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feature: Metadata Mapping pattern (#1932)

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Co-authored-by: Ilkka Seppälä <iluwatar@users.noreply.github.com>

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Co-authored-by: Ilkka Seppälä <iluwatar@users.noreply.github.com>
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---
layout: pattern
title: Metadata Mapping
folder: metadata-mapping
permalink: /patterns/metadata-mapping/
categories: Architectural
language: en
tags:
- Data access
---
## Intent
Holds details of object-relational mapping in the metadata.
## Explanation
Real world example
> Hibernate ORM Tool uses Metadata Mapping Pattern to specify the mapping between classes and tables either using XML or annotations in code.
In plain words
> Metadata Mapping specifies the mapping between classes and tables so that we could treat a table of any database like a Java class.
Wikipedia says
> Create a "virtual [object database](https://en.wikipedia.org/wiki/Object_database)" that can be used from within the programming language.
**Programmatic Example**
We give an example about visiting the information of `USER` table in `h2` database. Firstly, we create `USER` table with `h2`:
```java
@Slf4j
public class DatabaseUtil {
private static final String DB_URL = "jdbc:h2:mem:metamapping";
private static final String CREATE_SCHEMA_SQL = "DROP TABLE IF EXISTS `user`;"
+ "CREATE TABLE `user` (\n"
+ " `id` int(11) NOT NULL AUTO_INCREMENT,\n"
+ " `username` varchar(255) NOT NULL,\n"
+ " `password` varchar(255) NOT NULL,\n"
+ " PRIMARY KEY (`id`)\n"
+ ");";
/**
* Create database.
*/
static {
LOGGER.info("create h2 database");
var source = new JdbcDataSource();
source.setURL(DB_URL);
try (var statement = source.getConnection().createStatement()) {
statement.execute(CREATE_SCHEMA_SQL);
} catch (SQLException e) {
LOGGER.error("unable to create h2 data source", e);
}
}
}
```
Correspondingly, here's the basic `User` entity.
```java
@Setter
@Getter
@ToString
public class User {
private Integer id;
private String username;
private String password;
/**
* Get a user.
* @param username user name
* @param password user password
*/
public User(String username, String password) {
this.username = username;
this.password = password;
}
}
```
Then we write a `xml` file to show the mapping between the table and the object:
```xml
<?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="com.iluwatar.metamapping.model.User" table="user">
<id name="id" type="java.lang.Integer" column="id">
<generator class="native"/>
</id>
<property name="username" column="username" type="java.lang.String"/>
<property name="password" column="password" type="java.lang.String"/>
</class>
</hibernate-mapping>
```
We use `Hibernate` to resolve the mapping and connect to our database, here's its configuration:
```xml
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<session-factory>
<!-- JDBC Database connection settings -->
<property name="connection.url">jdbc:h2:mem:metamapping</property>
<property name="connection.driver_class">org.h2.Driver</property>
<!-- JDBC connection pool settings ... using built-in test pool -->
<property name="connection.pool_size">1</property>
<!-- Select our SQL dialect -->
<property name="dialect">org.hibernate.dialect.H2Dialect</property>
<!-- Echo the SQL to stdout -->
<property name="show_sql">false</property>
<!-- Drop and re-create the database schema on startup -->
<property name="hbm2ddl.auto">create-drop</property>
<mapping resource="com/iluwatar/metamapping/model/User.hbm.xml" />
</session-factory>
</hibernate-configuration>
```
Then we can get access to the table just like an object with `Hibernate`, here's some CRUDs:
```java
@Slf4j
public class UserService {
private static final SessionFactory factory = HibernateUtil.getSessionFactory();
/**
* List all users.
* @return list of users
*/
public List<User> listUser() {
LOGGER.info("list all users.");
List<User> users = new ArrayList<>();
try (var session = factory.openSession()) {
var tx = session.beginTransaction();
List<User> userIter = session.createQuery("FROM User").list();
for (var iterator = userIter.iterator(); iterator.hasNext();) {
users.add(iterator.next());
}
tx.commit();
} catch (HibernateException e) {
LOGGER.debug("fail to get users", e);
}
return users;
}
// other CRUDs ->
...
public void close() {
HibernateUtil.shutdown();
}
}
```
## Class diagram
![metamapping](etc/metamapping.png)
## Applicability
Use the Metadata Mapping when:
- you want reduce the amount of work needed to handle database mapping.
## Known uses
[Hibernate](https://hibernate.org/), [EclipseLink](https://www.eclipse.org/eclipselink/), [MyBatis](https://blog.mybatis.org/)......
## Credits
- [J2EE Design Patterns](https://www.amazon.com/gp/product/0596004273/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596004273&linkCode=as2&tag=javadesignpat-20&linkId=48d37c67fb3d845b802fa9b619ad8f31)

