3.5.2 声明
声明以<%!开始，并以%>结束。可以在JSP页面中声明一个变量或方法,以便在其他地方调用.
实例
declarationTest.jsp页面中声明了一个名为getTodaysDate的方法,如下所示
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| <%!
public String getTodaysDate() {
return new java.util.Date();
}
%>
<html>
<head><title>Declarations</title></head>
<body>
Today is <%=getTodaysDate()%>
</body>
</html>
|
声明可以出现的位置
在JSP页面中，一个声明可以出现在任何地方，并且一个页面可以有多个声明。
使用声明来重写JSP页面
可以使用声明来重写JSP页面，实现类的init和destroy方法。具体做法是通过声明jspInit方法，来重写init方法。通过声明jspDestroy方法，来重写destroy方法。
jspInit和jspDestroy方法说明
这两种方法说明如下：
jspInit。这个方法类似于 javax.servlet.Servlet的 init方法。JSP页面在初始化时调用jspInit。不同于init方法，jspInit没有参数。还可以通过隐式对象config访问ServletConfig对象。jspDestroy。这个方法类似于Servlet的destroy方法，在JSP页面将被销毁时调用。
实例
lifeCycle.jsp:
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| <%!
public void jspInit() {
System.out.println("jspInit ...");
}
public void jspDestroy() {
System.out.println("jspDestroy ...");
}
%>
<html>
<head><title>jspInit and jspDestroy</title></head>
<body>
Overriding jspInit and jspDestroy
</body>
</html>
|
lifeCycle.jsp页面会被转换成如下Servlet：
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|
package org.apache.jsp;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;
public final class lifeCycle_jsp
extends
org.apache.jasper.runtime.HttpJspBase
implements
org.apache.jasper.runtime.JspSourceDependent,
org.apache.jasper.runtime.JspSourceImports
{
public void
jspInit()
{
System.out.println("jspInit ...");
}
public void
jspDestroy()
{
System.out.println("jspDestroy ...");
}
private static final javax.servlet.jsp.JspFactory _jspxFactory = javax.servlet.jsp.JspFactory
.getDefaultFactory();
private static java.util.Map<java.lang.String,java.lang.Long> _jspx_dependants;
private static final java.util.Set<java.lang.String> _jspx_imports_packages;
private static final java.util.Set<java.lang.String> _jspx_imports_classes;
static {
_jspx_imports_packages = new java.util.HashSet<>();
_jspx_imports_packages.add("javax.servlet");
_jspx_imports_packages.add("javax.servlet.http");
_jspx_imports_packages.add("javax.servlet.jsp");
_jspx_imports_classes = null;
}
private volatile javax.el.ExpressionFactory _el_expressionfactory;
private volatile org.apache.tomcat.InstanceManager _jsp_instancemanager;
public java.util.Map<java.lang.String,java.lang.Long>
getDependants()
{
return _jspx_dependants;
}
public java.util.Set<java.lang.String>
getPackageImports()
{
return _jspx_imports_packages;
}
public java.util.Set<java.lang.String>
getClassImports()
{
return _jspx_imports_classes;
}
public javax.el.ExpressionFactory
_jsp_getExpressionFactory()
{
if (_el_expressionfactory == null) {
synchronized (this) {
if (_el_expressionfactory == null) {
_el_expressionfactory = _jspxFactory
.getJspApplicationContext(
getServletConfig().getServletContext())
.getExpressionFactory();
}
}
}
return _el_expressionfactory;
}
public org.apache.tomcat.InstanceManager
_jsp_getInstanceManager()
{
if (_jsp_instancemanager == null) {
synchronized (this) {
if (_jsp_instancemanager == null) {
_jsp_instancemanager = org.apache.jasper.runtime.InstanceManagerFactory
.getInstanceManager(getServletConfig());
}
}
}
return _jsp_instancemanager;
}
public void
_jspInit()
{}
public void
_jspDestroy()
{}
public void
_jspService(final javax.servlet.http.HttpServletRequest request,
final javax.servlet.http.HttpServletResponse response)
throws java.io.IOException, javax.servlet.ServletException
{
final java.lang.String _jspx_method = request.getMethod();
if (!"GET".equals(_jspx_method) && !"POST".equals(_jspx_method)
&& !"HEAD".equals(_jspx_method)
&& !javax.servlet.DispatcherType.ERROR
.equals(request.getDispatcherType())) {
response.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED,
"JSPs only permit GET POST or HEAD");
return;
}
final javax.servlet.jsp.PageContext pageContext;
javax.servlet.http.HttpSession session = null;
final javax.servlet.ServletContext application;
final javax.servlet.ServletConfig config;
javax.servlet.jsp.JspWriter out = null;
final java.lang.Object page = this;
javax.servlet.jsp.JspWriter _jspx_out = null;
javax.servlet.jsp.PageContext _jspx_page_context = null;
try {
response.setContentType("text/html; charset=UTF-8");
pageContext = _jspxFactory.getPageContext(this, request, response,
null, true, 8192, true);
_jspx_page_context = pageContext;
application = pageContext.getServletContext();
config = pageContext.getServletConfig();
session = pageContext.getSession();
out = pageContext.getOut();
_jspx_out = out;
out.write('\r');
out.write('\n');
out.write("\r\n");
out.write("<!DOCTYPE html>\r\n");
out.write("<html>\r\n");
out.write("<head>\r\n");
out.write("<title>测试jspInit和jspDestroy</title>\r\n");
out.write("</head>\r\n");
out.write("<body>\r\n");
out.write("<h2>测试jspInit和jspDestroy</h2>\r\n");
out.write("</body>\r\n");
out.write("</html>");
} catch (java.lang.Throwable t) {
if (!(t instanceof javax.servlet.jsp.SkipPageException)) {
out = _jspx_out;
if (out != null && out.getBufferSize() != 0)
try {
if (response.isCommitted()) {
out.flush();
} else {
out.clearBuffer();
}
} catch (java.io.IOException e) {}
if (_jspx_page_context != null)
_jspx_page_context.handlePageException(t);
else
throw new ServletException(t);
}
} finally {
_jspxFactory.releasePageContext(_jspx_page_context);
}
}
}
|
注意生成的Servlet类中的jspInit和jspDestroy方法。现在可以用如下URL访问lifeCycle.jsp:
http://localhost:8080/app03a/lifeCycle.jsp
第一次访问页面时，可以在控制台上看到“jspInit...”，以及在Servlet/JSP容器关闭时看到“jspDestory...”。
原文链接: 3.5.2 声明