In Hibernate, there are usually two ways to configure object-relational mapping, one is based on xml, the other is based on annotation of Hibernate Annotation library. In Hibernate version 3.2 and Java 5 environment, annotations are used to configure object-relational mapping. After using Hibernate annotations, the *.hbm.xml file corresponding to the persistent class can be written directly into the persistent class by annotations instead of defining the corresponding *.hbm.xml file.

Two differences between the two methods should be noted:
(1): In the hibernate.hbm.xml file, change the reference: x xxx.hbm.xml to the reference entity class:

(2): The way to obtain SessionFactory has changed:

Commonly used annotation labels

In the use of attribute annotations, you can maintain the persistence of both fields (annotations are written on member variables) and attributes (annotations are written on getter methods), but it is recommended not to introduce annotations on attributes, because attributes are private, if the introduction of annotations will destroy their encapsulation characteristics, so it is recommended that Add annotations to the getter method

@Entity

@Table

@Entity()
@Table(name="Student")
public class Student implements Serializable{
}

@Id

@GeneratedValue

@Id
@GeneratedValue(strategy=GenerationType.IDENTITY) 
private int sid;

Use of GenerationType.TABLE

@Id  
@GeneratedValue(strategy=GenerationType.TABLE,generator="strategyName")   
@TableGenerator(
    name = "strategyName",   // The name of the primary key generation policy, which is referenced in the "generator" value set in @GeneratedValue
    table="",   //Represents the table name persisted by the table generation strategy   
    catalog="",   //The directory name of the table
    schema="",   //Database name
    pkColumnName="",   //In the persistence table, the name of the key value corresponding to the primary key generation policy
    valueColumnName="",   //Represents the value currently generated by the primary key in the persistence table, whose value will accumulate with each creation  
    pkColumnValue="",   //Represents the primary key corresponding to this generation strategy in the persistence table  
    allocationSize=1   //Indicates the size of each increase in the primary key value, for example, set to 1, which automatically adds 1 after each creation of a new record. The default is 50.
)  

Use of Generation Type.SEQUENCE

@Id  
@GeneratedValue(strategy = GenerationType.SEQUENCE,generator="strategyName")   
@SequenceGenerator(
    name="strategyName", //The name attribute represents the name of the table's primary key generation policy, which is referenced in the "generator" value set in @GeneratedValue
    sequenceName=""  //Represents the name of the database sequence used for the generation policy
) 

More annotation-based Hibernate primary key generation strategies

@Transient

@Column

@Version

Some attributes

fetch

cascade

mappedBy

One-to-many Association

@OneToMany

@OneToMany(cascade={CascadeType.ALL},mappedBy="grade")
    public Set<Student> getStudents() {
        return students;
    }

Many-to-one Association

@ManyToOne

Bidirectional one-to-many and bidirectional many-to-one

/*
 * Student Responsibility Association
 */
@Entity
@Table(name="Student")
public class Student implements Serializable{
    private int sid;
    private String sex;
    private String name;
    private Grade grade;
    public Student() {

    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    @Id
    @GenericGenerator(name="generator",strategy="native")
    @GeneratedValue(generator="generator")
    public int getId() {
        return sid;
    }
    public void setId(int id) {
        this.sid = id;
    }
    @Column(name="sex")
    public String getSex() {
        return sex;
    }
    public void setSex(String sex) {
        this.sex = sex;
    }
    @ManyToOne(cascade={CascadeType.ALL})
    @JoinColumn(name="gid")
    public Grade getGrade() {
        return grade;
    }
    public void setGrade(Grade grade) {
        this.grade = grade;
    }
}

/*
 * Class Class
 */
@Entity
@Table(name="Grade")
public class Grade implements Serializable{
    private int gid;
    private String name;
    private Set<Student> students=new HashSet<Student>();
    public Grade() {
    }
    @Id
    @GenericGenerator(name="generator",strategy="native")
    @GeneratedValue(generator="generator")
    public int getGid() {
        return gid;
    }
    public void setGid(int gid) {
        this.gid = gid;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    @OneToMany(cascade={CascadeType.ALL},mappedBy="grade")
    public Set<Student> getStudents() {
        return students;
    }
    public void setStudents(Set<Student> students) {
        this.students = students;
    }
}

Many-to-many Association

 @JoinTable(
     name="student_course",
     joinColumns={@JoinColumn(name="")},
     inverseJoinColumns={@JoinColumn(name="")}
)

On the basis of the former, we should add courses and supplement students'classes.

/*
 * Student Responsibility Association
 */
@Entity
@Table(name="Student")
public class Student implements Serializable{
    private int sid;
    private String name;
    private String sex;
    private Grade grade;
    private Set<Course> courses=new HashSet<Course>();
    public Student() {

    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    @Id
    @GenericGenerator(name="generator",strategy="native")
    @GeneratedValue(generator="generator")
    public int getId() {
        return sid;
    }
    public void setId(int id) {
        this.sid = id;
    }
    @Column(name="sex")
    public String getSex() {
        return sex;
    }
    public void setSex(String sex) {
        this.sex = sex;
    }
    @ManyToOne(cascade={CascadeType.ALL})
    @JoinColumn(name="gid")
    public Grade getGrade() {
        return grade;
    }
    public void setGrade(Grade grade) {
        this.grade = grade;
    }
    @ManyToMany(cascade={CascadeType.ALL},fetch=FetchType.EAGER)
    @JoinTable(name="student_course",joinColumns={@JoinColumn(name="sid")},inverseJoinColumns={@JoinColumn(name="cid")})
    public Set<Course> getCourses() {
        return courses;
    }
    public void setCourses(Set<Course> courses) {
        this.courses = courses;
    }
}

@Entity
@Table(name="Course")
public class Course implements Serializable{

    private int cid;
    private String cName;
    private Set<Student> students=new HashSet<Student>();
    public Course() {
    }
    @Id
    @GenericGenerator(name="generator",strategy="native")
    @GeneratedValue(generator="generator")
    public int getCid() {
        return cid;
    }
    public void setStudents(Set<Student> students) {
        this.students = students;
    }
    public void setCid(int cid) {
        this.cid = cid;
    }
    public String getcName() {
        return cName;
    }
    public void setcName(String cName) {
        this.cName = cName;
    }
    @ManyToMany(cascade={CascadeType.ALL},fetch=FetchType.LAZY ,mappedBy="courses") 
    public Set<Student> getStudents() {
        return students;
    }
}