Serialization can extend the lifetime of objects, preventing their garbage collection. The
ObjectOutputStream ensures that each object is written to the stream only once by retaining a reference (or handle) to each object written to the stream. When a previously written object is subsequently written to the stream again, it is replaced with a reference to the originally written data in the stream. Note that this substitution takes place regardless whether the object's contents have changed in the interim. It requires a table of references to be maintained to keep track of previously serialized objects. This table of references prevents garbage collection of the previously serialized objects because the garbage collector cannot collect object instances referred to by live references.
This behavior is both desirable and correct for data that may contain arbitrary object graphs, especially when the graphs are fully allocated and constructed prior to serialization. However, it can lead to memory exhaustion when serializing data that lacks references to other objects being serialized and can be allocated in part or in full after serialization has begun. One such example is serializing a data stream from an external sensor. In such cases, programs must take additional action to avoid memory exhaustion. That is, programs reading in independent serialized data must reset the table of references between reads to prevent memory exhaustion.
This rule is a specific instance of the more general MSC05-J. Do not exhaust heap space.
Noncompliant Code Example
This noncompliant code example reads and serializes data from an external sensor. Each invocation of the
readSensorData() method returns a newly created
SensorData instance, each containing one megabyte of data.
SensorData instances are pure data streams, containing data and arrays but lacking references to other
As already described, the
ObjectOutputStream maintains a cache of previously written objects. Consequently, all
SensorData objects remain alive until the cache itself becomes garbage-collected. An
OutOfMemoryError can occure because the stream remains open while new objects are being written to it.
This compliant solution takes advantage of the known properties of the sensor data by resetting the output stream after each write. The reset clears the output stream's internal object cache; consequently, the cache no longer maintains references to previously written
SensorData objects. The garbage collector can collect
SensorData instances that are no longer needed.
Memory and resource leaks during serialization can result in a resource exhaustion attack or can crash the Java Virtual Machine.