Package org.apache.geode

Class Instantiator

java.lang.Object
org.apache.geode.Instantiator
Direct Known Subclasses:
CanonicalInstantiator
public abstract class Instantiator extends Object
Instantiator allows classes that implement DataSerializable to be registered with the data serialization framework. Knowledge of DataSerializable classes allows the framework to optimize how instances of those classes are data serialized.

Ordinarily, when a DataSerializable object is written using DataSerializer.writeObject(Object, java.io.DataOutput), a special marker class id is written to the stream followed by the class name of the DataSerializable object. After the marker class id is read by DataSerializer.readObject(java.io.DataInput) it performs the following operations,

  1. The class name is read
  2. The class is loaded using Class.forName(java.lang.String)
  3. An instance of the class is created using reflection
  4. DataSerializable.fromData(java.io.DataInput) is invoked on the newly-created object
However, if a DataSerializable class is registered with the data serialization framework and assigned a unique class id, an important optimization can be performed that avoid the expense of using reflection to instantiate the DataSerializable class. When the object is written using DataSerializer.writeObject(Object, java.io.DataOutput), the object's registered class id is written to the stream. Consequently, when the data is read from the stream, the newInstance() method of the appropriate Instantiator instance is invoked to create an "empty" instance of the DataSerializable instead of using reflection to create the new instance.

Commonly, a DataSerializable class will register itself with the Instantiator in a static initializer as shown in the below example code. 

public class User implements DataSerializable {
  private String name;
  private int userId;

  static {
    Instantiator.register(new Instantiator(User.class, 45) {
        public DataSerializable newInstance() {
          return new User();
        }
      });
  }

  public User(String name, int userId) {
    this.name = name;
    this.userId = userId;
  }

  /**
 Creates an "empty" User whose contents are filled in by
 invoking its toData() method
/
  private User() {

  }

  public void toData(DataOutput out) throws IOException {
    out.writeUTF(this.name);
    out.writeInt(this.userId);
  }

  public void fromData(DataInput in)
    throws IOException, ClassNotFoundException {
    this.name = in.readUTF();
    this.userId = in.readInt();
  }
}
Instantiators may be distributed to other members of the distributed system when they are registered. Consider the following scenario in which VM1 and VM2 are members of the same distributed system. Both VMs define the sameRegion and VM2's region replicates the contents of VM1's using replication. VM1 puts an instance of the above User class into the region. The act of instantiating User will load the User class and invoke its static initializer, thus registering the Instantiator with the data serialization framework. Because the region is a replicate, the User will be data serialized and sent to VM2. However, when VM2 attempts to data deserialize the User, its Instantiator will not necessarily be registered because User's static initializer may not have been invoked yet. As a result, an exception would be logged while deserializing the User and the replicate would not appear to have the new value. So, in order to ensure that the Instantiator is registered in VM2, the data serialization framework distributes a message to each member when an Instantiator is registered.

Note that the framework does not require that an Instantiator be Serializable, but it does require that it provide a two-argument constructor.

Since:
GemFire 3.5
See Also:

  • Constructor Details

    • Instantiator

      public Instantiator(Class<? extends DataSerializable> c, int classId)
      Creates a new Instantiator that instantiates a given class.
      Parameters:
      c - The DataSerializable class to register. This class must have a static initializer that registers this Instantiator.
      classId - A unique id for class c. The classId must not be zero. This has been an int since dsPhase1.
      Throws:
      IllegalArgumentException - If c does not implement DataSerializable, classId is less than or equal to zero.
      NullPointerException - If c is null

  • Method Details

    • register

      public static void register(Instantiator instantiator)
      Registers a DataSerializable class with the data serialization framework. This method is usually invoked from the static initializer of a class that implements DataSerializable.
      Parameters:
      instantiator - An Instantiator whose newInstance() method is invoked when an object is data deserialized.
      Throws:
      IllegalStateException - If class c is already registered with a different class id, or another class has already been registered with id classId
      NullPointerException - If instantiator is null.

    • register

      @Deprecated public static void register(Instantiator instantiator, boolean distribute)
      Deprecated.
      as of 9.0 use register(Instantiator) instead
      Registers a DataSerializable class with the data serialization framework. This method is usually invoked from the static initializer of a class that implements DataSerializable.
      Parameters:
      instantiator - An Instantiator whose newInstance() method is invoked when an object is data deserialized.
      distribute - True if the registered Instantiator has to be distributed to other members of the distributed system. Note that if distribute is set to false it may still be distributed in some cases.
      Throws:
      IllegalArgumentException - If class c is already registered with a different class id, or another class has already been registered with id classId
      NullPointerException - If instantiator is null.

    • newInstance

      public abstract DataSerializable newInstance()
      Creates a new "empty" instance of a DataSerializable class whose state will be filled in by invoking its fromData method.
      Returns:
      a new "empty" instance of a DataSerializable class
      See Also:

    • getInstantiatedClass

      public Class<? extends DataSerializable> getInstantiatedClass()
      Returns the DataSerializable class that is instantiated by this Instantiator.
      Returns:
      the DataSerializable class that is instantiated by this Instantiator

    • getId

      public int getId()
      Returns the unique id of this Instantiator.
      Returns:
      the unique id of this Instantiator

    • setEventId

      public void setEventId(Object eventId)
      sets the unique eventId of this Instantiator. For internal use only.
      Parameters:
      eventId - the unique eventId of this Instantiator

    • getEventId

      public Object getEventId()
      Returns the unique eventId of this Instantiator. For internal use only.
      Returns:
      the unique eventId of this Instantiator

    • setContext

      public void setContext(Object context)
      sets the context of this Instantiator. For internal use only.
      Parameters:
      context - the context of this Instantiator

    • getContext

      public Object getContext()
      Returns the context of this Instantiator. For internal use only.
      Returns:
      the context of this Instantiator