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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 regardless whether the object's contents have changed in the interim. This 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.

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This rule is a specific instance of the more general rule MSC05-J. Do not exhaust heap space.

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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 An OutOfMemoryError can occure because the stream remains open while new objects are being written to it.

class SensorData implements Serializable {
  // 1 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()) {
        // noteNote that each SensorData object is 1 MB in size
        SensorData sd = SensorData.readSensorData();
        out.writeObject(sd);
      }
    } finally {
      if (out != null) {
        out.close();
      }
    }
  }
}

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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()) {
        // noteNote that each SensorData object is 1 MB in size
        SensorData sd = SensorData.readSensorData();
        out.writeObject(sd);
        out.reset(); // resetReset the stream
      }
    } finally {
      if (out != null) {
        out.close();
      }
    }
  }
}

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

Related Guidelines

Bibliography

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