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：

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：

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：

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：

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。