Hibernate——Criteria

看完官方的文档提出几个问题

元组查询有什么优势？为什么要使用元组查询？

普通标准化查询和元组查询又有什么区别？

标准化查询是如何执行并生成的？

Criteria查询

实例查询

CriteriaBuilder builder = entityManager.getCriteriaBuilder();

CriteriaQuery<Person> criteria = builder.createQuery( Person.class );
Root<Person> root = criteria.from( Person.class );
criteria.select( root );
criteria.where( builder.equal( root.get( Person_.name ), "John Doe" ) );

List<Person> persons = entityManager.createQuery( criteria ).getResultList();

实体属性查询

CriteriaBuilder builder = entityManager.getCriteriaBuilder();

CriteriaQuery<String> criteria = builder.createQuery( String.class );
Root<Person> root = criteria.from( Person.class );
criteria.select( root.get( Person_.nickName ) );
criteria.where( builder.equal( root.get( Person_.name ), "John Doe" ) );

List<String> nickNames = entityManager.createQuery( criteria ).getResultList();

选择包装器查询（其实就是构造函数查询）

public class PersonWrapper {

    private final Long id;

    private final String nickName;

    public PersonWrapper(Long id, String nickName) {
        this.id = id;
        this.nickName = nickName;
    }

    public Long getId() {
        return id;
    }

    public String getNickName() {
        return nickName;
    }
}


CriteriaBuilder builder = entityManager.getCriteriaBuilder();

CriteriaQuery<PersonWrapper> criteria = builder.createQuery( PersonWrapper.class );
Root<Person> root = criteria.from( Person.class );

Path<Long> idPath = root.get( Person_.id );
Path<String> nickNamePath = root.get( Person_.nickName);

//builder.construct( PersonWrapper.class, idPath, nickNamePath )这个方法调用了构造函数
criteria.select( builder.construct( PersonWrapper.class, idPath, nickNamePath ) );
criteria.where( builder.equal( root.get( Person_.name ), "John Doe" ) );

List<PersonWrapper> wrappers = entityManager.createQuery( criteria ).getResultList();

在PersonWrapper 类中添加了构造函数并且使用生成器的builder.construct( PersonWrapper.class, idPath, nickNamePath )方法

生成并调用构造函数

使用元组进行查询

CriteriaBuilder builder = entityManager.getCriteriaBuilder();

CriteriaQuery<Tuple> criteria = builder.createQuery( Tuple.class );
Root<Person> root = criteria.from( Person.class );

Path<Long> idPath = root.get( Person_.id );
Path<String> nickNamePath = root.get( Person_.nickName);

criteria.multiselect( idPath, nickNamePath );
criteria.where( builder.equal( root.get( Person_.name ), "John Doe" ) );

List<Tuple> tuples = entityManager.createQuery( criteria ).getResultList();

for ( Tuple tuple : tuples ) {
	Long id = tuple.get( idPath );
	String nickName = tuple.get( nickNamePath );
}

//or using indices
for ( Tuple tuple : tuples ) {
	Long id = (Long) tuple.get( 0 );
	String nickName = (String) tuple.get( 1 );
}