尝试检索Spring bean，但不断获取NullPointerException

Question

I am trying to retrieve a BasicDataSource from a bean. 我正在尝试从bean检索BasicDataSource。 Whenever I try to access the bean, I get a NullPointerException 每当我尝试访问Bean时，我都会收到NullPointerException

My code: 我的代码：

<%@ page language="java" contentType="text/html; charset=ISO-8859-1" pageEncoding="ISO-8859-1" %>
<%@ page import="java.sql.*" %>
<%@ page import="org.apache.commons.dbcp2.BasicDataSource" %>
<%@ page import="org.postgresql.*" %>
<!DOCTYPE html >
<html xmlns:th="http://www.thymeleaf.org/">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Thank you for suscribing</title>
</head>
<body>
    <%  
        try {
            BasicDataSource source = (BasicDataSource)getServletContext().getAttribute("dataSource");
            out.write(source.toString());
        } catch(Exception e) {
            out.write(e.toString());
        }
    %>
    <p>${suscriber.name}<p/>
    <p>${suscriber.email}<p/>
</body>
</html>

My servlet context: 我的servlet上下文：

<?xml version="1.0" encoding="UTF-8"?>
<beans:beans xmlns="http://www.springframework.org/schema/mvc"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:beans="http://www.springframework.org/schema/beans"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc.xsd
        http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd">

    <!-- DispatcherServlet Context: defines this servlet's request-processing infrastructure -->

    <!-- Enables the Spring MVC @Controller programming model -->
    <annotation-driven />

    <!-- Handles HTTP GET requests for /resources/** by efficiently serving up static resources in the ${webappRoot}/resources directory -->
    <resources mapping="/resources/**" location="/resources/" />

    <!-- Resolves views selected for rendering by @Controllers to .jsp resources in the /WEB-INF/views directory -->
    <beans:bean class="org.springframework.web.servlet.view.InternalResourceViewResolver">
        <beans:property name="prefix" value="/WEB-INF/views/" />
        <beans:property name="suffix" value=".jsp" />
    </beans:bean>

    <context:component-scan base-package="com.edutainer4u.site" />



    <beans:bean id="dbURL" class="java.net.URI">
        <beans:constructor-arg
            value="'#{systemEnvironment['DATABASE_URL']}'">
        </beans:constructor-arg>
    </beans:bean>

    <beans:bean id="dataSource" 
        class="org.apache.commons.dbcp2.BasicDataSource">
        <beans:property name="url"
            value="jdbc:postgresql://localhost:10000/postgres">
        </beans:property>
        <beans:property value="blabla"
            name="username">
        </beans:property>
        <beans:property value="blabla"
            name="password">
        </beans:property>
    </beans:bean>
</beans:beans>

No matter what, I keep getting a NullPointerException. 无论如何，我一直收到NullPointerException。 What am I doing wrong? 我究竟做错了什么？

Answer 1

There is nothing in your question to indicate that you have put an attribute named dataSource in the ServletContext used here 您的问题中没有任何内容表明您已在此处使用的ServletContext放置了一个名为dataSource的属性。

BasicDataSource source = (BasicDataSource)getServletContext().getAttribute("dataSource");

You seem to be confused by the naming of the Servlet API type ServletContext and the Spring WebApplicationContext loaded by the DispatcherServlet , also commonly referred to as the (dispatcher) servlet context. 似乎对Servlet API类型ServletContext和DispatcherServlet加载的Spring WebApplicationContext的命名感到困惑， WebApplicationContext通常也称为（dispatcher）Servlet上下文。 Those are two completely different components. 这是两个完全不同的组件。

Here's some related content: 以下是一些相关内容：

Difference between applicationContext.xml and spring-servlet.xml in Spring Framework Spring Framework中applicationContext.xml和spring-servlet.xml之间的区别

You're probably better off using model attributes (request attributes). 使用模型属性（请求属性）可能更好。 To access a Spring bean from a JSP, you'll need to access the root WebApplicationContext or the servlet WebApplicationContext from the ServletContext using the appropriate attribute name, which may change from one version of Spring to the next, and access the bean on that. 要从JSP访问Spring bean，您需要使用适当的属性名从ServletContext访问根WebApplicationContext或Servlet WebApplicationContext ，这些属性名称可能会从Spring的一个版本更改为下一版本，并在该版本上访问bean。

Answer 2

In the view resolver configuration you can add the list of beans that you need to be available in JSP: 在视图解析器配置中，您可以添加需要在JSP中可用的bean列表：

<beans:bean class="org.springframework.web.servlet.view.InternalResourceViewResolver">
  <property name="exposedContextBeanNames">
      <list>
          <value>dataSource</value>
      </list>
  </property>
</beans:bean>

尝试检索Spring bean，但不断获取NullPointerException

问题描述

2 个解决方案

解决方案1
1 2014-11-12 23:10:52

解决方案2
0 2014-11-13 04:32:51

尝试检索Spring bean，但不断获取NullPointerException

问题描述

2 个解决方案

解决方案1 1 2014-11-12 23:10:52

解决方案2 0 2014-11-13 04:32:51

解决方案1
1 2014-11-12 23:10:52

解决方案2
0 2014-11-13 04:32:51