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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While a simple display of data is interesting in itself, most objects will also need to be modified and restored back to the database. Setting the ActivationDepth to 0 will cause nothing to be activated. This, along with the emergence of a viable alternative, has led to a renaissance of interest in object persistence and retrieval. You can use the same Store object command to update an object.
Note that there is a more efficient way to perform this style of query against the underlying objects stored on disk using the db4o SODA query API, but it’s slightly out of scope for this article, so I’ll leave it for a later discussion.
Walking the results is a simple exercise in using the Iterator interface implemented by ObjectSet.
Getting Started With db4o
It’s fairly easy to see how any moderately complicated query will rapidly become unworkable using this technique, particularly when compared against the simplicity of a query language like SQL. The default ActivationDepth in db4o is 5. When the environment is tutoriwl to run with reduced trust, all basic database operations require at least ReflectPermission MemberAccess and ReflectionEmit for the classes and types being persisted.
Actually, much of the utility method’s utility become apparent when placed onto the stored class itself, as shown in Listing Des tortues marines au long cours.
Queries, updates, and identity
You can also use the db. In order to use the interactive functionality a Java JRE 1. For strings, matching values will contain the supplied value anywhere within the match. When you unzip this file, you getthe following directory structure: Queries, updates, and identity. When you tutrial an ObjectContainer, you are in a transaction, when you commit or rollbackthe next transaction is started immediately.
NET Framework to be installed. Obviously, this would only be desirable during development — at runtime it tuttorial be preferable to log this failure to a log4j error stream or something equally less distracting to the user.
You will only need one of the following libraries, not all of them.
To close the transaction, use the Commit method:. You can then call the Activate object, depth method to manually activate the object as described above. Java developers in particular are frustrated with the so-called object-relational impedance mismatch, and impatient with the solutions that attempt to resolve it.
Astute readers of Tutirial Bloch’s Effective Java will notice that I’ve left out the hashCode implementation, a clear violation of Rule 8. First Glance Before diving straight into the first source code samples let’s get you familiar with some basics.
The db4o container doesn’t run into problems of identity here because the object in question has already been identified as one coming from the database, meaning its OID is already stored inside the db4o bookkeeping infrastructure. It’s important to know that db4o works best if you open the database file when you start working with data, and close the db when all posible operations are completed.
The yutorial values affect only the state of field indexes and when the evaluation is performed. Ttutorial Object Data Walks you through db4o’s basic operations, its various query types, and techniques for optimal data access performance.
By default, the db4o server runs in-process within your application.
To start a db4o server, place this code into your application:. Using the particular methods of Person would require a downcast on the objects returned by next. Walks you through db4o’s basic operations, its various query types, and techniques for optimal data access performance.
The busy Java developer’s guide to db4o: Queries, updates, and identity
When an object tuyorial stored into an object database, a unique key is created, called an Object identifier or OID pronounced similarly to the last syllable of avoidwhich uniquely identifies that object. As you’ll learn, it’s actually quite amazing what you can do once you’re freed from the constraints of SQL. Welcome db4o is the native Java. Store new Person “Petra” ; db. This multi-faceted approach to querying databases may strike you as complex and confusing and entirely different from how an RDBMS works.
I have a db4o database in which I have a type Person that I want to query based on a set of fields that have certain values in them. It is not required at all for embedded servers. Because QBE uses a prototype object as its template to search for data, there are a few simple rules regarding its usage. As it turns out, db4o does neither of these; instead, the principals behind db4o chose to take an interesting and innovative approach to native queries.
Query by Example QBE is a database query language that allows you to create queries by designing a “template” against which to do comparisons, rather than a titorial using predicate criteria as in SQL.