diff --git a/api/src/main/java/io/serverlessworkflow/api/mapper/WorkflowModule.java b/api/src/main/java/io/serverlessworkflow/api/mapper/WorkflowModule.java
index b2e2606e..5b42598d 100644
--- a/api/src/main/java/io/serverlessworkflow/api/mapper/WorkflowModule.java
+++ b/api/src/main/java/io/serverlessworkflow/api/mapper/WorkflowModule.java
@@ -42,11 +42,11 @@
 
 public class WorkflowModule extends SimpleModule {
 
-  private static final long serialVersionUID = 510l;
+  private static final long serialVersionUID = 510L;
 
-  private WorkflowPropertySource workflowPropertySource;
-  private ExtensionSerializer extensionSerializer;
-  private ExtensionDeserializer extensionDeserializer;
+  private final WorkflowPropertySource workflowPropertySource;
+  private final ExtensionSerializer extensionSerializer;
+  private final ExtensionDeserializer extensionDeserializer;
 
   public WorkflowModule() {
     this(null);
diff --git a/api/src/main/java/io/serverlessworkflow/api/serializers/FunctionRefSerializer.java b/api/src/main/java/io/serverlessworkflow/api/serializers/FunctionRefSerializer.java
index cf5ce81f..71f61606 100644
--- a/api/src/main/java/io/serverlessworkflow/api/serializers/FunctionRefSerializer.java
+++ b/api/src/main/java/io/serverlessworkflow/api/serializers/FunctionRefSerializer.java
@@ -41,12 +41,12 @@ public void serialize(FunctionRef functionRef, JsonGenerator gen, SerializerProv
           && (functionRef.getInvoke() == null
               || functionRef.getInvoke().equals(FunctionRef.Invoke.SYNC))
           && functionRef.getRefName() != null
-          && functionRef.getRefName().length() > 0) {
+          && !functionRef.getRefName().isEmpty()) {
         gen.writeString(functionRef.getRefName());
       } else {
         gen.writeStartObject();
 
-        if (functionRef.getRefName() != null && functionRef.getRefName().length() > 0) {
+        if (functionRef.getRefName() != null && !functionRef.getRefName().isEmpty()) {
           gen.writeStringField("refName", functionRef.getRefName());
         }
 
@@ -54,7 +54,7 @@ public void serialize(FunctionRef functionRef, JsonGenerator gen, SerializerProv
           gen.writeObjectField("arguments", functionRef.getArguments());
         }
 
-        if (functionRef.getSelectionSet() != null && functionRef.getSelectionSet().length() > 0) {
+        if (functionRef.getSelectionSet() != null && !functionRef.getSelectionSet().isEmpty()) {
           gen.writeStringField("selectionSet", functionRef.getSelectionSet());
         }
 
diff --git a/api/src/main/java/io/serverlessworkflow/api/serializers/WorkflowSerializer.java b/api/src/main/java/io/serverlessworkflow/api/serializers/WorkflowSerializer.java
index 4bbaf9c1..b5ac7cbb 100644
--- a/api/src/main/java/io/serverlessworkflow/api/serializers/WorkflowSerializer.java
+++ b/api/src/main/java/io/serverlessworkflow/api/serializers/WorkflowSerializer.java
@@ -15,187 +15,171 @@
  */
 package io.serverlessworkflow.api.serializers;
 
+import java.io.IOException;
+import java.security.MessageDigest;
+import java.util.List;
+import java.util.UUID;
+
 import com.fasterxml.jackson.core.JsonGenerator;
 import com.fasterxml.jackson.databind.SerializerProvider;
 import com.fasterxml.jackson.databind.ser.std.StdSerializer;
 import io.serverlessworkflow.api.Workflow;
-import io.serverlessworkflow.api.error.ErrorDefinition;
-import io.serverlessworkflow.api.events.EventDefinition;
-import io.serverlessworkflow.api.functions.FunctionDefinition;
 import io.serverlessworkflow.api.interfaces.Extension;
 import io.serverlessworkflow.api.interfaces.State;
-import io.serverlessworkflow.api.retry.RetryDefinition;
-import java.io.IOException;
-import java.security.MessageDigest;
-import java.util.UUID;
 
