JPA查询实体部分字段_JAVA_编程开发_程序员俱乐部

JPA本地查询中如果只想查询出实体的部分字段,可以通过返回方式为列模式的方法来实现！
首先建表(Oracle)：

create table NATIVEENTITY
(
  u_id     VARCHAR2(255) not null,
  u_email  VARCHAR2(255),
  password VARCHAR2(255),
  u_name   VARCHAR2(255)
)

首先是实体的定义

package com.test.jpa;

import javax.persistence.Column;
import javax.persistence.ColumnResult;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.NamedNativeQueries;
import javax.persistence.NamedNativeQuery;
import javax.persistence.SqlResultSetMapping;
import javax.persistence.SqlResultSetMappings;

@NamedNativeQueries({
	@NamedNativeQuery(
		name = "getNativeEntity",
		query = "select u_id,u_name,u_email from NativeEntity",
		resultSetMapping = "ReturnColumnEntityList"
	)
})

@SqlResultSetMappings({
	@SqlResultSetMapping(
		name = "ReturnColumnEntityList",
		entities = {},
		columns = {
			@ColumnResult(name = "u_id"),
			@ColumnResult(name = "u_name"),
			@ColumnResult(name = "u_email")
		}
	)
})
@Entity
public class NativeEntity {
	private String tid;
	
	private String username;
	
	private String password;
	
	private String email;

	public NativeEntity() {
	}

	public NativeEntity(String tid, String username) {
		super();
		this.tid = tid;
		this.username = username;
	}

	public NativeEntity(String tid, String username, String email) {
		super();
		this.tid = tid;
		this.username = username;
		this.email = email;
	}

	@Column(name = "u_email")
	public String getEmail() {
		return email;
	}

	public String getPassword() {
		return password;
	}

	@Id
	@Column(name="u_id")
	@GeneratedValue
	public String getTid() {
		return tid;
	}

	@Column(name="u_name")
	public String getUsername() {
		return username;
	}

	public void setEmail(String email) {
		this.email = email;
	}

	@Column
	public void setPassword(String password) {
		this.password = password;
	}

	public void setTid(String tid) {
		this.tid = tid;
	}

	public void setUsername(String username) {
		this.username = username;
	}
	
}

接下来是查询

package com.test.jpa;

import java.lang.reflect.Constructor;
import java.util.ArrayList;
import java.util.List;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.Query;
public class NativeSql {
	
	@SuppressWarnings("unchecked")
	public static void main(String[] args) throws Exception {
		EntityManagerFactory factory = Persistence.createEntityManagerFactory("manager1");
		EntityManager entityManager = factory.createEntityManager();
		Query query = entityManager.createNamedQuery("getNativeEntity");
		
		List<Object[]> list = (List<Object[]>)query.getResultList();
		
		List<NativeEntity> neList = castEntity(list, NativeEntity.class);
		for(NativeEntity ne : neList){
			System.out.println(ne.getTid() + ":" + ne.getUsername());
		}
		
//		persistEntity(factory, entityManager);
	}

	/**
	 * 通用实体转换方法,将JPA返回的数组转化成对应的实体集合,这里通过泛型和反射实现
	 * @param <T>
	 * @param list
	 * @param clazz 需要转化后的类型
	 * @return 
	 * @throws Exception
	 */
	@SuppressWarnings("unchecked")
	private static <T> List<T> castEntity(List<Object[]> list, Class<T> clazz) throws Exception {
		List<T> returnList = new ArrayList<T>();
		Object[] co = list.get(0);
		Class[]	c2 = new Class[co.length];
		
		//确定构造方法
		for(int i = 0; i < co.length; i++){
			c2[i] = co[i].getClass();
		}
		
		for(Object[] o : list){
			Constructor<T> constructor = clazz.getConstructor(c2);
			returnList.add(constructor.newInstance(o));
		}
		
		return returnList;
	}
	
	/**
	 * 保存实体
	 * @param factory
	 * @param entityManager
	 */
	@SuppressWarnings("unused")
	private static void persistEntity(EntityManagerFactory factory,
			EntityManager entityManager) {
		entityManager.getTransaction().begin();
		NativeEntity ne = new NativeEntity();
		ne.setUsername("lisi");
		ne.setPassword("3344");
		ne.setEmail("liuyong_0204@hotmail.com");
		
		entityManager.persist(ne);
		entityManager.getTransaction().commit();
		entityManager.close();
		factory.close();
	}
}

需要注意的几个点是:
1:需要查询的字段必须有相对应的构造方法,并且要一一对应，
比如说:

@ColumnResult(name = "u_id"),
@ColumnResult(name = "u_name"),
@ColumnResult(name = "u_email")

对应的构造方法必须是

public NativeEntity(String tid, String username, String email) {
		super();
		this.tid = tid;
		this.username = username;
		this.email = email;
	}

顺序不能改变!!!!!!
2:如果@SqlResultSetMappings中使用的返回模式为实体，是不能查询出实体的部分字段！必须指定全部实体字段……太恶心了！