Create constructors with zero argument for serialization

Change-Id: I503197a2dcfe1b71dc6a82d4c2ca4512465200c5

@@ -44,6 +44,12 @@ public class DefaultResource<S, T> implements Resource<S, T> {
         this.resource = checkNotNull(resource);
     }
 
+    // for serialization
+    private DefaultResource() {
+        this.subject = null;
+        this.resource = null;
+    }
+
     @Override
     public S subject() {
         return subject;

@@ -47,6 +47,13 @@ public class DefaultResourceAllocation<S, T> implements ResourceAllocation<S, T>
         this.consumer = consumer;
     }
 
+    // for serialization
+    private DefaultResourceAllocation() {
+        this.subject = null;
+        this.resource = null;
+        this.consumer = null;
+    }
+
     @Override
     public S subject() {
         return subject;