JavaWeb学习总结（四）：Servlet开发（二）
一、ServletConfig讲解
1.1、配置Servlet初始化参数
在Servlet的配置文件web.xml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：
1 <servlet>
2 <servlet-name>ServletConfigDemo1</servlet-name>
3 <servlet-class>gacl.servlet.study.ServletConfigDemo1</servlet-class>
4 <!--配置ServletConfigDemo1的初始化参数 -->
5 <init-param>
6 <param-name>name</param-name>
7 <param-value>gacl</param-value>
8 </init-param>
9 <init-param>
10 <param-name>password</param-name>
11 <param-value>123</param-value>
12 </init-param>
13 <init-param>
14 <param-name>charset</param-name>
15 <param-value>UTF-8</param-value>
16 </init-param>
17 </servlet>
1.2、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，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：
1 package gacl.servlet.study;
2
3 import java.io.IOException;
4 import java.util.Enumeration;
5 import javax.servlet.ServletConfig;
6 import javax.servlet.ServletException;
7 import javax.servlet.http.HttpServlet;
8 import javax.servlet.http.HttpServletRequest;
9 import javax.servlet.http.HttpServletResponse;
10
11 public class ServletConfigDemo1 extends HttpServlet {
12
13 /**
14 * 定义ServletConfig对象来接收配置的初始化参数
15 */
16 private ServletConfig config;
17
18 /**
19 * 当servlet配置了初始化参数后，web容器在创建servlet实例对象时，
20 * 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，
21 * 将ServletConfig对象传递给servlet。进而，程序员通过ServletConfig对象就可以
22 * 得到当前servlet的初始化参数信息。
23 */
24 @Override
25 public void init(ServletConfig config) throws ServletException {
26 this.config = config;
27 }
28
29 public void doGet(HttpServletRequest request, HttpServletResponse response)
30 throws ServletException, IOException {
31 //获取在web.xml中配置的初始化参数
32 String paramVal = this.config.getInitParameter("name");//获取指定的初始化参数
33 response.getWriter().print(paramVal);
34
35 response.getWriter().print("<hr/>");
36 //获取所有的初始化参数
37 Enumeration<String> e = config.getInitParameterNames();
38 while(e.hasMoreElements()){
39 String name = e.nextElement();
40 String value = config.getInitParameter(name);
41 response.getWriter().print(name + "=" + value + "<br/>");
42 }
43 }
44
45 public void doPost(HttpServletRequest request, HttpServletResponse response)
46 throws ServletException, IOException {
47 this.doGet(request, response);
48 }
49
50 }
运行结果如下：
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
1 package gacl.servlet.study;
2
3 import java.io.IOException;
4 import javax.servlet.ServletContext;
5 import javax.servlet.ServletException;
6 import javax.servlet.http.HttpServlet;
7 import javax.servlet.http.HttpServletRequest;
8 import javax.servlet.http.HttpServletResponse;
9
10 public class ServletContextDemo1 extends HttpServlet {
11
12 public void doGet(HttpServletRequest request, HttpServletResponse response)
13 throws ServletException, IOException {
14 String data = "xdp_gacl";
15 /**
16 * ServletConfig对象中维护了ServletContext对象的引用，开发人员在编写servlet时，
17 * 可以通过ServletConfig.getServletContext方法获得ServletContext对象。
18 */
19 ServletContext context = this.getServletConfig().getServletContext();//获得ServletContext对象
20 context.setAttribute("data", data); //将data存储到ServletContext对象中
21 }
22
23 public void doPost(HttpServletRequest request, HttpServletResponse response)
24 throws ServletException, IOException {
25 doGet(request, response);
26 }
27 }
1 package gacl.servlet.study;
2
3 import java.io.IOException;
4 import javax.servlet.ServletContext;
5 import javax.servlet.ServletException;
6 import javax.servlet.http.HttpServlet;
7 import javax.servlet.http.HttpServletRequest;
8 import javax.servlet.http.HttpServletResponse;
9
10 public class ServletContextDemo2 extends HttpServlet {
11
12 public void doGet(HttpServletRequest request, HttpServletResponse response)
13 throws ServletException, IOException {
14 ServletContext context = this.getServletContext();
15 String data = (String) context.getAttribute("data");//从ServletContext对象中取出数据
16 response.getWriter().print("data="+data);
17 }
18
19 public void doPost(HttpServletRequest request, HttpServletResponse response)
20 throws ServletException, IOException {
21 doGet(request, response);
22 }
23 }
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：
3.2、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，如下所示：
1 <?xml version="1.0" encoding="UTF-8"?>
