Serialization involves saving the current state of an object to a stream, and restoring an equivalent object from that stream. The stream functions as a container for the object. Its contents include a partial representation of the object's internal structure, including variable types, names, and values. The container may be transient (RAM-based) or persistent (disk-based). A transient container may be used to prepare an object for transmission from one computer to another. A persistent container, such as a file on disk, allows storage of the object after the current session is finished. In both cases theinformation stored in the container can later be used to construct an equivalent object containing the same data as the original.

For an object to be serialized, it must be an instance of a class that implements either the Serializable or Externalizable interface. Both interfaces only permit the saving of data associated with an object's variables. They depend on the class definition being available to the Java Virtual Machine at reconstruction time in order to construct the object. The Serializable interface relies on the Java runtime default mechanism to save an object's state. Writing an object is done via the writeObject() method in the ObjectOutputStream class (or the ObjectOutput interface). 

Sometimes you may wish to prevent certain fields from being stored in the serialized object. The Serializable interface allows the implementing class to specify that some of its fields do not get saved or restored. This is accomplished by placing the keyword transient before the data type in the variable declaration. In addition to those fields declared as transient, static fields are not serialized (written out), and so cannot be deserialized (read back in). 

Adding object persistence to Java applications using serialization is easy. Serialization allows you to save the current state of an object to a container, typically a file. At some later time, you can retrieve the saved data values and create an equivalent object. Depending on which interface you implement, you can choose to have the object Aand all its referenced objects saved and restored automatically, or you can specify which fields should be saved and restored. Java also provides several ways of protecting sensitive data in a serialized object, so objects loaded from a serialized representation should prove no less secure than those classes loaded at application startup.
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