Map映射
Map映射是一个java集合存储在键 - 值对的元素,并且不允许在列表中重复的元素。 Map接口提供三种collection视图,允许Map内容看作是一组键-值集合,或者设置键 - 值映射关系。
Map被映射到映射表中一个<map>元素和无序的地图可以在java.util.HashMap中被初始化。
定义RDBMS表:
考虑一个情况,我们需要员工记录存储在EMPLOYEE表,将有以下结构:
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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) ); |
此外,假设每个员工都可以有一个或多个与他/她相关的证书。我们将存储证书的相关信息在一个单独的表,该表具有以下结构:
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create table CERTIFICATE ( id INT NOT NULL auto_increment, certificate_type VARCHAR (40) default NULL , certificate_name VARCHAR (30) default NULL , employee_id INT default NULL , PRIMARY KEY (id) ); |
有一个对多(one-to-many )的关系在EMPLOYEE和证书对象之间。
定义POJO类:
让我们实现一个POJO类员工将被用于保存与EMPLOYEE表中的对象和有证书的列表变量的集合。
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import java.util.*; public class Employee { private int id; private String firstName; private String lastName; private int salary; private Map 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 Map getCertificates() { return certificates; } public void setCertificates( Map certificates ) { this .certificates = certificates; } } |
我们需要相应的证书表定义另一个POJO类,这样的证书对象可以存储和检索到的证书表。
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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; } } |
定义Hibernate映射文件:
让我们开发指示Hibernate如何定义的类映射到数据库表的映射文件。<map>元素将被用于定义所使用的映射的规则。
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<? 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 > < map name = "certificates" cascade = "all" > < key column = "employee_id" /> < index column = "certificate_type" type = "string" /> < one-to-many class = "Certificate" /> </ map > < 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 > |
应该保存的映射文件中的格式<classname>.hbm.xml。我们保存映射文件中的文件Employee.hbm.xml。已经熟悉了大部分的映射细节,但让我们再次看映射文件中的所有元素:
映射文档是具有<hibernate-mapping>为对应于每一个类包含2个<class>元素的根元素的XML文档。
<class>元素被用于定义数据库表从一个Java类特定的映射。 Java类名指定使用class元素的name属性和使用表属性数据库表名指定。
<meta>元素是可选元素,可以用来创建类的描述。
<id>元素映射在类中的唯一ID属性到数据库表的主键。 id元素的name属性是指属性的类和column属性是指在数据库表中的列。 type属性保存了Hibernate映射类型,这种类型的映射将会从Java转换为SQL数据类型。
id元素内的<generator>元素被用来自动生成的主键值。将生成元素的class属性设置为原产于让Hibernate拿起无论是identity,sequence或者hilo中的算法来创建主键根据底层数据库的支持能力。
<property>元素用于一个Java类的属性映射到数据库表中的列。元素的name属性是指属性的类和column属性是指在数据库表中的列。 type属性保存了Hibernate映射类型,这种类型的映射将会从Java转换为SQL数据类型。
<map>元素用于设置证书和Employee类之间的关系。我们使用cascade属性的<map>元素来告诉Hibernate来保存证书的对象,同时为Employee对象。 name属性设置为定义Mapvariable在父类中,在我们的案例中是证书。
<index>的元素用于表示键/值对地图的关键部位。该键将使用一个字符串类型的存储在列certificate_type。
<key>元素是包含外键的父对象,即在证书表中的列。表EMPLOYEE。
<one-to-many>元素表示一个Employee对象涉及到很多证书的对象,并因此,证书对象必须有与Employee父类有关。可以根据需要使用<one-to-one>,<many-to-one>或<many-to-many>这个元素。
创建应用程序类:
最后,创建应用程序类的main()方法来运行应用程序。使用这个应用程序来保存员工记录地连同证书列表,然后在应用上执行CRUD操作上的记录。
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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 */ HashMap set = new HashMap(); set.put("ComputerScience", new Certificate("MCA")); set.put("BusinessManagement", new Certificate("MBA")); set.put("ProjectManagement", new Certificate("PMP")); /* Add employee records in the database */ Integer empID = ME.addEmployee("Manoj", "Kumar", 4000, set); /* List down all the employees */ ME.listEmployees(); /* Update employee's salary records */ ME.updateEmployee(empID, 5000); /* 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, HashMap 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()); Map ec = employee.getCertificates(); System.out.println("Certificate: " + (((Certificate)ec.get("ComputerScience")).getName())); System.out.println("Certificate: " + (((Certificate)ec.get("BusinessManagement")).getName())); System.out.println("Certificate: " + (((Certificate)ec.get("ProjectManagement")).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(); } } } |
编译和执行:
下面是步骤来编译并运行上述应用程序。请确保您已在进行的编译和执行之前,已经适当地设置PATH和CLASSPATH。
- 创建hibernate.cfg.xml配置文件,在配置章节已经解释和学习。
- 创建Employee.hbm.xml映射文件,如上图所示。
- 创建Employee.java源文件,如上图所示,并编译它。
- 创建Certificate.java源文件,如上图所示,并编译它。
- 创建ManageEmployee.java源文件,如上图所示,并编译它。
- 执行ManageEmployee二进制文件来运行程序。
会在屏幕上获得以下结果,并同时创建记录在员工和证书表。
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$java ManageEmployee |
.......VARIOUS LOG MESSAGES WILL DISPLAY HERE........
