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According to the Java API [API 20112013], the writeUnshared() method

writes an "unshared" object to the ObjectOutputStream. This method is identical to writeObject, except that it always writes the given object as a new, unique object in the stream (as opposed to a back-reference pointing to a previously serialized instance).

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reads an "unshared" object from the ObjectInputStream. This method is identical to readObject, except that it prevents subsequent calls to readObject and readUnshared from returning additional references to the deserialized instance obtained via this call.

This means that Consequently, the writeUnshared() and readUnshared() methods cannot be used are unsuitable for round-trip serialization of data structures that require a one-to-one mapping between objects preserialization and objects postdeserialization. One common example of such a data structure is a graph or network of objects that may contain reference cycles. It is also important to note that calls to the writeUnshared() and readUnshared() methods prevent sharing only of the object referred to by (or returned by) the methods; sharing (or not) of objects reached from references in the unshared object remains unchanged.  

Consider the following code example:

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Professor and Students are types that extend the basic type Person. A student (that is, an object of type Student) has a tutor of type Professor. A professor (that is, an object of type Professor) has a list (actually, an ArrayList) of tutees (of type Student). The method checkTutees() checks whether all of the tutees of this professor have this professor as their tutor, returning true if that is the case and false otherwise. 

Suppose that Professor Jane has three tuteesstudents, Able, Baker, and Charlie, all of whom have Professor Jane as their tutor. Issues can arise if the writeUnshared() and readUnshared() methods are used with these classes, as demonstrated in the following noncompliant code example.

Noncompliant Code Example

This noncompliant code example attempts to serialize the data from the previous example using writeUnshared(). 

String filename = "serial";
try(ObjectOutputStream oos = new ObjectOutputStream(new 
      FileOutputStream(filename))) {
  // Serializing using writeUnshared
  oos.writeUnshared(jane);
} catch (Throwable e) {
  // Handle error
}

// Deserializing using readUnshared
try(ObjectInputStream ois = new ObjectInputStream(new 
      FileInputStream(filename))){
  Professor jane2 = (Professor)ois.readUnshared();
  System.out.println("checkTutees returns: " +
							                     jane2.checkTutees());
} catch (Throwable e) {
  // Handle error
}

However, when the data is deserialized using readUnshared(), the checkTutees() method no longer returns true because the tutor objects of the three students are different from the original Professor object.

Compliant Solution

This compliant solution overcomes the problem of the noncompliant code example by using uses the writeObject() and readObject(), ensuring methods to ensure that the tutor object referred to by the three students has a one-to-one mapping with the original Professor object. The checkTutees() method correctly returns true.

String filename = "serial";
try(ObjectOutputStream oos = new ObjectOutputStream(new 
      FileOutputStream(filename))) {
	// Serializing using writeUnshared
	oos.writeObject(jane);
} catch (Throwable e) {
    // Handle error
} 

// Deserializing using readUnshared
try(ObjectInputStream ois = new ObjectInputStream(new 
      FileInputStream(filename))) {
Professor jane2 = (Professor)ois.readObject();
System.out.println("checkTutees returns: " +
					                   jane2.checkTutees());
} catch (Throwable e) {
    // Handle error
} 

Applicability

Using the writeUnshared() and readUnshared() methods may produce unexpected results when used for the round-trip serialization of the data structures containing reference cycles.

Bibliography

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