2 <web-app version="3.0" 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
3 http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">
4 <display-name></display-name>
5 <!-- 配置WEB应用的初始化参数 -->
6 <context-param>
7 <param-name>url</param-name>
8 <param-value>jdbc:mysql://localhost:3306/test</param-value>
9 </context-param>
10
11 <welcome-file-list>
12 <welcome-file>index.jsp</welcome-file>
13 </welcome-file-list>
14 </web-app>
获取Web应用的初始化参数，代码如下：
1 package gacl.servlet.study;
2
3 import java.io.IOException;
4 import javax.servlet.ServletContext;
5 import javax.servlet.ServletException;
6 import javax.servlet.http.HttpServlet;
7 import javax.servlet.http.HttpServletRequest;
8 import javax.servlet.http.HttpServletResponse;
9
10
11 public class ServletContextDemo3 extends HttpServlet {
12
13 public void doGet(HttpServletRequest request, HttpServletResponse response)
14 throws ServletException, IOException {
15
16 ServletContext context = this.getServletContext();
17 //获取整个web站点的初始化参数
18 String contextInitParam = context.getInitParameter("url");
19 response.getWriter().print(contextInitParam);
20 }
21
22 public void doPost(HttpServletRequest request, HttpServletResponse response)
23 throws ServletException, IOException {
24 doGet(request, response);
25 }
26
27 }
运行结果：
3.3、用servletContext实现请求转发
ServletContextDemo4
1 package gacl.servlet.study;
2
3 import java.io.IOException;
4 import java.io.PrintWriter;
5 import javax.servlet.RequestDispatcher;
6 import javax.servlet.ServletContext;
7 import javax.servlet.ServletException;
8 import javax.servlet.http.HttpServlet;
9 import javax.servlet.http.HttpServletRequest;
10 import javax.servlet.http.HttpServletResponse;
11
12 public class ServletContextDemo4 extends HttpServlet {
13
14 public void doGet(HttpServletRequest request, HttpServletResponse response)
15 throws ServletException, IOException {
16 String data = "<h1><font color='red'>abcdefghjkl</font></h1>";
17 response.getOutputStream().write(data.getBytes());
18 ServletContext context = this.getServletContext();//获取ServletContext对象
19 RequestDispatcher rd = context.getRequestDispatcher("/servlet/ServletContextDemo5");//获取请求转发对象(RequestDispatcher)
20 rd.forward(request, response);//调用forward方法实现请求转发
21 }
22
23 public void doPost(HttpServletRequest request, HttpServletResponse response)
24 throws ServletException, IOException {
25 }
26 }
ServletContextDemo5
1 package gacl.servlet.study;
2
3 import java.io.IOException;
4 import javax.servlet.ServletException;
5 import javax.servlet.http.HttpServlet;
6 import javax.servlet.http.HttpServletRequest;
7 import javax.servlet.http.HttpServletResponse;
8
9 public class ServletContextDemo5 extends HttpServlet {
10
11 public void doGet(HttpServletRequest request, HttpServletResponse response)
12 throws ServletException, IOException {
13 response.getOutputStream().write("servletDemo5".getBytes());
14 }
15
16 public void doPost(HttpServletRequest request, HttpServletResponse response)
17 throws ServletException, IOException {
18 this.doGet(request, response);
19 }
20
21 }
运行结果：
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：
代码范例：使用servletContext读取资源文件
1 package gacl.servlet.study;
2
3 import java.io.FileInputStream;
4 import java.io.FileNotFoundException;
5 import java.io.IOException;
6 import java.io.InputStream;
7 import java.text.MessageFormat;
8 import java.util.Properties;
9 import javax.servlet.ServletException;
10 import javax.servlet.http.HttpServlet;
11 import javax.servlet.http.HttpServletRequest;
12 import javax.servlet.http.HttpServletResponse;
13
14 /**
15 * 使用servletContext读取资源文件
16 *
17 * @author gacl
18 *
19 */
20 public class ServletContextDemo6 extends HttpServlet {
21
22 public void doGet(HttpServletRequest request, HttpServletResponse response)
23 throws ServletException, IOException {
24 /**
25 * response.setContentType("text/html;charset=UTF-8");目的是控制浏览器用UTF-8进行解码；
26 * 这样就不会出现中文乱码了
27 */
28 response.setHeader("content-type","text/html;charset=UTF-8");
29 readSrcDirPropCfgFile(response);//读取src目录下的properties配置文件
30 response.getWriter().println("<hr/>");
31 readWebRootDirPropCfgFile(response);//读取WebRoot目录下的properties配置文件
32 response.getWriter().println("<hr/>");
33 readPropCfgFile(response);//读取src目录下的db.config包中的db3.properties配置文件
34 response.getWriter().println("<hr/>");
