一对一

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToOne;
import javax.persistence.Table;

@Table(name="JPA_DEPARTMENTS")
@Entity
public class Department {

    private Integer id;
    private String deptName;
    
    private Manager mgr;

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

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

    @Column(name="DEPT_NAME")
    public String getDeptName() {
        return deptName;
    }

    public void setDeptName(String deptName) {
        this.deptName = deptName;
    }

    //使用 @OneToOne 来映射 1-1 关联关系。
    //若需要在当前数据表中添加主键则需要使用 @JoinColumn 来进行映射. 注意, 1-1 关联关系, 所以需要添加 unique=true
    @JoinColumn(name="MGR_ID", unique=true)
    @OneToOne(fetch=FetchType.LAZY)
    public Manager getMgr() {
        return mgr;
    }

    public void setMgr(Manager mgr) {
        this.mgr = mgr;
    }
}

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.OneToOne;
import javax.persistence.Table;

@Table(name="JPA_MANAGERS")
@Entity
public class Manager {

    private Integer id;
    private String mgrName;
    
    private Department dept;

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

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

    @Column(name="MGR_NAME")
    public String getMgrName() {
        return mgrName;
    }

    public void setMgrName(String mgrName) {
        this.mgrName = mgrName;
    }

    //对于不维护关联关系, 没有外键的一方, 使用 @OneToOne 来进行映射, 建议设置 mappedBy=true
    @OneToOne(mappedBy="mgr")
    public Department getDept() {
        return dept;
    }

    public void setDept(Department dept) {
        this.dept = dept;
    }
}

一对多

import java.util.Date;
import java.util.HashSet;
import java.util.Set;

import javax.persistence.Cacheable;
import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.NamedQuery;
import javax.persistence.OneToMany;
import javax.persistence.Table;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;
import javax.persistence.Transient;
 
@NamedQuery(name="testNamedQuery", query="FROM Customer c WHERE c.id = ?")
@Cacheable(true)
@Table(name="JPA_CUTOMERS")
@Entity
public class Customer {

    private Integer id;
    private String lastName;

    private String email;
    private int age;
    
    private Date createdTime;
    private Date birth;
    
    public Customer() {
        // TODO Auto-generated constructor stub
    }
    
    public Customer(String lastName, int age) {
        super();
        this.lastName = lastName;
        this.age = age;
    }



    private Set<Order> orders = new HashSet<>();

//  @TableGenerator(name="ID_GENERATOR",
//          table="jpa_id_generators",
//          pkColumnName="PK_NAME",
//          pkColumnValue="CUSTOMER_ID",
//          valueColumnName="PK_VALUE",
//          allocationSize=100)
//  @GeneratedValue(strategy=GenerationType.TABLE,generator="ID_GENERATOR")
    @GeneratedValue(strategy=GenerationType.AUTO)
    @Id
    public Integer getId() {
        return id;
    }

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

    @Column(name="LAST_NAME",length=50,nullable=false)
    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public String getEmail() {
        return email;
    }

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

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
    
    @Temporal(TemporalType.TIMESTAMP)
    public Date getCreatedTime() {
        return createdTime;
    }

    public void setCreatedTime(Date createdTime) {
        this.createdTime = createdTime;
    }

    @Temporal(TemporalType.DATE)
    public Date getBirth() {
        return birth;
    }

    public void setBirth(Date birth) {
        this.birth = birth;
    }
    
    //映射单向 1-n 的关联关系
    //使用 @OneToMany 来映射 1-n 的关联关系
    //使用 @JoinColumn 来映射外键列的名称
    //可以使用 @OneToMany 的 fetch 属性来修改默认的加载策略
    //可以通过 @OneToMany 的 cascade 属性来修改默认的删除策略. 
    //注意: 若在 1 的一端的 @OneToMany 中使用 mappedBy 属性, 则 @OneToMany 端就不能再使用 @JoinColumn 属性了. 
//  @JoinColumn(name="CUSTOMER_ID")
    @OneToMany(fetch=FetchType.LAZY,cascade={CascadeType.REMOVE},mappedBy="customer")
    public Set<Order> getOrders() {
        return orders;
    }

    public void setOrders(Set<Order> orders) {
        this.orders = orders;
    }

    //工具方法. 不需要映射为数据表的一列. 
    @Transient
    public String getInfo(){
        return "lastName: " + lastName + ", email: " + email;
    }

    @Override
    public String toString() {
        return "Customer [id=" + id + ", lastName=" + lastName + ", email="
                + email + ", age=" + age + ", createdTime=" + createdTime
                + ", birth=" + birth + "]";
    }

}

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.Table;

@Table(name="JPA_ORDERS")
@Entity
public class Order {

    private Integer id;
    private String orderName;

    private Customer customer;

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

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

    @Column(name="ORDER_NAME")
    public String getOrderName() {
        return orderName;
    }

    public void setOrderName(String orderName) {
        this.orderName = orderName;
    }

    //映射单向 n-1 的关联关系
    //使用 @ManyToOne 来映射多对一的关联关系
    //使用 @JoinColumn 来映射外键. 
    //可使用 @ManyToOne 的 fetch 属性来修改默认的关联属性的加载策略
    @JoinColumn(name="CUSTOMER_ID")
    @ManyToOne(fetch=FetchType.LAZY)
    public Customer getCustomer() {
        return customer;
    }

    public void setCustomer(Customer customer) {
        this.customer = customer;
    }

}

多对多映射

@Table(name="JPA_CATEGORIES")
@Entity
public class Category {

    private Integer id;
    private String categoryName;
    
    private Set<Item> items = new HashSet<>();

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

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

    @Column(name="CATEGORY_NAME")
    public String getCategoryName() {
        return categoryName;
    }

    public void setCategoryName(String categoryName) {
        this.categoryName = categoryName;
    }

    @ManyToMany(mappedBy="categories")
    public Set<Item> getItems() {
        return items;
    }

    public void setItems(Set<Item> items) {
        this.items = items;
    }

import java.util.HashSet;
import java.util.Set;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

@Table(name="JPA_ITEMS")
@Entity
public class Item {

    private Integer id;
    private String itemName;
    
    private Set<Category> categories = new HashSet<>();

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

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

    @Column(name="ITEM_NAME")
    public String getItemName() {
        return itemName;
    }

    public void setItemName(String itemName) {
        this.itemName = itemName;
    }

    //使用 @ManyToMany 注解来映射多对多关联关系
    //使用 @JoinTable 来映射中间表
    //1. name 指向中间表的名字
    //2. joinColumns 映射当前类所在的表在中间表中的外键
    //2.1 name 指定外键列的列名
    //2.2 referencedColumnName 指定外键列关联当前表的哪一列
    //3. inverseJoinColumns 映射关联的类所在中间表的外键
    @JoinTable(name="ITEM_CATEGORY",
            joinColumns={@JoinColumn(name="ITEM_ID", referencedColumnName="ID")},
            inverseJoinColumns={@JoinColumn(name="CATEGORY_ID", referencedColumnName="ID")})
    @ManyToMany
    public Set<Category> getCategories() {
        return categories;
    }

    public void setCategories(Set<Category> categories) {
        this.categories = categories;
    }
}