大数据字段映射与字段延迟加载

当持久化类的属性为大字段时，String类型映射到数据库中最大的长度为255，满足不了需求，应该通过@Lob注解标注在String类型属性的get方法上，表示映射到数据库中为大文本类型；@Lob注解也可用于Byte[]类型，映射到数据库中为存放二进制数据类型

package cn.com.jyp.model;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Lob;
import javax.persistence.Table;

@Entity
@Table(name = "person")
public class Person {
	private Integer id;
	private String introduction;// 简介
	private Byte[] file;

	@Id
	@GeneratedValue
	public Integer getId() {
		return id;
	}

	public void setId(Integer id) {
		this.id = id;
	}

	@Lob
	public String getIntroduction() {
		return introduction;
	}

	public void setIntroduction(String introduction) {
		this.introduction = introduction;
	}

	@Lob
	public Byte[] getFile() {
		return file;
	}

	public void setFile(Byte[] file) {
		this.file = file;
	}

}

默认情况下，持久化类中的属性都会映射到数据库中。如果不想让持久化类中的某个属性映射到数据库，通过@Transient注解标注在属性的get方法上，表示该属性不被映射到数据库中。

package cn.com.jyp.model;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;
import javax.persistence.Transient;

@Entity
@Table(name = "person")
public class Person {
	private Integer id;
	private String name;
	private String address;

	@Id
	@GeneratedValue
	public Integer getId() {
		return id;
	}

	public void setId(Integer id) {
		this.id = id;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	@Transient
	public String getAddress() {
		return address;
	}

	public void setAddress(String address) {
		this.address = address;
	}

}

如果在加载一个持久化对象时，不想得到这个对象的大文本属性，可以通过@Basic(fetch = FetchType.LAZY)来实现，表示懒加载，查询对象时不查询该属性，只有在访问这个属性时才去查询数据库。

package cn.com.jyp.model;

import javax.persistence.Basic;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Lob;
import javax.persistence.Table;

@Entity
@Table(name = "person")
public class Person {
	private Integer id;
	private String name;
	private String introduction;// 简介

	@Id
	@GeneratedValue
	public Integer getId() {
		return id;
	}

	public void setId(Integer id) {
		this.id = id;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	@Lob
	@Basic(fetch = FetchType.LAZY)
	public String getIntroduction() {
		return introduction;
	}

	public void setIntroduction(String introduction) {
		this.introduction = introduction;
	}

}