Saving Collections in Hibernate with Example Show
What is collection mapping in Hibernate?Hibernate supports collection mapping as value type. In this mapping, the collection are mapped into a separate table. When we have to persist property of a class, which is of data type like integer, long, float etc. or objects of wrapper classes or Strings, these properties are stored in the table of class directly. What ways are available to you to map an object to database entities in Hibernate?Declaring Entity Mappings There are two ways: (1) using annotations in Java code, and (2) using XML files using either JPA standard format or Hibernate hbm. How do I save a JPA Spring data list?In Spring Data JPA Repository is a top-level interface in hierarchy. The saveAll() method has been defined as below. Iterable saveAll(Iterable entities) – used to save multiple entities. The saveAll() method internally annotated with @Transactional. What are the collection types in Hibernate?The persistent collections injected by Hibernate behave like HashMap , HashSet , TreeMap , TreeSet or ArrayList , depending on the interface type. Can we use JPA without hibernate?So for question – Can we use JPA without hibernate? The answer is no. We can’t use JPA alone. Note – We can’t use JPA without Hibernate, EclipseLink or any other framework. What is difference between save and saveAll in JPA?In our case, each time we call the save() method, a new transaction is created, whereas when we call saveAll(), only one transaction is created, and it’s reused later by save(). How to fetch multiple entities by id with Hibernate?With JPA and older Hibernate versions, you have 2 options to fetch multiple entities from the database: You call the EntityManager.find method for each primary key which forces Hibernate to execute a query for each of them or How does hibernate load all entities from the database?Hibernate loads all entities from the database and doesn’t check if some of them are already stored in the 1st level cache. You can, of course, handle all these issues in your code but that adds additional complexity and disturbs you from implementing the business logic. What do you mean by list mappings in hibernate?Hibernate – List Mappings – A List is a java collection that stores elements in sequence and allow duplicate elements. The user of this interface has precise control over where in the list When to use the list-index element in hibernate?The element is used to keep the position of the element and map with the index column in the collection table. The index of the persistent list starts at zero. You could change this, for example, with in your mapping. How do you save an object in hibernate?Hibernate save() method. Both save() methods take a transient object reference (which must not be null) as an argument. Second method takes an extra parameter ‘entityName’ which is useful in case you have mapped multiple entities to a Java class. Here you can specify which entity you are saving using save() method. How do you make a detached object persistence in hibernate?What is the proper way to re-attach detached objects in Hibernate…
How do you save a detached entity in JPA?1 of the JPA 1.0 spec) explicitly states that an EntityExistsException is thrown by the persist method when the object passed in is a detached entity….Putting it all together
What is @ElementCollection in Hibernate?In Hibernate Annotation, @ElementCollection is the feature which gets the columns values from another table without mapping two tables. We have taken two entity student and college. In college entity, we will fetch students without mapping student and college entity. What is the difference between GET and load in hibernate?In hibernate, get() and load() are two methods which is used to fetch data for the given identifier. Get() method return null, If no row is available in the session cache or the database for the given identifier whereas load() method throws object not found exception. How do I know if my entity is detached?2 Answers. To check if the given entity is managed by the current PersistenceContext you can use the EntityManager#contains(Object entity) . The above assumes that em is the EntityManager , Entity is the entity class and has a public id field that is a @GeneratedValue primary key. What is detached entity in hibernate?A detached entity is just an ordinary entity POJO whose identity value corresponds to a database row. The difference from a managed entity is that it’s not tracked anymore by any persistence context. An entity can become detached when the Session used to load it was closed, or when we call Session. A List is a java collection that stores elements in sequence and allow duplicate elements. The user of this interface has precise control over where in the list, each element is inserted. The user can access elements by their integer index, and search for elements in the list. More formally, lists typically allow pairs of elements e1 and e2 such that e1.equals(e2), and