Welcome. db4o is the native Java,.NET and Mono open source object database. This tutorial was written to get you started with db4o as quickly as possible. This documentation and tutorial is intended to get you started with db4o and to be a reliable The db4o object database engine consists of one single jar file. Introduction; Query by Example; Updates and identity; A search utility method; Advanced queries; Native queries in db4o; In conclusion.
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Simply register a listener Db4oQueryExecutionListener against the ObjectContainer itself to inform you if a native query cannot be optimized, as shown in Listing The db4o interface is supplied in two steps in two java packages, com.
Intuition isn’t always sufficient for anticipating a failure to optimize because the reasons can be entirely different from what a code review would imply. Setting the ActivationDepth to int. QBE is the preferred mechanism for simple query situations because it’s an easier API to work with, but it does require that your domain objects permit any or all of the fields containing data to be set dn4o null, which may violate some of your domain rules.
In our example this will be a Formula One F1 pilot whose attributes are his name and the F1 points he has already gained this season.
There is one more thing to be aware of: The drawback of OQL is that it wants to return Because QBE uses a prototype object as its template to search for data, there are a few simple rules regarding its usage.
It can be assembled again in the morning, but one eventually asks whether this is the most efficient way to park a car. While a db4o database file is open, it is locked and cannot be accessed by another application at the same time. NQ can do all this because they are expressed as native. When you run the query in Listing 3, the database reports three Brian s, two of them Brian Goetz.
Setting the UpdateDepth to int. Query Evaluation Mode This property controls when and how much of a query is executed. Loosely put, the db4o system sends a predicate to the database, where it performs bytecode analysis at runtime on the bytecode for the match method. These handlers are great for selectively activating, refreshing, or cascading db operations.
Information storage and retrieval has been nearly synonymous with RDBMS for about a decade now, but recently that has begun to change. It also lets you configure the db4o environment before opening a database. First we create a class to hold our data.
If concerned about consistency, use the Refresh object command to cause the objects to be refreshed with stored database values. The object returned will have none of its values loaded. This means that there is only ever 1 instance of an object in memory for each stored instance of the object.
Getting Started With db4o
The key thing to remember is that you can build convenience methods on top of the db4o infrastructure to make it easier to use. Two objects of note are com.
A similar effect would occur if the persons. The application developer should tune the number of indexes futorial the desired Query and Insert performance. After starting tutoriql db4o server instance, use either of the commands below to open a db4o client connection:. Refcard Getting Started With db4o. Rollback only undoes uncommitted changes in the database.
The busy Java developer’s guide to db4o: Queries, updates, and identity
Failing to include these libraries will not generate an error of any kind but will simply cause every native query to be unoptimized. This also points out a limitation of the QBE approach — zero cannot effectively be used as a value to search on. Always make sure that the object you are trying to update or delete was previously stored or retrieved in the database. Rather than force a complex query API onto developers or introduce a new “something-QL,” db4o offers a facility called native querieswhich is both powerful and remarkably easy to use, as you can begin to see in Listing Optimization is still done inside the database, but instead of text being returned real objects are sent back, ready for use.
But oftentimes, you will want to configure specific behaviors per-class-type, or per-field. Since the index snapshot is held in memory, the memory required varies greatly depending on what is being queried.
db4o tutorial – db4objects Inc. – Informatique
We will use a convenience method listResult to display the contents of our result: As you’ll learn, it’s actually quite amazing what you can do once you’re freed from the constraints of SQL. While you read through this tutorial it may be helpful to look into the API documentation occasionaly. For all but the simplest db4o use cases, you’ll probably want to specify one or more configuration settings when opening your db4o database:. Best for servers and concurrent environments.
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