Databases And Types
There are many vendors offering databases. Each dbms usually has some proprietary
The JDBC driver can enumerate a dbms' data types, and propose which Java data
types should be used for a given dbms type.
Some tables in databases may be of one type (VARCHAR for example), but are
known to hold a more specific kind of data (NUMERIC for example).
The wizard generates Java code to interact with the database. In order for
the wizard to be able to decide which Java types to use for each database entity
type, the wizard uses a type map.
The JDatabase wizard is equipped to deal with these situations. This section
describes how the JDatabase wizard can be used to finely control which Java data
types to use for every object in the database, or for individual columns of a
The code generated will contain data conversion routines where necessary.
The database tab is the first tab seen when flipping to the database view.
In most circumstances, it is not necessary to change information on this tab.
One exception we have come across is when using Microsoft Access databases (via
the JDBC-ODBC bridge). We have disabled catalogs, because the database does not typically
use catalog names in SQL queries.
- The product name displays the name of the database product, as provided
by the JDBC driver.
- The product version displays the version of the database product,
as reported by the JDBC driver.
- The catalog term reports what term the database product uses to refer
to database catalogs. A catalog is a JDBC term. Not all databases support
- The schema term reports what term the database product uses to refer
to database schemas. A schema is a JDBC term. Not all databases support schemas.
The above snapshot shows that Microsoft Access does not know of database schemas.
- The identifier quote string is used in database queries. For example,
should a table be called Big Customers, a select statement needs to put the
table name in quotes to avoid the problem a space character can cause: select
* from "Big Customers"
- The catalog separator defines what character is used when using catalog
names in queries. For example select * from Catalog.Schema.TableName The catalog
separator is the ',' between the catalog and the schema.
- Supports stored procedures specifies whether the DBMS supports stored
- Uses schemas in queries is used for generated SQL: if you would like
schemas to precede tables names in select statements, this option should be
selected. For example, on Oracle databases, the generated SQL would be of
the form "select * from SCOTT.EMP"
- Uses blank schemas in queries. This option is used if your dbms supports
schemas and catalogs. If you wish to use blank schemas in the generated SQL,
select this option. For example MS SQL server 6.5 and 7.0 should set this
option to getenrate select statements of the form: "select * from catalog..table".
- Uses no schemas in queries causes the generated SQL not to contain
schemas. For example, on Oracle databases, if you are connected as user scott,
you may wish to select this option to generate SQL of the form "select
* from EMP".
- Uses catalogs in queries controls whether the wizard uses catalog
names in SQL queries.
- Catalog starts query is used if the database has schemas and catalogs.
Either the schema appears first in SQL queries, or the catalog appears first.
- Should any changes have been made to the tab, the save button must
be pressed to save the changes.
The database type map tab allows the user to override the default data
mappings used when a new class is created. The wizard uses the type map to
determine which conversion, if any needs to be generated in the source code.
Type Map Controls:
- Use the "Type Name" combo control to select the database
- The "Get/Set Op." combo selects the ResultSet get/set operation
for this type.
- Using the "Procedure Get" combo, select the correct stored
procedure get for parameters. This combo is necessary, because some ResultSet
gets do not exist in the CallableStatement interface (for example getUnicodeStream
is not available in the CallableStatement interface).
- PreparedStatement setNull: when setting a column to NULL, JDBC requires
a java.sql.Types constant to be specified. This combo allows the type to be
- CallableStatement type: parameters need OUTPUT parameters and the
NULL to be assigned a type. This combo box allows a default java.sql.Types
constant to be specified.
- The Java type is the type you would like to work with in your Java
application. The wizard generates the code necessary to convert the data read
in the JDBC call to the java type you select here.
- By default types are Select, Insert and Update. This
means columns of this type can be selected, inserted and updated. It is possible
that some database types should be selected only (automatically incremented
data types), or inserted but not updated, or not used at all. Complete control
over the default operations is here. Please note that automatically updated
data types, where detected by the wizard are set to select only, this can
be changed here, should you disagree with this policy.
- Once the currently viewed database type's mapping has been altered, the
SAVE button should be pressed to save the changes for the currently
displayed database type.
Entity Type Maps
As stated in the introduction, some database
objects may be specified as of one type, when the data they contain is of
another type. The JDatabase wizard can be made to convert between types for a given
database object by using the class view tab.
We use the Property Editor to tell the JDatabase wizard what the "Property
Type" is, what ResultSet.get...() and set to call. The database JDBC type
is salso specified here.
The code generated will attempt to convert data types between the "get
set op" and the "property type", should they be diferent.
The sql access (select, insert, update) can be selected here.
The advanced tab allows server side expressions to be entered for insert and
update operations. The column access should include insert/update for the settings
below to take effect.
Please note that no question mark (?) character may be entered here.
The expressions can be used when the database does not allow default column
values to be set.
JDBC Types Supported
Version 1.0 of the JDatabase wizard supports the following JDBC types.
When creating a database class within the wizard, data class properties are
created. For example in a PERSON table, FIRST_NAME, LAST_NAME, table columns
have a corresponding property created for them.
When a class is created, the wizard can apply java styling to the column names.
This is usually a good idea. Database objects names do not always make good
java class property names. However, sometimes this may not be the preferred
When checked, the checkbox causes all new classes to use java styling. This
means that data class properties fit in with the java naming conventions. For
example a FIRST_NAME column becomes a FirstName data class property. If the
checkbox is not checked, any new generated data classes will use FIRST_NAME
as the java class property name.
This option only affects new classes created in the wizard. Any existing
classes in the wizard retain their original property names.
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