从配置文件读取连接数据库的配置信息

在webapps下新建Web应用程序的目录为conntest：
其目录结构如下：
webapps:
|-conntext
|  |-src
|  |  |-conntest
|  |       |-DBConfig.java
|  |       |-ConnDBServlet.java
|  |
|  |-WEB-INF
|     |-classes
|     |    |-conntest
|     |         |-database.properties
|     |         |-DBConfig.class
|     |         |-ConnDBServlet.class
|     |
|     |-lib
|     |  |-mysql-connector-java-5.1.26-bin.jar
|     |
|     |-web.xml

1.database.properties

#driverClass(数据库驱动类的类名)
#driverClass=oracle.jdbc.OracleDriver
driverClass=com.mysql.jdbc.Driver

#url(要连接数据库的地址)
#url=jdbc:oracle:thin:@192.168.1.180:1521:orcl
#url=jdbc:mysql://127.0.0.1:3306/test
url=jdbc:mysql://127.0.0.1:3306

#user(数据库用户)
#user=scott
user=root

#password(密码)
#password=tiger
password=root

2.web.xml

<?xml version="1.0" encoding="ISO-8859-1"?>
<web-app xmlns="http://java.sun.com/xml/ns/javaee"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
                      http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
  version="3.0"
  metadata-complete="true">
  <context-param>
      <param-name>driverClass</param-name>
      <param-value>com.mysql.jdbc.Driver</param-value>
  </context-param>
  <context-param>
      <param-name>url</param-name>
      <param-value>jdbc:mysql://localhost:3306/test</param-value>
  </context-param>
  <context-param>
      <param-name>user</param-name>
      <param-value>root</param-value>
  </context-param>
  <context-param>
      <param-name>password</param-name>
      <param-value>root</param-value>
  </context-param>
  
  <servlet>
    <servlet-name>ConnDBServlet</servlet-name>
    <servlet-class>conntest.ConnDBServlet</servlet-class>
    <init-param>
        <param-name>driverClass</param-name>
        <param-value>com.mysql.jdbc.Driver</param-value>
    </init-param>
    <init-param>
        <param-name>url</param-name>
        <param-value>jdbc:mysql://localhost:3306</param-value>
    </init-param>
    <init-param>
        <param-name>user</param-name>
        <param-value>root</param-value>
    </init-param>
    <init-param>
        <param-name>password</param-name>
        <param-value>root</param-value>
    </init-param>
  </servlet>
  <servlet-mapping>
    <servlet-name>ConnDBServlet</servlet-name>
    <url-pattern>/conndb</url-pattern>
  </servlet-mapping>
  
</web-app>

3.DBConfig.java

package conntest;

public class DBConfig{
    private String driverClass;
    private String url;
    private String user;
    private String password;
    public DBConfig(String driverClass, String url, String user, String password){
        this.driverClass = driverClass;
        this.url = url;
        this.user = user;
        this.password = password;
    }
    
    public String getDriverClass(){
        return this.driverClass;
    }
    
    public String getUrl(){
        return this.url;
    }
    
    public String getUser(){
        return this.user;
    }
    
    public String getPassWord(){
        return this.password;
    }
}

4.ConnDBServlet.java

package conntest;

import java.io.IOException;
import java.io.InputStream;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.PrintWriter;
import java.util.Properties;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import java.sql.DriverManager;
import java.sql.Connection;
import java.sql.SQLException;

public class ConnDBServlet extends HttpServlet{
    private DBConfig dbconfig;
    
    //读取database.properties配置文件的配置信息
    public DBConfig readDbConfig(String propFileName) throws IOException{
        try{
            //主要就是获取输入流
            //方法1：通过FileInputStream构造函数得到
            //String propFileName = "F:\\webapps\\conntest\\WEB-INF\\classes\\conntest\\database.properties";
            //FileInputStream in=new FileInputStream(propFileName);//FileNotFoundException
            
            //方法2：通过Class.getResourceAsStream()
            //String propFileName = "/conntest/database.properties";
            //InputStream in = this.getClass().getResourceAsStream(propFileName);
            
            //方法3：通过ServletContext.getResourceAsStream()
            //String propFileName = "/WEB-INF/classes/conntest/database.properties";
            InputStream in = this.getServletContext().getResourceAsStream(propFileName);
            
            Properties props = new Properties();            
            props.load(in);//IOException
            
            String driverClass = props.getProperty("driverClass");
            String url = props.getProperty("url");
            String user = props.getProperty("user");
            String password = props.getProperty("password");
            
            return new DBConfig(driverClass, url, user, password);
        }
        catch(FileNotFoundException e) {  
            new RuntimeException(e); 
        }
        catch(IOException e) {  
            new RuntimeException(e); 
        }
        return null;
    }
    
    //读取web.xml中的配置信息
    public DBConfig readDbConfig(){
        //<servlet>元素中的<init-param>
//        String driverClass = getInitParameter("driverClass");
//        String url = getInitParameter("url");
//        String user = getInitParameter("user");
//        String password = getInitParameter("password");
        
        //<context-param>
        String driverClass = getServletContext().getInitParameter("driverClass");
        String url = getServletContext().getInitParameter("url");
        String user = getServletContext().getInitParameter("user");
        String password = getServletContext().getInitParameter("password");
        return new DBConfig(driverClass, url, user, password);
    }
    
    public void init() throws ServletException{
        //String propFileName = "F:\\webapps\\conntest\\WEB-INF\\classes\\conntest\\database.properties";
        //String propFileName = "/conntest/database.properties";
        String propFileName = "/WEB-INF/classes/conntest/database.properties";
        //String propFileName = null;
        try{
            if(propFileName == null){
                dbconfig = readDbConfig();
            }
            else{
                dbconfig = readDbConfig(propFileName);
            }
            Class.forName(dbconfig.getDriverClass());//ClassNotFoundException
        }
        catch(ClassNotFoundException e){
            throw new ServletException("加载数据库驱动失败！");
        }
        catch(FileNotFoundException e) {  
            throw new ServletException("没有找到属性文件！"); 
        }
        catch(IOException e) {  
            throw new ServletException("加载属性文件失败！"); 
        }
    }
    
    public void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException{
        Connection conn = null;
        try{
            conn = DriverManager.getConnection(dbconfig.getUrl(), dbconfig.getUser(), dbconfig.getPassWord());
            res.setContentType("text/html; charset=UTF-8");
            PrintWriter out = res.getWriter();
            out.println("数据库连接成功！");
            out.close();
        }
        catch(SQLException e){
            throw new ServletException(e);
        }
        finally{
            if(conn != null){
                try{
                    conn.close();
                }
                catch(SQLException e){
                    e.printStackTrace();
                }
                conn = null;
            }
        }
    }
}

5.编译

F:\webapps\conntest>javac -d ./WEB-INF/classes ./src/conntest/DBConfig.java

F:\webapps\conntest>javac -classpath D:\web\tomcat\lib\servlet-api.jar;./WEB-INF/classes -d ./WEB-INF/classes ./src/conntest/ConnDBServlet.java

6.访问：
http:localhost:8080/conntest/conndb