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@startuml
interface com.iluwatar.metamapping.service.UserService {
+ List<User> listUser()
+ int createUser(User)
+ void updateUser(Integer,User)
+ void deleteUser(Integer)
+ User getUser(Integer)
+ void close()
}
class com.iluwatar.metamapping.utils.DatabaseUtil {
+ {static} void createDataSource()
}
class com.iluwatar.metamapping.model.User {
- Integer id
- String username
- String password
+ User(String username, String password)
}
class com.iluwatar.metamapping.utils.HibernateUtil {
+ {static} SessionFactory getSessionFactory()
+ {static} void shutdown()
}
class com.iluwatar.metamapping.App {
+ {static} void main(String[])
+ {static} List<User> generateSampleUsers()
}
com.iluwatar.metamapping.service.UserService <.. com.iluwatar.metamapping.App
com.iluwatar.metamapping.model.User <.. com.iluwatar.metamapping.service.UserService
com.iluwatar.metamapping.utils.HibernateUtil <.. com.iluwatar.metamapping.service.UserService
com.iluwatar.metamapping.utils.DatabaseUtil <-- com.iluwatar.metamapping.utils.HibernateUtil
@enduml

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<?xml version="1.0" encoding="UTF-8"?>
<!--
The MIT License
Copyright © 2014-2021 Ilkka Seppälä
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.
-->
<project xmlns="http://maven.apache.org/POM/4.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<parent>
<artifactId>java-design-patterns</artifactId>
<groupId>com.iluwatar</groupId>
<version>1.26.0-SNAPSHOT</version>
</parent>
<modelVersion>4.0.0</modelVersion>
<artifactId>metadata-mapping</artifactId>
<dependencies>
<dependency>
<groupId>org.junit.jupiter</groupId>
<artifactId>junit-jupiter-engine</artifactId>
<scope>test</scope>
</dependency>
<dependency>
<groupId>com.h2database</groupId>
<artifactId>h2</artifactId>
</dependency>
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-core</artifactId>
</dependency>
<dependency>
<groupId>com.h2database</groupId>
<artifactId>h2</artifactId>
</dependency>
<dependency>
<groupId>javax.xml.bind</groupId>
<artifactId>jaxb-api</artifactId>
</dependency>
<dependency>
<groupId>com.sun.xml.bind</groupId>
<artifactId>jaxb-impl</artifactId>
</dependency>
<dependency>
<groupId>com.sun.istack</groupId>
<artifactId>istack-commons-runtime</artifactId>
</dependency>
</dependencies>
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-assembly-plugin</artifactId>
<executions>
<execution>
<configuration>
<archive>
<manifest>
<mainClass>com.iluwatar.metamapping.App</mainClass>
</manifest>
</archive>
</configuration>
</execution>
</executions>
</plugin>
</plugins>
</build>
</project>

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package com.iluwatar.metamapping;
import com.iluwatar.metamapping.model.User;
import com.iluwatar.metamapping.service.UserService;
import com.iluwatar.metamapping.utils.DatabaseUtil;
import java.util.List;
import lombok.extern.slf4j.Slf4j;
import org.hibernate.service.ServiceRegistry;
/**
* Metadata Mapping specifies the mapping
* between classes and tables so that
* we could treat a table of any database like a Java class.
*
* <p>With hibernate, we achieve list/create/update/delete/get operations:
* 1)Create the H2 Database in {@link DatabaseUtil}.
* 2)Hibernate resolve hibernate.cfg.xml and generate service like save/list/get/delete.
* For learning metadata mapping pattern, we go deeper into Hibernate here:
* a)read properties from hibernate.cfg.xml and mapping from *.hbm.xml
* b)create session factory to generate session interacting with database
* c)generate session with factory pattern
* d)create query object or use basic api with session,
* hibernate will convert all query to database query according to metadata
* 3)We encapsulate hibernate service in {@link UserService} for our use.
* @see org.hibernate.cfg.Configuration#configure(String)
* @see org.hibernate.cfg.Configuration#buildSessionFactory(ServiceRegistry)
* @see org.hibernate.internal.SessionFactoryImpl#openSession()
*/
@Slf4j
public class App {
/**
* Program entry point.
*
* @param args command line args.
* @throws Exception if any error occurs.
*/
public static void main(String[] args) throws Exception {
// get service
var userService = new UserService();
// use create service to add users
for (var user: generateSampleUsers()) {
var id = userService.createUser(user);
LOGGER.info("Add user" + user + "at" + id + ".");
}
// use list service to get users
var users = userService.listUser();
LOGGER.info(String.valueOf(users));
// use get service to get a user
var user = userService.getUser(1);
LOGGER.info(String.valueOf(user));
// change password of user 1
user.setPassword("new123");
// use update service to update user 1
userService.updateUser(1, user);
// use delete service to delete user 2
userService.deleteUser(2);
// close service
userService.close();
}
/**
* Generate users.
*
* @return list of users.
*/
public static List<User> generateSampleUsers() {
final var user1 = new User("ZhangSan", "zhs123");
final var user2 = new User("LiSi", "ls123");
final var user3 = new User("WangWu", "ww123");
return List.of(user1, user2, user3);
}
}

