JPA通过DetachedCriteria实现分页

**

* 获取Session对象

*/

public Session getSession(){

EntityManager em=this.getJpaTemplate().getEntityManagerFactory().createEntityManager();

return (Session) em.getDelegate();

}

/**

* 创建DetachedCriteria对象

*/

@SuppressWarnings("hiding")

public <T> DetachedCriteria createDetachedCriteria() {

return DetachedCriteria.forClass(entityClass);

}

/**

* 分页查询函数，使用DetachedCriteria.

*

* @param startIndex 起始页,

* @param pageSize 每页显示条数

* @return PaginationSupport对象

*/

public PaginationSupport findPageBy(final DetachedCriteria detachedCriteria, final int pageSize, final int startIndex) {

JpaCallback jc=new JpaCallback() {

public Object doInJpa(EntityManager em) throws HibernateException {

Session session=(Session)em.getDelegate();

Criteria criteria = detachedCriteria.getExecutableCriteria(session);

int totalCount = ((Integer) criteria.setProjection(Projections.rowCount()).uniqueResult()).intValue();

criteria.setProjection(null);

List items = criteria.setFirstResult(startIndex).setMaxResults(pageSize).list();

PaginationSupport ps = new PaginationSupport(items, totalCount, pageSize, startIndex);

return ps;

}

};

return (PaginationSupport)getJpaTemplate().execute(jc, true);

}

public class PaginationSupport {

public final static int PAGESIZE = 30;

private int pageSize = PAGESIZE;

private List items;

private int totalCount;

private int[] indexes = new int[0];

private int startIndex = 0;

public PaginationSupport(List items, int totalCount) {

setPageSize(PAGESIZE);

setTotalCount(totalCount);

setItems(items);

setStartIndex(0);

}

public PaginationSupport(List items, int totalCount, int startIndex) {

setPageSize(PAGESIZE);

setTotalCount(totalCount);

setItems(items);

setStartIndex(startIndex);

}

public PaginationSupport(List items, int totalCount, int pageSize, int startIndex) {

setPageSize(pageSize);

setTotalCount(totalCount);

setItems(items);

setStartIndex(startIndex);

}

public List getItems() {

return items;

}

public void setItems(List items) {

this.items = items;

}

public int getPageSize() {

return pageSize;

}

public void setPageSize(int pageSize) {

this.pageSize = pageSize;

}

public int getTotalCount() {

return totalCount;

}

public void setTotalCount(int totalCount) {

if (totalCount > 0) {

this.totalCount = totalCount;

int count = totalCount / pageSize;

if (totalCount % pageSize > 0)

count++;

indexes = new int[count];

for (int i = 0; i < count; i++) {

indexes[i] = pageSize * i;

}

} else {

this.totalCount = 0;

}

}

public int[] getIndexes() {

return indexes;

}

public void setIndexes(int[] indexes) {

this.indexes = indexes;

}

public int getStartIndex() {

return startIndex;

}

public void setStartIndex(int startIndex) {

if (totalCount <= 0)

this.startIndex = 0;

else if (startIndex >= totalCount)

this.startIndex = indexes[indexes.length - 1];

else if (startIndex < 0)

this.startIndex = 0;

else {

this.startIndex = indexes[startIndex / pageSize];

}

}

public int getNextIndex() {

int nextIndex = getStartIndex() + pageSize;

if (nextIndex >= totalCount)

return getStartIndex();

else

return nextIndex;

}

public int getPreviousIndex() {

int previousIndex = getStartIndex() - pageSize;

if (previousIndex < 0)

return 0;

else

return previousIndex;

}

}

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space

at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:2931)

at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:2871)

at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3414)

代码：

public class PagingTest {

EntityManagerFactory emf = null;

private javax.persistence.EntityManager em;

/**

* @param args

*/

public static void main(String[] args) {

PagingTest pTest= new PagingTest();

pTest.testPaging();

}

public void testPaging(){

emf = EntityManagerHelper.getEmf();

em= emf.createEntityManager();

Query rowCountQuery = em.createQuery("SELECT COUNT(c) FROM Customer c"); // NOI18N

int size =((Long) rowCountQuery.getSingleResult()).intValue();

System.out.println(size);

final Query getRowsQuery = em.createQuery("select c from Customer c");

System.out.println("numer of requests to the database " + counter++);

Query query = getRowsQuery.setMaxResults(20).setFirstResult(1000000);//这里如果设置为setFirstResult(100000); 就可以运行。

//add the cache

List <Customer> resultList = query.getResultList();

System.out.println("=====query done " );

for(Customer cust:resultList){

System.out.println("cust:"cust.getCustomerId()","+cust.getCustomerName());

}

}

private javax.persistence.EntityManager entityManager1;

private javax.persistence.Query getRowsQuery;

private javax.persistence.Query rowCountQuery;

private List <Customer> getList() {

getRowsQuery = java.beans.Beans.isDesignTime() ? null : entityManager1.createQuery("SELECT c FROM Customer c"); // NOI18N

rowCountQuery = java.beans.Beans.isDesignTime() ? null : entityManager1.createQuery("SELECT COUNT(c) FROM Customer c"); // NOI18N

List <Customer> toReturn = new ResultListJPA <Customer>(rowCountQuery, getRowsQuery);

return toReturn;

}

private int counter=0;

}