QueryDSL介绍

1，QueryDSL仅仅是一个通用的查询框架，专注于通过Java API构建类型安全的SQL查询。

2，Querydsl可以通过一组通用的查询API为用户构建出适合不同类型ORM框架或者是SQL的查询语句，也就是说QueryDSL是基于各种ORM框架以及SQL之上的一个通用的查询框架。

3，借助QueryDSL可以在任何支持的ORM框架或者SQL平台上以一种通用的API方式来构建查询。目前QueryDSL支持的平台包括JPA,JDO,SQL,Java Collections,RDF,Lucene,Hibernate Search。

实例代码：

 @Schema("PUBLIC")
 @Table("MEMBER")
 public class QMember extends RelationalPathBase {

     private static final long        serialVersionUID = 448912875;

     public static final QMember      member           = new QMember("MEMBER");

     public final StringPath          address          = createString("ADDRESS");

     public final StringPath          city             = createString("CITY");

     public final StringPath          country          = createString("COUNTRY");

     public final StringPath          email            = createString("EMAIL");

     public final StringPath          firstName        = createString("FIRST_NAME");

     public final StringPath          lastName         = createString("LAST_NAME");

     public final StringPath          memberId         = createString("MEMBER_ID");

     public final StringPath          password         = createString("PASSWORD");

     public final StringPath          phone            = createString("PHONE");

     public final PrimaryKey memberPk         = createPrimaryKey(memberId);

     public QMember(String variable){
         super(QMember.class, forVariable(variable));
     }

     public QMember(BeanPathextends QMember> entity){
         super(entity.getType(), entity.getMetadata());
     }

     public QMember(PathMetadata metadata){
         super(QMember.class, metadata);
     }

 }

构建查询

 private static void queryFromDsl(Connection conn) {
     QMember qm = QMember.member;

     SQLTemplates dialect = new HSQLDBTemplates();
     SQLQuery query = new SQLQueryImpl(conn, dialect);
     List lastNames = query.from(qm).list(qm.lastName);
     // select last_name from member
     System.out.println(lastNames);

     SQLQuery query2 = new SQLQueryImpl(conn, dialect);
     System.out.println(query2.from(qm).count());
     // select count(*) from member

     SQLQuery query3 = new SQLQueryImpl(conn, dialect);
     List results = query3.from(qm).orderBy(qm.lastName.desc()).list(qm.firstName, qm.lastName);
     for (Object[] o : results) {
         System.out.println(ToStringBuilder.reflectionToString(o, ToStringStyle.SIMPLE_STYLE));
     }
     // select first_name, last_name from member order by last_name desc

     SQLQuery query4 = new SQLQueryImpl(conn, dialect);
     List members = query4.from(qm).where(qm.lastName.like("last%")).list(qm.all());
     for (Object[] o : members) {
         System.out.println(ToStringBuilder.reflectionToString(o, ToStringStyle.SIMPLE_STYLE));
     }
     // select * from member where last_name like 'last%'
 }