DATACMNS-113 - Introduced annotations to resolve custom conversion disambiguities.

Introduced @ReadingConverter and @WritingConverter to allow to resolve disambiguates that might arise when implementing custom Converters.

Suppose we implemented a Converter<String, Class> and the store is able to handle both String and Class as native types. In this case there's no way to determine whether the converter shall actually be used to convert String properties into Class instances when writing or to create Class instances from Strings when reading. Thus the @ReadingConverter comes in handy in this case to tell the infrastructure to consider the converter for reading only.

spring-data-commons-core/src/main/java/org/springframework/data/convert/ReadingConverter.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2011 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.convert;
+
+import static java.lang.annotation.ElementType.*;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+import org.springframework.core.convert.converter.Converter;
+
+/**
+ * Annotation to clarify intended usage of a {@link Converter} as reading converter in case the conversion types leave
+ * room for disambiguation.
+ * 
+ * @author Oliver Gierke
+ */
+@Target(TYPE)
+@Documented
+@Retention(RetentionPolicy.RUNTIME)
+public @interface ReadingConverter {
+
+}

spring-data-commons-core/src/main/java/org/springframework/data/convert/WritingConverter.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2011 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.convert;
+
+import static java.lang.annotation.ElementType.*;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+import org.springframework.core.convert.converter.Converter;
+
+/**
+ * Annotation to clarify intended usage of a {@link Converter} as writing converter in case the conversion types leave
+ * room for disambiguation.
+ * 
+ * @author Oliver Gierke
+ */
+@Target(TYPE)
+@Documented
+@Retention(RetentionPolicy.RUNTIME)
+public @interface WritingConverter {
+
+}