JSP监听器用法分析
什么是JSP监听器
JSP监听器是监听JSP页面对象(即JspPage对象)创建、销毁、初始化、属性发生更改和请求响应等事件的一种对象。通过在这些事件发生时执行自定义逻辑进行应用程序的初始化和维护。
JSP监听器的使用
步骤一:编写JSP监听器
- 实现javax.servlet.jsp.JspPageListener接口。
- 编写在Jsp创建、销毁、初始化、属性发生更改和请求响应时需要执行的逻辑。
下面是一个简单的示例:
import javax.servlet.jsp.JspException;
import javax.servlet.jsp.JspPage;
import javax.servlet.jsp.JspTagException;
import javax.servlet.jsp.PageContext;
import javax.servlet.jsp.tagext.Tag;
import javax.servlet.jsp.tagext.TagSupport;
public class MyJspListener implements javax.servlet.jsp.JspPageListener {
public void jspInit() {
//初始化逻辑
System.out.println("JSP页面初始化");
}
public void jspDestroy() {
//销毁逻辑
System.out.println("JSP页面销毁");
}
}
步骤二:在web.xml文件中注册JSP监听器
将在Step1中编写的JSP监听器在web.xml文件中进行注册。下面是一个web.xml的示例:
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns="http://java.sun.com/xml/ns/javaee"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
id="WebApp_ID" version="3.0">
<display-name>Example Project</display-name>
<!-- 配置JSP监听器 -->
<listener>
<listener-class>MyJspListener</listener-class>
</listener>
</web-app>
JSP监听器示例
示例一:统计JSP访问次数
在JSP的初始化时增加访问次数。
import javax.servlet.jsp.JspPage;
import javax.servlet.jsp.JspPageListener;
public class JspCounter implements JspPageListener {
private static int count = 0;
public static int getCount() {
return count;
}
@Override
public void jspInitialized(final JspPage arg0) {
count++;
}
@Override
public void jspDestroyed(final JspPage arg0) {
//do nothing
}
}
在web.xml中注册:
<listener>
<listener-class>example.servlet.JspCounter</listener-class>
</listener>
在JSP页面中进行调用:
<body>
<%int count = example.servlet.JspCounter.getCount(); %>
<h3>当前页面浏览次数:<%=count%></h3>
</body>
示例二:自定义标签库注册
在JSP页面的初始化时,将自定义标签库进行动态注册。
import javax.servlet.ServletContext;
import javax.servlet.jsp.JspException;
import javax.servlet.jsp.JspPage;
import javax.servlet.jsp.JspPageListener;
import javax.servlet.jsp.tagext.TagLibraryInfo;
import javax.servlet.jsp.tagext.TagLibraryValidator;
import javax.servlet.jsp.tagext.TagLibraryValidator.ValidationMessage;
import javax.servlet.jsp.tagext.TagSupport;
public class CustomTagLibraryListener implements JspPageListener {
private static final String PREFIX = "custom";
private static final String URI = "/WEB-INF/custom.tld";
private final TagLibraryInfo tagLibraryInfo;
public CustomTagLibraryListener(final ServletContext servletContext) {
final TagLibraryInfoBuilder builder = new TagLibraryInfoBuilder();
builder.setPrefix(PREFIX);
builder.setUri(URI);
builder.setDescription("Custom tag library.");
builder.addTag("hello", HelloTag.class.getName(), null);
tagLibraryInfo = builder.build();
TagSupport.setTagLibraryInfo(servletContext, PREFIX, tagLibraryInfo);
}
@Override
public void jspInitialized(final JspPage page) {
//首次访问时进行注册
page.getServletContext().setAttribute(PREFIX, tagLibraryInfo);
}
@Override
public void jspDestroyed(final JspPage page) {
}
private static class TagLibraryInfoBuilder {
private final TagLibraryInfo tagLibraryInfo;
public TagLibraryInfoBuilder() {
tagLibraryInfo = new TagLibraryInfo("Custom Tag Library", URI);
}
public void setPrefix(final String prefix) {
tagLibraryInfo.setPrefix(prefix);
}
public void setUri(final String uri) {
tagLibraryInfo.setURI(uri);
}
public void setDescription(final String description) {
tagLibraryInfo.setInfo(description);
}
public void addTag(final String tagName, final String tagClassName, final String bodyContent) {
tagLibraryInfo.addTagInfo(new TagLibraryInfo.TagInfo(tagName, tagClassName, bodyContent));
}
public TagLibraryInfo build() {
return tagLibraryInfo;
}
}
public static class HelloTag extends TagSupport {
String message;
public void setMessage(final String message) {
this.message = message;
}
@Override
public int doStartTag() throws JspException {
try {
pageContext.getOut().write("<h1>" + message + "</h1>");
} catch (final Exception e) {
throw new JspTagException(e.getMessage());
}
return super.doStartTag();
}
}
}
在web.xml中注册:
<listener>
<listener-class>example.servlet.CustomTagLibraryListener</listener-class>
</listener>
在JSP页面中进行调用:
<%@taglib prefix="custom" uri="/WEB-INF/custom.tld" %>
<html>
<head>
<title>Custom Tag Library Example</title>
</head>
<body>
<custom:hello message="Hello World"/>
</body>
</html>
综述
JSP监听器可以对JSP页面进行全面的初始化、销毁、属性更改等操作,让我们可以在应用程序的各个事件中进行适当的处理。使用JSP监听器时,需要实现JspPageListener接口并在web.xml文件中进行注册。同时,本文还通过两个示例,分别展示了如何统计JSP访问次数和实现自定义标签库的动态注册。
本站文章如无特殊说明,均为本站原创,如若转载,请注明出处:JSP监听器用法分析 - Python技术站