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Build an object database, Part 2: Object storage backend

Implement relational database storage for Java objects

By Michael Shoffner and Merlin Hughes, JavaWorld.com, 04/07/00

Page 2 of 6

Class StorageFields

As you'll no doubt recall, the StorageFields is a collection of the scattered fields, as well as the stored Java object's type information. The ObjectStorer implementation hands an instance of this class to the ObjectStorage implementation as a parameter to the put() method.

Class RetrievalFields

The RetrievalFields class is a collection of fields returned by the ObjectStorage implementation in response to a get() call. It is practically the same as the StorageFields class, except that there is no need to explicitly state type information since it is represented by the class of the object being returned.

We'll call our ObjectStorage implementation class SQLObjectStorage.

SQLObjectStorage's data model and constraints

Before we delve into the code, let's outline how our SQLObjectStorage's relational data model will work.

Figure 2. SQLObjectStorage's data model

The main concept behind this extremely simple data model is: SQLObjectStorage creates a relational table for each class to be stored in the database. SQLObjectStorage then stores instances of a given class in the table created to correspond to that class.

Each table includes a set of columns that correspond to the fields defined in the class. SQLObjectStorage dynamically creates tables and columns using a database-specific mapping from Java types to column types supported by the database.

A bit of overhead information -- the mapping from each key to its respective instance in the database -- is kept in the database in addition to the stored instances. To achieve this end, SQLObjectStorage creates a key column in each table to key the instances therein. It also creates a special key table in the database to store every key and the table where it resides.

In addition, SQLObjectStorage imposes the following constraints:

• Each database's keys are unique: A key uniquely identifies an object in a specific database. If a key/value pair exists in the database and another value is entered with the same key, the new value replaces the old value.
• There are no duplicate entries per key: Each key will store against only one value.
• One table exists per primary class type (classname): Each class is stored as an instance of its primary class, with all of its fields as well as superclass fields flattened out into the columns of the table that bears its name.
• Class and field names are escaped: In order to avoid illegal database characters, encode class names with escape characters to get table names. The same process applies to mapping fields to column names.
• Values are escaped: Field values are escaped to avoid problems with SQL statement syntax.

Abstract class SQLObjectStorage

So now we come to the ObjectStorage implementation. We'll look at just the important methods here. (For the complete source code, see Resources.)

First, we look at the ObjectStorage-interface method implementations, starting with put().

The put() implementation takes a key and a StorageFields object as arguments. It then removes any existing entries in the database under the key. If there is an object to store, it creates a table and stores the object's field values to it as follows: