Mark classes used by serializable classes also serializable

src/main/java/de/thm/arsnova/entities/Answer.java

@@ -21,13 +21,15 @@ import com.fasterxml.jackson.annotation.JsonIgnore;
 import io.swagger.annotations.ApiModel;
 import io.swagger.annotations.ApiModelProperty;
 
+import java.io.Serializable;
+
 /**
  * Both a regular (single choice, evaluation, etc.) as well as a freetext answer.
  *
  * This class has additional fields to transport generated answer statistics.
  */
 @ApiModel(value = "Answer", description = "the answer entity")
-public class Answer {
+public class Answer implements Serializable {
 
 	private String _id;
 	private String _rev;

src/main/java/de/thm/arsnova/entities/InterposedQuestion.java

@@ -21,11 +21,13 @@ import com.fasterxml.jackson.annotation.JsonIgnore;
 import io.swagger.annotations.ApiModel;
 import io.swagger.annotations.ApiModelProperty;
 
+import java.io.Serializable;
+
 /**
  * A question the user is asking the teacher. Also known as feedback or audience question.
  */
 @ApiModel(value = "audiencequestion", description = "the interposed question entity")
-public class InterposedQuestion {
+public class InterposedQuestion implements Serializable {
 
 	private String _id;
 	private String _rev;

src/main/java/de/thm/arsnova/entities/LearningProgressOptions.java

@@ -19,11 +19,14 @@ package de.thm.arsnova.entities;
 
 import io.swagger.annotations.ApiModel;
 import io.swagger.annotations.ApiModelProperty;
+
+import java.io.Serializable;
+
 /**
  * A session's settings regarding the calculation of the learning progress.
  */
 @ApiModel(value = "learning progress options", description = "the learning progress entity")
-public class LearningProgressOptions {
+public class LearningProgressOptions implements Serializable {
 
 	private String type = "questions";
 

src/main/java/de/thm/arsnova/entities/PossibleAnswer.java

@@ -20,11 +20,13 @@ package de.thm.arsnova.entities;
 import io.swagger.annotations.ApiModel;
 import io.swagger.annotations.ApiModelProperty;
 
+import java.io.Serializable;
+
 /**
  * This class represents an answer option of a question.
  */
 @ApiModel(value = "session/answer", description = "the possible answer entity")
-public class PossibleAnswer {
+public class PossibleAnswer implements Serializable {
 
 	private String id;
 	private String text;

src/main/java/de/thm/arsnova/entities/SessionFeature.java

@@ -16,14 +16,17 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 package de.thm.arsnova.entities;
+
 import io.swagger.annotations.ApiModel;
 import io.swagger.annotations.ApiModelProperty;
 
+import java.io.Serializable;
+
 /**
  * Contains fields that describe which specific feature is activated for a session.
  */
 @ApiModel(value = "session feature", description = "the session feature entity")
-public class SessionFeature {
+public class SessionFeature implements Serializable {
 
 	private boolean custom = true;
 	private boolean clicker = false;

src/main/java/de/thm/arsnova/entities/transport/Answer.java

@@ -23,6 +23,7 @@ import de.thm.arsnova.entities.User;
 import io.swagger.annotations.ApiModel;
 import io.swagger.annotations.ApiModelProperty;
 
+import java.io.Serializable;
 import java.util.Date;
 
 /**
@@ -30,7 +31,7 @@ import java.util.Date;
  */
 @JsonInclude(JsonInclude.Include.NON_DEFAULT)
 @ApiModel(value = "session/answer", description = "the Answer API")
-public class Answer {
+public class Answer implements Serializable {
 
 	private String answerSubject;