generate and print barcodes c# Reverse engineering a legacy database in Java

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Reverse engineering a legacy database
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Hibernate mapping metadata has many more options, most of which are related to reverse engineering, as to how XML mapping files, Java code, or even whole application skeletons can be generated automatically from an existing database schema. We ll first show you how to write an Ant target that can load an existing database into a Hibernate metadata model. Next, you ll apply various exporters and produce XML files, Java code, and other useful artifacts from the database tables and columns.
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Creating a database configuration
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Let s assume that you have a new WORKDIR with nothing but the lib directory (and its usual contents) and an empty src directory. To generate mappings and code from an existing database, you first need to create a configuration file that contains your database connection settings:
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hibernate.dialect = org.hibernate.dialect.HSQLDialect hibernate.connection.driver_class = org.hsqldb.jdbcDriver hibernate.connection.url = jdbc:hsqldb:hsql://localhost hibernate.connection.username = sa
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Store this file directly in WORKDIR, and name it helloworld.db.properties. The four lines shown here are the minimum that is required to connect to the database and read the metadata of all tables and columns. You could have created a Hibernate XML configuration file instead of hibernate.properties, but there is no reason to make this more complex than necessary. Write the Ant target next. In a build.xml file in your project, add the following code:
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<taskdef name="hibernatetool" classname="org.hibernate.tool.ant.HibernateToolTask" classpathref="project.classpath"/> <target name="reveng.hbmxml" description="Produces XML mapping files in src directory"> <hibernatetool destdir="${basedir}/src"> <jdbcconfiguration propertyfile="${basedir}/helloworld.db.properties" revengfile="${basedir}/helloworld.reveng.xml"/> <hbm2hbmxml/> <!-- Export Hibernate XML files --> <hbm2cfgxml/> <!-- Export a hibernate.cfg.xml file --> </hibernatetool> </target>
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The HibernateToolTask definition for Ant is the same as before. We assume that you ll reuse most of the build file introduced in previous sections, and that references such as project.classpath are the same. The <hibernatetool> task is set with WORKDIR/src as the default destination directory for all generated artifacts. A <jdbconfiguration> is a Hibernate tool configuration that can connect to a database via JDBC and read the JDBC metadata from the database catalog. You usually configure it with two options: database connection settings (the properties file) and an optional reverse-engineering customization file. The metadata produced by the tool configuration is then fed to exporters. The example Ant target names two such exporters: the hbm2hbmxml exporter, as you can guess from its name, takes Hibernate metadata (hbm) from a configuration, and generates Hibernate XML mapping files; the second exporter can prepare a hibernate.cfg.xml file that lists all the generated XML mapping files. Before we talk about these and various other exporters, let s spend a minute on the reverse-engineering customization file and what you can do with it.
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Customizing reverse engineering
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JDBC metadata that is, the information you can read from a database about itself via JDBC often isn t sufficient to create a perfect XML mapping file, let alone Java application code. The opposite may also be true: Your database may contain information that you want to ignore (such as particular tables or columns) or that you wish to transform with nondefault strategies. You can customize the reverseengineering procedure with a reverse-engineering configuration file, which uses an XML syntax. Let s assume that you re reverse-engineering the Hello World database you created earlier in this chapter, with its single MESSAGES table and only a few columns. With a helloworld.reveng.xml file, as shown in listing 2.17, you can customize this reverse engineering.
Listing 2.17 Configuration for customized reverse engineering
< xml version="1.0" encoding="UTF-8" > <!DOCTYPE hibernate-reverse-engineering SYSTEM "http://hibernate.sourceforge.net/ hibernate-reverse-engineering-3.0.dtd"> <hibernate-reverse-engineering> <table-filter match-name=".*" package="hello"/> <table name="MESSAGES" schema="PUBLIC" class="Message"> <primary-key>
Reverse engineering a legacy database
<generator class="increment"/> <key-column name="MESSAGE_ID" property="id" type="long"/> </primary-key>
<column name="MESSAGE_TEXT" property="text"/> <foreign-key constraint-name="FK_NEXT_MESSAGE"> <many-to-one property="nextMessage"/> <set exclude="true"/> </foreign-key> </table> </hibernate-reverse-engineering>
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This XML file has its own DTD for validation and autocompletion. A table filter can exclude tables by name with a regular expression. However, in this example, you define a a default package for all classes produced for the tables matching the regular expression. You can customize individual tables by name. The schema name is usually optional, but HSQLDB assigns the PUBLIC schema to all tables by default so this setting is needed to identify the table when the JDBC metadata is retrieved. You can also set a custom class name for the generated entity here. The primary key column generates a property named id, the default would be messageId. You also explicitly declare which Hibernate identifier generator should be used. An individual column can be excluded or, in this case, the name of the generated property can be specified the default would be messageText. If the foreign key constraint FK_NEXT_MESSAGE is retrieved from JDBC metadata, a many-to-one association is created by default to the target entity of that class. By matching the foreign key constraint by name, you can specify whether an inverse collection (one-to-many) should also be generated (the example excludes this) and what the name of the many-to-one property should be. If you now run the Ant target with this customization, it generates a Message.hbm.xml file in the hello package in your source directory. (You need to copy the Freemarker and jTidy JAR files into your library directory first.) The customizations you made result in the same Hibernate mapping file you wrote earlier by hand, shown in listing 2.2. In addition to the XML mapping file, the Ant target also generates a Hibernate XML configuration file in the source directory:
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