Add @Mandatory property feature.

owner/src/main/java/org/aeonbits/owner/Config.java

@@ -80,6 +80,16 @@ public interface Config extends Serializable {
         String value();
     }
 
+    /**
+	 * Indicates a mandatory property. If no property is found a {@link MissingMandatoryPropertyException} 
+	 * exception is thrown.
+	 */
+	@Retention(RUNTIME)
+	@Target(METHOD)
+	@Documented
+	@interface Mandatory {
+	}
+
     /**
      * The key used for lookup for the property.  If not present, the key will be generated based on the unqualified
      * method name.

owner/src/main/java/org/aeonbits/owner/MissingMandatoryPropertyException.java

@@ -0,0 +1,10 @@
+package org.aeonbits.owner;
+
+public class MissingMandatoryPropertyException extends RuntimeException {
+	private static final long serialVersionUID = 1L;
+
+	MissingMandatoryPropertyException(String message) {
+		super(message);
+	}
+
+}

owner/src/main/java/org/aeonbits/owner/PropertiesInvocationHandler.java

@@ -15,6 +15,8 @@
 import java.util.LinkedList;
 import java.util.List;
 
+import org.aeonbits.owner.Config.Mandatory;
+
 import static org.aeonbits.owner.Config.DisableableFeature.PARAMETER_FORMATTING;
 import static org.aeonbits.owner.Config.DisableableFeature.VARIABLE_EXPANSION;
 import static org.aeonbits.owner.Converters.SpecialValue.NULL;
@@ -81,8 +83,10 @@ private Object resolveProperty(Method method, Object... args) {
             String unexpandedKey = key(method);
             value = propertiesManager.getProperty(unexpandedKey);
         }
-        if (value == null)
+        if (value == null) {
+            throwIfMandatoryProperty(method, key);
             return null;
+        }
         // Before processing the value, we decrypt it if necessary.
         // It is a security hole store the decrypted value, so every time we need it it should be decrypted.
         value = this.propertiesManager.decryptIfNecessary( method, value );
@@ -92,6 +96,14 @@ private Object resolveProperty(Method method, Object... args) {
         return result;
     }
 
+    private void throwIfMandatoryProperty(Method method, String key) {
+        Mandatory mandatory = method.getAnnotation(Mandatory.class);
+        if (mandatory != null) {
+            String message = String.format("Missing mandatory property: '%s'", key);
+            throw new MissingMandatoryPropertyException(message);
+        }
+    }
+
     private String preProcess(Method method, String value) {
         List<Preprocessor> preprocessors = resolvePreprocessors(method);
         String result = value;

owner/src/test/java/org/aeonbits/owner/ConfigTest.java

@@ -8,7 +8,9 @@
 
 package org.aeonbits.owner;
 
+import org.junit.Rule;
 import org.junit.Test;
+import org.junit.rules.ExpectedException;
 import org.junit.runner.RunWith;
 import org.mockito.Mock;
 import org.mockito.runners.MockitoJUnitRunner;
@@ -26,7 +28,10 @@
 @RunWith(MockitoJUnitRunner.class)
 public class ConfigTest {
 
-    @Mock
+	@Rule
+	public ExpectedException expected = ExpectedException.none();
+
+	@Mock
     private ScheduledExecutorService scheduler;
 
     public interface SampleConfig extends Config {
@@ -85,6 +90,30 @@ public void testDefaultStringValue() {
         assertEquals("Hello Mr. Luigi!", config.helloMr("Luigi"));
     }
 
+	static interface WithMandatoryPropertyConfig extends Config {
+		@Mandatory
+		String missingMandatoryProperty();
+
+        @Mandatory
+		@DefaultValue("I'm here")
+        String mandatory();
+    }
+
+    @Test
+    public void shouldThrowWhenMissingMandatoryProperty() {
+		expected.expect(MissingMandatoryPropertyException.class);
+		expected.expectMessage("Missing mandatory property: 'missingMandatoryProperty'");
+
+    	WithMandatoryPropertyConfig config = ConfigFactory.create(WithMandatoryPropertyConfig.class);
+		config.missingMandatoryProperty();
+    }
+
+	@Test
+	public void testMandatoryStringValue() {
+    	WithMandatoryPropertyConfig config = ConfigFactory.create(WithMandatoryPropertyConfig.class);
+        assertEquals("I'm here", config.mandatory());
+    }
+
     @Test
     public void testDefaultPropertyOverridden() {
         SampleConfig config = ConfigFactory.create(SampleConfig.class);