diff --git a/src/main/java/eu/dnetlib/dhp/schema/oaf/Instance.java b/src/main/java/eu/dnetlib/dhp/schema/oaf/Instance.java
index 5907d78..1657f8f 100644
--- a/src/main/java/eu/dnetlib/dhp/schema/oaf/Instance.java
+++ b/src/main/java/eu/dnetlib/dhp/schema/oaf/Instance.java
@@ -4,6 +4,8 @@ package eu.dnetlib.dhp.schema.oaf;
 import java.io.Serializable;
 import java.util.List;
 
+import javax.validation.constraints.NotBlank;
+import javax.validation.constraints.NotEmpty;
 import javax.validation.constraints.NotNull;
 
 public class Instance implements Serializable {
@@ -19,7 +21,7 @@ public class Instance implements Serializable {
 	@NotNull
 	private KeyValue hostedby;
 
-	@NotNull
+	@NotEmpty
 	private List<String> url;
 
 	// other research products specifc
diff --git a/src/main/java/eu/dnetlib/dhp/schema/oaf/KeyValue.java b/src/main/java/eu/dnetlib/dhp/schema/oaf/KeyValue.java
index c274a96..8bc3707 100644
--- a/src/main/java/eu/dnetlib/dhp/schema/oaf/KeyValue.java
+++ b/src/main/java/eu/dnetlib/dhp/schema/oaf/KeyValue.java
@@ -3,6 +3,7 @@ package eu.dnetlib.dhp.schema.oaf;
 
 import java.io.Serializable;
 
+import javax.validation.constraints.NotBlank;
 import javax.validation.constraints.NotNull;
 
 import org.apache.commons.lang3.StringUtils;
@@ -11,10 +12,10 @@ import com.fasterxml.jackson.annotation.JsonIgnore;
 
 public class KeyValue implements Serializable {
 
-	@NotNull
+	@NotBlank
 	private String key;
 
-	@NotNull
+	@NotBlank
 	private String value;
 
 	private DataInfo dataInfo;
diff --git a/src/main/java/eu/dnetlib/dhp/schema/oaf/Oaf.java b/src/main/java/eu/dnetlib/dhp/schema/oaf/Oaf.java
index 8d8d4cd..9bbcac2 100644
--- a/src/main/java/eu/dnetlib/dhp/schema/oaf/Oaf.java
+++ b/src/main/java/eu/dnetlib/dhp/schema/oaf/Oaf.java
@@ -9,6 +9,8 @@ import java.util.Optional;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
+import javax.validation.Valid;
+import javax.validation.constraints.NotEmpty;
 import javax.validation.constraints.NotNull;
 
 public abstract class Oaf implements Serializable {
@@ -16,9 +18,11 @@ public abstract class Oaf implements Serializable {
 	/**
 	 * The list of datasource id/name pairs providing this relationship.
 	 */
-	@NotNull
+	@Valid
+	@NotEmpty
 	protected List<KeyValue> collectedfrom;
 
+	@Valid
 	@NotNull
 	private DataInfo dataInfo;
 
diff --git a/src/main/java/eu/dnetlib/dhp/schema/oaf/Result.java b/src/main/java/eu/dnetlib/dhp/schema/oaf/Result.java
index db8261a..0269fd4 100644
--- a/src/main/java/eu/dnetlib/dhp/schema/oaf/Result.java
+++ b/src/main/java/eu/dnetlib/dhp/schema/oaf/Result.java
@@ -5,9 +5,10 @@ import java.io.Serializable;
 import java.util.Comparator;
 import java.util.List;
 import java.util.Objects;
-import java.util.Optional;
 import java.util.stream.Collectors;
 
+import javax.validation.Valid;
+import javax.validation.constraints.NotEmpty;
 import javax.validation.constraints.NotNull;
 
 import eu.dnetlib.dhp.schema.common.AccessRightComparator;
@@ -20,6 +21,7 @@ public class Result extends OafEntity implements Serializable {
 	private List<Author> author;
 
 	// resulttype allows subclassing results into publications | datasets | software
+	@Valid
 	@NotNull
 	private Qualifier resulttype;
 
@@ -60,7 +62,8 @@ public class Result extends OafEntity implements Serializable {
 
 	private List<ExternalReference> externalReference;
 
-	@NotNull
+	@Valid
+	@NotEmpty
 	private List<Instance> instance;
 
 	public List<Measure> getMeasures() {