javaweb Servlet开发总结(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:
<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 + "<br/>"); } } 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。
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
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
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:
<?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
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
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:
代码范例:使用类装载器读取资源文件
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:
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); } }
以上就是本文的全部内容,希望对大家掌握javaweb servlet开发技术有所帮助。
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