與構造

Question

我從GlassFish3。所以開始的Java EE6嘗試一些例子的流程困惑，我創建了一個實體類，它看起來像這樣...

@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); 
    } 

} 

，我試圖堅持它像這樣...

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，但是當我跑，我看到一個輸出如下...

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，爲什麼有這麼多的默認構造函數調用時，沒有一個由我做... ...？

有人可以解釋我在樣品中的流動情況嗎？

Answer 1

JPA使用不帶參數的構造函數來實例化實體，然後將這些實體中的字段綁定到對應的映射表和列。

你看到的那些輸出是JPA每次處理實體時爲你做的調用。