 public class WorkflowSerializer extends StdSerializer<Workflow> {
 
-  public WorkflowSerializer() {
-    this(Workflow.class);
-  }
-
-  protected WorkflowSerializer(Class<Workflow> t) {
-    super(t);
-  }
-
-  private static final char[] hexArray = "0123456789ABCDEF".toCharArray();
-
-  @Override
-  public void serialize(Workflow workflow, JsonGenerator gen, SerializerProvider provider)
-      throws IOException {
-
-    gen.writeStartObject();
-
-    if (workflow.getId() != null && !workflow.getId().isEmpty()) {
-      gen.writeStringField("id", workflow.getId());
-    } else {
-      gen.writeStringField("id", generateUniqueId());
-    }
-
-    if (workflow.getKey() != null) {
-      gen.writeStringField("key", workflow.getKey());
-    }
-    gen.writeStringField("name", workflow.getName());
-
-    if (workflow.getDescription() != null && !workflow.getDescription().isEmpty()) {
-      gen.writeStringField("description", workflow.getDescription());
-    }
-
-    if (workflow.getVersion() != null && !workflow.getVersion().isEmpty()) {
-      gen.writeStringField("version", workflow.getVersion());
-    }
-
-    if (workflow.getAnnotations() != null && !workflow.getAnnotations().isEmpty()) {
-      gen.writeObjectField("annotations", workflow.getAnnotations());
-    }
-
-    if (workflow.getDataInputSchema() != null) {
-      if (workflow.getDataInputSchema().getSchema() != null
-          && workflow.getDataInputSchema().getSchema().length() > 0
-          && workflow.getDataInputSchema().isFailOnValidationErrors()) {
-        gen.writeStringField("dataInputSchema", workflow.getDataInputSchema().getSchema());
-
-      } else if (workflow.getDataInputSchema().getSchema() != null
-          && workflow.getDataInputSchema().getSchema().length() > 0
-          && !workflow.getDataInputSchema().isFailOnValidationErrors()) {
-        gen.writeObjectField("dataInputSchema", workflow.getDataInputSchema());
-      }
-    }
-
-    if (workflow.getStart() != null) {
-      gen.writeObjectField("start", workflow.getStart());
-    }
-
-    if (workflow.getSpecVersion() != null && !workflow.getSpecVersion().isEmpty()) {
-      gen.writeStringField("specVersion", workflow.getSpecVersion());
-    }
-
-    if (workflow.getExtensions() != null && !workflow.getExpressionLang().isEmpty()) {
-      gen.writeStringField("expressionLang", workflow.getExpressionLang());
-    }
-
-    if (workflow.isKeepActive()) {
-      gen.writeBooleanField("keepActive", workflow.isKeepActive());
-    }
-
-    if (workflow.isAutoRetries()) {
-      gen.writeBooleanField("autoRetries", workflow.isAutoRetries());
-    }
-
-    if (workflow.getMetadata() != null && !workflow.getMetadata().isEmpty()) {
-      gen.writeObjectField("metadata", workflow.getMetadata());
-    }
-
-    if (workflow.getEvents() != null && !workflow.getEvents().getEventDefs().isEmpty()) {
-      gen.writeArrayFieldStart("events");
-      for (EventDefinition eventDefinition : workflow.getEvents().getEventDefs()) {
-        gen.writeObject(eventDefinition);
-      }
-      gen.writeEndArray();
-    }
-
-    if (workflow.getFunctions() != null && !workflow.getFunctions().getFunctionDefs().isEmpty()) {
-      gen.writeArrayFieldStart("functions");
-      for (FunctionDefinition function : workflow.getFunctions().getFunctionDefs()) {
-        gen.writeObject(function);
-      }
-      gen.writeEndArray();
-    }
-
-    if (workflow.getRetries() != null && !workflow.getRetries().getRetryDefs().isEmpty()) {
-      gen.writeArrayFieldStart("retries");
-      for (RetryDefinition retry : workflow.getRetries().getRetryDefs()) {
-        gen.writeObject(retry);
-      }
-      gen.writeEndArray();
-    }
-
-    if (workflow.getErrors() != null && !workflow.getErrors().getErrorDefs().isEmpty()) {
-      gen.writeArrayFieldStart("errors");
-      for (ErrorDefinition error : workflow.getErrors().getErrorDefs()) {
-        gen.writeObject(error);
-      }
-      gen.writeEndArray();
-    }
-
-    if (workflow.getSecrets() != null && !workflow.getSecrets().getSecretDefs().isEmpty()) {
-      gen.writeArrayFieldStart("secrets");
-      for (String secretDef : workflow.getSecrets().getSecretDefs()) {
-        gen.writeString(secretDef);
-      }
-      gen.writeEndArray();
-    }
-
-    if (workflow.getConstants() != null && !workflow.getConstants().getConstantsDef().isEmpty()) {
-      gen.writeObjectField("constants", workflow.getConstants().getConstantsDef());
-    }
-
-    if (workflow.getTimeouts() != null) {
-      gen.writeObjectField("timeouts", workflow.getTimeouts());
-    }
-
-    if (workflow.getAuth() != null && !workflow.getAuth().getAuthDefs().isEmpty()) {
-      gen.writeObjectField("auth", workflow.getAuth().getAuthDefs());
-    }
-
-    if (workflow.getStates() != null && !workflow.getStates().isEmpty()) {
-      gen.writeArrayFieldStart("states");
-      for (State state : workflow.getStates()) {
-        gen.writeObject(state);
-      }
-      gen.writeEndArray();
-    }
-
-    if (workflow.getExtensions() != null && !workflow.getExtensions().isEmpty()) {
-      gen.writeArrayFieldStart("extensions");
-      for (Extension extension : workflow.getExtensions()) {
-        gen.writeObject(extension);
-      }
-      gen.writeEndArray();
-    }
-
-    gen.writeEndObject();
-  }
-
