Working with jpa

Working with jpa

• 1.
  Working with JPA Lecturer:Ondrej Mihályi
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  Contents ●JPA entities andpersistence units ●Mapping of JPA entities to SQL database ●Querying entities ●Lazy and eager loading ●Id generation ●Optimistic locking
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  JPA entities ●An entityis a lightweight persistence domain object ●Typically, an entity represents a table in a relational database, and each entity instance corresponds to a row in that table ●fields or properties use object/relational mapping annotations to map the entities and entity relationships to the relational data
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  Requirements for EntityClasses ●javax.persistence.Entity annotation ●Constructor without arguments ●Persistent values properties/fields may be of: l Java primitive values, their wrappers and String l Other Java standard number or datetime types l Enums, byta and char arrays l Other entities, collections of entities, embeddable classes
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  Entity persistence type ●PersistentFields l Mapped Fields are accessed directly ●Persistent Properties l Properties mapped via getter method accessed via setter/getter according to Java Beans convention ●Transient fields/properties l marked by transient or @Transient l their state is not saved to database
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  Validating integrity ofJPA entities ●Annotations of Java Bean Validation may be used to ensure integrity before committing transaction l javax.validation.constraints package ●Most used constraints: l @NotNull, @Pattern, @Past
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  Persistence Unit ●Persistence isturned on by presence of valid persistence.xml in META-INF directory ●persistence.xml defines groups of entities together with connection parameters ●Persistence unit represents a database ●Persistence context is tied to a persistence unit and represents a single connection to the database
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  Types of definitionof connection information ●Transaction-type l JTA – datasource to the DB and transaction are configured and handled by the application server – the datasource is provided by specified JNDI name in element jta-data-source l RESOURCE_LOCAL – connection information is specified directly and transactions are controlled by calling of appropriate methods
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  Entity Manager ●Component thatrepresents single connection ●API to control persistence context of JPA entities, search JPA entities and provide means to query DB and execute batch statements ●Entity Manager Factory l Represents information to reate connetions l Can be used to create Entity Managers l Provides information about database metadata
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  Lifecycle of aJPA entity ●to be synchronized with a DB, entity must be attached to an Entity Manager
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  Mapping of entities ●Entitymust be included in a persistence-unit ●Annotated by @Entity ●Must have a primary key specified ●All fields/properties of an entity are mapped to DB l Unless marked as transient or with @Transient annotation ●Name of columns in DB are inferred from field/property names
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  Mapping of entities– primitive types ●Primitive types are automatically mapped to a column of appropriate SQL type l String to VARCHAR l Numbers to NUMERIC, DECIMAL, … l Boolean to BOOLEAN, NUMERIC ●To refine mapping, additional JPA annotations are needed (@Lob, @Column)
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  Mapping of entities– date and time ●Mapping of Date and Calendar ●Needs annotation Temporal, with definition of type of data: l DATE l TIME l TIMESTAMP
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  Mapping of entities– enumerations ●Need to use @Enumerated ●Two options how to map to DB: l ORDINAL – not stable enum values are added/removed l STRING – not very convenient, if we want to map by id or to an existing enumeration table
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  Mapping of entities– collections ●According to direction l Uni-directional vs. Bi-directional ●According to cardinality l @OneToOne l @OneToMany l @ManyToOne l @ManyToMany (intermediary table is generated)
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  Mapping of entities– bi-directional relations ●Only one direction is mapped to a column l @JoinColumn overwrites default mapping ●Reverse direction is mapped to a Java field in the other entity ●In case of OneToOne and ManyToMany, annotations on both references ●Otherwise OneToMany is in one entity, ManyToOne in the other entity
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  Mapping of entities– collections of primitive types ●Need to use @ElementCollection ●A separate table to hold values of collection is generated
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  Mapping of entities– difference between collection types Collection type Description List Ordered list of elements. Order is either not persisted, or defined by additional integer column in DB. Only unique elements are stored in DB. Set Unordered collection of elements. Ensures that elements are unique. Very close to how data is stored in DB. Collection Similar to List, but can be represented also by a Set Map Essentially a Set of keys with attached values, not much used in practice
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  Customization of DBmapping ●JPA listeners ● @PrePersist, @PreRemove, @PostPersist, @PostRemove, @PreUpdate, @PostUpdate, @PostLoad
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  Customization of DBmapping ●Converters – new in Java EE 7 l @Convert to specify converter class l Converter class implements AttributeConverter l Converter class must be mentioned in orm.xml
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  Queries ●Two standard ways: l JPQL– text-based queries similar to SQL l Criteria – Java object based queries with syntax check and strict typing ●Also possible to execute a direct SQL query
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  JPQL ●SELECT b FROMBook b WHERE b.name like '%Web%' l Instead of table we use name of Java class l We assign alias to the entity to reference it l We may return both primitive types and complete entities (by the reference) l We may compare entities (their id will be compared) l We may access fields and make joins by dot notation
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  JPQL – namedqueries ●Queries tied with a JPA entity using @NamedQuery annotation ●Compiled and checked at application deployment ●Executed by a name ●Name of a named query must be unique in the whole persistence unit, not only within the entity
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  JPQL – joins ●Joinsare created when dot notation is used: l SELECT b.tags FROM Book b WHERE b.id = 2 ●When we need to reference properties of the joined entities, we must explicitely join them: l SELECT DISTINCT t.name FROM Book b INNER JOIN b.tags t l Only mapped properties can be joined
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  Criteria API ●Criteria queriesare written using Java APIs, are typesafe, and are portable ●However, too much of complex code needs for even simple queries ●Libraries exist to simplify working with criteria ●Criteria API is convenient to build dynamic queries based on parameters (for search dialogs, filters, ...)
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  Lazy loading ● Simpletypes are loaded from DB at the time when entity is loaded (EAGER) ● Collections are loaded only when a method on a collection is executed (LAZY) l Only reference to a proxy collection is injected to the entity ● This behavior can be changed by FetchType attribute on collection annotations ● When lazy collection is accessed when entity was disconnected from transaction, LazyLoadingException is thrown
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  Lazy loading behavior ●Defaultfetch behavior can be changed by FetchType attribute on collection annotations ●Lazy references can be loaded eagerly in JPQL by fetch join
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  Lazy loading problems ●Whenlazy collection is accessed when entity was disconnected from transaction, LazyLoadingException is thrown ●There are various strategies to avoid this: l Data Transfer Objects, Extended Persistence Context, ...
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  Id generation forprimary key ●an attribute of type long or String is marked with @Id ●Strategies to generate Id values: l IDENTITY – id generation by the DB l SEQUENCE – generation of id from a sequence l TABLE – emulates SEQUENCE using a table l AUTO – JPA provider will choose according to what is supported by the DB
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  Optimistic locking ●Technique toprevent conflicts without using DB locking mechanism l DB locks are often too restrictive, slow down the DB, may cause deadlocks ●Each row of a table stores its version number ●When JPA finds out that the transaction is trying to modify an entity with older version than in DB, it detects conflict and throws an exception with rollback
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  Optimistic locking –version attribute ●@Version marks entity property, which is mapped to the version column ●Version number is updated and used by JPA automatically