怎么使用EntityManager获得记录数

如下所示：

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首先是要获得jpa的entityManager:

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import javax.persistence.EntityManagerFactory;import javax.persistence.Persistence;public class JPAUtil {    private static EntityManagerFactory entityManagerFactory;    static {        try {            entityManagerFactory =                 Persistence.createEntityManagerFactory("mmsbackup");        }        catch(Throwable ex) {            throw new ExceptionInInitializerError(ex);        }    }     public static EntityManagerFactory getEntityManagerFactory() {        return entityManagerFactory;    }     public static void shutdown() {        getEntityManagerFactory().close();    }}

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然后编写实体：

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package net.kentop.mmsbackend.entity;import java.io.Serializable;import java.util.Date;import java.util.HashSet;import java.util.Set;import javax.persistence.CascadeType;import javax.persistence.Column;import javax.persistence.Entity;import javax.persistence.FetchType;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.Lob;import javax.persistence.OneToMany;import javax.persistence.Table;import javax.persistence.Temporal;import javax.persistence.Transient;import net.kentop.common.util.BlankUtil;import org.hibernate.annotations.GenericGenerator;/** * smil文件实体 * @author lhx1026 * */@Entity@Table(name="SMIL_FILES")public class SmilFile implements Serializable{/** * serialVersionUID */private static final long serialVersionUID = 1025423394267546744L;private String mmsid;private String title;private byte[] smil;private Date createDate;private String startDate;private String endDate;private Set<SmilMedia> medias = new HashSet<SmilMedia>();@Id@Column(length = 32)@GenericGenerator(name = "uuid", strategy = "uuid.hex")@GeneratedValue(generator = "uuid")    public String getMmsid() {return mmsid;}public void setMmsid(String mmsid) {this.mmsid = mmsid;}@Column(nullable=false)public String getTitle() {return title;}public void setTitle(String title) {this.title = title;}@Lob@Column(nullable=false)public byte[] getSmil() {return smil;}public void setSmil(byte[] smil) {this.smil = smil;}@Column(nullable=false)public Date getCreateDate() {return createDate;}public void setCreateDate(Date createDate) {this.createDate = createDate;}@Transientpublic String getStartDate() {return startDate;}public void setStartDate(String startDate) {this.startDate = startDate;}@Transientpublic String getEndDate() {return endDate;}public void setEndDate(String endDate) {this.endDate = endDate;}@OneToMany(fetch=FetchType.LAZY,mappedBy="smilFile",cascade={CascadeType.ALL})public Set<SmilMedia> getMedias() {return medias;}public void setMedias(Set<SmilMedia> medias) {this.medias = medias;}public SmilFile(){}public int hashCode() { return mmsid.hashCode();}public boolean equals(Object obj) {if (BlankUtil.isBlank(obj)) {return false;}if (this == obj) {return true;}if (!(obj instanceof SmilFile)) {return false;}SmilFile smil = (SmilFile) obj;if (BlankUtil.isBlank(this.mmsid) || BlankUtil.isBlank(smil.getMmsid())) {return false;}return this.mmsid.equals(smil.getMmsid());}public String toString() {return "id:"+mmsid+",title"+title;}}

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编写dao层中的获得记录数的方法：

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public static Long getCountsByConditions(SmilFile file){StringBuilder builder = new StringBuilder().append("select count(*) from SmilFile po where 1=1");if(!BlankUtil.isBlank(file)){if(!BlankUtil.isBlank(file.getTitle())){builder.append(" and po.title = :title");}}EntityManager em = JPAUtil.getEntityManagerFactory().createEntityManager();Query query = em.createQuery(builder.toString());if(!BlankUtil.isBlank(file.getTitle())){query.setParameter("title",file.getTitle());}Long counts = (Long)query.getSingleResult();//就是这个getSingleResult()方法em.close();return counts;}

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