Introduce Substitutable

pom.xml

@@ -78,7 +78,7 @@
         <dependency>
             <groupId>net.yetamine</groupId>
             <artifactId>net.yetamine.sova</artifactId>
-            <!-- Built with snapshot/1.0.0/20160317 -->
+            <!-- Built with snapshot/1.0.0/20160322 -->
             <version>1.0.0-SNAPSHOT</version>
         </dependency>
     </dependencies>

src/main/java/net/yetamine/sova/servlet/ServletAttributeSymbol.java

@@ -19,6 +19,8 @@
 import java.util.Objects;
 
 import net.yetamine.sova.AdaptationProvider;
+import net.yetamine.sova.Mappable;
+import net.yetamine.sova.Substitutable;
 import net.yetamine.sova.symbols.DelegatingSymbol;
 
 /**
@@ -28,10 +30,12 @@
  * @param <T>
  *            the type of resulting values
  */
-public abstract class ServletAttributeSymbol<T> extends DelegatingSymbol<T> {
+public abstract class ServletAttributeSymbol<T> extends DelegatingSymbol<T> implements Substitutable<Mappable<String, T>> {
 
     /** Name of the attribute. */
     private final String attribute;
+    /** Cached {@link #substitute()}. */
+    private Mappable<String, T> substitute;
 
     /**
      * Prepares a new instance.
@@ -86,4 +90,21 @@ public static String name(Class<?> qualifier, String identifier) {
     public final String attribute() {
         return attribute;
     }
+
+    /**
+     * @see net.yetamine.sova.symbols.PublicSymbol#substitute()
+     */
+    public final Mappable<String, T> substitute() {
+        // Using caching technique that uses out-of-thin air thread safety;
+        // this technique is alright here, because the instances are always
+        // behaving in the same way, therefore they are interchangeable
+        Mappable<String, T> result = substitute;
+        if (result != null) {
+            return result;
+        }
+
+        result = Mappable.of(attribute, this);
+        substitute = result;
+        return result;
+    }
 }

src/main/java/net/yetamine/sova/servlet/ServletSymbol.java

@@ -166,11 +166,7 @@ public T use(ServletRequest source) {
      */
     public Optional<T> find(ServletRequest source) {
         final T result = requestSymbol.get(source);
-        if (result != null) {
-            return Optional.of(result);
-        }
-
-        return contextSymbol.find(source.getServletContext());
+        return (result != null) ? Optional.of(result) : contextSymbol.find(source.getServletContext());
     }
 
     /**