如何迭代 JPA 查询结果而不将它们全部保留在内存中？
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【中文标题】如何迭代 JPA 查询结果而不将它们全部保留在内存中？【英文标题】：How do I iterate over JPA query results without retaining them all in memory? 【发布时间】：2013-10-07 00:46:43 【问题描述】：我正在使用 Oracle、Spring、Hibernate 和 JPA。我想迭代任意数量的数据库结果，而不必担心内存不足。
我试图滚动查询结果而不保留我得到的每个对象。
首先我创建原生查询：
Query q = getEm().createNativeQuery(sql,reportRowType);
q.setHint("org.hibernate.fetchSize",1000);
q.setHint("org.hibernate.cacheable",false);
然后我在自动装配对象上调用一个方法，该对象执行查询并对结果进行处理。对于测试，我完全忽略了结果并对其进行迭代。
@Transactional(readOnly = true, propagation = Propagation.REQUIRES_NEW)
public <T extends ResultRow> long run(EntityManager em, Query q)
ScrollableResults sr = q.unwrap(org.hibernate.Query.class)
.setReadOnly(true)
.setFetchSize(1000)
.setCacheable(false)
.setCacheMode(CacheMode.IGNORE)
.scroll(ScrollMode.FORWARD_ONLY);
try
while(sr.next())
T obj = (T)sr.get(0);
em.detach(obj);
// do something with the row here
finally
if(sr!=null)
sr.close();
我发现使用上面的代码我最终会耗尽内存（我的测试大约有 150 万个结果）。 Query 对象以某种方式保留了对象。
我发现即使我通过页面运行查询（使用 q.setFirstResult 和 q.setMaxResults）它实际上仍然保留了对象。
这样做的唯一方法是创建一个全新的 Query 对象，然后使用 setFirstResult 和 setMaxResults 获取结果 1 到 10000，然后是 10001 到 20000，等等。
我已经阅读过有关休眠 StatelessSession 的信息，但是让它工作起来看起来相当复杂。真的没有办法在不保留所有查询结果的情况下执行jpa查询吗？
【问题讨论】：
【参考方案1】：我最终想出了如何使用休眠无状态会话。这不会是可移植的，但如果你使用的是休眠，这样的东西会起作用。
import javax.persistence.EntityManager;
import java.util.Map;
import org.apache.log4j.Logger;
import org.hibernate.*;
import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Propagation;
import org.springframework.transaction.annotation.Transactional;
@Service
public class StatelessQueryRunner
/** Executes specified native sql in a stateless session. The consumer is given each row as it's received. */
@Transactional(readOnly = true, propagation = Propagation.REQUIRES_NEW)
public <T> long run(EntityManager em, String sql, Class<T> resultRowType, Map<String,Object> params, int fetchSize, Integer firstResult, Integer maxResults, QueryResultConsumer<T> consumer)
int totalResults = 0;
Session hibernateSession = em.unwrap(Session.class);
StatelessSession statelessSession = hibernateSession.getSessionFactory().openStatelessSession();
try
// create the query for the stateless session.
SQLQuery q = statelessSession.createSQLQuery(sql);
q.addEntity(resultRowType);
q.setFetchSize(1000);
JpaQueryWrapper qw = new JpaQueryWrapper(q);
if(params!=null)
for(Map.Entry<String,Object> entry : params.entrySet())
qw.setParameter(entry.getKey(),entry.getValue());
if(firstResult!=null)
q.setFirstResult(firstResult);
if(maxResults!=null)
q.setMaxResults(maxResults);
ScrollableResults sr = q
.setReadOnly(true)
.setFetchSize(fetchSize)
.setCacheable(false)
.scroll(ScrollMode.FORWARD_ONLY);
try
while(sr.next())
T obj = (T)sr.get(0);
em.detach(obj);
consumer.consume(obj);
++totalResults;
// if(totalResults % 100000 == 0)
// Logger.getLogger(getClass()).debug("totalResults="+ totalResults);
finally
if(sr!=null)
sr.close();
return totalResults;
finally
statelessSession.close();
/** You can't use annotations to start transactions when inside a stateless session, so if you want to do anything you'll need to call this method to run code in a separate session which uses the Session object to start a transaction and save things. */
public <T> void runInSession(EntityManager em, SessionRunnable<T> action)
Session hibernateSession = em.unwrap(Session.class);
Session session = hibernateSession.getSessionFactory().openSession();
try
action.run(session);
finally
session.close();
public interface SessionRunnable<T>
public void run(Session session);
import javax.persistence.*;
import java.util.*;
import org.hibernate.SQLQuery;
public class JpaQueryWrapper
implements javax.persistence.Query
private SQLQuery q;
public JpaQueryWrapper(SQLQuery q)
this.q = q;
@Override
public List getResultList() throw new UnsupportedOperationException();
@Override
public Object getSingleResult() throw new UnsupportedOperationException();
@Override
