Question

我正在尝试使用GlassFish3开始Java EE6的一些示例。所以，我创建了一个基本上看起来像这样的实体类......

@Entity
@Table(name="Book")
public class Book implements Serializable
{
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;
    @Column(nullable=false)
    private String name;
    @Column(nullable=false)
    private String isbn;
    private String description;

    public Book()
    {
        // Empty constructor to facilitate construction. 
        System.out.println("The variables have not been initialized...Please initialize them using the Setters or use the provided constructor");
    }

    public Book(String name, String isbn, String description) {
        this.name = name;
        this.isbn = isbn;
        this.description = description;
    }

    public String getIsbn() {
        return isbn;
    }

    public void setIsbn(String isbn) {
        this.isbn = isbn;
    }

    public String getName() {
        return name;
    }

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

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }


    @Override
    public String toString() {
        return this.name + " - " + this.isbn; 
    }

    @PrePersist
    public void printPrePersist(){
        System.out.println("Persisting the book "+this.name);
    }
    @PostPersist
    public void printPostPersist(){
        System.out.println("Persisted the book "+this.name);
    }

}

我试图坚持这样......

public class MainClass 
{
    public static void main(String[] args){
        Book book = new Book("Effective Java","ISBN - 1234415","A very good book on Java");
        Book book2 = new Book("Learning Java EE","ISBN - 1233415","A good book for Java EE beginners");

        // These are the necessary classes
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("PersistenceAppPU");
        EntityManager em = emf.createEntityManager();

        // Persist the book here
        EntityTransaction etx = em.getTransaction();
        etx.begin();
        em.persist(book);
        em.persist(book2);
        etx.commit();

        em.close();
        emf.close();

        System.out.println("The two books have been persisted");
    }
}

它仍然存在，但是当我跑步时，我看到的输出就像...

The variables have not been initialized...Please initialize them using the Setters or use the provided constructor
Persisting the book Effective Java
The variables have not been initialized...Please initialize them using the Setters or use the provided constructor
Persisting the book Learning Java EE
Persisted the book Learning Java EE
Persisted the book Effective Java
The variables have not been initialized...Please initialize them using the Setters or use the provided constructor
The variables have not been initialized...Please initialize them using the Setters or use the provided constructor
The variables have not been initialized...Please initialize them using the Setters or use the provided constructor
The variables have not been initialized...Please initialize them using the Setters or use the provided constructor
[EL Info]: 2012-05-10 12:01:19.623--ServerSession(17395905)--file:/C:/Users/raviteja.s/Documents/NetBeansProjects/PersistenceApp/src/_PersistenceAppPU logout successful
The two books have been persisted

我不明白，为什么有这么多默认的构造函数调用时，我没有一个...？

有人可以解释一下我的样本流量是怎样的吗？

Answer 1

JPA使用不带参数的构造函数来实例化您的实体，然后将这些实体中的字段绑定到对应的映射表和列。

您看到的那些输出是JPA每次操作您的实体时为您执行的调用。

与构造函数的流程混淆

1 个答案: