Java Enterprise Best Practices

Always Explicitly Initialize Transient Fields Inside Marshalling Methods

The transient keyword is very useful when you’re writing marshalling code. Very often, a first pass at making an object implement the Serializable interface consists of the following two steps:

1. Declare that the object implements the Serializable interface.

2. Declare all the fields you don’t want to be serialized to be transient variables.

Many programmers will look at this and wonder if it’s efficient enough. But very few will wonder whether it’s correct. And the sad truth is that it is often incorrect. The reason is that the transient keyword really means “Serialization shouldn’t pay attention to this field at all.”^[15] And that has serious consequences when you combine it with the notion of serialization’s extralinguistic constructor (as discussed earlier). If you implement the Serializable interface by following the previous two methods, here’s what will happen when your object is deserialized:

1. The instance will be created using the extralinguistic constructor (and hence, none of the initialization code in your constructors will be executed).

2. The serialization algorithm will completely ignore your transient fields.

The net result is that none of the transient fields will be initialized. This is rarely the expected outcome, and can result in bugs that are subtle and hard to track down.

The SerializationTest class illustrates the problem in detail. It has four integers (a, b, c, and d), which are initialized ...