they typically allow multiple null elements if they allow null elements at all. A List is mapped with a <list> element in the mapping table and initialized with java.util.ArrayList. Define RDBMS TablesConsider a situation where we need to store our employee records in EMPLOYEE table, which will have the following structure − create table EMPLOYEE ( id INT NOT NULL auto_increment, first_name VARCHAR(20) default NULL, last_name VARCHAR(20) default NULL, salary INT default NULL, PRIMARY KEY (id) );Further, assume each employee can have one or more certificate associated with him/her. A List collection mapping needs an index column in the collection table. The index column defines the position of the element in the collection. So, we will store certificate related information in a separate table having the following structure − create table CERTIFICATE ( id INT NOT NULL auto_increment, certificate_name VARCHAR(30) default NULL, idx INT default NULL, employee_id INT default NULL, PRIMARY KEY (id) );There will be one-to-many relationship between EMPLOYEE and CERTIFICATE objects. Define POJO ClassesLet us implement a POJO class Employee, which will be used to persist the objects related to EMPLOYEE table and having a collection of certificates in List variable. import java.util.*; public class Employee { private int id; private String firstName; private String lastName; private int salary; private List certificates; public Employee() {} public Employee(String fname, String lname, int salary) { this.firstName = fname; this.lastName = lname; this.salary = salary; } public int getId() { return id; } public void setId( int id ) { this.id = id; } public String getFirstName() { return firstName; } public void setFirstName( String first_name ) { this.firstName = first_name; } public String getLastName() { return lastName; } public void setLastName( String last_name ) { this.lastName = last_name; } public int getSalary() { return salary; } public void setSalary( int salary ) { this.salary = salary; } public List getCertificates() { return certificates; } public void setCertificates( List certificates ) { this.certificates = certificates; } }We need to define another POJO class corresponding to CERTIFICATE table so that certificate objects can be stored and retrieved into the CERTIFICATE table. public class Certificate{ private int id; private String name; public Certificate() {} public Certificate(String name) { this.name = name; } public int getId() { return id; } public void setId( int id ) { this.id = id; } public String getName() { return name; } public void setName( String name ) { this.name = name; } }Define Hibernate Mapping FileLet us develop our mapping file, which instructs Hibernate how to map the defined classes to the database tables. The <list> element will be used to define the rule for List collection used. The index of list is always of type integer and is mapped using the <list-index> element. <?xml version = "1.0" encoding = "utf-8"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"> <hibernate-mapping> <class name = "Employee" table = "EMPLOYEE"> <meta attribute = "class-description"> This class contains the employee detail. </meta> <id name = "id" type = "int" column = "id"> <generator class="native"/> </id> <list name = "certificates" cascade="all"> <key column = "employee_id"/> <list-index column = "idx"/> <one-to-many class="Certificate"/> </list> <property name = "firstName" column = "first_name" type = "string"/> <property name = "lastName" column = "last_name" type = "string"/> <property name = "salary" column = "salary" type = "int"/> </class> <class name = "Certificate" table = "CERTIFICATE"> <meta attribute = "class-description"> This class contains the certificate records. </meta> <id name = "id" type = "int" column = "id"> <generator class="native"/> </id> <property name = "name" column = "certificate_name" type = "string"/> </class> </hibernate-mapping>You should save the mapping document in a file with the format <classname>.hbm.xml. We saved our mapping document in the file Employee.hbm.xml. You are already familiar with most of the mapping detail, but let us see all the elements of mapping file once again −
Create Application ClassFinally, we will create our application class with the main() method to run the application. We will use this application to save few Employee's records along with their certificates and then we will apply CRUD operations on those records. import java.util.