javaweb学习总结（一）servlet开发

一、ServletConfig详解

1.1 配置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>ServletConfigDemo1</servlet-name>
    <servlet-class>rain.servlet.study.ServletConfigDemo1</servlet-  
  class>
<!-- 配置ServletConfigDemo1的初始化参数 -->
<init-param>
    <param-name>name</param-name>
    <param-value>rain</param-value>
</init-param>

<init-param>
    <param-name>score</param-name>
    <param-value>140</param-value>
</init-param>

<init-param>
    <param-name>age</param-name>
    <param-value>18</param-value>
</init-param>
</servlet>
1.2 通过ServletConfig获取初始化参数

当Servlet配置了初始化参数以后，web容器在创建servlet实例对象时，会自动将这些参数封装到ServletConfig对象中去，并且在调用Servlet的init方法时会将ServletConfig传递过去。因此我们可以通过ServletConfig对象就可以得到配置的初始化参数信息了。不过现在有了servlet3.0之后，有两种写法了。例如：

package rain.servlet.study;
import java.io.IOException;
import java.util.Enumeration;
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;

/**
 * Servlet implementation class ServletConfigDemo1
 */
//@WebServlet(urlPatterns="/ServletConfigDemo1",initParams= {
//      @WebInitParam(name="name",value="rain"),
//      @WebInitParam(name="score",value="100"),
//      @WebInitParam(name="age",value="18")
//})
public class ServletConfigDemo1 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    private ServletConfig config;

/**
 * 当servlet配置了初始化参数时，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，将改对象传递给servlet
 */
@Override
public void init(ServletConfig config) throws ServletException {
    // TODO Auto-generated method stub
//      super.init(config);
    this.config = config;
}

/**
 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
 */
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    // TODO Auto-generated method stub
//      response.getWriter().append("Served at: ").append(request.getContextPath());
    response.setContentType("text/html;charset=utf-8");
    String paramVal = this.config.getInitParameter("name");
    response.getWriter().print(paramVal);
    response.getWriter().print("<hr/>");
    
    //获取所有的初始化参数
    Enumeration<String> e = config.getInitParameterNames();
    while(e.hasMoreElements()) {
        String name = e.nextElement();
        String value = config.getInitParameter(name);
        response.getWriter().print(name + "=" + value + "<br/>");
    }
}

/**
 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
 */
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    // TODO Auto-generated method stub
    doGet(request, response);
  }

}

上面注释掉的部分就是Servlet 3.0之后采用注解的形式，可以看到还是挺方便的。运行结果如下：


image.png

二、ServletContext对象

web容器在启动时，它会为每个web应用创建一个对应的ServletContext对象，它代表当前web应用。
ServletConfig对象中维护了ServletContext对象的引用，可以通过ServletConfig.getServletContext获得。
由于一个web应用所有的servlet都共享同一个ServletContext，因此servlet对象之间可以实现通讯。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 来实现数据共享

下面是一个例子，两个servlet通过Servletc实现数据共享

package rain.servlet.study;

import java.io.IOException;

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;

/**
 * Servlet implementation class RainServletContextDemo1
 */
@WebServlet("/RainServletContextDemo1")
public class RainServletContextDemo1 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#doGet(HttpServletRequest request,         HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request,   HttpServletResponse response) throws ServletException,   IOException {
        // TODO Auto-generated method stub
//      response.getWriter().append("Served at:     ").append(request.getContextPath());
        String data = "rain_sample_data";
        /**
         * ServletConfig对象中维护了ServletContext的引用，可以通过
         * getServletConfig().getServletContext()来获取到。
         */
        ServletContext context = this.getServletConfig().getServletContext();
        context.setAttribute("data", data);
}

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

下面是另外一个Servlet

package rain.servlet.study;

import java.io.IOException;

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;

/**
 * Servlet implementation class RainServletContextDemo2
 */
@WebServlet("/RainServletContextDemo2")
public class RainServletContextDemo2 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
//      response.getWriter().append("Served at: ").append(request.getContextPath());
        ServletContext context = getServletContext();
        String data = (String) context.getAttribute("data");//从ServletContext对象中取出数据
        response.getWriter().print("data="+data);
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

先运行RainServletContextDemo1，然后再运行RainServletContextDemo2，即可以实现数据的共享。运行结果如下：

image.png
3.2 用ServletContext对象实现请求转发

RainServletContextDemo3

package rain.servlet.study;
import java.io.IOException;
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;

/**
 * Servlet implementation class RainServletContextDemo3
 */
@WebServlet("/RainServletContextDemo3")
public class RainServletContextDemo3 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
//      response.getWriter().append("Served at: ").append(request.getContextPath());
        String data = "i am servlet3";
        response.getOutputStream().write(data.getBytes());
        ServletContext context = getServletContext();
        RequestDispatcher dispatcher = context.getRequestDispatcher("/RainServletContextDemo4");
        dispatcher.forward(request, response);
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

