Page History

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 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 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 potentially contains 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 both 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 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 a one megabyte of data. SensorData instances are pure data streams, containing data and arrays but lacking references to other SensorData objects.

...

class SensorData implements Serializable {
  // 1MB1 MB 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 1 1MBMB in size
        SensorData sd = SensorData.readSensorData();
        out.writeObject(sd);
      }
    } finally {
      if (out != null) {
        out.close();
      }
    }
  }
}

...

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 1MB1 MB in size
        SensorData sd = SensorData.readSensorData();
        out.writeObject(sd);
        out.reset(); // reset the stream
      }
    } finally {
      if (out != null) {
        out.close();
      }
    }
  }
}

...

...