深入分析JavaWeb Item6 — servletConfig 与servletContext详解
来源:互联网 发布:linux主机 编辑:程序博客网 时间:2024/06/06 02:27
一、ServletConfig讲解
首先看ServletConfig API文档
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:
<servlet>
<servlet-name>ServletConfigDemo1</servlet-name>
<servlet-
class
>gacl.servlet.study.ServletConfigDemo1</servlet-
class
>
<!--配置ServletConfigDemo1的初始化参数 -->
<init-param>
<param-name>name</param-name>
<param-value>gacl</param-value>
</init-param>
<init-param>
<param-name>password</param-name>
<param-value>
123
</param-value>
</init-param>
<init-param>
<param-name>charset</param-name>
<param-value>UTF-
8
</param-value>
</init-param>
</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:
package
gacl.servlet.study;
import
java.io.IOException;
import
java.util.Enumeration;
import
javax.servlet.ServletConfig;
import
javax.servlet.ServletException;
import
javax.servlet.http.HttpServlet;
import
javax.servlet.http.HttpServletRequest;
import
javax.servlet.http.HttpServletResponse;
public
class
ServletConfigDemo1
extends
HttpServlet {
/**
* 定义ServletConfig对象来接收配置的初始化参数
*/
private
ServletConfig config;
/**
* 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,程序员通过ServletConfig对象就可以
* 得到当前servlet的初始化参数信息。
*/
@Override
public
void
init(ServletConfig config)
throws
ServletException {
this
.config = config;
}
public
void
doGet(HttpServletRequest request, HttpServletResponse response)
throws
ServletException, IOException {
//获取在web.xml中配置的初始化参数
String paramVal =
this
.config.getInitParameter(
"name"
);
//获取指定的初始化参数
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 +
""
);
}
}
public
void
doPost(HttpServletRequest request, HttpServletResponse response)
throws
ServletException, IOException {
this
.doGet(request, response);
}
}
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,但是还有更简洁的this.getServletContext()
方法;
<font color=”red”>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,是一个容器 2。作用范围是应用程序范围。
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
package
gacl.servlet.study;
import
java.io.IOException;
import
javax.servlet.ServletContext;
import
javax.servlet.ServletException;
import
javax.servlet.http.HttpServlet;
import
javax.servlet.http.HttpServletRequest;
import
javax.servlet.http.HttpServletResponse;
public
class
ServletContextDemo1
extends
HttpServlet {
public
void
doGet(HttpServletRequest request, HttpServletResponse response)
throws
ServletException, IOException {
String data =
"xdp_gacl"
;
/**
* 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。
*/
ServletContext context =
this
.getServletConfig().getServletContext();
//获得ServletContext对象
context.setAttribute(
"data"
, data);
//将data存储到ServletContext对象中
}
public
void
doPost(HttpServletRequest request, HttpServletResponse response)
throws
ServletException, IOException {
doGet(request, response);
}
}
package
gacl.servlet.study;
import
java.io.IOException;
import
javax.servlet.ServletContext;
import
javax.servlet.ServletException;
import
javax.servlet.http.HttpServlet;
import
javax.servlet.http.HttpServletRequest;
import
javax.servlet.http.HttpServletResponse;
public
class
ServletContextDemo2
extends
HttpServlet {
public
void
doGet(HttpServletRequest request, HttpServletResponse response)
throws
ServletException, IOException {
ServletContext context =
this
.getServletContext();
String data = (String) context.getAttribute(
"data"
);
//从ServletContext对象中取出数据
response.getWriter().print(
"data="
+data);
}
public
void
doPost(HttpServletRequest request, HttpServletResponse response)
throws
ServletException, IOException {
doGet(request, response);
}
}
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
如果想在所有的Servlet应用中都要配置并读取初始化参数,则可以在web.xml
文件的<web-app>
中使用<context-param>
标签配置WEB应用的初始化参数,如下所示:
<?xml version=
"1.0"
encoding=
"UTF-8"
?>
<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
http:
//java.sun.com/xml/ns/javaee/web-app_3_0.xsd">
<display-name></display-name>
<!-- 配置WEB应用的初始化参数 -->
<context-param>
<param-name>url</param-name>
<param-value>jdbc:mysql:
//localhost:3306/test</param-value>
</context-param>
<welcome-file-list>
<welcome-file>index.jsp</welcome-file>
</welcome-file-list>
</web-app>
获取Web应用的初始化参数,代码如下:
package
gacl.servlet.study;
import
java.io.IOException;
import
javax.servlet.ServletContext;
import
javax.servlet.ServletException;
import
javax.servlet.http.HttpServlet;
import
javax.servlet.http.HttpServletRequest;
import
javax.servlet.http.HttpServletResponse;
public
class
ServletContextDemo3
extends
HttpServlet {
public
void
doGet(HttpServletRequest request, HttpServletResponse response)
throws
ServletException, IOException {
ServletContext context =
this
.getServletContext();