public int executeUpdate() throw new UnsupportedOperationException();
@Override
public javax.persistence.Query setMaxResults(int maxResult) throw new UnsupportedOperationException();
@Override
public int getMaxResults() throw new UnsupportedOperationException();
@Override
public Query setFirstResult(int startPosition) throw new UnsupportedOperationException();
@Override
public int getFirstResult() throw new UnsupportedOperationException();
@Override
public javax.persistence.Query setHint(String hintName, Object value) throw new UnsupportedOperationException();
@Override
public Map<String, Object> getHints() throw new UnsupportedOperationException();
@Override
public <T> javax.persistence.Query setParameter(Parameter<T> param, T value) throw new UnsupportedOperationException();
@Override
public javax.persistence.Query setParameter(Parameter<Calendar> param, Calendar value, TemporalType temporalType) throw new UnsupportedOperationException();
@Override
public javax.persistence.Query setParameter(Parameter<Date> param, Date value, TemporalType temporalType) throw new UnsupportedOperationException();
@Override
public javax.persistence.Query setParameter(String name, Object value)
if(value instanceof Enum)
q.setParameter(name,((Enum) value).name());
else
q.setParameter(name,value);
return this;
@Override
public javax.persistence.Query setParameter(String name, Calendar value, TemporalType temporalType) return setParameter(name,(Date)(value==null?null:value.getTime()),temporalType);
@Override
public javax.persistence.Query setParameter(String name, Date value, TemporalType temporalType)
if(temporalType==TemporalType.DATE)
q.setDate(name,value);
else if(temporalType==TemporalType.TIME)
q.setTime(name,value);
else if(temporalType==TemporalType.TIMESTAMP)
q.setTimestamp(name,value);
else
throw new UnsupportedOperationException();
return this;
@Override
public javax.persistence.Query setParameter(int position, Object value) q.setParameter(position,value); return this;
@Override
public javax.persistence.Query setParameter(int position, Calendar value, TemporalType temporalType) throw new UnsupportedOperationException();
@Override
public javax.persistence.Query setParameter(int position, Date value, TemporalType temporalType) throw new UnsupportedOperationException();
@Override
public Set<Parameter<?>> getParameters() throw new UnsupportedOperationException();
@Override
public Parameter<?> getParameter(String name) throw new UnsupportedOperationException();
@Override
public <T> Parameter<T> getParameter(String name, Class<T> type) throw new UnsupportedOperationException();
@Override
public Parameter<?> getParameter(int position) throw new UnsupportedOperationException();
@Override
public <T> Parameter<T> getParameter(int position, Class<T> type) throw new UnsupportedOperationException();
@Override
public boolean isBound(Parameter<?> param) throw new UnsupportedOperationException();
@Override
public <T> T getParameterValue(Parameter<T> param) throw new UnsupportedOperationException();
@Override
public Object getParameterValue(String name) throw new UnsupportedOperationException();
@Override
public Object getParameterValue(int position) throw new UnsupportedOperationException();
@Override
public javax.persistence.Query setFlushMode(FlushModeType flushMode) throw new UnsupportedOperationException();
@Override
public FlushModeType getFlushMode() throw new UnsupportedOperationException();
@Override
public javax.persistence.Query setLockMode(LockModeType lockMode) throw new UnsupportedOperationException();
@Override
public LockModeType getLockMode() throw new UnsupportedOperationException();
@Override
public <T> T unwrap(Class <T> cls) throw new UnsupportedOperationException();
public interface QueryResultConsumer<T>
public void consume(T obj);
【讨论】：
【参考方案2】：据我所知，在 JPA 中，唯一的方法是使用多个查询，正如您所描述的。另一方面，您可以肯定，这是唯一使用的技术（如果有的话）。我的意思是：要么获取所有行，要么分页，每页有一个数据库查询。
当然，如果您更频繁地需要此功能，您可以创建一个特殊的 Iterator，它需要在其构造函数中使用 pageSize 和 QueryProvider。
【讨论】：
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