java - 使用hibernate读取大量数据时OutOfMemory

Question

我需要从数据库中导出大量数据。这是代表我的数据的类:

public class Product{
...

    @OneToMany
    @JoinColumn(name = "product_id")
    @Cascade({SAVE_UPDATE, DELETE_ORPHAN})
    List<ProductHtmlSource> htmlSources = new ArrayList<ProductHtmlSource>();

...

ProductHtmlSource - 包含我实际需要导出的大字符串。

由于导出数据的大小大于 JVM 内存，我正在按 block 读取数据。像这样:

final int batchSize = 1000;      
for (int i = 0; i < 50; i++) {
  ScrollableResults iterator = getProductIterator(batchSize * i, batchSize * (i + 1));
  while (iterator.getScrollableResults().next()) {
     Product product = (Product) iterator.getScrollableResults().get(0); 
     List<String> htmls = product.getHtmlSources();
     <some processing>
  }

}

getProductIterator代码:

public ScrollableResults getProductIterator(int offset, int limit) {
        Session session = getSession(true);
        session.setCacheMode(CacheMode.IGNORE);
        ScrollableResults iterator = session
                .createCriteria(Product.class)
                .add(Restrictions.eq("status", Product.Status.DONE))
                .setFirstResult(offset)
                .setMaxResults(limit)
                .scroll(ScrollMode.FORWARD_ONLY);
        session.flush();
        session.clear();

        return iterator;
    }

问题是，尽管我在读取每个数据 block 后清除了 session ，但 Product 对象在某处累积并且我得到了 OutOfMemory 异常。问题不在于处理代码块，即使没有它我也会收到内存错误。批处理的大小也不是问题，因为 1000 个对象很容易进入内存。

Profiler 显示对象在 org.hibernate.engine.StatefulPersistenceContext 类中累积。

堆栈跟踪:

Caused by: java.lang.OutOfMemoryError: Java heap space
    at java.lang.AbstractStringBuilder.expandCapacity(AbstractStringBuilder.java:99)
    at java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:518)
    at java.lang.StringBuffer.append(StringBuffer.java:307)
    at org.hibernate.type.TextType.get(TextType.java:41)
    at org.hibernate.type.NullableType.nullSafeGet(NullableType.java:163)
    at org.hibernate.type.NullableType.nullSafeGet(NullableType.java:154)
    at org.hibernate.type.AbstractType.hydrate(AbstractType.java:81)
    at org.hibernate.persister.entity.AbstractEntityPersister.hydrate(AbstractEntityPersister.java:2101)
    at org.hibernate.loader.Loader.loadFromResultSet(Loader.java:1380)
    at org.hibernate.loader.Loader.instanceNotYetLoaded(Loader.java:1308)
    at org.hibernate.loader.Loader.getRow(Loader.java:1206)
    at org.hibernate.loader.Loader.getRowFromResultSet(Loader.java:580)
    at org.hibernate.loader.Loader.doQuery(Loader.java:701)
    at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:236)
    at org.hibernate.loader.Loader.loadCollection(Loader.java:1994)
    at org.hibernate.loader.collection.CollectionLoader.initialize(CollectionLoader.java:36)
    at org.hibernate.persister.collection.AbstractCollectionPersister.initialize(AbstractCollectionPersister.java:565)
    at org.hibernate.event.def.DefaultInitializeCollectionEventListener.onInitializeCollection(DefaultInitializeCollectionEventListener.java:63)
    at org.hibernate.impl.SessionImpl.initializeCollection(SessionImpl.java:1716)
    at org.hibernate.collection.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:344)
    at org.hibernate.collection.AbstractPersistentCollection.read(AbstractPersistentCollection.java:86)
    at org.hibernate.collection.AbstractPersistentCollection.readSize(AbstractPersistentCollection.java:109)
    at org.hibernate.collection.PersistentBag.size(PersistentBag.java:225)
    **at com.rivalwatch.plum.model.Product.getHtmlSource(Product.java:76)
    at com.rivalwatch.plum.model.Product.getHtmlSourceText(Product.java:80)
    at com.rivalwatch.plum.readers.AbstractDataReader.getData(AbstractDataReader.java:64)**

Answer 1

您似乎在调用 getProductIterator() 时带有起始行号和结束行号，而 getProductIterator() 需要起始行和行数。随着您的“上限”越来越高，您正在读取更大的数据 block 。我认为您的意思是将 batchSize 作为第二个参数传递给 getProductIterator()。