MissingVoidConstructorOnExternalizable.qhelp

<!DOCTYPE qhelp PUBLIC
  "-//Semmle//qhelp//EN"
  "qhelp.dtd">
<qhelp>


<overview>
<p>
A class that implements <code>java.io.Externalizable</code> must have a public no-argument 
constructor. The constructor is used by the Java serialization framework when it creates the object 
during deserialization. If the class does not define such a constructor, the Java 
serialization framework throws an <code>InvalidClassException</code>.
</p>

<p>
The Java Development Kit API documentation for <code>Externalizable</code> states:
</p>

<blockquote>
<p>When an <code>Externalizable</code> object is reconstructed, an instance is created using the public no-arg constructor, 
then the <code>readExternal</code> method called.</p>
</blockquote>

</overview>
<recommendation>
<p>
Make sure that externalizable classes always have a no-argument constructor. 
</p>

</recommendation>
<example>
<p>In the following example, <code>WrongMemo</code> does not declare a public no-argument constructor. 
When the Java serialization framework tries to deserialize the object, an <code>InvalidClassException</code>
is thrown. However, <code>Memo</code> does declare a public no-argument constructor, so that the 
object is deserialized successfully.</p>
<sample src="MissingVoidConstructorOnExternalizable.java" />

</example>
<references>


<li>
  Java API Documentation:
  <a href="http://docs.oracle.com/javase/6/docs/api/java/io/Externalizable.html">Externalizable</a>.
</li>


</references>
</qhelp>