data matrix barcode generator java Mapping the columns in Java

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8.2.2 Mapping the columns
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The @Column annotation maps a persisted field or property to a table column. All of the fields used in listing 8.3 are annotated with @Column. For example, the userId field is mapped to the USER_ID column:
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@Column(name="USER_ID") protected Long userId;
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It is assumed that the USER_ID column belongs to the USERS table specified by the @Table annotation. Most often, this is as simple as your @Column annotation will look. At best, you might need to explicitly specify which table the persisted column belongs to (when you map your entity to multiple tables using the @SecondaryTable annotation) as we do for the picture field in listing 8.4:
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@Column(name="PICTURE", table="USER_PICTURES") ... protected byte[] picture;
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As you can see from the definition in listing 8.5, a number of other parameters exist for the annotation.
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Mapping entities
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Listing 8.5 The @Column annotation
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@Target({METHOD, FIELD}) @Retention(RUNTIME) public @interface Column { String name() default ""; Specifies unique constraint boolean unique() default false; boolean nullable() default true; Specifies if column boolean insertable() default true; allows nulls boolean updatable() default true; String columnDefinition() default ""; String table() default ""; Length of column int length() default 255; int precision() default 0; Decimal precision of column int scale() default 0; } Decimal scale of column
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The insertable and updatable parameters are used to control persistence behavior. If the insertable parameter is set to false, the field or property will not be included in the INSERT statement generated by the persistence provider to create a new record corresponding to the entity. Likewise, setting the updatable parameter to false excludes the field or property from being updated when the entity is saved. These two parameters are usually helpful in dealing with read-only data, like primary keys generated by the database. They could be applied to the userId field as follows:
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@Column(name="USER_ID", insertable=false, updatable=false) protected Long userId;
When a User entity is first created in the database, the persistence provider does not include the USER_ID as part of the generated INSERT statement. Instead, we could be populating the USER_ID column through an INSERT-induced trigger on the database server side. Similarly, since it does not make much sense to update a generated key, it is not included in the UPDATE statement for the entity either. The rest of the parameters of the @Column annotation are only used for automatic table generation and specify column creation data. The nullable parameter specifies whether the column supports null values C, the unique parameter b indicates if the column has a unique constraint, the length parameter D specifies the size of the database column, the precision parameter E specifies the precision of a decimal field, and the scale parameter F specifies the scale of a decimal column. Finally, the columnDefinition parameter allows you to specify the exact SQL to create the column.
Object-relational mapping
We won t cover these parameters much further than this basic information since we do not encourage automatic table creation. Note that the @Column annotation is optional. If omitted, a persistent field or property is saved to the table column matching the field or property name. For example, a property specified by the getName and setName methods will be saved into the NAME column of the table for the entity. Next, let s take a look at a few more annotations applied to entity data, starting with @Enumerated.
8.2.3 Using @Enumerated
Languages like C and Pascal have had enumerated data types for decades. Enumerations were finally introduced in Java 5. In case you are unfamiliar with them, we ll start with the basics. In listing 8.4, the user type field has a type of UserType. UserType is a Java enumeration that is defined as follows:
public enum UserType {SELLER, BIDDER, CSR, ADMIN};
This effectively means that any data type defined as UserType (like our persistent field in the User object) can only have the four values listed in the enumeration. Like an array, each element of the enumeration is associated with an index called the ordinal. For example, the UserType.SELLER value has an ordinal of 0, the UserType.BIDDER value has an ordinal of 1, and so on. The problem is determining how to store the value of enumerated data into the column. The Java Persistence API supports two options through the @Enumerated annotation. In our case, we specify that the ordinal value should be saved into the database:
@Enumerated(EnumType.ORDINAL) ... protected UserType userType;
This means that if the value of the field is set to UserType.SELLER, the value 0 will be stored into the database. Alternatively, you can specify that the enumeration value name should be stored as a String:
@Enumerated(EnumType.STRING) ... protected UserType userType;
In this case a UserType.ADMIN value would be saved into the database as "ADMIN". By default an enumerated field or property is saved as an ordinal. This would be the case if the @Enumerated annotation is omitted altogether, or no parameter to the annotation is specified.
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