Wednesday, December 30, 2009

Hibernate3 Tip#3: Methods You Should Override

Just some general best practices that have saved me considerable development time as I've worked with Hibernate3:
  • Always override equals(). Objects that are equal as plain Java objects are often not equal in Hibernate3 because of the identity problem. Hibernate considers the object's persistent state, database identity, and other factors to determine if two objects are equal. By overriding equals(), you set a standard of equality both Hibernate and Java can use.
  • It is a bad idea to override equals() in subclasses. Hibernate does not behave well.
  • Always override hashCode() and provide your own implementation based on the class's attributes that are expected to change rarely. I use the class identifier commonly. Actually, overriding equals() almost always requires overriding hashCode().
  • When you implement equals() and hashCode(), always use property accessors - getXX()/setXX(), POJO style. Hibernate allows the use of proxy classes or references that may no contain actual data if you call their properties directly. Believe me, this is a pain to debug!
  • Always make classes Comparable in case objects of the class end up in collections that require this feature (such as SortedSet or other collections that use a class-dependent Comparator). This gives you the flexibility to switch collection types whenever (e.g. from List to Set) without further base code changes.
  • I also always override toString() because I hate seeing memory addresses as representations of objects. I use it for debug information that I can print out and understand, and that uniquely identifies the object for me.
Pretty much every OOP project I've worked on, these are some of the first tasks that get done. If developers would just get in the habit of these best practices, I wouldn't dread inheriting their projects so much.

Tuesday, December 29, 2009

Hibernate3 Tip#2: Enable Automatic Dirty Checking

In database parlance, when you read a record from a table and update some of its fields in memory, the record is considered dirty. Dirty checking avoids unnecessary database write actions by performing SQL updates only on the modified fields of persistent objects. Hibernate3 JPA can do automatic dirty-checking for you via an annotation to the entity:
dynamicInsert=true, dynamicUpdate=true
The dynamicInsert property is really only necessary to prevent writing NULL values to database fields. But if the changes involve setting NULL values (bad practice), you should probably turn it off.
I can think of three possible implementations for dirty checking:
  1. Caching a clean copy of the record and maintaining an edit copy of the persistent object. When an SQL update is due, the copies are compared and changes extracted from the edit copy for update. Obviously memory-intensive as 2 copies of a record are maintained at all times.
  2. Keeping track of which fields changed.
  3. Doing a SQL query just before updating and doing the necessary reductions to determine what changed.
I do not know which strategy Hibernate3 uses. Either way, the performance hit is minimal compared to rewriting whole table rows in the database - depending on how many fields each record has. It is recommended when you have more than 50 fields where any given update affects less than 25% of the fields (if you want hard numbers).

Monday, December 28, 2009

Hibernate3 Tip#1: Persistent Lifecycles

For application developers that use the Hibernate3 JPA, it is crucial that you learn the different persistent states an object can be and what methods/functions cause them to transition to what other states. The states are: transient, persistent, detached, and removed. You must also know how a persistent context works, the ACID (atomicity, consistency, isolation, durability) principles for database transactions, and basics of object identity (Java's equality is NOT the same as database record equality).

You can find out more about Hibernate3 at