在执行选择和计数查询时,是否有更有效的方式在Hibernate中进行分页？

Question

通常分页查询看起来像这样.有没有更好的方法,而不是两个几乎相同的方法,其中一个执行"select*..."而另一个"count*..."？

public List<Cat> findCats(String name, int offset, int limit) {

    Query q = session.createQuery("from Cat where name=:name");

    q.setString("name", name);

    if (offset > 0) {
        q.setFirstResult(offset);
    }
    if (limit > 0) {
        q.setMaxResults(limit);
    }

    return q.list();

}

public Long countCats(String name) {
    Query q = session.createQuery("select count(*) from Cat where name=:name");
    q.setString("name", name);
    return (Long) q.uniqueResult();
}

Answer 1

MySQLPerformanceBlog.com的Baron Schwartz撰写了一篇关于此事的帖子.我希望这个问题有一个神奇的子弹,但没有.他提出的选项摘要:

1. 在第一个查询中,获取并缓存所有结果.
2. 不要显示所有结果.
3. 不显示总计数或其他页面的中间链接.仅显示"下一个"链接.
4. 估计有多少结果.

Answer 2

我的解决方案适用于Hibernate + Spring + MySQL的常见用例

与上述答案类似,我的解决方案基于Richard Kennar博士.但是,由于Hibernate经常与Spring一起使用,我希望我的解决方案能够很好地运行Spring和使用Hibernate的标准方法.因此,我的解决方案使用线程局部和单例bean的组合来实现结果.从技术上讲,拦截器是在SessionFactory的每个准备好的SQL语句上调用的,但它会跳过所有逻辑并且不会初始化任何ThreadLocal,除非它是专门设置为计算总行数的查询.

使用以下类,您的Spring配置如下所示:

<bean id="foundRowCalculator" class="my.hibernate.classes.MySQLCalcFoundRowsInterceptor" />
    <!-- p:sessionFactoryBeanName="mySessionFactory"/ -->

<bean id="mySessionFactory"
    class="org.springframework.orm.hibernate3.annotation.AnnotationSessionFactoryBean"
    p:dataSource-ref="dataSource"
    p:packagesToScan="my.hibernate.classes"
    p:entityInterceptor-ref="foundRowCalculator"/>

基本上,您必须声明拦截器bean,然后在SessionFactoryBean的"entityInterceptor"属性中引用它.如果Spring上下文中有多个SessionFactory,并且要引用的会话工厂不称为"sessionFactory",则只能设置"sessionFactoryBeanName".您无法设置引用的原因是这会导致无法解析的bean之间的相互依赖性.

使用包装bean来获得结果:

package my.hibernate.classes;

public class PagedResponse<T> {
    public final List<T> items;
    public final int total;
    public PagedResponse(List<T> items, int total) {
        this.items = items;
        this.total = total;
    }
}

然后使用抽象基类DAO类,在进行查询之前必须调用"setCalcFoundRows(true)",然后在[finally块中]之后调用"reset()"以确保调用它:

package my.hibernate.classes;

import org.hibernate.Criteria;
import org.hibernate.Query;
import org.springframework.beans.factory.annotation.Autowired;

public abstract class BaseDAO {

    @Autowired
    private MySQLCalcFoundRowsInterceptor rowCounter;

    public <T> PagedResponse<T> getPagedResponse(Criteria crit, int firstResult, int maxResults) {
        rowCounter.setCalcFoundRows(true);
        try {
            @SuppressWarnings("unchecked")
            return new PagedResponse<T>(
                crit.
                setFirstResult(firstResult).
                setMaxResults(maxResults).
                list(),
                rowCounter.getFoundRows());
        } finally {
            rowCounter.reset();
        }
    }

    public <T> PagedResponse<T> getPagedResponse(Query query, int firstResult, int maxResults) {
        rowCounter.setCalcFoundRows(true);
        try {
            @SuppressWarnings("unchecked")
            return new PagedResponse<T>(
                query.
                setFirstResult(firstResult).
                setMaxResults(maxResults).
                list(),
                rowCounter.getFoundRows());
        } finally {
            rowCounter.reset();
        }
    }
}

然后是一个名为MyEntity的@Entity的具体DAO类示例,其String属性为"prop":

package my.hibernate.classes;

import org.hibernate.SessionFactory;
import org.hibernate.criterion.Restrictions
import org.springframework.beans.factory.annotation.Autowired;

public class MyEntityDAO extends BaseDAO {

    @Autowired
    private SessionFactory sessionFactory;

    public PagedResponse<MyEntity> getPagedEntitiesWithPropertyValue(String propVal, int firstResult, int maxResults) {
        return getPagedResponse(
            sessionFactory.
            getCurrentSession().
            createCriteria(MyEntity.class).
            add(Restrictions.eq("prop", propVal)),
            firstResult, 
            maxResults);
    }
}

最后是执行所有工作的拦截器类:

package my.hibernate.classes;

import java.io.IOException;
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

import org.hibernate.EmptyInterceptor;
import org.hibernate.HibernateException;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.jdbc.Work;
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.BeanFactoryAware;

public class MySQLCalcFoundRowsInterceptor extends EmptyInterceptor implements BeanFactoryAware {



