如何使用Spring batch和Hibernate从@JoinColumn中获取值

Question

为了从数据库中提取一些数据，我试图设置一个基本的hibernate和spring批处理项目。目标是提供一个查询(HQL)，并基于该查询，spring batch应用程序将所有数据提取到一个平面文件中。

应用程序的一个要求是，用户不必配置列的映射。因此，我试图创建一个DynamicRecordProcessor，它计算输入并将输入（表，例如Address)，以平面文件项编写器可以使用PassthroughFieldExtractor的方式传递给编写器。

在读取器-处理器-写入器xml配置下面：

<!-- Standard Spring Hibernate Reader -->
<bean id="hibernateItemReader" class="org.springframework.batch.item.database.HibernateCursorItemReader">
    <property name="sessionFactory" ref="sessionFactory" />         
    <property name="queryString" value="from Address" />                
</bean> 

<!-- Custom Processor -->
<bean id="dynamicRecordProcessor" class="nl.sander.mieras.processor.DynamicRecordProcessor"/>   

<!-- Standard Spring Writer -->
<bean id="itemWriter" class="org.springframework.batch.item.file.FlatFileItemWriter">
    <property name="resource" value="file:target/extract/output.txt" />  
    <property name="lineAggregator">
        <bean class="org.springframework.batch.item.file.transform.DelimitedLineAggregator">
            <property name="delimiter" value="|"/>
            <property name="fieldExtractor">
                <bean class="org.springframework.batch.item.file.transform.PassThroughFieldExtractor"/>                                   
            </property>             
        </bean>
    </property>
</bean>

<bean id="jobLauncher" class="org.springframework.batch.core.launch.support.SimpleJobLauncher">
    <property name="jobRepository" ref="jobRepository" />
</bean>

<bean id="jobRepository" class="org.springframework.batch.core.repository.support.MapJobRepositoryFactoryBean">
    <property name="transactionManager" ref="transactionManager"/>
</bean> 

<bean id="sessionFactory" class="org.springframework.orm.hibernate5.LocalSessionFactoryBean">
    <property name="configLocation" value="classpath:hibernate.cfg.xml"/>           
    <property name="cacheableMappingLocations" value="classpath*:META-INF/mappings/*.hbm.xml"/>    
</bean> 

<bean id="transactionManager" class="org.springframework.orm.hibernate5.HibernateTransactionManager" lazy-init="true">
    <property name="sessionFactory" ref="sessionFactory" />
</bean>

public class DynamicRecordProcessor<Input,Output> implements ItemProcessor<Input,Output> {  

    private static final String DELIMITER = "|";
    private boolean areNamesSetup = false;

    private List<String> names = new ArrayList<String>();
    private Input item;

    @SuppressWarnings("unchecked")
    @Override
    public Output process(Input item) throws Exception {
        this.item = item;       
        initMapping();              
        return (Output) extract();      
    }

    private void initMapping() {
        if (!areNamesSetup) {
            mapColumns();
        }
        areNamesSetup = true;
    }

    private void mapColumns() {     
        Field[] allFields = item.getClass().getDeclaredFields();
        for (Field field : allFields) {
            if (!field.getType().equals(Set.class) && Modifier.isPrivate(field.getModifiers())) {               
                names.add(field.getName());                 
            }
        }
    }   

    private Object extract() {
        List<Object> values = new ArrayList<Object>();
        BeanWrapper bw = new BeanWrapperImpl(item);     
        for (String propertyName : this.names) {                        
            values.add(bw.getPropertyValue(propertyName));          
        }       
        return StringUtils.collectionToDelimitedString(values, DELIMITER);      
    }   
}

表地址有以下字段：

@ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="city_id", nullable=false)
    public City getCity() {
        return this.city;
    }

和城市中的相应栏：

@Column(name="city_id", unique=true, nullable=false)
    public Short getCityId() {
        return this.cityId;
    }

当使用values.add(bw.getPropertyValue（propertyName））；propertyName为“city”时，会出现以下异常：

org.hibernate.SessionException: proxies cannot be fetched by a stateless session
    at org.hibernate.internal.StatelessSessionImpl.immediateLoad(StatelessSessionImpl.java:292)
    at org.hibernate.proxy.AbstractLazyInitializer.initialize(AbstractLazyInitializer.java:156)
    at org.hibernate.proxy.AbstractLazyInitializer.getImplementation(AbstractLazyInitializer.java:260)
    at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.invoke(JavassistLazyInitializer.java:68)
    at nl.sander.mieras.localhost.sakila.City_$$_jvstc2c_d.toString(City_$$_jvstc2c_d.java)
    at java.lang.String.valueOf(String.java:2982)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at org.springframework.util.StringUtils.collectionToDelimitedString(StringUtils.java:1132)
    at org.springframework.util.StringUtils.collectionToDelimitedString(StringUtils.java:1148)
    at nl.sander.mieras.processor.DynamicRecordProcessor.extract(DynamicRecordProcessor.java:52)
    at nl.sander.mieras.processor.DynamicRecordProcessor.process(DynamicRecordProcessor.java:27)

我已经尝试使用反射API获得值，但我无法达到我想要获得的实际值...

我已经设置了一个公开回购。但是，您仍然需要一个本地数据库来准确地再现问题。https://github.com/weirdfishees/hibernate-batch-example。任何关于如何进一步隔离这个问题的建议都是非常受欢迎的。

Answer 1

只需使用UseStateLessSession属性将阅读器翻转为状态满而不是无状态：

<!-- Standard Spring Hibernate Reader -->
<bean id="hibernateItemReader" class="org.springframework.batch.item.database.HibernateCursorItemReader">
    <property name="sessionFactory" ref="sessionFactory" />         
    <property name="queryString" value="from Address" />
    <property name="useStatelessSession" value="false" />       
</bean>