*; import org.hibernate.HibernateException; import org.hibernate.Session; import org.hibernate.Transaction; import org.hibernate.SessionFactory; import org.hibernate.cfg.Configuration; public class ManageEmployee { private static SessionFactory factory; public static void main(String[] args) { try { factory = new Configuration().configure().buildSessionFactory(); } catch (Throwable ex) { System.err.println("Failed to create sessionFactory object." + ex); throw new ExceptionInInitializerError(ex); } ManageEmployee ME = new ManageEmployee(); /* Let us have a set of certificates for the first employee */ ArrayList set1 = new ArrayList(); set1.add(new Certificate("MCA")); set1.add(new Certificate("MBA")); set1.add(new Certificate("PMP")); /* Add employee records in the database */ Integer empID1 = ME.addEmployee("Manoj", "Kumar", 4000, set1); /* Another set of certificates for the second employee */ ArrayList set2 = new ArrayList(); set2.add(new Certificate("BCA")); set2.add(new Certificate("BA")); /* Add another employee record in the database */ Integer empID2 = ME.addEmployee("Dilip", "Kumar", 3000, set2); /* List down all the employees */ ME.listEmployees(); /* Update employee's salary records */ ME.updateEmployee(empID1, 5000); /* Delete an employee from the database */ ME.deleteEmployee(empID2); /* List down all the employees */ ME.listEmployees(); } /* Method to add an employee record in the database */ public Integer addEmployee(String fname, String lname, int salary, ArrayList cert){ Session session = factory.openSession(); Transaction tx = null; Integer employeeID = null; try { tx = session.beginTransaction(); Employee employee = new Employee(fname, lname, salary); employee.setCertificates(cert); employeeID = (Integer) session.save(employee); tx.commit(); } catch (HibernateException e) { if (tx!=null) tx.rollback(); e.printStackTrace(); } finally { session.close(); } return employeeID; } /* Method to list all the employees detail */ public void listEmployees( ){ Session session = factory.openSession(); Transaction tx = null; try { tx = session.beginTransaction(); List employees = session.createQuery("FROM Employee").list(); for (Iterator iterator1 = employees.iterator(); iterator1.hasNext();){ Employee employee = (Employee) iterator1.next(); System.out.print("First Name: " + employee.getFirstName()); System.out.print(" Last Name: " + employee.getLastName()); System.out.println(" Salary: " + employee.getSalary()); List certificates = employee.getCertificates(); for (Iterator iterator2 = certificates.iterator(); iterator2.hasNext();){ Certificate certName = (Certificate) iterator2.next(); System.out.println("Certificate: " + certName.getName()); } } tx.commit(); } catch (HibernateException e) { if (tx!=null) tx.rollback(); e.printStackTrace(); } finally { session.close(); } } /* Method to update salary for an employee */ public void updateEmployee(Integer EmployeeID, int salary ){ Session session = factory.openSession(); Transaction tx = null; try { tx = session.beginTransaction(); Employee employee = (Employee)session.get(Employee.class, EmployeeID); employee.setSalary( salary ); session.update(employee); tx.commit(); } catch (HibernateException e) { if (tx!=null) tx.rollback(); e.printStackTrace(); } finally { session.close(); } } /* Method to delete an employee from the records */ public void deleteEmployee(Integer EmployeeID){ Session session = factory.openSession(); Transaction tx = null; try { tx = session.beginTransaction(); Employee employee = (Employee)session.get(Employee.class, EmployeeID); session.delete(employee); tx.commit(); } catch (HibernateException e) { if (tx!=null) tx.rollback(); e.printStackTrace(); } finally { session.close(); } } }Compilation and ExecutionHere are the steps to compile and run the above application. Make sure you have set PATH and CLASSPATH appropriately before proceeding for the compilation and execution.
You would get the following result on the screen, and same time records would be created in EMPLOYEE and CERTIFICATE tables. You can see, certificates have been sorted in reverse order. You can try by changing your mapping file, simply set sort="natural" and execute your program and compare the results. $java ManageEmployee .......VARIOUS LOG MESSAGES WILL DISPLAY HERE........ First Name: Manoj Last Name: Kumar Salary: 4000 Certificate: MCA Certificate: MBA Certificate: PMP First Name: Dilip Last Name: Kumar Salary: 3000 Certificate: BCA Certificate: BA First Name: Manoj Last Name: Kumar Salary: 5000 Certificate: MCA Certificate: MBA Certificate: PMPIf you check your EMPLOYEE and CERTIFICATE tables, they should have the following records − mysql> select * from EMPLOYEE; +----+------------+-----------+--------+ | id | first_name | last_name | salary | +----+------------+-----------+--------+ | 51 | Manoj | Kumar | 5000 | +----+------------+-----------+--------+ 1 row in set (0.00 sec) mysql> select * from CERTIFICATE; +----+------------------+------+-------------+ | id | certificate_name | idx | employee_id | +----+------------------+------+-------------+ | 6 | MCA | 0 | 51 | | 7 | MBA | 1 | 51 | | 8 | PMP | 2 | 51 | +----+------------------+------+-------------+ 3 rows in set (0.00 sec mysql>Alternatively, you could map a Java array instead of a list. An array mapping is virtually identical to the previous example, except with different element and attribute names (<array> and <array-index>). However, for reasons explained earlier, Hibernate applications rarely use arrays. hibernate_or_mappings.htm |
