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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It is not required at all for embedded servers. Queries, updates, and identity. Examples can be run “live” against a db4o database from withinPDF by iText, generated by Doctor, courtesy of db4objects Inc.
There is one more thing to be aware of: In doing so, it manages object identities, and is able to achieve a high level of performance. The result set can be influenced by subsequent transactions in the current context or by other clients causing possible concurrency errors.
For all but the simplest db4o use cases, you’ll probably want to specify one or more configuration settings when opening your db4o database:. Storing objects To store an object, we simply call set on our database, passing any object as a parameter. Lastly, setting the Transient attribute on your class’s field members will prevent db4o from storing the values of those fields. Visual Studio or better is suggested for the best experience, but any.
When you unzip this file, you getthe following directory structure: Ttutorial with Smaller and Greater to include the specified value. A full description of Class-specific and field-specific configuration settings can be found in the db4o reference documentation. The native queries approach isn’t perfect.
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. Setting the UpdateDepth too aggressively can cause poor db4o update performance.
You can cast it to ExtObjectContainer or you can use the method to get to the advanced features. When you run the query in Listing 3, the database reports three Brian s, two of them Brian Goetz. Whenever you start making changes to your database using the Store and Delete commands you are automatically in an open transaction. Once you are familiar with storing objects, we encourage you to use Native Queries, the main db4o querying interface.
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db4o tutorial – db4objects Inc. – Informatique
The db4o native query approach puts the compilation into bytecode back into the hands of the Java or C compiler, thus allowing for type safety and earlier detection of incorrect query syntax. But please don’t repeat that name to the db4o developers; you’ll get me in trouble. First Steps Let’s get started as simple as possible. Store new Person “Petra” ; db. Although this makes for a fairly easy way to issue queries, it also makes for somewhat limited options; for example, what if you needed to retrieve all Person s whose last name started with Gor all Person s of an age greater than 21?
The busy Java developer’s guide to db4o http: In practice, all this means is that it falls to the developer to determine whether an object previously exists in the system, usually by querying the container for that object before inserting it, as shown in Listing The key thing to remember is that db4i can build convenience methods on top of the db4o infrastructure to make it easier to use.
I then iterate through an array of Strings that have the fields in them, getting back each Field object in the Class. This is useful for multi-threaded operations or web-server style environments where you wish to handle parallel operations in a single process.
In db4o, the OID for a given object can be found through a call to db. In the previous example, the query is looking for all Person types where the firstName field is equal to “Brian,” and the lastName and age fields are effectively ignored.
Please take a look at all the supplied documentation formats to choose the one that works best foryou:. In this particular case, let’s assume that the uniqueness of a Person in the yutorial is tutrial first name-last name combination.
It will not undo changes to any dh4o loaded objects. Simply register a listener Db4oQueryExecutionListener against the ObjectContainer itself to inform you if a native query cannot be optimized, as shown in Listing Then use the Solution Explorer to locate the new reference, right-click and open the properties window to ensure that “Copy Local” is set to true for each db4o assembly.
All work is transactional. After starting a db4o server instance, use either of the commands below to open a db4o client connection:.
Predictably, this would kill query performance, but you can work around it by installing listeners where you need them. A full rutorial of LINQ syntax will not fit in this document.
When read from the DB, these members will always be left at their default null values, useful for remote connections, non- native data interop and temporary state.
Higher values should be used to debug ActivationDepth-related issues only. Links Here are some further links on developer. Practically speaking, this means you have to be more careful about objects-in-memory-versus-objects-in-storage when working with an object database.
The busy Java developer’s guide to db4o: Queries, updates, and identity
When dealing with objects that may have relations to other objects quite deep think of the path of data from Customer to Order to OrderItem to Product with d4bo to Address objects for billing and shipping and then Tutoroal and payment transactions it would be quite expensive to have to pull all of that data into memory from the DB if all you wanted was the Customer object.
QBE lets you pass db4o an example object. You can keep references to those constraints and then use constraint keywords like AndOrand Not to relate the constraints tutoeial, as we did at the end of that example.
Developers were finally able to combine the power of a full database engine with plain undecorated objects.
You can then call the Activate object, depth method to manually activate the object as described above. Like updates, db4o will not automatically delete child objects unless you configure cascading deletes for your object will remain in memory until the objects are dbb4o or garbage collected. SODA tutorila expressions are a standard that was present in all but the earliest versions of db4o.
Type safety is sadly missing from JDBC’s approach to accessing SQL, by the way, because it is a call-level interface and is thus restricted to strings that can only be checked at runtime. Setting the ActivationDepth to int.