{"id":19705,"date":"2013-12-18T22:00:27","date_gmt":"2013-12-18T20:00:27","guid":{"rendered":"http:\/\/www.javacodegeeks.com\/?p=19705"},"modified":"2013-12-18T09:42:37","modified_gmt":"2013-12-18T07:42:37","slug":"what-is-spring-data","status":"publish","type":"post","link":"https:\/\/www.javacodegeeks.com\/2013\/12\/what-is-spring-data.html","title":{"rendered":"What is Spring Data?"},"content":{"rendered":"

(This is related to Project Student<\/a> and I will revisit this topic later.)<\/p>\n

Spring Data<\/a> came up in passing in several recent interviews. What is Spring Data<\/a>?<\/p>\n

To answer that let\u2019s consider the standard approach to persistence \u2013 all access is made via Data Access Objects<\/a> (DAOs). This completely isolates the rest of the system from the specific details of the persistence mechanisms. This sounds easy but anyone who has ever done a non-trivial project knows there\u2019s a big headache.<\/p>\n

DAO code is boring.<\/p>\n

It\u2019s monotonous, it has a lot of very similar code, and a slight error can cause a lot of damage. Worse it violates the Don\u2019t Repeat Yourself<\/a> (DRY) principle since most of the information is already captured in JPA annotations.<\/p>\n

This is such a big problem that there have been code generation tools for years. In theory they solve the problems but in practice they introduce their own. E.g., the need for custom configuration files or annotations.<\/p>\n

Using an interface as a DRY contract<\/h2>\n

What is the ultimate DRY contract in the Java world? That\u2019s easy \u2013 it\u2019s an Interface. Given an interface and a template we can use CGLib to generate the necessary classes on the fly during application startup. There\u2019s a slight performance hit but it\u2019s modest when compared to the benefits.

<\/div> <\/div><\/p>\n

Student.java<\/strong><\/p>\n

@Entity\r\npublic class Student {\r\n    private Integer id;\r\n    private String uuid;\r\n    private String name;\r\n    private String emailAddress;\r\n    private Integer creditHours;\r\n\r\n    @Id\r\n    public Integer getId() { return id; }\r\n    public void setId(Integer id) { this.id = id; }\r\n\r\n    @Column(unique=true)\r\n    public String getUuid() { return uuid; }\r\n    public void setUuid(String uuid) { this.uuid = uuid; }\r\n\r\n    @Column\r\n    public String getName() { return name; }\r\n    public void setName(String name) { this.name = name; }\r\n\r\n    @Column(unique=true)\r\n    public String getEmailAddress() { return emailAddress; }\r\n    public void setEmailAddress(String emailAddress) { this.emailAddress; }\r\n\r\n    @Column\r\n    public Integer getCreditHours() { return creditHours; }\r\n    public void setCreditHours(Integer creditHours) { this.creditHours = creditHours; }\r\n}<\/pre>\n
What could our Interface look like?<\/p>\n
repository\/StudentRepository.java<\/strong><\/p>\n
import org.springframework.data.jpa.repository.JpaRepository;\r\nimport org.springframework.stereotype.Repository;\r\n\r\n@Repository\r\npublic interface StudentRepository extends CrudRepository<Student, Integer> {\r\n    \/\/ this could also be getByUuid() or fetchByUuid() - all are recognized\r\n    Student findStudentByUuid(String uuid);\r\n\r\n    Student findStudentByEmailAddress(String emailAddress);\r\n\r\n    List<Student> findStudentsByNameLike(String pattern);\r\n\r\n    \/\/ we can use a custom query\r\n    @Query(\"select s from Student s where s.creditHours < 15\")\r\n    List<Student> findFreshmen();\r\n}<\/pre>\n
and so on. In all there are 15 predicates that can be used alone or in combination.<\/p>\n