Hibernate注解详解：从入门到精通

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"该文档是Hibernate注解的英文版参考指南，版本3.5.1-Final，由Emmanuel Bernard编写。它详细介绍了如何设置Hibernate注解项目，以及各种实体映射、查询映射和Hibernate特定的注解扩展。" 在 Hibernate 中，注解是一种强大的工具，用于声明式地配置对象-关系映射（ORM），使得Java对象可以直接与数据库交互而无需编写大量的SQL代码。以下是关键知识点的概述： 1. **设置注解项目** - **需求**：使用Hibernate注解需要Java SE环境，Hibernate库，以及对JPA（Java Persistence API）的基本理解。 - **配置**：包括引入Hibernate和相关的依赖库，以及在项目中配置persistence.xml文件来指定持久化单元。 - **属性**：配置文件中可以设置各种属性，如数据库连接信息、方言、缓存策略等。 - **日志**：配置日志系统，如Log4j或Java Util Logging，以便在开发过程中获取必要的调试信息。 2. **映射实体** - **Intro**：实体是数据库中的表的Java类表示，可以通过注解将它们关联起来。 - **JPA注解**：例如`@Entity`用于标记一个类为持久化实体，`@Table`定义实体对应的数据库表名。 - **简单属性**：使用`@Column`注解映射字段到表列，`@Basic`定义基本类型属性的映射。 - **标识符属性**：`@Id`用于标记主键字段，可以结合`@GeneratedValue`自动生成主键。 - **继承映射**：`@Inheritance`处理类的继承关系，如单表继承（SINGLE_TABLE）、分表继承（TABLE_PER_CLASS）和联合继承（JOINED）。 - **关联映射**：`@ManyToOne`, `@OneToOne`, `@OneToMany`, `@ManyToMany`处理不同类型的实体关联。 - **复合主键和外键**：使用`@EmbeddedId`和`@Embeddable`注解映射复合主键，`@AssociationOverride`处理复杂的关联映射。 - **二级表**：`@SecondaryTable`注解允许实体映射到多个表。 - **缓存**：`@Cacheable`和`@Cache`注解实现实体的缓存策略。 3. **映射查询** - **JP-QL/HQL查询**：`@NamedQuery`和`@NamedNativeQuery`用于预定义JP-QL（Java Persistence Query Language）和原生SQL查询。 - **原生查询**：`@SqlResultSetMapping`和`@NamedNativeQuery`配合处理原生SQL查询结果的映射。 4. **Hibernate注解扩展** - Hibernate提供了许多特有的注解，如`@EntityListeners`用于注册实体监听器，`@Temporal`处理日期和时间类型，以及`@TableGenerator`自定义主键生成器等。以上只是文档的一部分内容，实际指南中还涵盖了更多细节，包括但不限于级联操作（`@Cascade`），过滤器（`@Filter`），自定义SQL操作（`@SqlInsert`, `@SqlUpdate`, `@SqlDelete`），元数据处理（`@Tuplizer`）和fetch profile（`@FetchProfile`）等。通过这个指南，开发者能够深入理解和使用Hibernate注解，高效地实现Java应用与数据库之间的数据持久化。

Mapping simple properties

@Entity

public class Flight implements Serializable {

...

@Version

@Column(name="OPTLOCK")

public Integer getVersion() { ... }

}

The version property will be mapped to the OPTLOCK column, and the entity manager will use it to

detect conflicting updates (preventing lost updates you might otherwise see with the last-commit-

wins strategy).

The version column may be a numeric (the recommended solution) or a timestamp.

Hibernate supports any kind of type provided that you define and implement the appropriate

UserVersionType.

The application must not alter the version number set up by Hibernate in

any way. To artificially increase the version number, check in Hibernate Entity

Manager's reference documentation LockModeType.OPTIMISTIC_FORCE_INCREMENT or

LockModeType.PESSIMISTIC_FORCE_INCREMENT.

2.2.2. Mapping simple properties

2.2.2.1. Declaring basic property mappings

Every non static non transient property (field or method depending on the access type) of an entity

is considered persistent, unless you annotate it as @Transient. Not having an annotation for your

property is equivalent to the appropriate @Basic annotation. The @Basic annotation allows you to

declare the fetching strategy for a property:

public transient int counter; //transient property

private String firstname; //persistent property

@Transient

String getLengthInMeter() { ... } //transient property

String getName() {... } // persistent property

@Basic

int getLength() { ... } // persistent property

@Basic(fetch = FetchType.LAZY)

String getDetailedComment() { ... } // persistent property

@Temporal(TemporalType.TIME)

java.util.Date getDepartureTime() { ... } // persistent property

@Enumerated(EnumType.STRING)

Starred getNote() { ... } //enum persisted as String in database

Chapter 2. Mapping Entities

counter, a transient field, and lengthInMeter, a method annotated as @Transient, and will be

ignored by the entity manager. name, length, and firstname properties are mapped persistent

and eagerly fetched (the default for simple properties). The detailedComment property value will

be lazily fetched from the database once a lazy property of the entity is accessed for the first time.

Usually you don't need to lazy simple properties (not to be confused with lazy association fetching).

Note

To enable property level lazy fetching, your classes have to be instrumented:

bytecode is added to the original class to enable such feature, please refer to the

Hibernate reference documentation. If your classes are not instrumented, property

level lazy loading is silently ignored.

The recommended alternative is to use the projection capability of JP-QL (Java Persistence Query

Language) or Criteria queries.

JPA support property mapping of all basic types supported by Hibernate (all basic Java types ,

their respective wrappers and serializable classes). Hibernate Annotations support out of the box

enum type mapping either into a ordinal column (saving the enum ordinal) or a string based column

(saving the enum string representation): the persistence representation, defaulted to ordinal, can

be overridden through the @Enumerated annotation as shown in the note property example.

In plain Java APIs, the temporal precision of time is not defined. When dealing with temporal

data you might want to describe the expected precision in database. Temporal data can have

DATE, TIME, or TIMESTAMP precision (ie the actual date, only the time, or both). Use the @Temporal

annotation to fine tune that.

@Lob indicates that the property should be persisted in a Blob or a Clob depending on the property

type: java.sql.Clob, Character[], char[] and java.lang.String will be persisted in a Clob.

java.sql.Blob, Byte[], byte[] and serializable type will be persisted in a Blob.

@Lob

public String getFullText() {

return fullText;

}

@Lob

public byte[] getFullCode() {

return fullCode;

}

If the property type implements java.io.Serializable and is not a basic type, and if the property

is not annotated with @Lob, then the Hibernate serializable type is used.

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