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First Name: Manoj Last Name: Kumar Salary: 4000 Certificate: MCA Certificate: MBA Certificate: PMP First Name: Manoj Last Name: Kumar Salary: 5000 Certificate: MCA Certificate: MBA Certificate: PMP |
如果检查员工和证书表,就应该记录下了:
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mysql> select * from EMPLOYEE; |
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+----+------------+-----------+--------+ | id | first_name | last_name | salary | +----+------------+-----------+--------+ | 60 | Manoj | Kumar | 5000 | +----+------------+-----------+--------+ 1 row in set (0.00 sec) |
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mysql> select * from CERTIFICATE; |
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+----+--------------------+------------------+-------------+ | id | certificate_type | certificate_name | employee_id | +----+--------------------+------------------+-------------+ | 16 | ProjectManagement | PMP | 60 | | 17 | BusinessManagement | MBA | 60 | | 18 | ComputerScience | MCA | 60 | +----+--------------------+------------------+-------------+ 3 rows in set (0.00 sec) |
SortedMap映射
SortedMap是存储在键 - 值对的元素,并提供键的总体排序类似java的集合作为映射。重复的元素未在映射不允许的。该映射是根据其键的自然顺序进行排序,或者通过提供通常在有序映射的创建时间比较。
SortedMap被映射在该映射表中的<map>元素和有序映射可以在java.util.TreeMap中被初始化。
我们依然使用上面定义的RDBMS表和POJO类来讲下面的例子:
定义Hibernate映射文件:
让我们开发指示Hibernate如何定义的类映射到数据库表的映射文件。<map>元素将被用于定义所使用的映射的规则。
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<? 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 > < map name = "certificates" cascade = "all" sort = "MyClass" > < key column = "employee_id" /> < index column = "certificate_type" type = "string" /> < one-to-many class = "Certificate" /> </ map > < 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 > |
应该保存的映射文件中的格式<classname>.hbm.xml。我们保存映射文件Employee.hbm.xml。前面已经熟悉了大部分的映射细节,但让我们再次看看映射文件中的所有元素:
映射文档是具有<hibernate-mapping>对应于每一个类包含2个<class>元素的根元素的XML文档。
<class>元素被用于定义数据库表从一个Java类特定的映射。 Java类名指定使用class元素的name属性和使用表属性数据库表名指定。
<meta>元素是可选元素,可以用来创建类的描述。
<id>元素映射在类中的唯一ID属性到数据库表的主键。 id元素的name属性是指属性的类和column属性是指在数据库表中的列。 type属性保存了Hibernate映射类型,这种类型的映射将会从Java转换为SQL数据类型。
id元素内的<generator>元素被用来自动生成的主键值。将生成元素的class属性设置让Hibernate对应于identity,sequence或者hilo中的算法来创建主键根据底层数据库的支持能力。
<property>元素用于一个Java类的属性映射到数据库表中的列。元素的name属性是指属性的类和column属性是指在数据库表中的列。 type属性保存了Hibernate映射类型,这种类型的映射将会从Java转换为SQL数据类型。
<map>元素用于设置证书和Employee类之间的关系。我们使用cascade属性的<map>元素来告诉Hibernate来保存证书的对象,同时为Employee对象。 name属性被设置为在父类中的定义的SortedMap变量,在我们的情况下,它是证书。排序属性可以设置为自然有自然排序,也可以设置为自定义类实现为java.util.Comparator。我们已经使用了一个类MyClass,它实现为java.util.Comparator扭转证书类实现的排序顺序。
<index>元素用于表示键/值对映射的键部分。该键将使用一个字符串类型的存储在列certificate_type。
<key>元素是包含外键的父对象,即在证书表中的列。表EMPLOYEE。
<one-to-many>元素表示一个Employee对象涉及到很多证书的对象,并因此,证书对象必须有与Employee父有关联。可以根据需要使用<one-to-one>,<many-to-one>或<many-to-many>这个元素。
如果使用sort="natural",然后我们并不需要创建一个单独的类,因为证书类已经实现了Comparable接口和hibernate会使用compareTo在证书类定义为比较的SortedMap()方法。但是,我们使用的是在我们的映射文件自定义的比较器类MyClass,所以我们必须创建这个类的基础上我们的排序算法。让我们做降在映射上可用的按键排序。