    /**
     * 
     */
    private static final long serialVersionUID = 2745492452467374139L;

    //
    // Private statics
    //

    private final static String SELECT_PREFIX = "select ";

    private final static String CALC_FOUND_ROWS_HINT = "SQL_CALC_FOUND_ROWS ";

    private final static String SELECT_FOUND_ROWS = "select FOUND_ROWS()";

    //
    // Private members
    //
    private SessionFactory sessionFactory;

    private BeanFactory beanFactory;

    private String sessionFactoryBeanName;

    private ThreadLocal<Boolean> mCalcFoundRows = new ThreadLocal<Boolean>();

    private ThreadLocal<Integer> mSQLStatementsPrepared = new ThreadLocal<Integer>() {
        @Override
        protected Integer initialValue() {
            return Integer.valueOf(0);
        }
    };

    private ThreadLocal<Integer> mFoundRows = new ThreadLocal<Integer>();



    private void init() {
        if (sessionFactory == null) {
            if (sessionFactoryBeanName != null) {
                sessionFactory = beanFactory.getBean(sessionFactoryBeanName, SessionFactory.class);
            } else {
                try {
                    sessionFactory = beanFactory.getBean("sessionFactory", SessionFactory.class);
                } catch (RuntimeException exp) {

                }
                if (sessionFactory == null) {
                    sessionFactory = beanFactory.getBean(SessionFactory.class); 
                }
            }
        }
    }

    @Override
    public String onPrepareStatement(String sql) {
        if (mCalcFoundRows.get() == null || !mCalcFoundRows.get().booleanValue()) {
            return sql;
        }
        switch (mSQLStatementsPrepared.get()) {

        case 0: {
            mSQLStatementsPrepared.set(mSQLStatementsPrepared.get() + 1);

            // First time, prefix CALC_FOUND_ROWS_HINT

            StringBuilder builder = new StringBuilder(sql);
            int indexOf = builder.indexOf(SELECT_PREFIX);

            if (indexOf == -1) {
                throw new HibernateException("First SQL statement did not contain '" + SELECT_PREFIX + "'");
            }

            builder.insert(indexOf + SELECT_PREFIX.length(), CALC_FOUND_ROWS_HINT);
            return builder.toString();
        }

        case 1: {
            mSQLStatementsPrepared.set(mSQLStatementsPrepared.get() + 1);

            // Before any secondary selects, capture FOUND_ROWS. If no secondary
            // selects are
            // ever executed, getFoundRows() will capture FOUND_ROWS
            // just-in-time when called
            // directly

            captureFoundRows();
            return sql;
        }

        default:
            // Pass-through untouched
            return sql;
        }
    }

    public void reset() {
        if (mCalcFoundRows.get() != null && mCalcFoundRows.get().booleanValue()) {
            mSQLStatementsPrepared.remove();
            mFoundRows.remove();
            mCalcFoundRows.remove();
        }
    }

    @Override
    public void afterTransactionCompletion(Transaction tx) {
        reset();
    }

    public void setCalcFoundRows(boolean calc) {
        if (calc) {
            mCalcFoundRows.set(Boolean.TRUE);
        } else {
            reset();
        }
    }

    public int getFoundRows() {
        if (mCalcFoundRows.get() == null || !mCalcFoundRows.get().booleanValue()) {
            throw new IllegalStateException("Attempted to getFoundRows without first calling 'setCalcFoundRows'");
        }
        if (mFoundRows.get() == null) {
            captureFoundRows();
        }

        return mFoundRows.get();
    }

    //
    // Private methods
    //

    private void captureFoundRows() {
        init();

        // Sanity checks

        if (mFoundRows.get() != null) {
            throw new HibernateException("'" + SELECT_FOUND_ROWS + "' called more than once");
        }

        if (mSQLStatementsPrepared.get() < 1) {
            throw new HibernateException("'" + SELECT_FOUND_ROWS + "' called before '" + SELECT_PREFIX + CALC_FOUND_ROWS_HINT + "'");
        }

        // Fetch the total number of rows

        sessionFactory.getCurrentSession().doWork(new Work() {
            @Override
            public void execute(Connection connection) throws SQLException {
                final Statement stmt = connection.createStatement();
                ResultSet rs = null;
                try {
                    rs = stmt.executeQuery(SELECT_FOUND_ROWS);
                    if (rs.next()) {
                        mFoundRows.set(rs.getInt(1));
                    } else {
                        mFoundRows.set(0);
                    }
                } finally {
                    if (rs != null) {
                        rs.close();
                    }
                    try {
                        stmt.close();
                    } catch (RuntimeException exp) {

                    }
                }
            }
        });
    }

    public void setSessionFactoryBeanName(String sessionFactoryBeanName) {
        this.sessionFactoryBeanName = sessionFactoryBeanName;
    }

    @Override
    public void setBeanFactory(BeanFactory arg0) throws BeansException {
        this.beanFactory = arg0;
    }

}

Answer 3

如果您不需要显示总页数,那么我不确定您是否需要计数查询.包括谷歌在内的很多网站都没有在分页结果中显示总数.相反,他们只是说"下一步".