Hibernate bidirectional 1-n (detailed analysis)

• Bidirectional 1-n and bidirectional n-1 are exactly the same two situations
• Bidirectional 1-n need to be at one end of the 1 to access the end of N, and vice versa.
• Domain Model: A Many-to-one bidirectional association from order to customer needs to define a customer property in the Order class, and in the Customer class, you define the collection property that holds the order object
  
  
• Relational data Model: The CUSTOMER_ID in the ORDERS table references the primary key of the CUSTOMER table
  
  
• Attention:

When a

Session loads a Java collection from a database, it creates an instance of the Hibernate built-in collection class, so you must declare the property as a Java interface type when defining the collection property in a persisted class

Hibernate's built-in collection classes have a collection agent feature that supports deferred retrieval strategies in fact, Hibernate's built-in collection classes encapsulate the collection classes in the JDK, which allows Hibernate to dirty-check the collection objects in the cache and update the database synchronously by the state of the collection object.

When you define a collection property, you typically initialize it to an instance of the collection implementation class. This can improve the robustness of the program and prevent the application from accessing a null-valued collection of methods thrown NullPointerException

• Hibernate uses the <set> element to map the properties of a set type
  
  
• Set
  
• Key
• One-to-many:
  
• 
  

Customer.java

Package Com.atguigu.hibernate.entities.n21.both;import Java.util.hashset;import Java.util.set;public class Customer  {Private Integer customerid;private String customername;/* * 1. When declaring a collection type, the interface type is used, because hibernate returns hibernate when it gets the * collection type Built-in collection type, rather than javase a standard * collection implementation.  * 2. The collection needs to be initialized to prevent null pointer exceptions */private set<order> orders = new hashset<> ();p ublic Integer Getcustomerid () {return CustomerId;} public void Setcustomerid (Integer customerId) {this.customerid = customerId;} Public String Getcustomername () {return customerName;} public void Setcustomername (String customerName) {this.customername = CustomerName;} Public set<order> getorders () {return orders;} public void Setorders (set<order> orders) {this.orders = orders;}}

Order.java

Package Com.atguigu.hibernate.entities.n21.both;public class Order {private Integer orderid;private String ordername; Private Customer customer;public Integer Getorderid () {return orderId;} public void Setorderid (Integer orderId) {this.orderid = orderId;} Public String Getordername () {return ordername;} public void Setordername (String ordername) {this.ordername = Ordername;} Public Customer GetCustomer () {return customer;} public void Setcustomer (customer customer) {This.customer = customer;}}

Customer.hbm.xml

<?xml version= "1.0"? ><! DOCTYPE hibernate-mapping Public "-//hibernate/hibernate mapping DTD 3.0//en" "http://hibernate.sourceforge.net/ Hibernate-mapping-3.0.dtd ">
Order.hbm.xml
<?xml version= "1.0"? ><! DOCTYPE hibernate-mapping Public "-//hibernate/hibernate mapping DTD 3.0//en" "http://hibernate.sourceforge.net/ Hibernate-mapping-3.0.dtd ">

Package Com.atguigu.hibernate.entities.n21.both;import Org.hibernate.session;import org.hibernate.SessionFactory; Import Org.hibernate.transaction;import Org.hibernate.cfg.configuration;import Org.hibernate.service.serviceregistry;import Org.hibernate.service.serviceregistrybuilder;import Org.junit.After ; Import Org.junit.before;import Org.junit.test;public class Hibernatetest {private Sessionfactory sessionfactory; Private Session session;private Transaction Transaction; @Beforepublic void init () {Configuration configuration = new Configuration (). Configure ();                            Serviceregistry serviceregistry = new Serviceregistrybuilder (). Applysettings (Configuration.getproperties ()) . Buildserviceregistry (); sessionfactory = Configuration.buildsessionfactory (serviceregistry); session = Session Factory.opensession (); transaction = Session.begintransaction ();} @Afterpublic void Destroy () {transaction.commit (); Session.close (); Sessionfactory.close ();} @Testpublic void Testcascade () {Customer CUStomer = (Customer) session.get (Customer.class, 3); Customer.getorders (). Clear (); @Testpublic void Testdelete () {//In the case that there is no cascading relationship, and the object at this end of 1 has an object of n being referenced, you cannot directly delete the object at the end of the 1 customer customer = (customer) session. Get (Customer.class, 1); Session.delete (Customer); } @Testpublic void TestUpdat2 () {Customer customer = (customer) session.get (Customer.class, 1); Customer.getorders (). Iterator (). Next (). Setordername ("GGG"); } @Testpublic void Testupdate () {Order order = (order) Session.get (Order.class, 1); Order.getcustomer (). Setcustomername ( "AAA");} @Testpublic void Testone2manyget () {//1. Use lazy loading for a collection of one end of n the customer customer = (customer) session.get (Customer.class, 7); System.out.println (Customer.getcustomername ()); 2. Returns a collection of multiple ends when Hibernate is built into the collection type. This type has the ability to delay loading and storing proxy objects. System.out.println (Customer.getorders (). GetClass ()); Session.close ();//3. Lazyinitializationexception exceptions may be thrown System.out.println (Customer.getorders (). Size ()); 4. You need to initialize the elements in the collection again. } @Testpublic void Testmany2oneget () {//1. If you query an object at one end of a multiple, the defaultCase, only the objects at one end of the query are queried. Without querying the object associated with that end of the//1! Order order = (order) Session.get (Order.class, 1); System.out.println (Order.getordername ()); System.out.println (Order.getcustomer (). GetClass (). GetName ()); Session.close ();//2. The corresponding SQL statement is sent when the associated object needs to be used. Customer customer = Order.getcustomer (); System.out.println (Customer.getcustomername ()); 3. When querying the Customer object, the lazyinitializationexception exception//4 will occur by default//If the one end of the multiple is navigated to one end of 1//If the session is closed at this time. When you get the Order object, by default, its associated Customer object is a proxy object!} @Testpublic void Testmany2onesave () {Customer customer = new Customer (); Customer.setcustomername ("AA"); Order order1 = New Order (); Order1.setordername ("ORDER-1"); Order Order2 = New Order (); Order2.setordername ("ORDER-2");// Sets the association relationship order1.setcustomer (customer); Order2.setcustomer (customer); Customer.getorders (). Add (Order1); Customer.getorders (). Add (order2);//Perform the save operation: First insert Customer, then insert Order, 3 inserts, 2 update//because one end of 1 and the other side of N maintain the correlation relationship. So there will be more update//. You can specify inverse=true on the set node at one end of the 1 to give the end of the 1 to the Maintenance association!//it is recommended that set inVerse=true, it is recommended to first insert one end of the 1, and then insert many of the end//benefit is not more than the UPDATE statement session.save (customer);//session.save (order1);//session.save ( ORDER2);//First insert Order, then insert Cusomer, 3 Insert, 4 update//session.save (order1);//session.save (order2);////session.save ( customer);}}