jsp:useBean的用法

有几种用法，下面逐一讲解：
1》
<%@page import="MyBean.*"%>
<jsp:useBean class="MyBean.TestBean" id="user" scope="session"/>
生成的java代码如下：
Java代码:

MyBean.TestBean user = null;

synchronized (session) {

user = (MyBean.TestBean) _jspx_page_context.getAttribute("user", javax.servlet.jsp.PageContext.SESSION_SCOPE);

if (user == null){

user = new MyBean.TestBean();

_jspx_page_context.setAttribute("user", user, javax.servlet.jsp.PageContext.SESSION_SCOPE);

}

}

先从session范围内查找user属性是否存在，如果不存在，user = new MyBean.TestBean()，然后把user追加到session中。存在的话，啥也不做。
2》
<%@page import="MyBean.*"%>
<jsp:useBean type="BaseBean" class="MyBean.TestBean" id="user" scope="session"/>
生成的java代码如下：
Java代码:

BaseBean user = null;

synchronized (session) {

user = (BaseBean) _jspx_page_context.getAttribute("user", javax.servlet.jsp.PageContext.SESSION_SCOPE);

if (user == null){

user = new MyBean.TestBean();

_jspx_page_context.setAttribute("user", user, javax.servlet.jsp.PageContext.SESSION_SCOPE);

}

}

BaseBean是TestBean的父类。
除了user的类型是BaseBean，其它的处理逻辑和1》相同。
3》
<%@page import="MyBean.*"%>
<jsp:useBean type="MyBean.BaseBean" beanName="MyBean.TestBean" id="user" scope="session"/>
生成的java代码如下：
Java代码:

MyBean.BaseBean user = null;

synchronized (session) {

user = (MyBean.BaseBean) _jspx_page_context.getAttribute("user", javax.servlet.jsp.PageContext.SESSION_SCOPE);

if (user == null){

try {

user = (MyBean.BaseBean) java.beans.Beans.instantiate(this.getClass().getClassLoader(), "MyBean.TestBean");

} catch (java.lang.ClassNotFoundException exc) {

throw new InstantiationException(exc.getMessage());

} catch (java.lang.Exception exc) {

throw new javax.servlet.ServletException("Cannot create bean of class " + "MyBean.TestBean", exc);

}

_jspx_page_context.setAttribute("user", user, javax.servlet.jsp.PageContext.SESSION_SCOPE);

}

}

和2》的区别如下：
1>不使用new关键字进行创建，而是使用Beans.instantiate来加载并实例化对象
2>对象创建完以后进行了强制类型转换(MyBean.BaseBean)
4》
<%@page import="MyBean.*"%>
<jsp:useBean type="MyBean.BaseBean" id="user" scope="session"/>
生成的java代码如下：
Java代码:

MyBean.BaseBean user = null;

synchronized (session) {

user = (MyBean.BaseBean) _jspx_page_context.getAttribute("user", javax.servlet.jsp.PageContext.SESSION_SCOPE);

if (user == null){

throw new java.lang.InstantiationException("bean user not found within scope");

}

}

先从session范围内查找user属性，如果不存在就直接抛出异常，存在的话直接引用。

原文链接：http://www.software8.co/wzjs/jsp/2697.html