35 readPropCfgFile2(response);//读取src目录下的gacl.servlet.study包中的db4.properties配置文件
36
37 }
38
39 /**
40 * 读取src目录下的gacl.servlet.study包中的db4.properties配置文件
41 * @param response
42 * @throws IOException
43 */
44 private void readPropCfgFile2(HttpServletResponse response)
45 throws IOException {
46 InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/gacl/servlet/study/db4.properties");
47 Properties prop = new Properties();
48 prop.load(in);
49 String driver = prop.getProperty("driver");
50 String url = prop.getProperty("url");
51 String username = prop.getProperty("username");
52 String password = prop.getProperty("password");
53 response.getWriter().println("读取src目录下的gacl.servlet.study包中的db4.properties配置文件：");
54 response.getWriter().println(
55 MessageFormat.format(
56 "driver={0},url={1},username={2},password={3}",
57 driver,url, username, password));
58 }
59
60 /**
61 * 读取src目录下的db.config包中的db3.properties配置文件
62 * @param response
63 * @throws FileNotFoundException
64 * @throws IOException
65 */
66 private void readPropCfgFile(HttpServletResponse response)
67 throws FileNotFoundException, IOException {
68 //通过ServletContext获取web资源的绝对路径
69 String path = this.getServletContext().getRealPath("/WEB-INF/classes/db/config/db3.properties");
70 InputStream in = new FileInputStream(path);
71 Properties prop = new Properties();
72 prop.load(in);
73 String driver = prop.getProperty("driver");
74 String url = prop.getProperty("url");
75 String username = prop.getProperty("username");
76 String password = prop.getProperty("password");
77 response.getWriter().println("读取src目录下的db.config包中的db3.properties配置文件：");
78 response.getWriter().println(
79 MessageFormat.format(
80 "driver={0},url={1},username={2},password={3}",
81 driver,url, username, password));
82 }
83
84 /**
85 * 通过ServletContext对象读取WebRoot目录下的properties配置文件
86 * @param response
87 * @throws IOException
88 */
89 private void readWebRootDirPropCfgFile(HttpServletResponse response)
90 throws IOException {
91 /**
92 * 通过ServletContext对象读取WebRoot目录下的properties配置文件
93 * “/”代表的是项目根目录
94 */
95 InputStream in = this.getServletContext().getResourceAsStream("/db2.properties");
96 Properties prop = new Properties();
97 prop.load(in);
98 String driver = prop.getProperty("driver");
99 String url = prop.getProperty("url");
100 String username = prop.getProperty("username");
101 String password = prop.getProperty("password");
102 response.getWriter().println("读取WebRoot目录下的db2.properties配置文件：");
103 response.getWriter().print(
104 MessageFormat.format(
105 "driver={0},url={1},username={2},password={3}",
106 driver,url, username, password));
107 }
108
109 /**
110 * 通过ServletContext对象读取src目录下的properties配置文件
111 * @param response
112 * @throws IOException
113 */
114 private void readSrcDirPropCfgFile(HttpServletResponse response) throws IOException {
115 /**
116 * 通过ServletContext对象读取src目录下的db1.properties配置文件
117 */
118 InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db1.properties");
119 Properties prop = new Properties();
120 prop.load(in);
121 String driver = prop.getProperty("driver");
122 String url = prop.getProperty("url");
123 String username = prop.getProperty("username");
124 String password = prop.getProperty("password");
125 response.getWriter().println("读取src目录下的db1.properties配置文件：");
126 response.getWriter().println(
127 MessageFormat.format(
128 "driver={0},url={1},username={2},password={3}",
129 driver,url, username, password));
130 }
131
132 public void doPost(HttpServletRequest request, HttpServletResponse response)
133 throws ServletException, IOException {
134 this.doGet(request, response);
135 }
136
137 }
运行结果如下：
代码范例：使用类装载器读取资源文件
1 package gacl.servlet.study;
2
3 import java.io.FileOutputStream;
4 import java.io.IOException;
5 import java.io.InputStream;
6 import java.io.OutputStream;
7 import java.text.MessageFormat;
8 import java.util.Properties;
9
10 import javax.servlet.ServletException;
11 import javax.servlet.http.HttpServlet;
12 import javax.servlet.http.HttpServletRequest;
13 import javax.servlet.http.HttpServletResponse;
14
15 /**
16 * 用类装载器读取资源文件
17 * 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
18 * @author gacl
19 *
20 */
21 public class ServletContextDemo7 extends HttpServlet {
22
23 public void doGet(HttpServletRequest request, HttpServletResponse response)
