@Column in Java

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Using the @Column annotation, we set the name property s column name to be employee_name and not nullable, and we set its database type to be an integer. We also set the VARCHAR length to 20. This is often important to save RAM in the database and
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keep things moving efficiently; if you don t need the extra space, it s a good idea to restrict your field types to take up the least amount of data as possible. The @javax.persistence.Column annotation describes how a particular field or property is mapped to a specific column in a table:
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public @interface Column { String name() default ""; boolean unique() default false; boolean nullable() default true; boolean insertable() default true; boolean updatable() default true; String columnDefinition() default ""; String table() default ""; int length() default 255; int precision() default 0; int scale() default 0; }
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The name() attribute obviously specifies the column name. If it is unspecified, the column name defaults to the property or field name. The table() attribute is used for multitable mappings, which we cover later in this chapter. The rest of the attributes are used when you are autogenerating the schema from vendor-provided tools. If you are mapping to an existing schema, you do not need to define any of these attributes. The unique() and nullable() attributes define constraints you want placed on the column. You can specify whether you want this column to be included in SQL INSERT or UPDATE by using insertable() and updatable(), respectively. The columnDefinition() attribute allows you to define the exact DDL used to define the column type. The length() attribute determines the length of a VARCHAR when you have a String property. For numeric properties, you can define the scale() and precision() attributes.
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Primary Keys
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A primary key is the identity of a given entity bean. Every entity bean must have a primary key, and it must be unique. Primary keys can map to one or more properties and must map to one of the following types: any Java primitive type (including wrappers), java.lang.String, or a primary-key class composed of primitives and/or strings. Let s first focus on simple one-property primary keys.
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@Id
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The @javax.persistence.Id annotation identifies one or more properties that make up the primary key for your table:
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package javax.persistence; @Target({METHOD, FIELD}) @Retention(RUNTIME) public @interface Id
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{ }
You can generate the primary key for your entity beans manually or have the persistence provider do it for you. When you want provider-generated keys, you have to use the @javax.persistence.GeneratedValue annotation:
package javax.persistence; @Target({METHOD, FIELD}) @Retention(RUNTIME) public @interface GeneratedValue { GenerationType strategy() default AUTO; String generator() default ""; } public enum GenerationType { TABLE, SEQUENCE, IDENTITY, AUTO }
Persistence providers are required to provide key generation for primitive primary keys. You can define the type of primary generator you would like to have using the strategy() attribute. The GeneratorType.AUTO strategy is the most commonly used configuration, and it is the default:
/** * Primary key */ @Id @GeneratedValue private Long id;
Table Generators
The TABLE strategy designates a user-defined relational table from which the numeric keys will be generated. A relational table with the following logical structure is used:
create table GENERATOR_TABLE ( PRIMARY_KEY_COLUMN VARCHAR not null, VALUE_COLUMN long not null );
The PRIMARY_KEY_COLUMN holds a value that is used to match the primary key you are generating for. The VALUE_COLUMN holds the value of the counter. To use this strategy, you must have already defined a table generator using the @javax.persistence.TableGenerator annotation. This annotation can be applied to a class or to the method or field of the primary key:
@Target({ TYPE, METHOD, FIELD }) @Retention(RUNTIME)
public @interface TableGenerator { String name(); String table() default ""; String catalog() default ""; String schema() default ""; String pkColumnName() default ""; String valueColumnName() default ""; String pkColumnValue() default ""; int initialValue() default 0; int allocationSize() default 50; UniqueConstraint[] uniqueConstraints() default { }; }
The name() attribute defines the name of the @TableGenerator and is the name referenced in the @Id.generator() attribute. The table(), catalog(), and schema() attributes describe the table definition of the generator table. The pkColumnName() attribute is the name of the column that identifies the specific entity primary key you are generating for. The valueColumnName() attribute specifies the name of the column that will hold the counter for the generated primary key. pkColumnValue() is the value used to match up with the primary key you are generating for. The allocationSize() attribute is how much the counter will be incremented when the persistence provider queries the table for a new value. This allows the provider to cache blocks so that it doesn t have to go to the database every time it needs a new ID. If you are autogenerating this table, then you can also define some constraints using the uniqueConstraints() attribute. Let s look at how you would actually use this generator on the Employee entity:
@Entity public class Employee implements java.io.Serializable { @TableGenerator(name="MY_GENERATOR" table="GENERATOR_TABLE" pkColumnName="PRIMARY_KEY_COLUMN" valueColumnName="VALUE_COLUMN" pkColumnValue="EMPLOYEE_ID" allocationSize=10) @Id @GeneratedValue (strategy=GenerationType.TABLE, generator="MY_GENERATOR") private long id; ... }
Now if you allocate and persist() an Employee entity, the id property will be autogenerated when the persist() operation is called.
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