Fix for #112

Cleans content of each found directory when unpacking a new java_vm.
Fixes the problem when java_wm was build using standard jar. 
For pack200 this remains unresolved.

core/src/main/java/com/threerings/getdown/data/Resource.java

@@ -254,20 +254,21 @@ public class Resource implements Comparable<Resource>
     /**
      * Installs the {@code getLocalNew} version of this resource to {@code getLocal}.
      */
-    public void install () throws IOException {
+    public void install (boolean cleanExistingDirs) throws IOException {
         File source = getLocalNew(), dest = getLocal();
         log.info("- " + source);
         if (!FileUtil.renameTo(source, dest)) {
             throw new IOException("Failed to rename " + source + " to " + dest);
         }
-        applyAttrs();
+        applyAttrs(cleanExistingDirs);
         markAsValid();
     }
 
+
     /**
      * Unpacks this resource file into the directory that contains it.
      */
-    public void unpack () throws IOException
+    public void unpack (boolean cleanExistingDirs) throws IOException
     {
         // sanity check
         if (!_isJar && !_isPacked200Jar) {
@@ -275,26 +276,35 @@ public class Resource implements Comparable<Resource>
         }
         if (_isJar) {
             try (JarFile jar = new JarFile(_local)) {
-                FileUtil.unpackJar(jar, _unpacked);
+                FileUtil.unpackJar(jar, _unpacked, cleanExistingDirs);
             }
         } else {
             FileUtil.unpackPacked200Jar(_local, _unpacked);
         }
     }
+    
+    public void unpack () throws IOException
+    {
+    	unpack(false);
+    }
 
     /**
      * Applies this resources special attributes: unpacks this resource if needed, marks it as
      * executable if needed.
      */
-    public void applyAttrs () throws IOException {
+    public void applyAttrs (boolean cleanExistingDirs) throws IOException {
         if (shouldUnpack()) {
-            unpack();
+            unpack(cleanExistingDirs);
         }
         if (_attrs.contains(Attr.EXEC)) {
             FileUtil.makeExecutable(_local);
         }
     }
 
+    public void applyAttrs () throws IOException {
+    	applyAttrs(false);
+    }
+    
     /**
      * Wipes this resource file along with any "validated" marker file that may be associated with
      * it.

core/src/main/java/com/threerings/getdown/util/FileUtil.java

@@ -94,9 +94,25 @@ public class FileUtil
     /**
      * Unpacks the specified jar file into the specified target directory.
      */
-    public static void unpackJar (JarFile jar, File target) throws IOException
+    public static void unpackJar (JarFile jar, File target, boolean cleanExistingDirs) throws IOException
     {
         Enumeration<?> entries = jar.entries();
+        if (cleanExistingDirs)
+        {
+			while (entries.hasMoreElements()) {
+				JarEntry entry = (JarEntry) entries.nextElement();
+				if (entry.isDirectory()) {
+					File efile = new File(target, entry.getName());
+					 if (efile.exists()) {
+						 for (File f : efile.listFiles()) {
+							 if (!f.isDirectory())
+								 f.delete();
+						 }
+					 }
+				}
+			}
+        }
+        entries = jar.entries();
         while (entries.hasMoreElements()) {
             JarEntry entry = (JarEntry)entries.nextElement();
             File efile = new File(target, entry.getName());