diff --git a/CHANGES.txt b/CHANGES.txt
index 12add5121e..cd5b1096e5 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -151,6 +151,11 @@ Trunk (unreleased changes)
     HADOOP-6536. Fixes FileUtil.fullyDelete() not to delete the contents of
     the sym-linked directory. (Ravi Gummadi via amareshwari)
 
+    HADOOP-6881. Make WritableComparator intialize classes when
+    looking for their raw comparator, as classes often register raw
+    comparators in initializers, which are no longer automatically run
+    in Java 6 when a class is referenced. (cutting via omalley)
+
 Release 0.21.0 - Unreleased
 
   INCOMPATIBLE CHANGES
diff --git a/src/java/org/apache/hadoop/io/WritableComparator.java b/src/java/org/apache/hadoop/io/WritableComparator.java
index e55036c6c0..bfb1673222 100644
--- a/src/java/org/apache/hadoop/io/WritableComparator.java
+++ b/src/java/org/apache/hadoop/io/WritableComparator.java
@@ -42,13 +42,38 @@ public class WritableComparator implements RawComparator {
     new HashMap<Class, WritableComparator>(); // registry
 
   /** Get a comparator for a {@link WritableComparable} implementation. */
-  public static synchronized WritableComparator get(Class<? extends WritableComparable> c) {
+  public static synchronized 
+  WritableComparator get(Class<? extends WritableComparable> c) {
     WritableComparator comparator = comparators.get(c);
-    if (comparator == null)
-      comparator = new WritableComparator(c, true);
+    if (comparator == null) {
+      // force the static initializers to run
+      forceInit(c);
+      // look to see if it is defined now
+      comparator = comparators.get(c);
+      // if not, use the generic one
+      if (comparator == null) {
+        comparator = new WritableComparator(c, true);
+        comparators.put(c, comparator);
+      }
+    }
     return comparator;
   }
 
+  /**
+   * Force initialization of the static members.
+   * As of Java 5, referencing a class doesn't force it to initialize. Since
+   * this class requires that the classes be initialized to declare their
+   * comparators, we force that initialization to happen.
+   * @param cls the class to initialize
+   */
+  private static void forceInit(Class<?> cls) {
+    try {
+      Class.forName(cls.getName(), true, cls.getClassLoader());
+    } catch (ClassNotFoundException e) {
+      throw new IllegalArgumentException("Can't initialize class " + cls, e);
+    }
+  } 
+
   /** Register an optimized comparator for a {@link WritableComparable}
    * implementation. */
   public static synchronized void define(Class c,
diff --git a/src/test/core/org/apache/hadoop/io/TestWritable.java b/src/test/core/org/apache/hadoop/io/TestWritable.java
index 9e7de1e633..a60ef1b8d2 100644
--- a/src/test/core/org/apache/hadoop/io/TestWritable.java
+++ b/src/test/core/org/apache/hadoop/io/TestWritable.java
@@ -96,4 +96,27 @@ public static Writable testWritable(Writable before
     return after;
   }
 	
+  private static class FrobComparator extends WritableComparator {
+    public FrobComparator() { super(Frob.class); }
+    @Override public int compare(byte[] b1, int s1, int l1,
+                                 byte[] b2, int s2, int l2) {
+      return 0;
+    }
+  }
+
+  private static class Frob implements WritableComparable {
+    static {                                     // register default comparator
+      WritableComparator.define(Frob.class, new FrobComparator());
+    }
+    @Override public void write(DataOutput out) throws IOException {}
+    @Override public void readFields(DataInput in) throws IOException {}
+    @Override public int compareTo(Object o) { return 0; }
+  }
+
+  /** Test that comparator is defined. */
+  public static void testGetComparator() throws Exception {
+    assert(WritableComparator.get(Frob.class) instanceof FrobComparator);
+  }
+
+
 }