24 throws ServletException, IOException {
25 /**
26 * response.setContentType("text/html;charset=UTF-8");目的是控制浏览器用UTF-8进行解码；
27 * 这样就不会出现中文乱码了
28 */
29 response.setHeader("content-type","text/html;charset=UTF-8");
30 test1(response);
31 response.getWriter().println("<hr/>");
32 test2(response);
33 response.getWriter().println("<hr/>");
34 //test3();
35 test4();
36
37 }
38
39 /**
40 * 读取类路径下的资源文件
41 * @param response
42 * @throws IOException
43 */
44 private void test1(HttpServletResponse response) throws IOException {
45 //获取到装载当前类的类装载器
46 ClassLoader loader = ServletContextDemo7.class.getClassLoader();
47 //用类装载器读取src目录下的db1.properties配置文件
48 InputStream in = loader.getResourceAsStream("db1.properties");
49 Properties prop = new Properties();
50 prop.load(in);
51 String driver = prop.getProperty("driver");
52 String url = prop.getProperty("url");
53 String username = prop.getProperty("username");
54 String password = prop.getProperty("password");
55 response.getWriter().println("用类装载器读取src目录下的db1.properties配置文件：");
56 response.getWriter().println(
57 MessageFormat.format(
58 "driver={0},url={1},username={2},password={3}",
59 driver,url, username, password));
60 }
61
62 /**
63 * 读取类路径下面、包下面的资源文件
64 * @param response
65 * @throws IOException
66 */
67 private void test2(HttpServletResponse response) throws IOException {
68 //获取到装载当前类的类装载器
69 ClassLoader loader = ServletContextDemo7.class.getClassLoader();
70 //用类装载器读取src目录下的gacl.servlet.study包中的db4.properties配置文件
71 InputStream in = loader.getResourceAsStream("gacl/servlet/study/db4.properties");
72 Properties prop = new Properties();
73 prop.load(in);
74 String driver = prop.getProperty("driver");
75 String url = prop.getProperty("url");
76 String username = prop.getProperty("username");
77 String password = prop.getProperty("password");
78 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：");
79 response.getWriter().println(
80 MessageFormat.format(
81 "driver={0},url={1},username={2},password={3}",
82 driver,url, username, password));
83 }
84
85 /**
86 * 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
87 */
88 public void test3() {
89 /**
90 * 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：
91 * java.lang.OutOfMemoryError: Java heap space
92 */
93 InputStream in = ServletContextDemo7.class.getClassLoader().getResourceAsStream("01.avi");
94 System.out.println(in);
95 }
96
97 /**
98 * 读取01.avi,并拷贝到e:\根目录下
99 * 01.avi文件太大，只能用servletContext去读取
100 * @throws IOException
101 */
102 public void test4() throws IOException {
103 // path=G:\Java学习视频\JavaWeb学习视频\JavaWeb\day05视频\01.avi
104 // path=01.avi
105 String path = this.getServletContext().getRealPath("/WEB-INF/classes/01.avi");
106 /**
107 * path.lastIndexOf("\\") + 1是一个非常绝妙的写法
108 */
109 String filename = path.substring(path.lastIndexOf("\\") + 1);//获取文件名
110 InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/01.avi");
111 byte buffer[] = new byte[1024];
112 int len = 0;
113 OutputStream out = new FileOutputStream("e:\\" + filename);
114 while ((len = in.read(buffer)) > 0) {
115 out.write(buffer, 0, len);
116 }
117 out.close();
118 in.close();
119 }
120
121 public void doPost(HttpServletRequest request, HttpServletResponse response)
122 throws ServletException, IOException {
123
124 this.doGet(request, response);
125 }
126
127 }
运行结果如下：
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：
1 package gacl.servlet.study;
2
3 import java.io.IOException;
4
5 import javax.servlet.ServletException;
6 import javax.servlet.http.HttpServlet;
7 import javax.servlet.http.HttpServletRequest;
8 import javax.servlet.http.HttpServletResponse;
9
10 public class ServletDemo5 extends HttpServlet {
11
12 public void doGet(HttpServletRequest request, HttpServletResponse response)
13 throws ServletException, IOException {
14 String data = "abcddfwerwesfasfsadf";
15 /**
16 * 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
17 * 这里是将数据的缓存时间设置为1天
18 */
19 response.setDateHeader("expires",System.currentTimeMillis() + 24 * 3600 * 1000);
20 response.getOutputStream().write(data.getBytes());
21 }
22
23 public void doPost(HttpServletRequest request, HttpServletResponse response)
24 throws ServletException, IOException {
25
26 this.doGet(request, response);
27 }
28
29 }