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import java.util.Comparator; public class MyClass implements Comparator <String>{ public int compare(String o1, String o2) { final int BEFORE = - 1 ; final int AFTER = 1 ; /* To reverse the sorting order, multiple by -1 */ if (o2 == null ) { return BEFORE * - 1 ; } Comparable thisCertificate = o1; Comparable thatCertificate = o2; if (thisCertificate == null ) { return AFTER * 1 ; } else if (thatCertificate == null ) { return BEFORE * - 1 ; } else { return thisCertificate.compareTo(thatCertificate) * - 1 ; } } } |
最后,我们将创建应用程序类的main()方法来运行应用程序。我们将使用这个应用程序,以节省一些员工的记录地连同的证书,然后我们将提交CRUD操作上面的记录。
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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 */ TreeMap set1 = new TreeMap(); set1.put("ComputerScience", new Certificate("MCA")); set1.put("BusinessManagement", new Certificate("MBA")); set1.put("ProjectManagement", 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 */ TreeMap set2 = new TreeMap(); set2.put("ComputerScience", new Certificate("MCA")); set2.put("BusinessManagement", new Certificate("MBA")); /* 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, TreeMap 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()); SortedMap<String, Certificate> map = employee.getCertificates(); for(Map.Entry<String,Certificate> entry : map.entrySet()){ System.out.print("\tCertificate Type: " + entry.getKey()); System.out.println(", Name: " + (entry.getValue()).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(); } } } |
编译和执行:
可以看到证书已排序顺序相反。可以通过改变映射文件试试,只需设置sort="natural"和执行程序,并比较结果。
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$java ManageEmployee |
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.......VARIOUS LOG MESSAGES WILL DISPLAY HERE........ First Name: Manoj Last Name: Kumar Salary: 4000 Certificate Type: ProjectManagement, Name: PMP Certificate Type: ComputerScience, Name: MCA Certificate Type: BusinessManagement, Name: MBA First Name: Dilip Last Name: Kumar Salary: 3000 Certificate Type: ComputerScience, Name: MCA Certificate Type: BusinessManagement, Name: MBA First Name: Manoj Last Name: Kumar Salary: 5000 Certificate Type: ProjectManagement, Name: PMP Certificate Type: ComputerScience, Name: MCA Certificate Type: BusinessManagement, Name: MBA |
如果检查员工和证书表,就应该记录下了:
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mysql> select * from EMPLOYEE; |
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+----+------------+-----------+--------+ | id | first_name | last_name | salary | +----+------------+-----------+--------+ | 74 | Manoj | Kumar | 5000 | +----+------------+-----------+--------+ 1 row in set (0.00 sec) |
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mysql> select * from CERTIFICATE; |
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+----+--------------------+------------------+-------------+ | id | certificate_type | certificate_name | employee_id | +----+--------------------+------------------+-------------+ | 52 | BusinessManagement | MBA | 74 | | 53 | ComputerScience | MCA | 74 | | 54 | ProjectManagement | PMP | 74 | +----+--------------------+------------------+-------------+ 3 rows in set (0.00 sec) |