diff --git a/src/main/java/com/jcabi/manifests/Manifests.java b/src/main/java/com/jcabi/manifests/Manifests.java
index fb9cff8..82a9d05 100644
--- a/src/main/java/com/jcabi/manifests/Manifests.java
+++ b/src/main/java/com/jcabi/manifests/Manifests.java
@@ -35,11 +35,13 @@
 import java.io.InputStream;
 import java.util.Collection;
 import java.util.HashMap;
+import java.util.HashSet;
 import java.util.Map;
 import java.util.Set;
 import java.util.TreeSet;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.atomic.AtomicReference;
 import java.util.jar.Attributes;
 import java.util.jar.Manifest;
 import javax.servlet.ServletContext;
@@ -126,7 +128,8 @@ public final class Manifests implements MfMap {
     /**
      * Default singleton.
      */
-    public static final MfMap DEFAULT = new Manifests();
+    private static final AtomicReference<MfMap> DEFAULT =
+        new AtomicReference<MfMap>(new Manifests());
 
     /**
      * Attributes retrieved.
@@ -135,7 +138,11 @@ public final class Manifests implements MfMap {
 
     static {
         try {
-            Manifests.DEFAULT.append(new ClasspathMfs());
+            final MfMap currentInstance = Manifests.DEFAULT.get();
+            Manifests.DEFAULT.compareAndSet(
+                currentInstance,
+                currentInstance.append(new ClasspathMfs())
+            );
         } catch (final IOException ex) {
             Logger.error(
                 Manifests.class,
@@ -173,57 +180,34 @@ public boolean isEmpty() {
     }
 
     @Override
-    public boolean containsKey(final Object key) {
+    public boolean containsKey(final String key) {
         return this.attributes.containsKey(key);
     }
 
     @Override
-    public boolean containsValue(final Object value) {
+    public boolean containsValue(final String value) {
         return this.attributes.containsValue(value);
     }
 
     @Override
-    public String get(final Object key) {
+    public String get(final String key) {
         return this.attributes.get(key);
     }
 
     @Override
-    public String put(final String key, final String value) {
-        return this.attributes.put(key, value);
-    }
-
-    @Override
-    public String remove(final Object key) {
-        return this.attributes.remove(key);
-    }
-
-    @Override
-    public void putAll(final Map<? extends String, ? extends String> attrs) {
-        this.attributes.putAll(attrs);
-    }
-
-    @Override
-    public void clear() {
-        this.attributes.clear();
+    public Map<String, String> getAsMap() {
+        return new HashMap<String, String>(this.attributes);
     }
 
     @Override
     public Set<String> keySet() {
-        return this.attributes.keySet();
-    }
-
-    @Override
-    public Collection<String> values() {
-        return this.attributes.values();
-    }
-
-    @Override
-    public Set<Map.Entry<String, String>> entrySet() {
-        return this.attributes.entrySet();
+        return new HashSet<String>(this.attributes.keySet());
     }
 
     @Override
     public MfMap append(final Mfs streams) throws IOException {
+        final ConcurrentHashMap<String, String> joinedAttributes =
+            new ConcurrentHashMap<String, String>(this.attributes);
         final long start = System.currentTimeMillis();
         final Collection<InputStream> list = streams.fetch();
         int saved = 0;
@@ -231,10 +215,10 @@ public MfMap append(final Mfs streams) throws IOException {
         for (final InputStream stream : list) {
             for (final Map.Entry<String, String> attr
                 : Manifests.load(stream).entrySet()) {
-                if (this.attributes.containsKey(attr.getKey())) {
+                if (joinedAttributes.containsKey(attr.getKey())) {
                     ++ignored;
                 } else {
-                    this.attributes.put(attr.getKey(), attr.getValue());
+                    joinedAttributes.put(attr.getKey(), attr.getValue());
                     ++saved;
                 }
             }
@@ -248,7 +232,7 @@ public MfMap append(final Mfs streams) throws IOException {
             saved, ignored,
             new TreeSet<String>(this.attributes.keySet())
         );
-        return this;
+        return new Manifests(joinedAttributes);
     }
 
     /**
@@ -276,12 +260,12 @@ public static String read(final String name) {
                     // @checkstyle LineLength (1 line)
                     "Attribute '%s' not found in MANIFEST.MF file(s) among %d other attribute(s): %[list]s",
                     name,
-                    Manifests.DEFAULT.size(),
-                    new TreeSet<String>(Manifests.DEFAULT.keySet())
+                    Manifests.DEFAULT.get().size(),
+                    new TreeSet<String>(Manifests.DEFAULT.get().keySet())
                 )
             );
         }
-        return Manifests.DEFAULT.get(name);
+        return Manifests.DEFAULT.get().get(name);
     }
 
     /**
@@ -302,7 +286,7 @@ public static boolean exists(final String name) {
         if (name.isEmpty()) {
             throw new IllegalArgumentException("attribute name can't be empty");
         }
-        return Manifests.DEFAULT.containsKey(name);
+        return Manifests.DEFAULT.get().containsKey(name);
     }
 
