"An inner class is a nested class that is not explicitly or implicitly declared {{static}}" \[[JLS 2005|AA. Bibliography#JLS 05]\]. Serialization of inner classes (including local and anonymous classes) is error prone. According to the Serialization Specification \[[Sun 2006|AA. Bibliography#Sun 06]\]

• Because inner classes declared in non-static contexts contain implicit non-transient references to enclosing class instances, serializing such an inner class instance results in serialization of its associated outer class instance.

• Synthetic fields generated by javac (or other Java ^TM compilers) to implement inner classes are implementation dependent and may vary between compilers; differences in such fields can disrupt compatibility as well as result in conflicting default serialVersionUID values. The names assigned to local and anonymous inner classes are also implementation dependent and may differ between compilers.

• Because inner classes cannot declare static members other than compile-time constant fields, they cannot use the serialPersistentFields mechanism to designate serializable fields.

• Finally, because inner classes associated with outer instances do not have zero-argument constructors (constructors of such inner classes implicitly accept the enclosing instance as a prepended parameter), they cannot implement Externalizable. The Externalizable interface requires the implementing object to manually save and restore its state using the writeExternal() and readExternal() methods.

Consequently, programs are forbidden to serialize inner classes.

Note, however, that none of the above issues apply to static member classes. Consequently, serialization of static member classes is permitted.

Noncompliant Code Example

In this noncompliant code example, the fields contained within the outer class are also serialized when the inner class is serialized.

public class OuterSer implements Serializable {
  private int rank;
  class InnerSer implements Serializable {
    protected String name;
    //...
  }
}

Compliant Solution

This compliant solution omits implementation of the Serializable interface in the InnerSer class.

public class OuterSer implements Serializable {
  private int rank;
  class InnerSer {
    protected String name;
    //...
  }
}

Compliant Solution

It is allowable to declare the inner class as static to prevent its serialization. It is also permissible for a static inner class to implement Serializable.

public class OuterSer implements Serializable {
  private int rank;
  static class InnerSer implements Serializable {
    protected String name;
    //...
  }
}

Risk Assessment

Attempts to serialize inner classes can introduce platform dependencies and can cause serialization of instances of the outer class.

Automated Detection

Detection of inner classes that implement serialization appears to be straightforward.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this guideline on the CERT website.

Bibliography

\[[API 2006|AA. Bibliography#API 06]\] 
\[[Bloch 2008|AA. Bibliography#Bloch 08]\] Item 74: "Implement serialization judiciously"
\[[JLS 2005|AA. Bibliography#JLS 05]\] [Section 8.1.3, Inner Classes and Enclosing Instances|http://java.sun.com/docs/books/jls/third_edition/html/classes.html]
\[[Sun 2006|AA. Bibliography#Sun 06]\] "Serialization specification:  

SER05-J. Do not allow serialization and deserialization to bypass the Security Manager      16. Serialization (SER)      SER07-J. Make defensive copies of private mutable components