Spring和auto-wiring:NullPointerException

时间:2011-11-23 14:29:51

标签: spring autowired

我正试图抓住Spring中的自动连接,但我似乎无法正确实例化bean(DocumentBuilder)。我已经创建了一个自定义JSP标记:

public class MyTag extends SimpleTagSupport {

    @Autowired
    private DocumentBuilder documentBuilder;

    public void setBuilder(DocumentBuilder builder) {
        this.documentBuilder = builder;
    }

    @Override
    public void doTag() throws IOException {
        // documentBuilder is null in here!
    }
}

这是servlet配置:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:p="http://www.springframework.org/schema/p"
       xmlns:context="http://www.springframework.org/schema/context"
       xmlns:mvc="http://www.springframework.org/schema/mvc"
       xsi:schemaLocation="http://www.springframework.org/schema/mvc
        http://www.springframework.org/schema/mvc/spring-mvc-3.0.xsd
        http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
        http://www.springframework.org/schema/context
        http://www.springframework.org/schema/context/spring-context-3.0.xsd">

    <!-- Scan for HTTP/REST controllers -->
    <context:component-scan base-package="the.right.package" />

    <context:annotation-config/>

    <bean id="documentBuilderFactory"
          class="javax.xml.parsers.DocumentBuilderFactory"
          factory-method="newInstance">
        <property name="validating" value="false" />
        <property name="ignoringElementContentWhitespace" value="true" />
    </bean>

    <bean id="documentBuilder" class="javax.xml.parsers.DocumentBuilder"
          factory-bean="documentBuilderFactory"
          factory-method="newDocumentBuilder">
    </bean>

</beans>

有什么想法吗?

3 个答案:

答案 0 :(得分:4)

你只能注入春豆!但是Jsp-Tags不是Spring Beans,因此Autowird注释将被完全忽略,因此该字段为空。

有两种解决方案:

答案 1 :(得分:0)

尝试修改这样的代码

public class MyTag extends SimpleTagSupport {

    private DocumentBuilder documentBuilder;

    @Autowired
    public void setBuilder(DocumentBuilder builder) {
        this.documentBuilder = builder;
    }

    @Override
    public void doTag() throws IOException {
        // documentBuilder is null in here!
    }
}

答案 2 :(得分:0)

如果将标记类标记为Spring bean,则可以使用@Autowired。但它很愚蠢,因为简单的标签不是通过容器缓存的。每个请求都会创建自己的标记实例,但只有上下文才会启动。