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<h1>Final Fields Specifics</h1>
<p>Db4o uses reflection to store and retrieve objects from the
database file. In the case of final fields db4o needs a successful call to
java.lang.Field#setAccessible to allow write access to those fields.
Unfortunately different Java versions produce different results in this case.
To be more specific:</p>
<ul>
<li value="1">In (Sun) JDK 1.-1.2
java.lang.Field#setAccessible call will be successful for the fields with
the final modifier. </li>
<li value="2">This behavior was changed for
JDK1.3-1.4 as the API documentation for java.lang.Field#set() made a quite
clear distinction between 'Java language access control' (visibility
modifiers, affected by setAccessible()) and final fields (not affected by
setAccessible()). For more information refer to <a href="http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4250960">java bug
4250960</a>.</li>
<li value="3">The
behavior of java.lang.Field#setAccessible method was changed again for
JDK5 and JDK6. The access to final fields was made manageable by
setAccessible() call to accommodate for the extended semantics of the
final modifier for the revised Java memory model. The API documentation of
java.lang.Field#set() was changed accordingly. See <a href="http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=5044412">java bug
5044412</a>.</li>
</ul>
<p>You can use the following example code to check final fields
behavior with different java versions:</p>
<pre class="prettyprint" xml:space="preserve">TestFinal.java
/**//* Copyright (C) 2007 Versant Inc. http://www.db4o.com */
package com.db4odoc.finalfields;
import java.io.File;
import com.db4o.Db4o;
import com.db4o.ObjectContainer;
import com.db4o.ObjectSet;
public class TestFinal
{
private static final String DB4O_FILE_NAME = "reference.db4o";
// non-final fields
public int _i;
public String _s;
// final fields storing the same values as above
public final int _final_i;
public final String _final_s;
public static void main(String[] args)
{
new File(DB4O_FILE_NAME).delete();
ObjectContainer container = Db4o.openFile(DB4O_FILE_NAME);
try {
TestFinal test = new TestFinal(1,"test");
container.store(test);
System.out.println("Added: " + test);
} finally {
// Close does implicit commit and refreshes the reference cache
container.close();
}
container = Db4o.openFile(DB4O_FILE_NAME);
try {
ObjectSet result = container.queryByExample(null);
listResult(result);
} finally {
container.close();
}
}
// end main
public TestFinal(int i, String s)
{
// initialize final and non-final fields with the same values
_i = i;
_s = s;
_final_i = i;
_final_s = s;
}
// end TestFinal
public String toString()
{
return "Int - " + _i + "; String - " + _s + "; FINAL Int - "
+ _final_i + "; FINAL String - " + _final_s;
}
// end toString
private static void listResult(ObjectSet result)
{
while(result.hasNext()) {
System.out.println(result.next());
}
}
// end listResult
}</pre>
<p>If you are using Eclipse it is easy to switch between java
versions - you can switch to the versions lower than the one installed on your
computer without having to install them all. For example if you are using JDK6
you can easily test your project on JDK1.1 - 1.4 and JDK5. Just go to the project
properties, select "Java Build Path" on the left panel and "Libraries" tab on
the right panel. Remove the system library currently used. Select "Add
library->JRE System Library"; on the next screen check the "Execution
Environment" radio button and select the desired environment from the list.</p>Don't forget to use the appropriate db4o version for
the selected java environment version. See
db4o on Java Platformsfor more information.
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