Hibernate supports two primary methods for configuring object-relational mappings: XML-based configuration and annotation-based configuration. This article focuses on the annotation approach, which offers a more concise and integrated solution. Startnig from Hibernate 4, annotation support is included by default; for Hibernate 3, you need to add the annotation JARs separately.

Basic Entity Mapping

To map a simple entity, use annotations to define the database table and fields. Here's an example with a Customer class:

@Entity
@Table(name = "customer_table")
public class Customer {
    private Integer customerId;
    private String customerName;
    private String email;
    private LocalDate signupDate;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "customer_id")
    public Integer getCustomerId() {
        return customerId;
    }

    public void setCustomerId(Integer customerId) {
        this.customerId = customerId;
    }

    @Column(name = "customer_name", length = 100, nullable = false)
    public String getCustomerName() {
        return customerName;
    }

    public void setCustomerName(String customerName) {
        this.customerName = customerName;
    }

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

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

    @Column(name = "signup_date")
    public LocalDate getSignupDate() {
        return signupDate;
    }

    public void setSignupDate(LocalDate signupDate) {
        this.signupDate = signupDate;
    }
}

In your hibernate.cfg.xml, include the entity class:

<mapping class="com.example.model.Customer"/>

One-to-Many Relationship

For a one-to-many relasionship, such as between Department and Employee, define the mapping as follows:

Department class:

@Entity
@Table(name = "department")
public class Department {
    private Integer deptId;
    private String deptName;
    private Set<Employee> employees = new HashSet<>();

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "dept_id")
    public Integer getDeptId() {
        return deptId;
    }

    public void setDeptId(Integer deptId) {
        this.deptId = deptId;
    }

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

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

    @OneToMany(mappedBy = "department")
    @LazyCollection(LazyCollectionOption.EXTRA)
    public Set<Employee> getEmployees() {
        return employees;
    }

    public void setEmployees(Set<Employee> employees) {
        this.employees = employees;
    }
}

Employee class:

@Entity
@Table(name = "employee")
public class Employee {
    private Integer empId;
    private String empName;
    private Department department;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "emp_id")
    public Integer getEmpId() {
        return empId;
    }

    public void setEmpId(Integer empId) {
        this.empId = empId;
    }

    @Column(name = "emp_name")
    public String getEmpName() {
        return empName;
    }

    public void setEmpName(String empName) {
        this.empName = empName;
    }

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "dept_id")
    public Department getDepartment() {
        return department;
    }

    public void setDepartment(Department department) {
        this.department = department;
    }
}

One-to-One Relationship

For a one-to-one relationship, such as between User and Profile, map it like this:

User class:

@Entity
@Table(name = "user_table")
public class User {
    private Integer userId;
    private String userName;
    private Profile profile;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "user_id")
    public Integer getUserId() {
        return userId;
    }

    public void setUserId(Integer userId) {
        this.userId = userId;
    }

    @Column(name = "user_name")
    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName;
    }

    @OneToOne(mappedBy = "user")
    public Profile getProfile() {
        return profile;
    }

    public void setProfile(Profile profile) {
        this.profile = profile;
    }
}

Profile class:

@Entity
@Table(name = "profile")
public class Profile {
    private Integer profileId;
    private String bio;
    private User user;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "profile_id")
    public Integer getProfileId() {
        return profileId;
    }

    public void setProfileId(Integer profileId) {
        this.profileId = profileId;
    }

    @Column(name = "bio")
    public String getBio() {
        return bio;
    }

    public void setBio(String bio) {
        this.bio = bio;
    }

    @OneToOne
    @JoinColumn(name = "user_id")
    public User getUser() {
        return user;
    }

    public void setUser(User user) {
        this.user = user;
    }
}

Many-to-Many Relationship

For a many-to-many relationship, such as between Product and Category, you can use a join table:

Product class:

@Entity
@Table(name = "product")
public class Product {
    private Integer productId;
    private String productName;
    private Set<Category> categories = new HashSet<>();

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "product_id")
    public Integer getProductId() {
        return productId;
    }

    public void setProductId(Integer productId) {
        this.productId = productId;
    }

    @Column(name = "product_name")
    public String getProductName() {
        return productName;
    }

    public void setProductName(String productName) {
        this.productName = productName;
    }

    @ManyToMany
    @JoinTable(name = "product_category",
               joinColumns = @JoinColumn(name = "product_id"),
               inverseJoinColumns = @JoinColumn(name = "category_id"))
    public Set<Category> getCategories() {
        return categories;
    }

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

Category class:

@Entity
@Table(name = "category")
public class Category {
    private Integer categoryId;
    private String categoryName;
    private Set<Product> products = new HashSet<>();

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "category_id")
    public Integer getCategoryId() {
        return categoryId;
    }

    public void setCategoryId(Integer categoryId) {
        this.categoryId = categoryId;
    }

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

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

    @ManyToMany(mappedBy = "categories")
    public Set<Product> getProducts() {
        return products;
    }

    public void setProducts(Set<Product> products) {
        this.products = products;
    }
}

Alternatively, you can decompose a many-to-many into two one-to-many relationships using an intermediate entity, such as Order and Item with OrderItem:

Order class:

@Entity
@Table(name = "orders")
public class Order {
    private Integer orderId;
    private String orderNumber;
    private Set<OrderItem> orderItems = new HashSet<>();

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "order_id")
    public Integer getOrderId() {
        return orderId;
    }

    public void setOrderId(Integer orderId) {
        this.orderId = orderId;
    }

    @Column(name = "order_number")
    public String getOrderNumber() {
        return orderNumber;
    }

    public void setOrderNumber(String orderNumber) {
        this.orderNumber = orderNumber;
    }

    @OneToMany(mappedBy = "order")
    public Set<OrderItem> getOrderItems() {
        return orderItems;
    }

    public void setOrderItems(Set<OrderItem> orderItems) {
        this.orderItems = orderItems;
    }
}

Item class:

@Entity
@Table(name = "item")
public class Item {
    private Integer itemId;
    private String itemName;
    private Set<OrderItem> orderItems = new HashSet<>();

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "item_id")
    public Integer getItemId() {
        return itemId;
    }

    public void setItemId(Integer itemId) {
        this.itemId = itemId;
    }

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

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

    @OneToMany(mappedBy = "item")
    public Set<OrderItem> getOrderItems() {
        return orderItems;
    }

    public void setOrderItems(Set<OrderItem> orderItems) {
        this.orderItems = orderItems;
    }
}

OrderItem class:

@Entity
@Table(name = "order_item")
public class OrderItem {
    private Integer orderItemId;
    private Integer quantity;
    private Order order;
    private Item item;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "order_item_id")
    public Integer getOrderItemId() {
        return orderItemId;
    }

    public void setOrderItemId(Integer orderItemId) {
        this.orderItemId = orderItemId;
    }

    @Column(name = "quantity")
    public Integer getQuantity() {
        return quantity;
    }

    public void setQuantity(Integer quantity) {
        this.quantity = quantity;
    }

    @ManyToOne
    @JoinColumn(name = "order_id")
    public Order getOrder() {
        return order;
    }

    public void setOrder(Order order) {
        this.order = order;
    }

    @ManyToOne
    @JoinColumn(name = "item_id")
    public Item getItem() {
        return item;
    }

    public void setItem(Item item) {
        this.item = item;
    }
}

When insetring data, save the entity that does not define the foreign key first, then the one that does. For example, in a one-to-one relationship:

User user = new User();
user.setUserName("john_doe");
session.save(user);

Profile profile = new Profile();
profile.setBio("Software developer");
profile.setUser(user);
session.save(profile);