[COLLECTIONS-576] MultiKey subclassing has deserialization problem since COLLECTIONS-266: either declare protected readResolve() or MultiKey must be final

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 4.0
• Fix Version/s: 4.1
• Component/s: KeyValue
• Assignee: Unassigned
• Reporter: Stephan Roch
• Created: 03/Sep/15 08:44
• Updated: 27/Nov/15 21:11
• Resolved: 28/Sep/15 08:54

Description

MultiKey from collections 4 provides a transient hashCode and a private readResolve to resolve COLLECTIONS-266: Issue with MultiKey when serialized/deserialized via RMI.

Unfortunately the solution does not work in case of subclassing: readResolve in MultiKey should be declared protected readResolve() to be called during deserialization of the subclass. Otherwise MultiKey must be final to avoid such subclassing.

Testcase:

MultiKeySerializationTest.java

package de.ivu.test.common.collections4;

import static org.junit.Assert.assertEquals;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;

import org.apache.commons.collections4.keyvalue.MultiKey;
import org.junit.Test;

public class MultiKeySerializationTest {

    @Test
    @SuppressWarnings("unchecked")
    public void testReadResolveEqualHashCode()
            throws IOException, ClassNotFoundException {
        class MultiKey2<A, B>
                extends MultiKey {

            private static final long serialVersionUID = 1928896152249821416L;

            public MultiKey2(A key1, B key2) {
                super(key1, key2);
            }

            public A getFirst() {
                return (A) getKey(0);
            }

            public B getSecond() {
                return (B) getKey(1);
            }
            
            // FIXME: MultiKey should either declare protected readResolve() or must be final.
        }
        MultiKey2<String, String> one = new MultiKey2<>("bla", "blub");
        System.out.println(one.hashCode());
        ByteArrayOutputStream byteOut = new ByteArrayOutputStream();
        ObjectOutputStream out = new ObjectOutputStream(byteOut);
        out.writeObject(one);
        out.close();
        byte[] serialized = byteOut.toByteArray();
        ByteArrayInputStream byteIn = new ByteArrayInputStream(serialized);
        ObjectInputStream in = new ObjectInputStream(byteIn);
        MultiKey2<String, String> two = (MultiKey2<String, String>) in.readObject();
        System.out.println(two.hashCode());
        assertEquals("hashCode must be equal - please check for protected readResolve in MultiKey*", one.hashCode(),
            two.hashCode());
    }
}

Fix:

MultiKey.java

@@ -274,7 +274,7 @@
      * only stable for the same process).
      * @return the instance with recalculated hash code
      */
-    private Object readResolve() {
+    protected Object readResolve() {
         calculateHashCode(keys);
         return this;
     }

Issue Links

• is related to: COLLECTIONS-266 Issue with MultiKey when serialized/deserialized via RMI