ORMLite教程：映射与持久化详解

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ORMLite是一款轻量级的对象关系映射（Object-Relational Mapping, ORM）工具，用于简化Java应用程序与数据库的交互。这个文档是版本4.40，发布于2012年4月，由GrayWatson提供，遵循Creative Commons Attribution-Share Alike 3.0许可证。它详细介绍了如何在Java项目中集成ORMLite，包括设置、配置、数据类型支持、连接管理、DAO（Data Access Object）的使用、表和模式创建、自增ID处理、索引和原始SQL查询等关键知识点。 1. **开始使用**: - 下载ORMLite Jar包：文档提供了下载链接，确保将其添加到项目的构建路径中。 - 配置类和DAO: 学习如何为要映射的实体类添加ORMLite注解，以及如何配置DAO来执行数据库操作。 2. **配置和数据类型**: - 添加ORMLite注解: 使用`@Table`、`@Column`等注解来指定表结构和字段属性。 - `javax.persistence`注解兼容性: 提供了对标准JPA注解的支持，可以结合使用。 - 必须包含无参构造函数：ORMLite需要无参构造函数来实例化对象。 3. **连接管理**: - 数据库连接源：可以使用内置的连接池或自定义的连接源，如JDBC URL、DataSource或Connection对象。 4. **DAO的使用**: - 创建和使用DAO: DAO提供了CRUD（Create, Read, Update, Delete）操作接口，简化了数据库操作的封装。 5. **数据库支持**: - 支持的数据库：ORMLite支持多种数据库，包括SQLite、MySQL、PostgreSQL等，可能需要额外的驱动程序。 6. **表和模式创建**: - TableUtils和TableCreator类：这两个工具分别用于动态创建表和获取表信息，简化了数据库初始化过程。 - 自动识别主键：ORMLite能自动识别`id`字段作为主键，有三种模式可供选择：字段类型、自动生成ID和序列。 7. **高级特性**: - 索引字段：提高查询性能，可通过注解指定需要索引的字段。 - 原生SQL查询：允许在ORM之外直接执行SQL语句，增强灵活性。通过阅读这份文档，开发人员可以快速上手ORMLite，并了解如何在实际项目中有效地管理和操作数据库，从而提升代码的可维护性和性能。学习并掌握这些知识点对于Java开发者来说是至关重要的，因为它能够简化数据库操作，降低编程复杂度，尤其是在处理大量数据和频繁的数据交互场景中。

10Chapter 2: How to Use 16 April 2012

In our example, the foreign ﬁelds in Question and Answer could be set to auto-

refresh. If this is true then with the maxForeignAutoRefreshLevel set to 1,

when you query for a Question, the Answer ﬁeld will be auto-refreshed, but

the Question ﬁeld on the answer will only have its id ﬁeld set. It will not be

auto-refreshed.

allowGeneratedIdInsert

If this is set to true (default is false) then inserting an object with the ID ﬁeld

already set will not override it with a generated-id. This is useful when you have

a table where you are inserting items that sometimes have IDs and sometimes

need them generated. This only works if the database supports this behavior

and if generatedId is also true for the ﬁeld.

columnDefinition

If this is set with a string then it will be used to deﬁne the column in the CREATE

TABLE statement. By default the database type is used to auto-generate the SQL

necessary to create the column. The column name is provided by ORMLite.

For example:

@DatabaseField(columnDefinition = "LONGBLOB not null",

dataType = DataType.BYTE_ARRAY)

public byte[] bigBunchOfBytes;

foreignAutoCreate

Set this to be true (default false) to have the foreign ﬁeld automatically created

using an internal DAO if its ID ﬁeld is not set (null or 0). So when you call

dao.create() on the parent object, any foreign ﬁeld that has this set to true

will possibly generate an additonal create call via an internal DAO. By default

you have to create the object using its DAO directly. By default you have to

create the object using its DAO directly. This only works if generatedId is

also set to true.

// the account field in Order has foreignAutoCreate=true

Order order1 = new Order();

// account1 has not been created in the db yet, id field is null

order1.account = account1;

// this will create order1 in the order table _and_

// pass order1.account to the internal accountDao.create().

orderDao.create(order1);

version

Set this to true (default false) to have this ﬁeld be a version for the row. A

version ﬁeld adds restrictions when an object is updated to the datbase that

protects against data corruption when multiple entites are updating the row

at the same time. This is very useful in distributed database scenarios if (for

example) a utility process is updating an Account and you don’t want to protect

against overwriting a user update from the web-server.

1. The current-value of the version ﬁeld is read from the object

2. The update statement is augmented with a "WHERE version = current-

value"

11Chapter 2: How to Use 16 April 2012

3. The new-value being updated in the database is changed by ORMLite to

be the current-value + 1 or the current java.util.Date

4. If the row has been updated by another entity then the update will not

change the database since the current-value will not match the version

column in the database and 0 rows changed will be returned by the update

method

5. However, if the current-value does match the version ﬁeld in the database

then the row will be updated, the object’s version ﬁeld will be set by

ORMLite to the new-value, 1 will be returned from update

Only the following ﬁeld types are appropriate for version ﬁelds: short, integer,

long, and Date ﬁelds.

foreignColumnName

Name of the foreign object’s ﬁeld that is tied to this table. This does not need to

be speciﬁed if you are using the ID of the foreign object which is recommended.

For example, if you have an Order object with a foreign Account then you may

want to key oﬀ of the Account name instead of the ID.

NOTE: Setting this implies foreignAutoRefresh is also set to true because

there is no way to refresh the object since the id ﬁeld is not stored in the

database. So when this is set, the ﬁeld will be automatically refreshed in another

database query.

2.1.2 Using javax.persistence Annotations

Instead of using the ORMLite annotations (see

Section 2.1.1 [Local Annotations],

page 5), you can use the more standard JPA annotations from the javax.persistence

package. In place of the @DatabaseTable annotation, you can use the javax.persistence

@Entity annotation. For example:

@Entity(name = "accounts")

public class Account {

...

The @Entity annotations can have an optional name argument which speciﬁes the table

name. If not speciﬁed, the class name with normalized case is used by default.

Instead of using the @DatabaseField annotation on each of the ﬁelds, you can use

the javax.persistence annotations: @Column, @Id, @GeneratedValue, @OneToOne,

@ManyToOne, @JoinColumn, and @Version. For example:

@Entity(name = "accounts")

public class Account {

@Id

private String name;

@Column(nullable = false)

private String password;

...

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ORMLite教程：映射与持久化详解

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