CHAPTER 6 (Best web design) n MAPPING WITH ANNOTATIONS 113 n
CHAPTER 6 n MAPPING WITH ANNOTATIONS 113 n MAPPING WITH ANNOTATIONS 113 nullable permits the column to be marked NOT NULL when the schema is generated. The default is that fields should be permitted to be null; however, it is common to override this when a field is, or ought to be, mandatory. unique permits the column to be marked as containing only unique values. This defaults to false, but commonly would be set for a value that might not be a primary key but would still cause problems if duplicated (such as username). We have marked up the title field of our Book entity using the @Column entity to show how three of these attributes would be applied: @Column(name=”working_title”,length=200,nullable=false) public String getTitle() { return title; } The remaining attributes, less commonly used, are as follows: table is used when the owning entity has been mapped across one or more secondary tables. By default, the value is assumed to be drawn from the primary table, but the name of one of the secondary tables can be substituted here (see the @SecondaryTable annotation example earlier in this chapter). insertable defaults to true, but if set to false, the annotated field will be omitted from insert statements generated by Hibernate (i.e., it won t be persisted). updatable defaults to true, but if set to false, the annotated field will be omitted from update statements generated by Hibernate (i.e., it won t be altered once it has been persisted). columnDefinition can be set to an appropriate DDL fragment to be used when generating the column in the database. This can only be used during schema generation from the annotated entity, and should be avoided if possible, since it is likely to reduce the portability of your application between database dialects. precision permits the precision of decimal numeric columns to be specified for schema generation, and will be ignored when a non-decimal value is persisted. The value given represents the number of digits in the number (usually requiring a minimum length of n+1, where n is the scale). scale permits the scale of decimal numeric columns to be specified for schema generation and will be ignored where a non-decimal value is persisted. The value given represents the number of places after the decimal point. Modeling Entity Relationships Naturally, annotations also allow you to model associations between entities. EJB 3 supports one-to-one, one-to-many, many-to-one, and many-to-many associations. Each of these has its corresponding annotation. We discussed the various ways in which these mappings can be established in the tables in Chapter 5. In this section, we will show how the various mappings are requested using the annotations.
Note: If you are looking for best quality webspace to host and run your tomcat application check Vision shared web hosting services