     /**
@@ -319,7 +303,11 @@ public static boolean exists(final String name) {
      */
     @Deprecated
     public static void append(final ServletContext ctx) throws IOException {
-        Manifests.DEFAULT.append(new ServletMfs(ctx));
+        final MfMap currentInstance = Manifests.DEFAULT.get();
+        Manifests.DEFAULT.compareAndSet(
+            currentInstance,
+            currentInstance.append(new ServletMfs(ctx))
+        );
     }
 
     /**
@@ -338,7 +326,11 @@ public static void append(final File file) throws IOException {
         if (file == null) {
             throw new IllegalArgumentException("file can't be NULL");
         }
-        Manifests.DEFAULT.append(new FilesMfs(file));
+        final MfMap currentInstance = Manifests.DEFAULT.get();
+        Manifests.DEFAULT.compareAndSet(
+            currentInstance,
+            currentInstance.append(new FilesMfs(file))
+        );
     }
 
     /**
@@ -358,7 +350,11 @@ public static void append(final InputStream stream) throws IOException {
         if (stream == null) {
             throw new IllegalArgumentException("input stream can't be NULL");
         }
-        Manifests.DEFAULT.append(new StreamsMfs(stream));
+        final MfMap currentInstance = Manifests.DEFAULT.get();
+        Manifests.DEFAULT.compareAndSet(
+            currentInstance,
+            currentInstance.append(new StreamsMfs(stream))
+        );
     }
 
     /**
@@ -403,5 +399,4 @@ private static Map<String, String> load(final InputStream stream)
         }
         return props;
     }
-
 }
diff --git a/src/main/java/com/jcabi/manifests/MfMap.java b/src/main/java/com/jcabi/manifests/MfMap.java
index c96b0e0..9191b7e 100644
--- a/src/main/java/com/jcabi/manifests/MfMap.java
+++ b/src/main/java/com/jcabi/manifests/MfMap.java
@@ -31,6 +31,7 @@
 
 import java.io.IOException;
 import java.util.Map;
+import java.util.Set;
 
 /**
  * Map of manifest attributes.
@@ -40,15 +41,67 @@
  * @since 1.1
  * @see Manifests
  */
-public interface MfMap extends Map<String, String> {
+public interface MfMap {
+
+    /**
+     * Get size of attributes map.
+     * @return Size of attributes map
+     * @since 2.0
+     */
+    int size();
+
+    /**
+     * Check if attributes map is empty.
+     * @return True if attributes map is empty and false otherwise
+     * @since 2.0
+     */
+    boolean isEmpty();
+
+    /**
+     * Check if attributes map contains the given key.
+     * @param key Attribute name
+     * @return True if attributes map contains the given key and false otherwise
+     * @since 2.0
+     */
+    boolean containsKey(String key);
+
+    /**
+     * Check if attributes map contains the given value.
+     * @param value Attribute value
+     * @return True if attributes map contains the given value and false otherwise
+     * @since 2.0
+     */
+    boolean containsValue(String value);
+
+    /**
+     * Get attribute value by its key.
+     * @param key Attribute name
+     * @return Value of the attribute and null if attribute not found
+     */
+    String get(String key);
+
+    /**
+     * Get a copy of attributes map.
+     * @return Copy of attributes map
+     * @since 2.0
+     */
+    Map<String, String> getAsMap();
+
+    /**
+     * Get a copy of a set of attributes keys.
+     * @return Copy of a set of attributes keys
+     * @since 2.0
+     */
+    Set<String> keySet();
 
     /**
      * Append this collection of MANIFEST.MF files.
+     * This method may not change the original instance
+     * and should create a new instance and return it as a result.
      * @param streams Files to append
-     * @return This
-     * @since 1.0
+     * @return New instance that contains original and appended attributes
      * @throws IOException If fails on I/O problem
+     * @since 1.0
      */
     MfMap append(Mfs streams) throws IOException;
-
 }
diff --git a/src/test/java/com/jcabi/manifests/ManifestsTest.java b/src/test/java/com/jcabi/manifests/ManifestsTest.java
index 5507600..76cc77a 100644
--- a/src/test/java/com/jcabi/manifests/ManifestsTest.java
+++ b/src/test/java/com/jcabi/manifests/ManifestsTest.java
@@ -102,11 +102,10 @@ public void appendsAttributesFromFile() throws Exception {
             file,
             Logger.format("%s: %s\n", name, value)
         );
-        final Manifests mfs = new Manifests();
-        mfs.append(new FilesMfs(file));
+        final MfMap manifests = new Manifests().append(new FilesMfs(file));
         MatcherAssert.assertThat(
             "loaded from file",
-            mfs.containsKey(name) && mfs.get(name).equals(value)
+            manifests.containsKey(name) && manifests.get(name).equals(value)
         );
     }
 
@@ -117,12 +116,11 @@ public void appendsAttributesFromFile() throws Exception {
      */
     @Test
     public void appendsAttributesFromInputStream() throws Exception {
-        final Manifests mfs = new Manifests();
-        mfs.append(
-            new StreamsMfs(this.getClass().getResourceAsStream("test.mf"))
+        final MfMap manifests = new Manifests().append(
+                new StreamsMfs(this.getClass().getResourceAsStream("test.mf"))
         );
         MatcherAssert.assertThat(
-            mfs.get("From-File"),
+            manifests.get("From-File"),
             Matchers.equalTo("some test attribute")
         );
     }