java持久化jpa笔记

@Entity//标识这个pojo是一个jpa实体@Table(name = "users") //指定表名为userspublic class Users implements Serializable {@Id//设置主键private String userId;//设置字段类型通过@Column注解设置，包含的设置如下//name：字段名//unique：是否唯一//nullable：是否可以为空//inserttable：是否可以插入//updateable：是否可以更新//columnDefinition: 定义建表时创建此列的DDL//secondaryTable: 从表名。如果此列不建在主表上（默认建在主表），该属性定义该列所在从表的名字。@Column(name = "user_code", nullable = false, length=32)//设置属性userCode对应的字段为user_code，长度为32，非空private String userCode;@Column(name = "user_wages", nullable = true, precision=12, scale=2)//设置属性wages对应的字段为user_wages，12位数字可保留两位小数，可以为空private double wages;@Temporal(TemporalType.DATE)//设置为时间类型private Date joinDate;@OrderBy(name = "group_name ASC, name DESC")//[字段排序]在加载数据的时候可以为其指定顺序，使用@OrderBy注解实现private List books = new ArrayList();}

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//主键生成策略public class Users implements Serializable {    @Id    @GeneratedValue(strategy=GenerationType.IDENTITY)//主键自增，注意，这种方式依赖于具体的数据库，如果数据库不支持自增主键，那么这个类型是没法用的    @Column(name = "user_id", nullable = false)    private int userId;}public class Users implements Serializable {    @Id    @GeneratedValue(strategy=GenerationType.TABLE)//通过一个表来实现主键id的自增，这种方式不依赖于具体的数据库，可以解决数据迁移的问题    @Column(name = "user_code", nullable = false)    private String userCode;}public class Users implements Serializable {    @Id    @GeneratedValue(strategy=GenerationType.SEQUENCE)//通过Sequence来实现表主键自增，这种方式依赖于数据库是否有SEQUENCE，如果没有就不能用    @SequenceGenerator(name="seq_user")    @Column(name = "user_id", nullable = false)    private int userId; }

//一对多映射关系//有T_One和T_Many两个表，他们是一对多的关系，注解范例如下//主Pojo@Entity@Table(name = "T_ONE")public class One implements Serializable {private static final long serialVersionUID = 1L;@Id@Column(name = "ONE_ID", nullable = false)private String oneId;@Column(name = "DESCRIPTION")private String description;@OneToMany(cascade = CascadeType.ALL, mappedBy = "oneId")//指向多的那方的pojo的关联外键字段   private Collection<Many> manyCollection;   //子Pojo@Entity@Table(name = "T_MANY")public class Many implements Serializable {private static final long serialVersionUID = 1L;@Id@Column(name = "MANY_ID", nullable = false)private String manyId;@Column(name = "DESCRIPTION")private String description;@JoinColumn(name = "ONE_ID", referencedColumnName = "ONE_ID")//设置对应数据表的列名和引用的数据表的列名@ManyToOne//设置在“一方”pojo的外键字段上private One oneId; 

//多对多映射关系//貌似多对多关系不需要设置级联，以前用hibernate的时候着实为多对多的级联头疼了一阵子，JPA的多对多还需要实际的尝试一下才能有所体会。//估计JPA的多对多也是可以转换成两个一对多的。//第一个Pojo@Entity@Table(name = "T_MANYA")public class ManyA implements Serializable {private static final long serialVersionUID = 1L;@Id@Column(name = "MANYA_ID", nullable = false)private String manyaId;@Column(name = "DESCRIPTION")private String description;@ManyToMany@JoinTable(name = "TMANY1_TMANY2", joinColumns = {@JoinColumn(name = "MANYA_ID", referencedColumnName = "MANYA_ID")}, inverseJoinColumns = {@JoinColumn(name = "MANYB_ID", referencedColumnName = "MANYB_ID")})private Collection<ManyB> manybIdCollection;   //第二个Pojo@Entity@Table(name = "T_MANYB")public class ManyB implements Serializable {private static final long serialVersionUID = 1L;@Id@Column(name = "MANYB_ID", nullable = false)private String manybId;@Column(name = "DESCRIPTION")private String description;@ManyToMany(mappedBy = "manybIdCollection")private Collection<ManyA> manyaIdCollection;

//一对一映射关系//主Pojo@Entity@Table(name = "T_ONEA")public class OneA implements Serializable {private static final long serialVersionUID = 1L;@Id@Column(name = "ONEA_ID", nullable = false)private String oneaId;@Column(name = "DESCRIPTION")private String description;@OneToOne(cascade = CascadeType.ALL, mappedBy = "oneA")//主Pojo这方的设置比较简单，只要设置好级联和映射到从Pojo的外键就可以了。  private OneB oneB;//从Pojo@Entity@Table(name = "T_ONEB")public class OneB implements Serializable {private static final long serialVersionUID = 1L;@Id@Column(name = "ONEA_ID", nullable = false)private String oneaId;@Column(name = "DESCRIPTION")private String description;@JoinColumn(name = "ONEA_ID", referencedColumnName = "ONEA_ID", insertable = false, updatable = false)//设置从方指向主方的关联外键，这个ONEA_ID其实是表T_ONEA的主键@OneToOneprivate OneA oneA;

//大字段[图像２制制]@Lob //对应Blob字段类型@Column(name = "PHOTO")private Serializable photo;@Lob //对应Clob字段类型@Column(name = "DESCRIPTION")private String description;

//瞬时字段[不需要与数据库映射的字段，在保存的时候不需要保存倒数据库]@Transientprivate int tempValue;public int getTempValue(){return tempValue;}public void setTempValue(int value){this.tempValue = value;}