Index: lucene/CHANGES.txt
===================================================================
--- lucene/CHANGES.txt	(revision 933613)
+++ lucene/CHANGES.txt	(working copy)
@@ -111,6 +111,11 @@
   behavior you can call writer.commit()/close() immediately after you create it.
   (Shai Erera, Mike McCandless)
   
+* LUCENE-2316: Directory.fileLength contract was clarified - it returns the
+  actual file's length if the file exists, and throws FileNotFoundException
+  otherwise. Returning length=0 for a non-existent file is no longer allowed. If
+  you relied on that, make sure to catch the exception. (Shai Erera)
+  
 Changes in runtime behavior
 
 * LUCENE-1923: Made IndexReader.toString() produce something
Index: lucene/contrib/db/bdb-je/src/java/org/apache/lucene/store/je/JEDirectory.java
===================================================================
--- lucene/contrib/db/bdb-je/src/java/org/apache/lucene/store/je/JEDirectory.java	(revision 933591)
+++ lucene/contrib/db/bdb-je/src/java/org/apache/lucene/store/je/JEDirectory.java	(working copy)
@@ -19,6 +19,7 @@
 
 import java.io.ByteArrayInputStream;
 import java.io.DataInputStream;
+import java.io.FileNotFoundException;
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Collections;
@@ -133,7 +134,7 @@
         if (file.exists(this))
             return file.getLength();
 
-        throw new IOException("File does not exist: " + name);
+        throw new FileNotFoundException(name);
     }
 
     @Override
Index: lucene/contrib/db/bdb/src/java/org/apache/lucene/store/db/DbDirectory.java
===================================================================
--- lucene/contrib/db/bdb/src/java/org/apache/lucene/store/db/DbDirectory.java	(revision 933591)
+++ lucene/contrib/db/bdb/src/java/org/apache/lucene/store/db/DbDirectory.java	(working copy)
@@ -17,6 +17,7 @@
  * limitations under the License.
  */
 
+import java.io.FileNotFoundException;
 import java.io.IOException;
 import java.io.ByteArrayInputStream;
 import java.io.DataInputStream;
@@ -140,15 +141,13 @@
     }
 
     @Override
-    public long fileLength(String name)
-        throws IOException
-    {
+    public long fileLength(String name) throws IOException {
         File file = new File(name);
 
         if (file.exists(this))
             return file.getLength();
 
-        throw new IOException("File does not exist: " + name);
+        throw new FileNotFoundException(name);
     }
     
     @Override
Index: lucene/src/java/org/apache/lucene/index/CompoundFileReader.java
===================================================================
--- lucene/src/java/org/apache/lucene/index/CompoundFileReader.java	(revision 933591)
+++ lucene/src/java/org/apache/lucene/index/CompoundFileReader.java	(working copy)
@@ -24,6 +24,7 @@
 import org.apache.lucene.store.Lock;
 
 import java.util.HashMap;
+import java.io.FileNotFoundException;
 import java.io.IOException;
 
 
@@ -183,12 +184,10 @@
     /** Returns the length of a file in the directory.
      * @throws IOException if the file does not exist */
     @Override
-    public long fileLength(String name)
-    throws IOException
-    {
+    public long fileLength(String name) throws IOException {
         FileEntry e = entries.get(name);
         if (e == null)
-            throw new IOException("File " + name + " does not exist");
+            throw new FileNotFoundException(name);
         return e.length;
     }
 
Index: lucene/src/java/org/apache/lucene/store/Directory.java
===================================================================
--- lucene/src/java/org/apache/lucene/store/Directory.java	(revision 933613)
+++ lucene/src/java/org/apache/lucene/store/Directory.java	(working copy)
@@ -17,6 +17,7 @@
  * limitations under the License.
  */
 
+import java.io.FileNotFoundException;
 import java.io.IOException;
 import java.io.Closeable;
 import java.util.Collection; // for javadocs
@@ -73,9 +74,20 @@
   public abstract void deleteFile(String name)
        throws IOException;
 
-  /** Returns the length of a file in the directory. */
-  public abstract long fileLength(String name)
-       throws IOException;
+  /**
+   * Returns the length of a file in the directory. This method follows the
+   * following contract:
+   * <ul>
+   * <li>Throws {@link FileNotFoundException} if the file does not exist
+   * <li>Returns a value &ge;0 if the file exists, which specifies its length.
+   * </ul>
+   * 
+   * @param name the name of the file for which to return the length.
+   * @throws FileNotFoundException if the file does not exist.
+   * @throws IOException if there was an IO error while retrieving the file's
+   *         length.
+   */
+  public abstract long fileLength(String name) throws IOException;
 
 
   /** Creates a new, empty file in the directory with the given name.
Index: lucene/src/java/org/apache/lucene/store/FSDirectory.java
===================================================================
--- lucene/src/java/org/apache/lucene/store/FSDirectory.java	(revision 933591)
+++ lucene/src/java/org/apache/lucene/store/FSDirectory.java	(working copy)
@@ -19,6 +19,7 @@
 
 import java.io.File;
 import java.io.FileInputStream;
+import java.io.FileNotFoundException;
 import java.io.FileOutputStream;
 import java.io.FilenameFilter;
 import java.io.IOException;
@@ -265,9 +266,12 @@
 
   /** Returns the length in bytes of a file in the directory. */
   @Override
-  public long fileLength(String name) {
+  public long fileLength(String name) throws IOException {
     ensureOpen();
     File file = new File(directory, name);
+    if (!file.exists()) {
+      throw new FileNotFoundException(name);
+    }
     return file.length();
   }