JavaWeb学习总结(六)--Servlet开发(二)

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[TOC]

一、ServletConfig讲解

1.1、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，我们通过ServletConfig对象就可以得到当前servlet的初始化参数信息。

​ 例如:

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package org.wuqinghua.java.ch01;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebInitParam;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.Enumeration;
/**
 * Created by wuqinghua on 17/6/30.
 */
@WebServlet(name = "servletConfigDemo01",urlPatterns = {"/servletConfigDemo01"},initParams
        ={@WebInitParam(name = "name",value = "gacl"),@WebInitParam(name = "password",value =
        "123"),@WebInitParam(name = "charset",value = "UTF-8")} )
public class ServletConfigDemo01 extends HttpServlet {
    //定义ServletConfig对象来接受配置的初始化参数
    private ServletConfig servletConfig;
    /**
     *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对象就可以得到初始化数据
     */
    @Override
    public void init(ServletConfig config) throws ServletException {
        this.servletConfig = config;
    }
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取配置的参数
        String paramVal = this.servletConfig.getInitParameter("name");
        resp.getWriter().print(paramVal);
        resp.getWriter().print("<hr />");
        //获取所有初始化参数
        Enumeration<String> e = this.servletConfig.getInitParameterNames();
        while (e.hasMoreElements()){
            String name = e.nextElement();
            String value = this.servletConfig.getInitParameter(name);
            resp.getWriter().print(name+"-"+value+"<br />");
        }
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doGet(req, resp);
    }
}

运行结果如下：

　　img

二、ServletContext对象

　　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。

三、ServletContext的应用

　　3.1、多个Servlet通过ServletContext对象实现数据共享

　　范例：ServletContextDemo1和ServletContextDemo2通过ServletContext对象实现数据共享

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package org.wuqinghua.java.ch01;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebInitParam;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
/**
 * Created by wuqinghua on 17/6/30.
 */
@WebServlet(urlPatterns = "/servletContextDemo01",initParams = @WebInitParam(name = "name",value
        = "hello"))
public class ServletContextDemo01 extends HttpServlet {
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String data = "xdp_gacl";
        /**
         * 获取ServletContext
         */
        ServletContext servletContext =
                this.getServletConfig().getServletContext();
        ServletContext servletContext1 = this.getServletContext();
        System.out.println(servletContext == servletContext1);
        System.out.println(this.getServletConfig().getInitParameter("name"));
        servletContext.setAttribute("data", data);
    }
}
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package org.wuqinghua.java.ch01;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
/**
 * Created by wuqinghua on 17/6/30.
 */
@WebServlet(urlPatterns = "/servletContextDemo02")
public class ServletContextDemo02 extends HttpServlet {
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext servletContext = this.getServletContext();
        String data = (String) servletContext.getAttribute("data");
        resp.getWriter().print("data="+data);
    }
}

先运行ServletContextDemo1，将数据data存储到ServletContext对象中，然后运行ServletContextDemo2就可以从ServletContext对象中取出数据了，这样就实现了数据共享，如下图所示：

　　img

　　3.2、获取WEB应用的初始化参数

　　在web.xml文件中使用标签配置WEB应用的初始化参数，如下所示：

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<context-param>
       <param-name>url</param-name>
       <param-value>jdbc:mysql://localhost:3306/test</param-value>
</context-param>

获取参数代码：

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package org.wuqinghua.java.ch01;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
/**
 * Created by wuqinghua on 17/6/30.
 */
@WebServlet("/servletContextDemo03")
public class ServletContextDemo03 extends HttpServlet {
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取servletContext
        ServletContext servletContext = this.getServletContext();
        String url = servletContext.getInitParameter("url");
        resp.getWriter().print(url);
    }
}

运行结果：

　　img

　　3.3、用servletContext实现请求转发

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package org.wuqinghua.java.ch01;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
/**
 * Created by wuqinghua on 17/6/30.
 */
@WebServlet(urlPatterns = {"/servletContextDemo04"})
public class ServletContextDemo04 extends HttpServlet {
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String data = "<h1><font color='red'>abcdefghijkl</font></h1>";
        resp.getOutputStream().write(data.getBytes());
        ServletContext servletContext = this.getServletContext();
        RequestDispatcher rd = servletContext.getRequestDispatcher("/servletContextDemo05");
        rd.forward(req,resp);
    }
}
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package org.wuqinghua.java.ch01;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
/**
 * Created by wuqinghua on 17/6/30.
 */
@WebServlet(urlPatterns = "/servletContextDemo05")
public class ServletContextDemo05 extends HttpServlet{
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.getOutputStream().write("servletContextDemo05".getBytes());
    }
}

