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Serialization can extend the lifetime of objects, consequently preventing their garbage collection of those objects. 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 without regard to whether the object's contents have changed in the interim. This 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 written serialized objects because the garbage collector cannot collect live references.

This behavior is both desirable and correct for data that potentially contains 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 both lacks references to other objects being serialized and also 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 object cache table of references between reads to prevent memory exhaustion.

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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. This can result in an OutOfMemoryError because the stream remains open while new objects are being written to it.

class SensorData implements Serializable {
  // 1MB of data per instance!
  ... 
  public static SensorData readSensorData() {...}
  public static boolean isAvailable() {...}
}

class SerializeSensorData {
  public static void main(String[] args) throws IOException {
    ObjectOutputStream out = null;
    try {
      out = new ObjectOutputStream(
          new BufferedOutputStream(new FileOutputStream("ser.dat")));
      while (SensorData.isAvailable()) {
        // note that each SensorData object is 1MB in size
        SensorData sd = SensorData.readSensorData();
        out.writeObject(sd);
      }
    } finally {
      if (out != null) {
        out.close();
      }
    }
  }
}

Compliant Solution

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.

class SerializeSensorData {
  public static void main(String[] args) throws IOException {
    ObjectOutputStream out = null;
    try {
      out = new ObjectOutputStream(
          new BufferedOutputStream(new FileOutputStream("ser.dat")));
      while (SensorData.isAvailable()) {
        // note that each SensorData object is 1MB in size
        SensorData sd = SensorData.readSensorData();
        out.writeObject(sd);
        out.reset(); // reset the stream
      }
    } finally {
      if (out != null) {
        out.close();
      }
    }
  }
}

Risk Assessment

Memory and resource leaks during serialization can result in a resource exhaustion attack or crash the JVM.

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Related Guidelines

Bibliography

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