Question

我有一个带对象关联的POJO类，如下所示。

当我执行我的主类时，得到Null指针异常。

有人请帮我理解这里出了什么问题。

第一个Pojo类

@Entity
@Table(name = "club")

public class Club implements Serializable{
     @Id
     @GeneratedValue
     @Column(name = "clubId")
    private Integer clubId;

     @Column(name="name")
    private String name;

     @OneToMany(fetch = FetchType.LAZY,mappedBy = "club")
        private Set<Team> team=new HashSet<Team>(0);

第二个Pojo类

@Entity
@Table(name="team")
public class Team implements Serializable{

    @Id
    @GeneratedValue
    @Column(name="teamid")
    private Integer teamId;

    @Column(name="teamname")
    private String teamName;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "clubid", nullable = false, insertable = false, updatable = false)
    private Club club;

主类：

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.util.List;
import java.util.Set;

import org.hibernate.Criteria;
import org.hibernate.FetchMode;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.criterion.Projection;
import org.hibernate.criterion.Projections;

public class CriteriaExecution {

    public static void main(String[] args) {

        Session session=HibernateUtil.getSessionFactory().openSession();
        //Transaction tx = session.beginTransaction();
        Criteria criteria = session.createCriteria(Club.class);
        criteria.setFetchMode("Team", FetchMode.JOIN);
            List list=criteria.list();
            for(int i=0;i<list.size();i++)
        {
            System.out.println("Values"+list.get(i));
        }
       // tx.commit();
        session.close();

    }

}

eclipse控制台的Stacktrace：

Exception in thread "main" java.lang.NullPointerException
    at com.mysql.jdbc.PreparedStatement.asSql(PreparedStatement.java:649)
    at com.mysql.jdbc.PreparedStatement.asSql(PreparedStatement.java:587)
    at com.mysql.jdbc.PreparedStatement.toString(PreparedStatement.java:4068)
    at java.lang.String.valueOf(Unknown Source)
    at java.lang.StringBuffer.append(Unknown Source)
    at com.mysql.jdbc.trace.Tracer.printParameters(Tracer.aj:240)
    at com.mysql.jdbc.trace.Tracer.printEntering(Tracer.aj:167)
    at com.mysql.jdbc.trace.Tracer.entry(Tracer.aj:126)
    at com.mysql.jdbc.trace.Tracer.ajc$before$com_mysql_jdbc_trace_Tracer$1$f51c62b8(Tracer.aj:45)
    at com.mysql.jdbc.Connection.registerStatement(Connection.java)
    at com.mysql.jdbc.Statement.<init>(Statement.java:270)
    at com.mysql.jdbc.PreparedStatement.<init>(PreparedStatement.java:500)
    at com.mysql.jdbc.Connection.clientPrepareStatement(Connection.java:2187)
    at com.mysql.jdbc.Connection.prepareStatement(Connection.java:4829)
    at com.mysql.jdbc.Connection.prepareStatement(Connection.java:4734)
    at org.hibernate.engine.jdbc.internal.StatementPreparerImpl$5.doPrepare(StatementPreparerImpl.java:161)
    at org.hibernate.engine.jdbc.internal.StatementPreparerImpl$StatementPreparationTemplate.prepareStatement(StatementPreparerImpl.java:182)
    at org.hibernate.engine.jdbc.internal.StatementPreparerImpl.prepareQueryStatement(StatementPreparerImpl.java:159)
    at org.hibernate.loader.Loader.prepareQueryStatement(Loader.java:1859)
    at org.hibernate.loader.Loader.executeQueryStatement(Loader.java:1836)
    at org.hibernate.loader.Loader.executeQueryStatement(Loader.java:1816)
    at org.hibernate.loader.Loader.doQuery(Loader.java:900)
    at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:342)
    at org.hibernate.loader.Loader.doList(Loader.java:2526)
    at org.hibernate.loader.Loader.doList(Loader.java:2512)
    at org.hibernate.loader.Loader.listIgnoreQueryCache(Loader.java:2342)
    at org.hibernate.loader.Loader.list(Loader.java:2337)
    at org.hibernate.loader.criteria.CriteriaLoader.list(CriteriaLoader.java:124)
    at org.hibernate.internal.SessionImpl.list(SessionImpl.java:1662)
    at org.hibernate.internal.CriteriaImpl.list(CriteriaImpl.java:374)
    at com.user.CriteriaExecution.main(CriteriaExecution.java:47)

Answer 1

您应该指明字段，而不是标准中的类型：

criteria.setFetchMode("team", FetchMode.JOIN);

空指针异常criteria.list（）

1 个答案: