Jpa EclipseLink的executeObjectLevelReadQuery中出现空指针异常

我在项目中使用eclipseLink 2.5.2，有时会出现以下异常：

java.lang.NullPointerException
    at org.eclipse.persistence.queries.ReadAllQuery.executeObjectLevelReadQuery(ReadAllQuery.java:431)
    at org.eclipse.persistence.queries.ObjectLevelReadQuery.executeDatabaseQuery(ObjectLevelReadQuery.java:1168)
    at org.eclipse.persistence.queries.DatabaseQuery.execute(DatabaseQuery.java:899)
    at org.eclipse.persistence.queries.ObjectLevelReadQuery.execute(ObjectLevelReadQuery.java:1127)
    at org.eclipse.persistence.queries.ReadAllQuery.execute(ReadAllQuery.java:403)
    at org.eclipse.persistence.internal.sessions.AbstractSession.internalExecuteQuery(AbstractSession.java:3214)
    at org.eclipse.persistence.internal.sessions.AbstractSession.executeQuery(AbstractSession.java:1804)
    at org.eclipse.persistence.internal.sessions.AbstractSession.executeQuery(AbstractSession.java:1786)
    at org.eclipse.persistence.mappings.CollectionMapping.executeBatchQuery(CollectionMapping.java:974)
    at org.eclipse.persistence.mappings.ForeignReferenceMapping.extractResultFromBatchQuery(ForeignReferenceMapping.java:643)
    at org.eclipse.persistence.mappings.CollectionMapping.extractResultFromBatchQuery(CollectionMapping.java:955)
    at org.eclipse.persistence.internal.indirection.BatchValueHolder.instantiate(BatchValueHolder.java:58)
    at org.eclipse.persistence.internal.indirection.QueryBasedValueHolder.instantiate(QueryBasedValueHolder.java:116)
    at org.eclipse.persistence.internal.indirection.DatabaseValueHolder.getValue(DatabaseValueHolder.java:89)
    at org.eclipse.persistence.internal.indirection.UnitOfWorkValueHolder.instantiateImpl(UnitOfWorkValueHolder.java:173)
    at org.eclipse.persistence.internal.indirection.UnitOfWorkValueHolder.instantiate(UnitOfWorkValueHolder.java:234)
    at org.eclipse.persistence.internal.indirection.DatabaseValueHolder.getValue(DatabaseValueHolder.java:89)
    at org.eclipse.persistence.indirection.IndirectSet.buildDelegate(IndirectSet.java:211)
    at org.eclipse.persistence.indirection.IndirectSet.getDelegate(IndirectSet.java:375)
    at org.eclipse.persistence.indirection.IndirectSet.contains(IndirectSet.java:349)
    at com.services.CategoryServiceImpl.categoryContainsCareType(CategoryServiceImpl.java:305)
    at 

这是定义careType和category之间关系的部分，调用

category.getCareTypes（）

时会发生异常：

@ManyToMany(targetEntity=CareTypeJPAImpl.class) @JoinTable(
        name="J_CARE_TYPE_CATEGORY",
        joinColumns={@JoinColumn(name="CATEGORY_ID", referencedColumnName="ID")},
        inverseJoinColumns={@JoinColumn(name="CARE_TYPE_ID", referencedColumnName="ID")})
@org.eclipse.persistence.annotations.BatchFetch(value=org.eclipse.persistence.annotations.BatchFetchType.JOIN,size=990)
private Set<CareType> careTypes = new HashSet<CareType>();

@ManyToMany（targetEntity=CareTypeJPAImpl.class）@JoinTable(
name=“J_护理类型_类别”，
joinColumns={@JoinColumn（name=“CATEGORY\u ID”，referencedColumnName=“ID”）}，
inverseJoinColumns={@JoinColumn（name=“CARE\u TYPE\u ID”，referencedColumnName=“ID”）}）
@org.eclipse.persistence.annotations.BatchFetch（value=org.eclipse.persistence.annotations.BatchFetchType.JOIN，size=990）
private Set careTypes=new HashSet（）；

我看了这本书，但不太明白