Difference between revisions of "Hibernate"
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Revision as of 11:11, 5 June 2008
Before you begin
Since ORM architecture isn't obvious, this document will explain some important things you need to know in order to analyze a Hibernate application in a security context. This document assumes some SQL and database knowledge.
A note about SQL injection
Since it is the hot topic, I will address it now but discuss in detail later.
- Hibernate does not grant immunity to SQL Injection, one can misuse the api as they please.
- There is nothing special about HQL (Hibernates subset of SQL) that makes it any more or less susceptible.
- Functions such as createQuery(String query) and createSQLQuery(String query) create a Query object that will be executed when the call to commit() is made. If the query string is tainted you have sql injection. The details of these functions are covered later.
- StatelessSession?'s insert(), update(), and delete() methods do NOT wait for a call to commit(), they are equated with direct database communication.
The problem Hibernate addresses
- In object oriented systems, we represent entities as objects, and use a database to persist those objects. Generally these objects are considered non-scalar values (non-primitive types).
- But many databases can only store and manipulate scalar values such as integers and strings organized in tables.
- The crux of the problem is translating those objects to forms which can be stored in the database, and which can later be retrieved easily, while preserving the properties of the objects and their relationships; these objects are then said to be persistent.
- Hibernate attempts to address this problem with Object/Relational Mapping (O/R M) by mapping attributes of objects we wish to persist to columns of a database.
- Generally Hibernate applications use POJOs to represent objects we want to persist in a database.
- They are usually implemented in the style of JavaBeans.
- These objects should have an identifier property (i.e private class variable) used to represent it's primary key value in the database.
- This identifier will be mapped in a hibernate mapping file which defaults to the extension .hbm.xml, you will see this mapping in later examples with the <id> element.
- This mapping file will also map all other object properties we wish to preserve to columns in a database table, along with their data types and other optional metadata.
- There are ways to map object properties programmatically, but this will not be covered until I find reason to include.
- Once objects are mapped, Hibernate provides the mechanism for you to store and access them via org.hibernate.Session and org.hibernate.Transaction objects.
- The Session object has methods to save() objects to a session, load() objects from a database and createQuery()s to be executed against the database.
- The Transaction object often wraps a database transaction, allowing one to begin() transactions, commit() changes, and rollback() to a previous state.
- Other classes worth mentioning: SessionFactory, TransactionFactory, and Query.
- Hibernate's main configuration file, extension .cfg.xml, provides basic setup for things like datasource, dialect, mapping files, etc.
- Transient - The instance is not associated with a Session, has no persistent representation in the database and no identifier assigned. An object that has just been instantiated with the new operator is said to be transient.
- Persistent - Is associated with a Session, has a representation in the database and has been assigned an identifier. Hibernate synchronizes changes on a persistent object with its representation in the database when it completes a unit of work.
- Detatched - was once in a persistent state, but its session has been closed. The reference is still valid and the object may be modified and even reattached to a new session later.