Fixed an NPE in ImplicitPropertyAccessor due to putting null value in ConcurrentHashMap

src/main/java/net/abhinavsarkar/spelhelper/ImplicitConstructorResolver.java

@@ -27,8 +27,7 @@ import org.springframework.expression.ConstructorResolver;
 import org.springframework.expression.EvaluationContext;
 import org.springframework.expression.spel.support.ReflectiveConstructorResolver;
 
-final class ImplicitConstructorResolver implements
+final class ImplicitConstructorResolver implements ConstructorResolver {
-        ConstructorResolver {
 
     private final ReflectiveConstructorResolver delegate = new ReflectiveConstructorResolver();
 

src/main/java/net/abhinavsarkar/spelhelper/ImplicitMethodResolver.java

@@ -39,20 +39,21 @@ final class ImplicitMethodResolver implements MethodResolver {
     private final ReflectiveMethodResolver delegate = new ReflectiveMethodResolver();
 
     private static final MethodExecutor NULL_ME = new MethodExecutor() {
+        @Override
         public TypedValue execute(final EvaluationContext context, final Object target,
                 final Object... arguments) throws AccessException {
-            return null;
+            throw new UnsupportedOperationException("This method should never be called");
         }
     };
 
-    private static final class ImplicitMethodExecutor implements
+    private static final class ImplicitMethodExecutor implements MethodExecutor {
-            MethodExecutor {
         private final MethodExecutor executor;
 
         public ImplicitMethodExecutor(final MethodExecutor executor) {
             this.executor = executor;
         }
 
+        @Override
         public TypedValue execute(final EvaluationContext context, final Object target,
                 final Object... arguments) throws AccessException {
             Object[] modifiedArguments = new Object[arguments.length + 1];
@@ -66,7 +67,7 @@ final class ImplicitMethodResolver implements MethodResolver {
     public MethodExecutor resolve(
             final EvaluationContext context, final Object targetObject,
             final String name, final List<TypeDescriptor> argumentTypes)
-    throws AccessException {
+        throws AccessException {
         if (targetObject == null) {
             return null;
         }

src/main/java/net/abhinavsarkar/spelhelper/ImplicitPropertyAccessor.java

@@ -34,13 +34,22 @@ final class ImplicitPropertyAccessor extends ReadOnlyGenericPropertyAccessor {
     private static final ConcurrentHashMap<String, MethodExecutor> CACHE =
         new ConcurrentHashMap<String, MethodExecutor>();
 
+    private static final MethodExecutor NULL_ME = new MethodExecutor() {
+        @Override
+        public TypedValue execute(final EvaluationContext context, final Object target,
+                final Object... arguments) throws AccessException {
+            throw new UnsupportedOperationException("This method should never be called");
+        }
+    };
+
+    @Override
     public boolean canRead(final EvaluationContext context,
             final Object target, final String name)
             throws AccessException {
         Assert.notNull(target, "target is null");
         String cacheKey = target.getClass().getName() + "." + name;
         if (CACHE.containsKey(cacheKey)) {
-            return CACHE.get(cacheKey) != null;
+            return CACHE.get(cacheKey) != NULL_ME;
         }
 
         for (MethodResolver mr : context.getMethodResolvers()) {
@@ -52,10 +61,11 @@ final class ImplicitPropertyAccessor extends ReadOnlyGenericPropertyAccessor {
             }
         }
 
-        CACHE.putIfAbsent(cacheKey, null);
+        CACHE.putIfAbsent(cacheKey, NULL_ME);
         return false;
     }
 
+    @Override
     public TypedValue read(final EvaluationContext context,
             final Object target, final String name)
             throws AccessException {

src/main/java/net/abhinavsarkar/spelhelper/ReadOnlyGenericPropertyAccessor.java

@@ -23,19 +23,21 @@ import org.springframework.expression.AccessException;
 import org.springframework.expression.EvaluationContext;
 import org.springframework.expression.PropertyAccessor;
 
-abstract class ReadOnlyGenericPropertyAccessor implements
+abstract class ReadOnlyGenericPropertyAccessor implements PropertyAccessor {
-        PropertyAccessor {
 
+    @Override
     public final boolean canWrite(final EvaluationContext context,
             final Object target, final String name) throws AccessException {
         return false;
     }
 
+    @Override
     @SuppressWarnings("rawtypes")
     public final Class[] getSpecificTargetClasses() {
         return null;
     }
 
+    @Override
     public final void write(final EvaluationContext context, final Object target,
             final String name, final Object newValue) throws AccessException {
         throw new AccessException(MessageFormat.format(