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Classes that require special handling during object serialization and deserialization must implement special methods with exactly the following signatures [API 2006]:

private void writeObject(java.io.ObjectOutputStream out) 
    throws IOException;
private void readObject(java.io.ObjectInputStream in)
    throws IOException, ClassNotFoundException;
private void readObjectNoData() throws ObjectStreamException;

Note that these methods must be declared private for any serializable class. Serializable classes may also implement the readResolve() and writeReplace() methods.
According to the Serialization Specification [Sun 2006], readResolve() and writeReplace() method documentation:

For Serializable and Externalizable classes, the readResolve method allows a class to replace/resolve the object read from the stream before it is returned to the caller. By implementing the readResolve method, a class can directly control the types and instances of its own instances being deserialized.

For Serializable and Externalizable classes, the writeReplace method allows a class of an object to nominate its own replacement in the stream before the object is written. By implementing the writeReplace method, a class can directly control the types and instances of its own instances being serialized.

It is possible to add any access-specifier to the readResolve() and writeReplace() methods. However, if these methods are declared private, extending classes cannot invoke or override them. Similarly, if these methods are declared static, extending classes cannot override these methods; they can only hide them.

Deviating from these method signatures produces a method that is not invoked during object serialization or deserialization. Such methods, especially if declared public, might be accessible to untrusted code.

Unlike most interfaces, Serializable does not define the method signatures it requires. Interfaces allow only public fields and methods, whereas readObject(), readObjectNoData, and writeObject() must be declared private. Similarly, the Serializable interface does not prevent readResolve() and writeReplace() methods from being declared static, public, or private. Consequently, the Java serialization mechanism fails to let the compiler identify an incorrect method signature for any of these methods.

Noncompliant Code Example (readObject(), writeObject())

This noncompliant code example shows a class Ser with a private constructor, indicating that code external to the class should be unable to create instances of it. The class implements java.io.Serializable and defines public readObject() and writeObject() methods. Consequently, untrusted code can obtain the reconstituted objects by using readObject() and can write to the stream by using writeObject().

public class Ser implements Serializable {
  private final long serialVersionUID = 123456789;
  private Ser() {
    // initialize
  }
  public static void writeObject(final ObjectOutputStream stream)
    throws IOException {
    stream.defaultWriteObject();
  }	
  public static void readObject(final ObjectInputStream stream)
      throws IOException, ClassNotFoundException {
    stream.defaultReadObject();
  }
}

Similarly, omitting the static keyword is insufficient to make this example secure; the JVM will not detect the two methods, resulting in failure to use the custom serialized form.

Compliant Solution (readObject(), writeObject())

This compliant solution declares the readObject() and writeObject() methods private and nonstatic to limit their accessibility.

private void writeObject(final ObjectOutputStream stream)
    throws IOException {
  stream.defaultWriteObject();
}

private void readObject(final ObjectInputStream stream)
    throws IOException, ClassNotFoundException {
  stream.defaultReadObject();
}

Reducing the accessibility also prevents malicious overriding of the two methods.

Noncompliant Code Example (readResolve(), writeReplace())

This noncompliant code example declares the readResolve() and writeReplace() methods as private.

class Extendable implements Serializable {
  private Object readResolve() {
    // ...
  }

  private Object writeReplace() {
    // ...
  }
}

Noncompliant Code Example (readResolve(), writeReplace())

This noncompliant code example declares the readResolve() and writeReplace() methods as static.

class Extendable implements Serializable {
  protected static Object readResolve() {
    // ...
  }

  protected static Object writeReplace() {
    // ...
  }
}

Compliant Solution (readResolve(), writeReplace())

This compliant solution declares the two methods protected while eliminating the static keyword so that subclasses can inherit them.

class Extendable implements Serializable {
  protected Object readResolve() {
    // ...
  }

  protected Object writeReplace() {
    // ...
  }
}

Risk Assessment

Deviating from the proper signatures of serialization methods can lead to unexpected behavior. Failure to limit the accessibility of the readObject() and writeObject() methods can leave code vulnerable to untrusted invocations. Declaring readResolve() and writeReplace() methods to be static or private can force subclasses to silently ignore them, while declaring them public allows them to be invoked by untrusted code.

Related Guidelines

Bibliography

      13. Serialization (SER)      SER02-J. Sign then seal sensitive objects before sending them outside a trust boundary