运行结果：

　　img

　　访问的是ServletContextDemo4，浏览器显示的却是ServletContextDemo5的内容，这就是使用ServletContext实现了请求转发

　　3.4、利用ServletContext对象读取资源文件

项目目录结构如下：

　　img

代码范例：使用servletContext读取资源文件

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package gacl.servlet.study;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.text.MessageFormat;
import java.util.Properties;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
 * 使用servletContext读取资源文件
 * 
 * @author gacl
 * 
 */
public class ServletContextDemo6 extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException { 
        /**
         * response.setContentType("text/html;charset=UTF-8");目的是控制浏览器用UTF-8进行解码；
         * 这样就不会出现中文乱码了
         */
        response.setHeader("content-type","text/html;charset=UTF-8");
        readSrcDirPropCfgFile(response);//读取src目录下的properties配置文件
        response.getWriter().println("<hr/>");
        readWebRootDirPropCfgFile(response);//读取WebRoot目录下的properties配置文件
        response.getWriter().println("<hr/>");
        readPropCfgFile(response);//读取src目录下的db.config包中的db3.properties配置文件
        response.getWriter().println("<hr/>");
        readPropCfgFile2(response);//读取src目录下的gacl.servlet.study包中的db4.properties配置文件
        
    }
    /**
     * 读取src目录下的gacl.servlet.study包中的db4.properties配置文件
     * @param response
     * @throws IOException
     */
    private void readPropCfgFile2(HttpServletResponse response)
            throws IOException {
        InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/gacl/servlet/study/db4.properties");
        Properties prop = new Properties();
        prop.load(in);
        String driver = prop.getProperty("driver");
        String url = prop.getProperty("url");
        String username = prop.getProperty("username");
        String password = prop.getProperty("password");
        response.getWriter().println("读取src目录下的gacl.servlet.study包中的db4.properties配置文件：");
        response.getWriter().println(
                MessageFormat.format(
                        "driver={0},url={1},username={2},password={3}", 
                        driver,url, username, password));
    }
    /**
     * 读取src目录下的db.config包中的db3.properties配置文件
     * @param response
     * @throws FileNotFoundException
     * @throws IOException
     */
    private void readPropCfgFile(HttpServletResponse response)
            throws FileNotFoundException, IOException {
        //通过ServletContext获取web资源的绝对路径
        String path = this.getServletContext().getRealPath("/WEB-INF/classes/db/config/db3.properties");
        InputStream in = new FileInputStream(path);
        Properties prop = new Properties();
        prop.load(in);
        String driver = prop.getProperty("driver");
        String url = prop.getProperty("url");
        String username = prop.getProperty("username");
        String password = prop.getProperty("password");
        response.getWriter().println("读取src目录下的db.config包中的db3.properties配置文件：");
        response.getWriter().println(
                MessageFormat.format(
                        "driver={0},url={1},username={2},password={3}", 
                        driver,url, username, password));
    }
    /**
     * 通过ServletContext对象读取WebRoot目录下的properties配置文件
     * @param response
     * @throws IOException
     */
    private void readWebRootDirPropCfgFile(HttpServletResponse response)
            throws IOException {
        /**
         * 通过ServletContext对象读取WebRoot目录下的properties配置文件
         * “/”代表的是项目根目录
         */
        InputStream in = this.getServletContext().getResourceAsStream("/db2.properties");
        Properties prop = new Properties();
        prop.load(in);
        String driver = prop.getProperty("driver");
        String url = prop.getProperty("url");
        String username = prop.getProperty("username");
        String password = prop.getProperty("password");
        response.getWriter().println("读取WebRoot目录下的db2.properties配置文件：");
        response.getWriter().print(
                MessageFormat.format(
                        "driver={0},url={1},username={2},password={3}", 
                        driver,url, username, password));
    }
    /**
     * 通过ServletContext对象读取src目录下的properties配置文件
     * @param response
     * @throws IOException
     */
    private void readSrcDirPropCfgFile(HttpServletResponse response) throws IOException {
        /**
         * 通过ServletContext对象读取src目录下的db1.properties配置文件
         */
        InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db1.properties");
        Properties prop = new Properties();
        prop.load(in);
        String driver = prop.getProperty("driver");
        String url = prop.getProperty("url");
        String username = prop.getProperty("username");
        String password = prop.getProperty("password");
        response.getWriter().println("读取src目录下的db1.properties配置文件：");
        response.getWriter().println(
                MessageFormat.format(
                        "driver={0},url={1},username={2},password={3}", 
                        driver,url, username, password));
    }
    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        this.doGet(request, response);
    }
}