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package com.iluwatar.metamapping.model;
import lombok.Getter;
import lombok.Setter;
import lombok.ToString;
/**
* User Entity.
*/
@Setter
@Getter
@ToString
public class User {
private Integer id;
private String username;
private String password;
public User() {}
/**
* Get a user.
* @param username user name
* @param password user password
*/
public User(String username, String password) {
this.username = username;
this.password = password;
}
}

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package com.iluwatar.metamapping.service;
import com.iluwatar.metamapping.model.User;
import com.iluwatar.metamapping.utils.HibernateUtil;
import java.util.ArrayList;
import java.util.List;
import lombok.extern.slf4j.Slf4j;
import org.hibernate.HibernateException;
import org.hibernate.SessionFactory;
/**
* Service layer for user.
*/
@Slf4j
public class UserService {
private static final SessionFactory factory = HibernateUtil.getSessionFactory();
/**
* List all users.
* @return list of users
*/
public List<User> listUser() {
LOGGER.info("list all users.");
List<User> users = new ArrayList<>();
try (var session = factory.openSession()) {
var tx = session.beginTransaction();
List<User> userIter = session.createQuery("FROM User").list();
for (var iterator = userIter.iterator(); iterator.hasNext();) {
users.add(iterator.next());
}
tx.commit();
} catch (HibernateException e) {
LOGGER.debug("fail to get users", e);
}
return users;
}
/**
* Add a user.
* @param user user entity
* @return user id
*/
public int createUser(User user) {
LOGGER.info("create user: " + user.getUsername());
var id = -1;
try (var session = factory.openSession()) {
var tx = session.beginTransaction();
id = (Integer) session.save(user);
tx.commit();
} catch (HibernateException e) {
LOGGER.debug("fail to create user", e);
}
LOGGER.info("create user " + user.getUsername() + " at " + id);
return id;
}
/**
* Update user.
* @param id user id
* @param user new user entity
*/
public void updateUser(Integer id, User user) {
LOGGER.info("update user at " + id);
try (var session = factory.openSession()) {
var tx = session.beginTransaction();
user.setId(id);
session.update(user);
tx.commit();
} catch (HibernateException e) {
LOGGER.debug("fail to update user", e);
}
}
/**
* Delete user.
* @param id user id
*/
public void deleteUser(Integer id) {
LOGGER.info("delete user at: " + id);
try (var session = factory.openSession()) {
var tx = session.beginTransaction();
var user = session.get(User.class, id);
session.delete(user);
tx.commit();
} catch (HibernateException e) {
LOGGER.debug("fail to delete user", e);
}
}
/**
* Get user.
* @param id user id
* @return deleted user
*/
public User getUser(Integer id) {
LOGGER.info("get user at: " + id);
User user = null;
try (var session = factory.openSession()) {
var tx = session.beginTransaction();
user = session.get(User.class, id);
tx.commit();
} catch (HibernateException e) {
LOGGER.debug("fail to get user", e);
}
return user;
}
/**
* Close hibernate.
*/
public void close() {
HibernateUtil.shutdown();
}
}

