JEST provides RESTful access to Java Persistence API (JPA) data by representing managed object graphs as resources accessible via HTTP requests. It uses the JPA metamodel to generate self-describing representations in XML or JSON formats. JEST maps HTTP verbs to JPA operations like find(), merge(), persist(), and remove(). This allows REST clients to access and modify persistent objects without knowledge of the underlying schema or JPA implementation.

1. JEST: REST on OpenJPA Pinaki Poddar, PhD Java Persistence Evangelist [email_address]

3. JEST: REST on OpenJPA JEST is RESTful interaction with OpenJPA Runtime http JDBC XML/JSON Java Object Graph

7. REST: RE presentational S tate T ransfer C0 C0 C1’ C1’ S1 S2 S1 renders C1 modifies C2 renders Response 1 Request 2 Request 1 Response 2 @ rest State Transition 1 2 3 4 5 6

10. JPA is Model-View-Controller for Persistence View Control Model /** * A persistent entity **/ @Entity public class Actor { @Id private String name ; } /** * Find Actor by name **/ public Actor findByName( String name ) { EntityManager em = …; return em.find( Actor . class , name); } POJO sees uses updates modifies Java Persistence API Relational Database

11. JPA: access-detach-merge programming model C0 C0 C1’ C1’ em1 em2 renders C1 modifies C2 renders State Transition SELECT … detach find merge commit UPDATE/INSERT/DELETE … 1 2 3 5 6 4

12. REST and JPA: brothers separated at birth? Stateless Server Detached Transaction Identifiable Resource Persistent Identity Coherent Representation Persistent Closure Self-descriptive JPA 2.0 Metamodel REST JPA

15. Identifiable Resource: URI for persistent object? Actor p1 = em1.find ( Actor . class , “ John ”); Actor p2 = em2.find ( Actor . class , “ John ”); name :” John ” hash : 56789 p1 name :” John ” hash : 56789 p2 p1.equals(p2) p1.hashCode() == p2.hashCode() p1 != p2 public class Actor { @Id String name ; } Persistent Identity carries the notion that both p1 and p2 refer to the same database record. EntityManager EntityManager … Robert … John ACTOR

17. OpenJPA Fetch Plan @Entity @FetchGroups ({ @FetchGroup ( name =&quot; OnlyName &quot;, attributes ={ @FetchAttribute ( name =&quot; firstName &quot;), @FetchAttribute ( name =&quot; lastName &quot;) }) }) public class Actor { @Id private String id ; private String firstName ; private String lastName ; private Gender gender ; private Date dob ; @OneToOne private Actor partner ; @OneToMany private Set<Movie> movies ; // getters and setters }

19. JPA Metamodel: Persistent Type System Class Type<X> ManagedType<X> BasicType<X> IdentifiableType<X> EmbeddableType<X> EntityType<X> MappedSuperclassType<X> package javax.persistence.metamodel package java.lang <X>: Type<X> represents Java class X

20. JPA Metamodel : Persistent Type System Field Attribute<X,Y> SingularAttribute<X,Y> PluralAttribute<X,C,E> ListAttribute<X,E> CollectionAttribute<X,E> SetAttribute<X> MapAttribute<X,K,V> package javax.persistence.metamodel package java.lang.reflect <X>: Attribute<X,Y> declared in Java class X <Y>: Attribute<X,Y> is of type Y <C>: PluralAttribute<X,C,E> is of Java collection type C <E>: PluralAttribute<X,C,E> contains elements of type E

21. JPA Metamodel: Persistent Scope java.lang.Class java.lang.reflect.Field Metamodel ManagedType<X> Attribute<X,Y> java.lang.ClassLoader A classloader defines a scope for a set of Java classes A metamodel defines a scope for a set of managed persistent types package javax.persistence.metamodel

24. JEST URI Syntax identifies JPA resources. maps to JPA operation decorates JPA operation JEST resolves argument strings to strong types, e.g. “ type=Actor ” to Actor .class . scheme authority path query context action qualifiers arguments OpenJPAEntityManager em = getPersistenceContext(); em.addFetchPlan(“ basic ”); Actor a = em.find( Actor . class , “m1”); find plan=basic type=Actor ? jest / / & id=m1 http 8080 openjpa.com / : ://

25. JEST URI Examples find an Actor with primary key m1 using fetch plan ‘basic’ http :// openjpa.com:8080 / jest / find / plan = basic ? type = Actor & m1 Query for a single Actor by name John http :// openjpa.com:8080 / jest / query / single ? q = select p from Actor p where p.name=:n & n = John Get the metamodel http :// openjpa.com:8080 / jest / domain

26. JEST translates HTTP verbs to JPA operations GET HTTP POST PUT DELETE find() getMetamodel() createQuery().getResultList() getProperties() /find / query / domain / properties merge() persist() remove() JEST Action JPA Operation non-transactional transactional

27. JEST Deployment < web-app > < servlet > < servlet-name > demo </ servlet-name > < servlet-class > demo.SimpleApp </ servlet-class > </ servlet > < servlet-mapping > < servlet-name > demo </ servlet-name > < url-pattern > /* </ url-pattern > </ servlet-mapping > <!-- Deployment descriptor for JESTServlet. --> < servlet > < servlet-name > jest </ servlet-name > < servlet-class > org.apache.openjpa.persistence.jest.JESTServlet </ servlet-class > < init-param > < param-name > persistence.unit </ param-name > < param-value > SimplePU </ param-value > </ init-param > </ servlet > < servlet-mapping > < servlet-name > jest </ servlet-name > < url-pattern > /jest/* </ url-pattern > </ servlet-mapping > </ web-app > Deployed as HTTP Servlet within the same module scope Identifies the module by persistence unit name

28. Deployment Modes for JEST Servlet JESTServlet emf Module X emf JESTServlet instantiates discovers instantiates or injected primary mode auxiliary mode default

36. JEST: JPA Detached Transaction C1 C1 C1’ C1’ S1 S2 S1 renders C1 modifies C2 renders A1 A1’ R1 Response 2 @ rest State Transition find() detach() represent() parse() merge() commit()

37. REST is successful in real world Success has many friends… “ Amazon has both SOAP and REST interfaces to their web services, and 85% of their usage is of the REST interface. ” Jeff Barr, Web Services Evangelist @ Amazon Reference: REST vs SOAP at Amazon

38. Does JEST pass the RESTness est? YES Q5 . Can server transfer logic to clients? NO Q4 . Can client determine the exact server endpoint? YES Q3 . Does response describe their cacheability for the client? NO Q2 . Does server store client context between requests? YES Q1 . Are clients separated from servers by a uniform interface?

39. Does JEST pass Uniform Interface Test? YES Q9 . Are related resources accessible via received representation? YES Q8 . Does representation carry enough information to be processed by the client? YES Q7 . Can client manipulate received resource representation? YES Q6 . Does request identify a resource?