Object Relational Mapping and Persistence Layer

Object Relational Mapping and a Persistence Layer

• This presentation discusses object-relational mapping (O-R mapping) and persistence layers.
• The connection to the database must be closed.
• The DBMS may start refusing connections if the connection is not closed.
• Result sets and statements should be closed to avoid issues.
• The presentation recommends using a Java try-with-resources block for efficient resource management.

Integration with high-level languages

• The presentation describes building applications with persistent storage using high-level languages.
• A persistence layer acts as a model.
• The persistence layer lives in the database but is also in memory.
• A controller provides access to the model.

Last time

• The previous lesson introduced JDBC as a Client-Server Interface (CLI).
• Handling responses from the Database Management System (DBMS) was not covered.

The ResultSet object

• The ResultSet object contains metadata about the columns.
• The getMetaData() method accesses this metadata.
• The metadata helps determine field types.
• The ResultSet implements a concept of a "current record".
• The cursor is initially positioned before the first record.
• Methods exist to move the cursor forward, such as next().
• This allows iteration over the result set.
• SQL ordered lists are converted to a Java iterable collection.
• The ResultSet object offers methods to retrieve data from its rows, like getString(i).

Accessing the ResultSet

• The example code shows how to retrieve ResultSet data.
• It iterates through the columns.

The metadata

• The presentation demonstrates code to extract metadata of the ResultSet.
• The code counts columns (ColumnCount).
• It iterates through, printing names and types.
• Demonstrates the getColumnName() and getColumnTypeName() methods.

The data

• The code shows how to access the data in the ResultSet.
• The example uses a while loop and resultSet.next() to move to the next row.
• The inner for loop accesses each column in a given row.
• The method resultSet.getString(i) is used to extract column values.

Closing

• Databases need explicit closing of connections.
• DBMS start refusing newer connections if older connections are not closed.
• To ensure the integrity of your application close your connections and statements.
• The try-with-resources statement in Java is used to ensure these are closed.

Review Simple Example

• The code shows a complete example of using JDBC.
• It iterates over the ResultSet data.
• It closes the resources (connection, statement, and result set).

What does a driver do?

• Drivers implement interfaces like Connection and Statement to interact with the DBMS.
• A JDBC Driver handles communication with different DBMS.
• Connection metadata, such as the connection details, is provided.
• Drivers send commands to the DBMS.
• Not all DBMSs support all facilities, such as scrolling back.

Transactions

• The default connection mode is "auto-commit".
• Each statement is committed separately.
• setAutoCommit(false) makes later commit needed for transaction.
• connection.commit() commits changes.
• connection.rollback() can revert changes.
• setAutoCommit(true) returns to auto-commit mode.

Isolation levels

• JDBC defines five isolation levels, inherited from SQL (NONE, READ-COMMITTED, UNCOMMITTED, REPEATABLE-READ, SERIALIZABLE).
• Isolation levels allow you to set the integrity of your data.
• This can control how the DBMS handles transactions.
• Different DBMS may not support all the specified levels.

JDBC as a CLI

• JDBC acts as a package to integrate with different DBMS.
• JDBC is also a class.
• JDBC is designed for DBMS independence.
• It offers methods such as executeUpdate, executeQuery, etc.
• It provides a ResultSet for data access.
• Data types (string, integer, etc.) can be extracted and converted to an appropriate Java type.

Observations on CLI

• JDBC handles SQL commands as strings.
• It does not employ a domain-specific construct.
• Data access (copy in/out) is specifically programmed.

JDBC Documentation

• The documentation provides references for understanding JDBC.

The JPA

• JPA is a Java Persistence API.
• It's a framework used to interact with databases in Java code.
• It abstracts away database interaction details.
• JPA implementation options include Hibernate and EclipseLink.
• This tool is designed for use across desktop and web applications.
• It has O-R mapping, entity managers, and query mechanisms.

This material

• This material highlights that O-R mapping is supported in other object-oriented languages.
• It uses Java as an example.

O-R Mapping

• O-R mapping translates between object-oriented representations and relational databases.
• It defines components like tables in a database and Objects within the code.
• In essence, it translates the structure of a database into a programming language.
• The presentation shows examples of converting database tables to classes.

Entities

• JavaBeans are reusable software components fitting conventions in Java.

Column annotations

• Parameters to annotations can specify data types or constraints in database columns.
• Fields in the class representing database columns must be private.
• Getters and setters are used to access and modify these.
• Annotations such as @Id, @Column, @NotNull and @Transient are used to control column behavior (e.g., primary key, name, non-null, etc).

Entity example

• Demonstrates using the @Entity annotation and @Table annotation.
• The @Id annotation defines a primary key for the entity.

Column annotations (continued)

• Explains annotating database columns with @Column, @Transient to affect how data is handled and which data in a class should be mapped to which columns in a database table.
• The code includes examples and annotations for constraints.

Validation

• Annotation examples like @NotNull or @Pattern are used for validating data.

Mappings examples

• The presentation provides specific mapping examples with JPA annotations.

What types can be stored?

• Supported types include Java primitives and objects (wrappers, java.util.*, custom objects, collections) for storage in the database.

Primary Keys

• Primary keys can be generated automatically or manually.
• Composite primary keys are possible.
• Java primitive types, primitives' wrapper objects, and specific Java objects conform to their representation in the database.
• The code has an example showing @GeneratedValue.

Summary

• JPA (Java Persistence API) is used as an example of O-R mapping.
• It consists of Entities, an Entity manager and Query mechanisms.
• These parts manage data, entities, and other components of JPA.
• Defining these elements maps objects in the code to relations.

This quiz covers the concepts of object-relational mapping and how persistence layers function within applications. It emphasizes the importance of properly managing database connections and resource usage in high-level programming languages, with specific notes on Java's try-with-resources structure. Understand the role of the ResultSet object and its metadata in database interactions.