In a response to a recent JavaLobby thread, Tom Hawtin looks at optimization of serialization and decides that you shouldn't do it.
The JavaLobby post describes a few different options for optimization and found that by using the Externalizable interface, instead of Serializable, yields significant performance gains (up to 55% in one version of the JDK). Hatwin noted some flaws in the microbenchmark, most significantly:
Serializable objects that implement Externalizable do not have fields included in their class descriptors. Not a lot of people know that. The descriptions are used if there is no writeObject/readObject method and for defaultReadObject and readFields. So, for fairness, the fields for the writeObject/readObject version should be marked as transient.
After running his updated microbenchmark he finds four conclusions:
- Don't use Externalizable
- Do reuse streams
- Implementing writeObject and readObject by hand can improve performance
- JVMs get better at serialization with each release
In a follow up to his benchmark, Hatwin finds a small optimization that is valuable. He notes that calling ObjectStreamClass.lookup on all classes that you are going to serialize, and assigning the results to static fields, will provide a boost. The boost comes from the fact that the soft reference cache used by serialization will never miss if you hold on to a reference. He has not yet updated his benchmarks to show the impact on it.
