org.springframework.dao.EmptyResultDataAccessException

 　　今天在做定时任务获取数据的时候遇到下面的错误:

org.springframework.dao.EmptyResultDataAccessException: Incorrect result size: expected 1, actual 0
    at org.springframework.dao.support.DataAccessUtils.requiredSingleResult(DataAccessUtils.java:71)
    at org.springframework.jdbc.core.JdbcTemplate.queryForObject(JdbcTemplate.java:472)
    at org.springframework.jdbc.core.JdbcTemplate.queryForMap(JdbcTemplate.java:467)
    at zd.record.utils.AbstractRecordDBUtils.getRecordByUniqueProperty(AbstractRecordDBUtils.java)
    at zd.record.utils.RecordDBUtils.getRecordByUniqueProperty(RecordDBUtils.java)
    at zd.dms.service.ebuy.BaseInfoServiceImpl.saveOrUpdateData(BaseInfoServiceImpl.java:283)
    at zd.dms.service.ebuy.BaseInfoServiceImpl.batchFetchAndDisposeMerchantInfos(BaseInfoServiceImpl.java:242)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:307)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:183)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:150)
    at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:107)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:172)
    at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:202)
    at com.sun.proxy.$Proxy171.batchFetchAndDisposeMerchantInfos(Unknown Source)
    at zd.dms.job.ebuy.FetchBaseInfosJob.work(FetchBaseInfosJob.java:30)
    at zd.dms.utils.AbstractHibernateQuartzJob.execute(AbstractHibernateQuartzJob.java)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at org.springframework.util.MethodInvoker.invoke(MethodInvoker.java:273)
    at org.springframework.scheduling.quartz.MethodInvokingJobDetailFactoryBean$MethodInvokingJob.executeInternal(MethodInvokingJobDetailFactoryBean.java:264)
    at org.springframework.scheduling.quartz.QuartzJobBean.execute(QuartzJobBean.java:86)
    at org.quartz.core.JobRunShell.run(JobRunShell.java:223)
    at org.quartz.simpl.SimpleThreadPool$WorkerThread.run(SimpleThreadPool.java:549)

 解释:上面的错误是Spring中使用JdbcTemplate的queryForObject方法，当查不到数据时会抛出异常。如下源码:

    public static <T> T requiredSingleResult(Collection<T> results) throws IncorrectResultSizeDataAccessException {
         int size = (results != null ? results.size() : 0);
         if (size == 0) {
             throw new EmptyResultDataAccessException(1);
         }
         if (results.size() > 1) {
             throw new IncorrectResultSizeDataAccessException(1, size);
         }
         return results.iterator().next();

　　可以看出，当results为空时，就会抛出EmptyResultDataAccessException异常，Spring这样做的目的是为了防止程序员不对空值进行判断，保证了程序的健壮性。另外，当results的size大于1时，还会抛出IncorrectResultSizeDataAccessException异常，以保证返回的记录只有一条。如果我们想查询结果为空时，返回null而不是抛出异常，该怎么办呢？

　　很简单，只需要在捕获EmptyResultDataAccessException，然后返回null，代码如下：

 　　　　Object object = null;
        try {
            object = jdbcTemplate.queryForObject();
        } catch (EmptyResultDataAccessException e) {
            return null;
        }
        return object;