1. First Glance

Before diving straight into the first source code samples let's get you familiar with some basics.


    1.1. The db4o engine...

    The db4o object database engine consists of one single jar file. This is all that you need to program against. The versions supplied with the distribution can be found in /db4o-6.1/lib/. You will only need one of the following libraries, not all of them.

    db4o-6.1-java1.1.jar
    will run with most Java JDKs that supply JDK 1.1.x functionality such as reflection and Exception handling. That includes many IBM J9 configurations, Symbian and Savaje.

    db4o-6.1-java1.2.jar
    is built for all Java JDKs between 1.2 and 1.4.

    db4o-6.1-java5.jar
    is built for Java JDK 5.


    1.2. Installation

    If you add one of the above db4o-*.jar files to your CLASSPATH db4o is installed. In case you work with an integrated development environment like Eclipse you would copy the db4o-*.jar to the /lib/ folder under your project and add db4o to your project as a library.



    1.3. API Overview

    Do not forget the API documentation while reading through this tutorial. It provides an organized view of the API, looking from a java package perspective and you may find related functionality to the theme you are currently reading up on.

    For starters, the java packages com.db4o and com.db4o.query are all that you need to worry about.

    com.db4o

    The com.db4o java package contains almost all of the functionality you will commonly need when using db4o. Two objects of note are com.db4o.Db4o, and the com.db4o.ObjectContainer interface.

    The com.db4o.Db4o factory is your starting point. Static methods in this class allow you to open a database file, start a server, or connect to an existing server. It also lets you configure the db4o environment before opening a database.

    The most important interface, and the one that you will be using 99% of the time is
    com.db4o.ObjectContainer: This is your db4o database.
    - An ObjectContainer can either be a database in single-user mode or a client connection to a db4o server.
    - Every ObjectContainer owns one transaction. All work is transactional. When you open an ObjectContainer, you are in a transaction, when you commit() or rollback(), the next transaction is started immediately.
    - Every ObjectContainer maintains it's own references to stored and instantiated objects. In doing so, it manages object identities, and is able to achieve a high level of performance.
    - ObjectContainers are intended to be kept open as long as you work against them. When you close an ObjectContainer, all database references to objects in RAM will be discarded.

    com.db4o.ext

    In case you wonder why you only see very few methods in an ObjectContainer, here is why: The db4o interface is supplied in two steps in two java packages, com.db4o and com.db4o.ext for the following reasons:
    - It's easier to get started, because the important methods are emphasized.
    - It will be easier for other products to copy the basic db4o interface.
    - It is an example of how a lightweight version of db4o could look.

    Every com.db4o.ObjectContainer object is also an com.db4o.ext.ExtObjectContainer. You can cast it to ExtObjectContainer or you can use the method  to get to the advanced features.

    com.db4o.config

    The com.db4o.config java package contains types and classes necessary to configure db4o. The objects and interfaces within are discussed in the Configuration section.

    com.db4o.query

    The com.db4o.query java package contains the Predicate class to construct Native Queries. The Native Query interface is the primary db4o querying interface and should be preferred over the Soda Query API.




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    Doctor courtesy of db4objects Inc.