties配置文件内容
     * @param resp
     * @param fileUrl
     * @throws IOException
     * @创建人 zender
     */
    protected void readProperties(HttpServletResponse resp, String fileUrl) throws IOException{
        //2种方式都能获取到config.properties
        //InputStream in = new FileInputStream(this.getServletContext().getRealPath(fileUrl));
        InputStream in = this.getServletContext().getResourceAsStream(fileUrl);
        Properties prop = new Properties();
        prop.load(in);
        String url = prop.getProperty("database.url");
        String username = prop.getProperty("database.username");
        String password = prop.getProperty("database.password");
        resp.getWriter().println(
                    MessageFormat.format("url={0},username={1},password={2}", url,username, password)
                );
    }
}

Web.xml：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns="http://java.sun.com/xml/ns/javaee"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
    id="WebApp_ID" version="3.0">
    <display-name>ServletDemo</display-name>
    <welcome-file-list>
        <welcome-file>index.jsp</welcome-file>
    </welcome-file-list>
 
    <servlet>
        <servlet-name>GetProperties</servlet-name><!-- servlet名称 -->
        <servlet-class>com.zender.servlet.GetProperties</servlet-class><!-- servlet关联的类 -->
    </servlet>
    <servlet-mapping>
        <servlet-name>GetProperties</servlet-name>
        <url-pattern>/GetProperties.html</url-pattern><!-- servlet访问路径 -->
    </servlet-mapping>
</web-app>

访问路径：http://localhost:8081/ServletDemo/GetProperties.html