Issue #108: Opened constructor and fixed NPE

In some use cases it is required to pass in all three parameters into
`DynamoDBTemplate` - therefore made the internal constructor public (that
was used by all other constructors anyway)

Also passing in a combination of `null`/non-`null` parameters could lead
to NPEs down the road - ie. by calling `getOverrideTableName`.
The constructor now initalizes all fields with non-`null` objects and does
not relay on AWS constructors to pick some proper default configs.

src/main/java/org/socialsignin/spring/data/dynamodb/core/DynamoDBTemplate.java

@@ -25,6 +25,7 @@
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.ApplicationContextAware;
 import org.springframework.context.ApplicationEventPublisher;
+import org.springframework.util.Assert;
 
 import java.util.List;
 import java.util.Map;
@@ -36,36 +37,45 @@ public class DynamoDBTemplate implements DynamoDBOperations, ApplicationContextA
 	private final DynamoDBMapperConfig dynamoDBMapperConfig;
 	private ApplicationEventPublisher eventPublisher;
 
+	/** Convenient constructor to use the default {@link DynamoDBMapper#DynamoDBMapper(AmazonDynamoDB)} */
 	public DynamoDBTemplate(AmazonDynamoDB amazonDynamoDB, DynamoDBMapperConfig dynamoDBMapperConfig)
 	{
 	    this(amazonDynamoDB, dynamoDBMapperConfig, null);
 	}
 
+	/** Convenient constructor to use the {@link DynamoDBMapperConfig.DEFAULT} */
+
     public DynamoDBTemplate(AmazonDynamoDB amazonDynamoDB, DynamoDBMapper dynamoDBMapper)
     {
         this(amazonDynamoDB, null, dynamoDBMapper);
     }
 
+    /** Convenient construcotr to thse the {@link DynamoDBMapperConfig.DEFAULT} and default {@link DynamoDBMapper#DynamoDBMapper(AmazonDynamoDB)} */
     public DynamoDBTemplate(AmazonDynamoDB amazonDynamoDB)
     {
         this(amazonDynamoDB, null, null);
     }
 
-	DynamoDBTemplate(AmazonDynamoDB amazonDynamoDB, DynamoDBMapperConfig dynamoDBMapperConfig, DynamoDBMapper dynamoDBMapper) {
-       this.amazonDynamoDB = amazonDynamoDB;
-       if (dynamoDBMapper == null && dynamoDBMapperConfig == null) {
-            this.dynamoDBMapperConfig = DynamoDBMapperConfig.DEFAULT;
-            this.dynamoDBMapper = new DynamoDBMapper(amazonDynamoDB);
-            // Mapper must be null as it could not have been constructed without a Config
-            assert dynamoDBMapper == null;
-        } else {
-            this.dynamoDBMapperConfig = dynamoDBMapperConfig;
-            if (dynamoDBMapper == null) {
-                this.dynamoDBMapper = new DynamoDBMapper(amazonDynamoDB, dynamoDBMapperConfig);
-            } else {
-                this.dynamoDBMapper = dynamoDBMapper;
-            }
-        }
+    /** Initializes a new {@code DynamoDBTemplate}.
+     * The following combinations are valid:
+     * @param amazonDynamoDB must not be {@code null}
+     * @param dynamoDBMapperConfig can be {@code null} - {@link DynamoDBMapperConfig.DEFAULT} is used if {@code null} is passed in
+     * @param dynamoDBMapper can be {@code null} - {@link DynamoDBMapper#DynamoDBMapper(AmazonDynamoDB, DynamoDBMapperConfig)} is used if {@code null} is passed in */
+	public DynamoDBTemplate(AmazonDynamoDB amazonDynamoDB, DynamoDBMapperConfig dynamoDBMapperConfig, DynamoDBMapper dynamoDBMapper) {
+       Assert.notNull(amazonDynamoDB, "amazonDynamoDB must not be null!");
+	   this.amazonDynamoDB = amazonDynamoDB;
+
+	  if (dynamoDBMapperConfig == null) {
+		  this.dynamoDBMapperConfig = DynamoDBMapperConfig.DEFAULT;		  
+	  } else {
+		  this.dynamoDBMapperConfig = dynamoDBMapperConfig;
+	  }
+
+	  if (dynamoDBMapper == null) {
+          this.dynamoDBMapper = new DynamoDBMapper(amazonDynamoDB, dynamoDBMapperConfig);
+	  } else {
+          this.dynamoDBMapper = dynamoDBMapper;
+	  }
     }
 
 	@Override

src/test/java/org/socialsignin/spring/data/dynamodb/core/DynamoDBTemplateUnitTest.java

@@ -4,9 +4,13 @@
 import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBMapper;
 import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBMapperConfig;
 import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBMapperConfig.TableNameResolver;
+import com.amazonaws.services.dynamodbv2.document.DynamoDB;
+
 import org.junit.Assert;
 import org.junit.Before;
+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.Mockito;
@@ -17,11 +21,13 @@
 import java.util.ArrayList;
 import java.util.List;
 
+import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertTrue;
 
 @RunWith(MockitoJUnitRunner.class)
 public class DynamoDBTemplateUnitTest {
-
+	@Rule
+	public ExpectedException expectedException = ExpectedException.none();
 	@Mock
 	private DynamoDBMapper dynamoDBMapper;
 	@Mock
@@ -35,9 +41,25 @@ public class DynamoDBTemplateUnitTest {
 	public void setUp() {
 		this.dynamoDBTemplate = new DynamoDBTemplate(dynamoDB, dynamoDBMapperConfig, dynamoDBMapper);
 	}
+
+	@Test
+	public void testConstructorMandatory() {
+		expectedException.expect(IllegalArgumentException.class);
+		expectedException.expectMessage("must not be null!");
+		new DynamoDBTemplate(null);
+	}
 
 	@Test
-	public void testPreconfiguredDynamoDBMapper() {
+	public void testConstructorOptionalAllNull() {
+		dynamoDBTemplate = new DynamoDBTemplate(dynamoDB, null, null);
+
+		// check that the defaults are properly initialized - #108
+		String userTableName = dynamoDBTemplate.getOverriddenTableName(User.class, "UserTable");
+		assertEquals("user", userTableName);
+	}
+
+	@Test
+	public void testConstructorOptionalPreconfiguredDynamoDBMapper() {
 		// Introduced constructor via #91 should not fail its assert
 		DynamoDBTemplate usePreconfiguredDynamoDBMapper = new DynamoDBTemplate(dynamoDB, dynamoDBMapper);