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feature: Implement Serialized Entity Pattern (#2150)
* Add files: - App.java - Country.java - CountryDao.java - CountrySchemaSql.java - CountryTest.java - README.md - serialize-entity.urm.puml - pom.xml (inherit from parent pom.xml) Update files: - pom.xml (parent pom.xml, add 1 module called serialized-entity) * Fix duplicated error in pom.xml * Update: - pom.xml (module servant, changed by error, now fixed) - pom.xml (module serialized-entity) * Update: - pom.xml (changed, artifactId) * Update: - pom.xml Add: AppTest.java * Resolved changes required by the reviewer Co-authored-by: Ilkka Seppälä <iluwatar@users.noreply.github.com>
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JurenXu
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---
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title: Serialized Entity Pattern
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categories: Architectural
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language: en
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tags:
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- Data access
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---
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## Intent
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To easily persist Java objects to the database.
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## Explanation
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Java serialization allow us to convert the object to a set of bytes. We can store these bytes into database as
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BLOB(binary long objects) and read them at any time and reconstruct them into Java objects.
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**Programmatic Example**
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Walking through our customers example, here's the basic `Customer` entity.
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```java
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@Getter
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@Setter
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@EqualsAndHashCode
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@ToString
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@AllArgsConstructor
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public class Country implements Serializable {
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private int code;
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private String name;
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private String continents;
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private String language;
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public static final long serialVersionUID = 7149851;
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// Constructor ->
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// getters and setters ->
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}
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```
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Here is `CountrySchemaSql`, this class have method allow us to serialize `Country` object and insert it into the
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database, also have a method that read serialized data from the database and deserialize it to `Country` object.
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```java
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@Slf4j
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public class CountrySchemaSql {
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public static final String CREATE_SCHEMA_SQL = "CREATE TABLE IF NOT EXISTS WORLD (ID INT PRIMARY KEY, COUNTRY BLOB)";
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public static final String DELETE_SCHEMA_SQL = "DROP TABLE WORLD IF EXISTS";
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private Country country;
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private DataSource dataSource;
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/**
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* Public constructor.
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*
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* @param dataSource datasource
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* @param country country
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*/
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public CountrySchemaSql(Country country, DataSource dataSource) {
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this.country = new Country(
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country.getCode(),
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country.getName(),
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country.getContinents(),
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country.getLanguage()
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);
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this.dataSource = dataSource;
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}
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/**
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* This method will serialize a Country object and store it to database.
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* @return int type, if successfully insert a serialized object to database then return country code, else return -1.
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* @throws IOException if any.
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*/
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public int insertCountry() throws IOException {
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var sql = "INSERT INTO WORLD (ID, COUNTRY) VALUES (?, ?)";
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try (var connection = dataSource.getConnection();
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var preparedStatement = connection.prepareStatement(sql);
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ByteArrayOutputStream baos = new ByteArrayOutputStream();
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ObjectOutputStream oss = new ObjectOutputStream(baos)) {
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oss.writeObject(country);
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oss.flush();
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preparedStatement.setInt(1, country.getCode());
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preparedStatement.setBlob(2, new ByteArrayInputStream(baos.toByteArray()));
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preparedStatement.execute();
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return country.getCode();
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} catch (SQLException e) {
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LOGGER.info("Exception thrown " + e.getMessage());
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}
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return -1;
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}
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/**
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* This method will select a data item from database and deserialize it.
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* @return int type, if successfully select and deserialized object from database then return country code,
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* else return -1.
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* @throws IOException if any.
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* @throws ClassNotFoundException if any.
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*/
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public int selectCountry() throws IOException, ClassNotFoundException {
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var sql = "SELECT ID, COUNTRY FROM WORLD WHERE ID = ?";
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try (var connection = dataSource.getConnection();
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var preparedStatement = connection.prepareStatement(sql)) {
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preparedStatement.setInt(1, country.getCode());
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try (ResultSet rs = preparedStatement.executeQuery()) {
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if (rs.next()) {
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Blob countryBlob = rs.getBlob("country");
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ByteArrayInputStream baos = new ByteArrayInputStream(countryBlob.getBytes(1, (int) countryBlob.length()));
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ObjectInputStream ois = new ObjectInputStream(baos);
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country = (Country) ois.readObject();
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LOGGER.info("Country: " + country);
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}
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return rs.getInt("id");
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}
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} catch (SQLException e) {
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LOGGER.info("Exception thrown " + e.getMessage());
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}
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return -1;
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}
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}
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```
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This `App.java` will first delete a World table from the h2 database(if there is one) and create a new table called
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`WORLD` to ensure we have a table we want.
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It will then create two `Country` objects called `China` and `UnitedArabEmirates`, then create two `CountrySchemaSql`
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objects and use objects `China` and `UnitedArabEmirates` as arguments.
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Finally, call `SerializedChina.insertCountry()` and `serializedUnitedArabEmirates.insertCountry()` to serialize and
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store them to database, and call `SerializedChina.selectCountry()` and `serializedUnitedArabEmirates.selectCountry()`
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methods to read and deserialize data items to `Country` objects.
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```java
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@Slf4j
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public class App {
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private static final String DB_URL = "jdbc:h2:~/test";
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private App() {
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}
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/**
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* Program entry point.
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* @param args command line args.
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* @throws IOException if any
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* @throws ClassNotFoundException if any
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*/
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public static void main(String[] args) throws IOException, ClassNotFoundException {
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final var dataSource = createDataSource();
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deleteSchema(dataSource);
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createSchema(dataSource);
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final var China = new Country(
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86,
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"China",
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"Asia",
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"Chinese"
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);
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final var UnitedArabEmirates = new Country(
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971,
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"United Arab Emirates",
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"Asia",
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"Arabic"
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);
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final var serializedChina = new CountrySchemaSql(China, dataSource);
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final var serializedUnitedArabEmirates = new CountrySchemaSql(UnitedArabEmirates, dataSource);
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serializedChina.insertCountry();
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serializedUnitedArabEmirates.insertCountry();
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serializedChina.selectCountry();
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serializedUnitedArabEmirates.selectCountry();
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}
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private static void deleteSchema(DataSource dataSource) {
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try (var connection = dataSource.getConnection();
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var statement = connection.createStatement()) {
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statement.execute(CountrySchemaSql.DELETE_SCHEMA_SQL);
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} catch (SQLException e) {
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LOGGER.info("Exception thrown " + e.getMessage());
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}
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}
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private static void createSchema(DataSource dataSource) {
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try (var connection = dataSource.getConnection();
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var statement = connection.createStatement()) {
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statement.execute(CountrySchemaSql.CREATE_SCHEMA_SQL);
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} catch (SQLException e) {
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LOGGER.info("Exception thrown " + e.getMessage());
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}
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}
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private static DataSource createDataSource() {
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var dataSource = new JdbcDataSource();
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dataSource.setURL(DB_URL);
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return dataSource;
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}
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}
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```
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## Class diagram
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## Applicability
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Use the Serialized Entity pattern when
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* You want to easily persist Java objects to the database.
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## Related patterns
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[Data Access Object](https://github.com/iluwatar/java-design-patterns/tree/master/dao)
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## Credits
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* [J2EE Design Patterns by William Crawford, Jonathan Kaplan](https://www.oreilly.com/library/view/j2ee-design-patterns/0596004273/re21.html)
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* [komminen](https://github.com/komminen/java-design-patterns) (His attempts of serialized-entity inspired me and learned a lot from his code)
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