Java Still Struggles With Persistence
By Allen Holub
July 15, 2004 —
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When it comes to persistence-storing an object in some place external to your program, ideally in a database-there's a strong need for simplicity.
The Holy Grail of persistence is transparent object storage. I just want to put some object in the persistent store and pull it out again later, as if I were storing the object in a local data structure like a hash table.
I don't want to know anything about databases, XML files or whatever physical storage is used under the covers. Achieving this level of simplicity is very difficult. Sometimes I need to use complex criteria to find a particular object or group of objects, but I want the complexity out of the way when it's not relevant.
You can find proof of the difficulty of building a simple-to-use but powerful persistence system by looking at all the bad implementation floating around.
Roll-your-own systems built on JDBC are too complex, and require specialized knowledge of databases. Object databases looked promising for a while, but they were never trusted enough to be commonly used or sufficiently standardized, and forced you to learn complex new query languages.
I don't want to open the Enterprise JavaBeans can of worms in this column, but unfortunately, many misguided people used EJB to implement persistence-at a huge cost in up-front development time, long-term maintenance problems and runtime performance.
An entity bean is not the same thing as a persistent object. EJB-based O/R-mapping systems such as Thought Inc.'s CocoBase cover the zits with a thin layer of makeup, but they don't solve the problems inherent in the underlying EJB system.
Java's other persistence solutions have also been far from ideal. JDBC doesn't get along very well with the notion of objects, it doesn't provide enough in the way of APIs to let you write in a database-independent way, and at the same time doesn't do many common database-related tasks (like transactions) very well.
The various XML APIs don't talk to relational databases, and introducing an XML-to-database translation layer adds too much inefficiency. Java Data Objects is a step in the right direction, but JDO is way too complicated for my taste, and requires a post-processor that manipulates your bytecode to make an object persistable-something that's bound to cause maintenance problems and also ties your code to a specific vendor.