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package com.iluwatar.metamapping.utils;
import java.sql.SQLException;
import lombok.extern.slf4j.Slf4j;
import org.h2.jdbcx.JdbcDataSource;
/**
* Create h2 database.
*/
@Slf4j
public class DatabaseUtil {
private static final String DB_URL = "jdbc:h2:mem:metamapping";
private static final String CREATE_SCHEMA_SQL = "DROP TABLE IF EXISTS `user`;"
+ "CREATE TABLE `user` (\n"
+ " `id` int(11) NOT NULL AUTO_INCREMENT,\n"
+ " `username` varchar(255) NOT NULL,\n"
+ " `password` varchar(255) NOT NULL,\n"
+ " PRIMARY KEY (`id`)\n"
+ ");";
/**
* Hide constructor.
*/
private DatabaseUtil() {}
/**
* Create database.
*/
static {
LOGGER.info("create h2 database");
var source = new JdbcDataSource();
source.setURL(DB_URL);
try (var statement = source.getConnection().createStatement()) {
statement.execute(CREATE_SCHEMA_SQL);
} catch (SQLException e) {
LOGGER.error("unable to create h2 data source", e);
}
}
}

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package com.iluwatar.metamapping.utils;
import lombok.extern.slf4j.Slf4j;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
/**
* Manage hibernate.
*/
@Slf4j
public class HibernateUtil {
private static final SessionFactory sessionFactory = buildSessionFactory();
/**
* Hide constructor.
*/
private HibernateUtil() {}
/**
* Build session factory.
* @return session factory
*/
private static SessionFactory buildSessionFactory() {
// Create the SessionFactory from hibernate.cfg.xml
return new Configuration().configure().buildSessionFactory();
}
/**
* Get session factory.
* @return session factory
*/
public static SessionFactory getSessionFactory() {
return sessionFactory;
}
/**
* Close session factory.
*/
public static void shutdown() {
// Close caches and connection pools
getSessionFactory().close();
}
}

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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="com.iluwatar.metamapping.model.User" table="user">
<id name="id" type="java.lang.Integer" column="id">
<generator class="native"/>
</id>
<property name="username" column="username" type="java.lang.String"/>
<property name="password" column="password" type="java.lang.String"/>
</class>
</hibernate-mapping>

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<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<session-factory>
<!-- JDBC Database connection settings -->
<property name="connection.url">jdbc:h2:mem:metamapping</property>
<property name="connection.driver_class">org.h2.Driver</property>
<!-- JDBC connection pool settings ... using built-in test pool -->
<property name="connection.pool_size">1</property>
<!-- Select our SQL dialect -->
<property name="dialect">org.hibernate.dialect.H2Dialect</property>
<!-- Echo the SQL to stdout -->
<property name="show_sql">false</property>
<!-- Drop and re-create the database schema on startup -->
<property name="hbm2ddl.auto">create-drop</property>
<mapping resource="com/iluwatar/metamapping/model/User.hbm.xml" />
</session-factory>
</hibernate-configuration>

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package com.iluwatar.metamapping;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
/**
* Tests that metadata mapping example runs without errors.
*/
class AppTest {
/**
* Issue: Add at least one assertion to this test case.
*
* Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
* throws an exception.
*/
@Test
void shouldExecuteMetaMappingWithoutException() {
assertDoesNotThrow(() -> App.main(new String[]{}));
}
}

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<license-maven-plugin.version>3.0</license-maven-plugin.version> <license-maven-plugin.version>3.0</license-maven-plugin.version>
<urm-maven-plugin.version>1.4.8</urm-maven-plugin.version> <urm-maven-plugin.version>1.4.8</urm-maven-plugin.version>
<commons-io.version>2.7</commons-io.version> <commons-io.version>2.7</commons-io.version>
<istack-commons-runtime.version>4.0.1</istack-commons-runtime.version>
<!-- SonarCloud --> <!-- SonarCloud -->
<sonar.host.url>https://sonarcloud.io</sonar.host.url> <sonar.host.url>https://sonarcloud.io</sonar.host.url>
<sonar.organization>iluwatar</sonar.organization> <sonar.organization>iluwatar</sonar.organization>
@ -227,6 +228,7 @@
<module>lockable-object</module> <module>lockable-object</module>
<module>fanout-fanin</module> <module>fanout-fanin</module>
<module>domain-model</module> <module>domain-model</module>
<module>metadata-mapping</module>
</modules> </modules>
<repositories> <repositories>
<repository> <repository>
@ -377,6 +379,11 @@
<artifactId>commons-io</artifactId> <artifactId>commons-io</artifactId>
<version>${commons-io.version}</version> <version>${commons-io.version}</version>
</dependency> </dependency>
<dependency>
<groupId>com.sun.istack</groupId>
<artifactId>istack-commons-runtime</artifactId>
<version>${istack-commons-runtime.version}</version>
</dependency>
</dependencies> </dependencies>
</dependencyManagement> </dependencyManagement>
<dependencies> <dependencies>