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###### 22. [Java Persistence Query Language (JPQL)](https://www.tutorialspoint.com/jpa/jpa_jpql.htm)
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###### 23. [Web API development using Spring REST with Spring Boot](https://www.tutorialspoint.com/spring_boot/spring_boot_building_restful_web_services.htm)
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The persistence layer is the layer which interacts with the relational database and service layer. It gets data from service layer, performs operations on database and sends back results to service layer. In this layer the code to interact with database is implemented.
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#### 17. Spring JDBC:
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You have already learned how to use JDBC for data access. But using JDBC is challenging because of following reasons:
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-> Connection Management
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-> Exception Handling
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-> Code Duplication
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So to make use of JDBC easier Spring provided one layer of abstraction, called Spring JDBC, on top of existing JDBC technology.[Learn More](https://www.baeldung.com/spring-jdbc-jdbctemplate)
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#### 18. [Data Access Using JDBC Template](https://www.vogella.com/tutorials/SpringJDBC/article.html)
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#### 19. Object Relational Mapping (ORM):
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You have learned how to use Spring JDBC for developing persistence layer. But Spring JDBC code involves both Java objects (object model) and SQL queries (relational model) which makes it difficult to use. This is because the way data is stored in objects is different from the way it is stored in tables and so some sort of translation is required between object model and relational model. But this translation is not easy because of paradigm mismatch between object and relational model. This paradigm mismatch is called as Object-Relational Impedance Mismatch and it exposes following problems :
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-> Problem of Granularity
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-> Problem of Inheritance
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-> Problem of Identity
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-> Problem of Associations
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-> Problem of Data Navigation
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To handle these problems, a technique called as Object-Relational Mapping (ORM) came into market. It handles object relational impedance mismatch by providing a way to map objects directly to tables so that object model can be automatically translated to relation model and vice versa, This let developers focus only on the object model.[Learn More](https://docs.spring.io/spring/docs/4.2.x/spring-framework-reference/html/orm.html)
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#### 20. Introduction to JPA:
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The Java Persistence API (JPA) is Java EE specification that defines how data persistence‐related tasks are handled using object‐relational mapping (ORM) frameworks in Java applications.[Learn More](https://www.javatpoint.com/jpa-introduction)
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#### 21. Spring JPA:
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The Spring Framework provides support for JPA, Hibernate, and other ORM frameworks. You can easily integrate these frameworks with Spring.[Learn how to use JPA with Spring Boot](https://spring.io/guides/gs/accessing-data-jpa/)
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#### 22. Java Persistence Query Language (JPQL):
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JPA provides a query language called as Java Persistence Query Language (JPQL). Its syntax is very similar to SQL but these queries are defined using entity classes and its attributes instead of tables and columns. This makes it easy for Java developers to use it. But since database uses SQL JPA implementations translates the JPQL query into SQL using query translator.[Learn More](https://www.tutorialspoint.com/jpa/jpa_jpql.htm)
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## • __Presentation Layer__
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The presentation layer of an enterprise application handles the client requests and provide clients a visual view of the information. It also allows them to perform business functions provided and managed by the application.
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#### 23. [Web API development using Spring REST with Spring Boot](https://www.tutorialspoint.com/spring_boot/spring_boot_building_restful_web_services.htm)
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## • [__Testing Spring Boot Applications using JUnit and Mockito__](https://howtodoinjava.com/spring-boot2/testing/spring-boot-mockito-junit-example/)
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