//获取整个web站点的初始化参数
String contextInitParam = context.getInitParameter(
"url"
);
response.getWriter().print(contextInitParam);
}
public
void
doPost(HttpServletRequest request, HttpServletResponse response)
throws
ServletException, IOException {
doGet(request, response);
}
}
运行结果:
3.3、用servletContext实现请求转发
实现Servlet的转发。
- ServletContextDemo4
package
gacl.servlet.study;
import
java.io.IOException;
import
java.io.PrintWriter;
import
javax.servlet.RequestDispatcher;
import
javax.servlet.ServletContext;
import
javax.servlet.ServletException;
import
javax.servlet.http.HttpServlet;
import
javax.servlet.http.HttpServletRequest;
import
javax.servlet.http.HttpServletResponse;
public
class
ServletContextDemo4
extends
HttpServlet {
public
void
doGet(HttpServletRequest request, HttpServletResponse response)
throws
ServletException, IOException {
String data =
"<h1><font color='red'>abcdefghjkl</font></h1>"
;
response.getOutputStream().write(data.getBytes());
ServletContext context =
this
.getServletContext();
//获取ServletContext对象
RequestDispatcher rd = context.getRequestDispatcher(
"/servlet/ServletContextDemo5"
);
//获取请求转发对象(RequestDispatcher)
rd.forward(request, response);
//调用forward方法实现请求转发
}
public
void
doPost(HttpServletRequest request, HttpServletResponse response)
throws
ServletException, IOException {
}
}
- ServletContextDemo5
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
ServletContextDemo5
extends
HttpServlet {
public
void
doGet(HttpServletRequest request, HttpServletResponse response)
throws
ServletException, IOException {
response.getOutputStream().write(
"servletDemo5"
.getBytes());
}
public
void
doPost(HttpServletRequest request, HttpServletResponse response)
throws
ServletException, IOException {
this
.doGet(request, response);
}
}
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
利用ServletContext对象读取资源文件,因为文件的位置不同,所有读取的方式也不同,一般来说分为两种情况:
- 在Servlet的context域中读取文件,工程目录下的src目录发布到服务器中,会映射到“/WEB-INF/classes”文件夹下。所以要一一对应。而且这个是相对目录,相对于web服务器的目录。如果要用传统的文件读取文件,则要使用绝对路劲
PrintWriter out = response.getWriter();
ServletContext context =
this
.getServletContext();
String path = context.getRealPath(
"/WEB-INF/classes/itcast.properties"
);
InputStream in =
new
FileInputStream(path);
Properties pro =
new
Properties();
pro.load(in);
- 如果是非servlet中读取配置文件,则要使用类加载器去读取。稍后讲到
项目目录结构如下:
代码范例:使用servletContext读取资源文件
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:
使用类装载器读取资源文件
我们在非servlet中读取资源文件时(比如在数据库的dao层读取配置文件),采用类装载器 classLoader,你可以先采用servlet服务先读取,然后在把servlet传递给dao,这样虽然可以实现,但是,这样损坏了我们编代码的设计原则,就是层之间不能有交织在一起的东西。
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:
使用类装载器读取资源文件,存在的问题是;类装载器,每次只会装载一次。
//如果读取资源文件的程序不是servlet的话,
//就只能通过类转载器去读了,文件不能太大
//用传递参数方法不好,耦合性高
public
class
UserDao {
private
static
Properties dbconfig=
new
Properties();
static
{
InputStream in=UserDao.
class
.getClassLoader().getResourceAsStream(
"db.properties"
);
try
{
dbconfig.load(in);
}
catch
(IOException e) {
throw
new
ExceptionInInitializerError(e);
}
//上面代码类装载器只装载一次,下面代码用类装载方式得到文件位置
URL url=UserDao.
class
.getClassLoader().getResource(
"db.properties"
);
String str=url.getPath();
//file:/C:/apache-tomcat-7.0.22/webapps/day05/WEB-INF/classes/db.properties
try
{
InputStream in2=
new
FileInputStream(str);
try
{
dbconfig.load(in2);
}
catch
(IOException e) {
throw
new
ExceptionInInitializerError(e);
}
}
catch
(FileNotFoundException e1) {
throw
new
ExceptionInInitializerError(e1);
}
}
public
void
update() {
System.out.println(dbconfig.get(
"url"
));
}
}
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:
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);
}
}
- 深入分析JavaWeb Item6 — servletConfig 与servletContext详解
- 深入分析JavaWeb Item6 -- servletConfig 与servletContext详解
- 深入分析JavaWeb 6 -- servletConfig 与servletContext详解
- 深入分析JavaWeb servletConfig 与servletContext
- javaweb-servlet- ServletConfig与ServletContext
- ServletContext与ServletConfig分析
- servletcontext 与 servletconfig 分析
- ServletConfig与ServletContext详解
- javaweb基础学习(一)<ServletConfig与ServletContext对象详解>
- javaweb—ServletContext和ServletConfig
- JavaWeb基础(6)—— ServletContext 与 ServletConfig
- ServletContext与ServletConfig的分析
- ServletContext与ServletConfig的分析
- ServletContext与ServletConfig的分析
- ServletConfig与ServletContext对象详解
- ServletConfig与ServletContext对象详解
- ServletConfig与ServletContext对象详解
- ServletConfig与ServletContext对象详解
- 统计文本中出现最高的词频数并存储为excel
- eclipse快捷键常用命令总结
- redis无法写入新数据
- 常用ACSII码总结
- httpclient模拟表单上传参数和文件
- 深入分析JavaWeb Item6 — servletConfig 与servletContext详解
- 括号匹配
- 记录下首次开通博客的时刻
- 记开发时遇到的两个问题以及解决方案
- 实现达到 60FPS 的高性能交互动画
- 大白话讲解Promise(一)
- 指定Keepalived输出日志文件
- 最全iOS数据存储方法介绍:FMDB,SQLite3 ,Core Data,Plist,Preference偏好设置
- ZIGBEE的硬件介绍和应用工程