RainServletContextDemo4

package rain.servlet.study;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class RainServletContextDemo4
 */
@WebServlet("/RainServletContextDemo4")
public class RainServletContextDemo4 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        response.getOutputStream().write("i am servlet4 hia!!!".getBytes());
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}
3.3 用ServletContext对象读取资源文件
项目的目录结构：
image.png
代码：使用Servlet读取资源文件
package rain.servlet.study;

import java.io.IOException;
import java.io.InputStream;
import java.text.MessageFormat;
import java.util.Properties;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class RainServletContextDemo5
 * 使用ServletContext读取资源文件
 */
@WebServlet("/RainServletContextDemo5")
public class RainServletContextDemo5 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
//      response.getWriter().append("Served at: ").append(request.getContextPath());
        response.setHeader("content-type", "text/html;charset=UTF-8");
        readSrcDirFile(response);//读取src目录下面的文件
        response.getWriter().write("<hr/>");
        readWebContentFile(response);//读取webcontent目录下面的文件
        response.getWriter().write("<hr/>");
        readPackageFile(response);//读取rain.servlet.study目录下的文件
        response.getWriter().write("<hr/>");
        readDataConfigFile(response);//读取data.config下的文件 
    }

    /**
     * 读取data.config下的文件 data1.properties
     * @param response
     * @throws IOException 
     */
    private void readDataConfigFile(HttpServletResponse response) throws IOException {
        InputStream in = getServletContext().getResourceAsStream(
            "/WEB-INF/classes/data/config/data1.properties");
        Properties prop = new Properties();
        prop.load(in);
        String name = prop.getProperty("name");
        String data = prop.getProperty("data");
        String time = prop.getProperty("time");
        response.getWriter().println("读取的是data.config包下面的data2.properties:");
        response.getWriter().println(
            MessageFormat.format("name={0},data={1},time={2}", 
                    name,data,time));
}

/**
 * readPackageFile(response);//读取rain.servlet.study目录下的文件  data2.properties
 * @param response
 * @throws IOException 
 */
private void readPackageFile(HttpServletResponse response) throws IOException {
    InputStream in = getServletContext().getResourceAsStream(
            "/WEB-INF/classes/rain/servlet/study/data2.properties");
    Properties prop = new Properties();
    prop.load(in);
    String name = prop.getProperty("name");
    String data = prop.getProperty("data");
    String time = prop.getProperty("time");
    response.getWriter().println("读取的是src目录下面rain.servlet.study包下面的data2.properties:");
    response.getWriter().println(
            MessageFormat.format("name={0},data={1},time={2}", 
                    name,data,time));
}

/**
 * 读取webcontent目录下面的文件 data4.properties
 * @param response
 * @throws IOException 
 */
private void readWebContentFile(HttpServletResponse response) throws IOException {
    InputStream in = getServletContext().getResourceAsStream(
            "/data4.properties");
    Properties prop = new Properties();
    prop.load(in);
    String name = prop.getProperty("name");
    String data = prop.getProperty("data");
    String time = prop.getProperty("time");
    response.getWriter().println("读取的是webcontent目录下面的data4.properties:");
    response.getWriter().println(
            MessageFormat.format("name={0},data={1},time={2}", 
                    name,data,time));
}

/**
 * 读取src目录下面的文件 data3.properties
 * @param response
 * @throws IOException 
 */
private void readSrcDirFile(HttpServletResponse response) throws IOException {
    InputStream in = getServletContext().getResourceAsStream(
            "/WEB-INF/classes/data3.properties");
    Properties prop = new Properties();
    prop.load(in);
    String name = prop.getProperty("name");
    String data = prop.getProperty("data");
    String time = prop.getProperty("time");
    response.getWriter().println("读取的是src目录下面的data3.properties:");
    response.getWriter().println(
            MessageFormat.format("name={0},data={1},time={2}", 
                    name,data,time));
}

/**
 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
 */
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    // TODO Auto-generated method stub
        doGet(request, response);
    }

}
运行结果如下：
image.png

四、在客户端缓存Servlet的输出

对于不是经常变化的数据，可以在服务器端设置一个缓存时间，以避免浏览器频繁的向服务器端发起请求，提高服务器的性能。

代码如下：
package rain.servlet.study;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class RainServletDemo6
 */
@WebServlet("/RainServletDemo6")
public class RainServletDemo6 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
//      response.getWriter().append("Served at: ").append(request.getContextPath());
        String data = "asdffffdsgsgsfgsgsgs";
        response.setDateHeader("expires", System.currentTimeMillis()+24*3600*1000);
        response.getWriter().println(data);
}

/**
 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
 */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}
运行结果
image.png
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