运行结果如下：

　　img

代码范例：使用类装载器读取资源文件

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package gacl.servlet.study;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.text.MessageFormat;
import java.util.Properties;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
 * 用类装载器读取资源文件
 * 通过类装载器读取资源文件的注意事项:不适合装载大文件，否则会导致jvm内存溢出
 * @author gacl
 *
 */
public class ServletContextDemo7 extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        /**
         * response.setContentType("text/html;charset=UTF-8");目的是控制浏览器用UTF-8进行解码；
         * 这样就不会出现中文乱码了
         */
        response.setHeader("content-type","text/html;charset=UTF-8");
        test1(response);
        response.getWriter().println("<hr/>");
        test2(response);
        response.getWriter().println("<hr/>");
        //test3();
        test4();
        
    }
    
    /**
     * 读取类路径下的资源文件
     * @param response
     * @throws IOException
     */
    private void test1(HttpServletResponse response) throws IOException {
        //获取到装载当前类的类装载器
        ClassLoader loader = ServletContextDemo7.class.getClassLoader();
        //用类装载器读取src目录下的db1.properties配置文件
        InputStream in = loader.getResourceAsStream("db1.properties");
        Properties prop = new Properties();
        prop.load(in);
        String driver = prop.getProperty("driver");
        String url = prop.getProperty("url");
        String username = prop.getProperty("username");
        String password = prop.getProperty("password");
        response.getWriter().println("用类装载器读取src目录下的db1.properties配置文件：");
        response.getWriter().println(
                MessageFormat.format(
                        "driver={0},url={1},username={2},password={3}", 
                        driver,url, username, password));
    }
    /**
     * 读取类路径下面、包下面的资源文件
     * @param response
     * @throws IOException
     */
    private void test2(HttpServletResponse response) throws IOException {
        //获取到装载当前类的类装载器
        ClassLoader loader = ServletContextDemo7.class.getClassLoader();
        //用类装载器读取src目录下的gacl.servlet.study包中的db4.properties配置文件
        InputStream in = loader.getResourceAsStream("gacl/servlet/study/db4.properties");
        Properties prop = new Properties();
        prop.load(in);
        String driver = prop.getProperty("driver");
        String url = prop.getProperty("url");
        String username = prop.getProperty("username");
        String password = prop.getProperty("password");
        response.getWriter().println("用类装载器读取src目录下的gacl.servlet.study包中的db4.properties配置文件：");
        response.getWriter().println(
                MessageFormat.format(
                        "driver={0},url={1},username={2},password={3}", 
                        driver,url, username, password));
    }
    
    /**
     * 通过类装载器读取资源文件的注意事项:不适合装载大文件，否则会导致jvm内存溢出
     */
    public void test3() {
        /**
         * 01.avi是一个150多M的文件，使用类加载器去读取这个大文件时会导致内存溢出：
         * java.lang.OutOfMemoryError: Java heap space
         */
        InputStream in = ServletContextDemo7.class.getClassLoader().getResourceAsStream("01.avi");
        System.out.println(in);
    }
    
    /**
     * 读取01.avi,并拷贝到e:\根目录下
     * 01.avi文件太大，只能用servletContext去读取
     * @throws IOException
     */
    public void test4() throws IOException {
        // path=G:\Java学习视频\JavaWeb学习视频\JavaWeb\day05视频\01.avi
        // path=01.avi
        String path = this.getServletContext().getRealPath("/WEB-INF/classes/01.avi");
        /**
         * path.lastIndexOf("\\") + 1是一个非常绝妙的写法
         */
        String filename = path.substring(path.lastIndexOf("\\") + 1);//获取文件名
        InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/01.avi");
        byte buffer[] = new byte[1024];
        int len = 0;
        OutputStream out = new FileOutputStream("e:\\" + filename);
        while ((len = in.read(buffer)) > 0) {
            out.write(buffer, 0, len);
        }
        out.close();
        in.close();
    }
    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        this.doGet(request, response);
    }
}

　运行结果如下：

　　img

四、在客户端缓存Servlet的输出

　　对于不经常变化的数据，在servlet中可以为其设置合理的缓存时间值，以避免浏览器频繁向服务器发送请求，提升服务器的性能。例如：

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package gacl.servlet.study;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class ServletDemo5 extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String data = "abcddfwerwesfasfsadf";
        /**
         * 设置数据合理的缓存时间值，以避免浏览器频繁向服务器发送请求，提升服务器的性能
         * 这里是将数据的缓存时间设置为1天
         */
        response.setDateHeader("expires",System.currentTimeMillis() + 24 * 3600 * 1000);
        response.getOutputStream().write(data.getBytes());
    }
    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        this.doGet(request, response);
    }
}