Spring和空指針異常

Question

我正在嘗試這樣做：

@Controller    
public class HomeController {

        @Autowired
        private TemplatesService templatesService;    

        public final String TEST = templatesService.getUri();

getUri ：

@SuppressWarnings("unchecked")
    public String getUri() {

        return (String) sessionFactory.getCurrentSession()
                .createQuery("select uri from Templates WHERE state=1")
                .uniqueResult();
    }

如果我將以任何方法聲明TEST ，它將正常工作。 但是現在我有了這個：

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'homeController' defined in file [C:\Users\anton\springsource\vfabric-tc-server-developer-2.8.2.RELEASE\base-instance\wtpwebapps\blog\WEB-INF\classes\net\babobka\blog\controller\HomeController.class]: Instantiation of bean failed; nested exception is org.springframework.beans.BeanInstantiationException: Could not instantiate bean class [net.babobka.blog.controller.HomeController]: Constructor threw exception; nested exception is java.lang.NullPointerException
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:997)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:943)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:485)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:456)
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:294)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:225)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:291)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:193)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:585)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:913)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:464)
    at org.springframework.web.context.ContextLoader.configureAndRefreshWebApplicationContext(ContextLoader.java:385)
    at org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:284)
    at org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:111) 

我的根上下文（我認為這沒有幫助）：

    <context:annotation-config />


    <context:component-scan base-package="net.babobka.blog" />
    <mvc:resources mapping="/resources/**" location="/resources/" />
    <cache:annotation-driven />

    <bean id="cacheManager" class="org.springframework.cache.support.SimpleCacheManager">
        <property name="caches">
            <set>
                <bean
                    class="org.springframework.cache.concurrent.ConcurrentMapCacheFactoryBean"
                    p:name="template" />
                <bean
                    class="org.springframework.cache.concurrent.ConcurrentMapCacheFactoryBean"
                    p:name="headHunter" />

            </set>
        </property>
    </bean>

    <bean id="converter" class="net.babobka.blog.headHunter.Converter" />


    <bean id="propertyConfigurer"
        class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer"
        p:location="/WEB-INF/db/jdbc.properties" />

    <bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource"
        destroy-method="close" p:driverClassName="${jdbc.driverClassName}"
        p:url="${jdbc.databaseurl}" p:username="${jdbc.username}" p:password="${jdbc.password}" />
    <bean id="sessionFactory"
        class="org.springframework.orm.hibernate3.LocalSessionFactoryBean">
        <property name="dataSource" ref="dataSource" />
        <property name="configLocation">
            <value>/WEB-INF/db/hibernate.cfg.xml</value>
        </property>
        <property name="configurationClass">
            <value>org.hibernate.cfg.AnnotationConfiguration</value>
        </property>
        <property name="hibernateProperties">
            <props>
                <prop key="hibernate.dialect">${jdbc.dialect}</prop>
                <prop key="hibernate.show_sql">${jdbc.show_sql}</prop>
                <prop key="hibernate.connection.useUnicode">true</prop>
                <prop key="hibernate.connection.characterEncoding">UTF-8</prop>
                <prop key="hibernate.connection.charSet">UTF-8</prop>
            </props>
        </property>
    </bean>




    <bean id="transactionManager"
        class="org.springframework.orm.hibernate3.HibernateTransactionManager">
        <property name="sessionFactory" ref="sessionFactory" />
    </bean>
    <mvc:annotation-driven />
    <tx:annotation-driven />
    <task:annotation-driven />



    <bean id="Backupper" class="net.babobka.blog.backupper.Backupper"></bean>

    <bean id="OldDataRemoval" class="net.babobka.blog.termination.OldDataRemoval"></bean>

    <bean id="HeadHunterImport" class="net.babobka.blog.headHunter.HeadHunterImport"></bean>

    <bean id="urlForwardController"
        class="org.springframework.web.servlet.mvc.UrlFilenameViewController" />

    <bean id="tilesConfigurer"
        class="org.springframework.web.servlet.view.tiles2.TilesConfigurer">
        <property name="definitions">
            <list>
                <value>/WEB-INF/tiles.xml</value>
            </list>
        </property>
    </bean>

    <bean id="viewResolver"
        class="org.springframework.web.servlet.view.UrlBasedViewResolver">
        <property name="viewClass"
            value="org.springframework.web.servlet.view.tiles2.TilesView" />
    </bean>





</beans>

怎么了？

Answer 1

它不起作用，因為在您引用它來設置TEST時還沒有注入templatesService 。 字段注入僅在實例化對象之后才有效。 您需要解決的是使用@PostConstruct設置測試。

@Controller    
public class HomeController {

    @Autowired
    private TemplatesService templatesService;    

    public String test;

    @PostConstruct
    public void init() {
       this.test = templatesService.getUri();
    }

如果仍要使用final，則可以按構造函數使用注入，然后設置TEST。

  @Controller    
  public class HomeController {

    private TemplatesService templatesService;    

    public final String TEST;

    @Autowired
    public HomeController(TemplatesService templatesService) {
        this.templatesService = templatesService;
        this.TEST = templatesService.getUri();
    }

Answer 2

這是初始化順序的問題。 Spring會在調用bean的構造函數之后將依賴項注入@Autowired字段中，但是在對象構造期間（因此，在字段自動裝配之前）將執行字段初始化程序。 因此，該字段仍為null ，這就是為什么嘗試調用方法會引發NullPointerException 。

備擇方案：

• 通過構造函數注入傳遞TemplatesService ，並從構造函數而不是字段初始值設定項分配final字段
• 繼續通過字段注入來注入TemplatesService ，但是將Bean的初始化移動到@PostConstruct方法中。 您將必須刪除final才能使編譯器正常運行。
• 通過setter注入來注入TemplatesService，並在setter中執行必要的初始化。 您將必須刪除final才能使編譯器正常運行。