-  protected static String generateUniqueId() {
-    try {
-      MessageDigest salt = MessageDigest.getInstance("SHA-256");
-
-      salt.update(UUID.randomUUID().toString().getBytes("UTF-8"));
-      return bytesToHex(salt.digest());
-    } catch (Exception e) {
-      return UUID.randomUUID().toString();
-    }
-  }
-
-  protected static String bytesToHex(byte[] bytes) {
-    char[] hexChars = new char[bytes.length * 2];
-    for (int j = 0; j < bytes.length; j++) {
-      int v = bytes[j] & 0xFF;
-      hexChars[j * 2] = hexArray[v >>> 4];
-      hexChars[j * 2 + 1] = hexArray[v & 0x0F];
+    private static final char[] hexArray = "0123456789ABCDEF".toCharArray();
+
+    public WorkflowSerializer() {
+        this(Workflow.class);
+    }
+
+    protected WorkflowSerializer(Class<Workflow> t) {
+        super(t);
+    }
+
+    protected static String generateUniqueId() {
+        try {
+            MessageDigest salt = MessageDigest.getInstance("SHA-256");
+            salt.update(UUID.randomUUID().toString().getBytes("UTF-8"));
+            return bytesToHex(salt.digest());
+        } catch (Exception e) {
+            return UUID.randomUUID().toString();
+        }
+    }
+
+    protected static String bytesToHex(byte[] bytes) {
+        char[] hexChars = new char[bytes.length * 2];
+        for (int j = 0; j < bytes.length; j++) {
+            int v = bytes[j] & 0xFF;
+            hexChars[j * 2] = hexArray[v >>> 4];
+            hexChars[j * 2 + 1] = hexArray[v & 0x0F];
+        }
+        return new String(hexChars);
+    }
+
+    // Helper to write either an array of items or a single string reference
+    private <T> void writeListOrRef(
+            JsonGenerator gen,
+            SerializerProvider prov,
+            String fieldName,
+            List<T> items,
+            String refValue) throws IOException {
+        if (items != null && !items.isEmpty()) {
+            gen.writeArrayFieldStart(fieldName);
+            for (T item : items) {
+                prov.defaultSerializeValue(item, gen);
+            }
+            gen.writeEndArray();
+        } else if (refValue != null) {
+            gen.writeStringField(fieldName, refValue);
+        }
+    }
+
+    @Override
+    public void serialize(Workflow workflow, JsonGenerator gen, SerializerProvider provider)
+            throws IOException {
+
+        gen.writeStartObject();
+
+        // --- ID / key / basic fields ---
+        if (workflow.getId() != null && !workflow.getId().isEmpty()) {
+            gen.writeStringField("id", workflow.getId());
+        } else {
+            gen.writeStringField("id", generateUniqueId());
+        }
+        if (workflow.getKey() != null) {
+            gen.writeStringField("key", workflow.getKey());
+        }
+        gen.writeStringField("name", workflow.getName());
+        if (workflow.getDescription() != null && !workflow.getDescription().isEmpty()) {
+            gen.writeStringField("description", workflow.getDescription());
+        }
+        if (workflow.getVersion() != null && !workflow.getVersion().isEmpty()) {
+            gen.writeStringField("version", workflow.getVersion());
+        }
+        if (workflow.getAnnotations() != null && !workflow.getAnnotations().isEmpty()) {
+            gen.writeObjectField("annotations", workflow.getAnnotations());
+        }
+        if (workflow.getDataInputSchema() != null) {
+            if (workflow.getDataInputSchema().getSchema() != null
+                    && !workflow.getDataInputSchema().getSchema().isEmpty()
+                    && workflow.getDataInputSchema().isFailOnValidationErrors()) {
+                gen.writeStringField("dataInputSchema", workflow.getDataInputSchema().getSchema());
+            } else if (workflow.getDataInputSchema().getSchema() != null
+                    && !workflow.getDataInputSchema().getSchema().isEmpty()
+                    && !workflow.getDataInputSchema().isFailOnValidationErrors()) {
+                gen.writeObjectField("dataInputSchema", workflow.getDataInputSchema());
+            }
+        }
+        if (workflow.getStart() != null) {
+            gen.writeObjectField("start", workflow.getStart());
+        }
+        if (workflow.getSpecVersion() != null && !workflow.getSpecVersion().isEmpty()) {
+            gen.writeStringField("specVersion", workflow.getSpecVersion());
+        }
+        if (workflow.getExpressionLang() != null && !workflow.getExpressionLang().isEmpty()) {
+            gen.writeStringField("expressionLang", workflow.getExpressionLang());
+        }
+        if (workflow.isKeepActive()) {
+            gen.writeBooleanField("keepActive", workflow.isKeepActive());
+        }
+        if (workflow.isAutoRetries()) {
+            gen.writeBooleanField("autoRetries", workflow.isAutoRetries());
+        }
+        if (workflow.getMetadata() != null && !workflow.getMetadata().isEmpty()) {
+            gen.writeObjectField("metadata", workflow.getMetadata());
+        }
+
+        // --- Collections or references ---
+        if (workflow.getEvents() != null) {
+            writeListOrRef(gen, provider,
+                    "events",
+                    workflow.getEvents().getEventDefs(),
+                    workflow.getEvents().getRefValue());
+        }
+        if (workflow.getFunctions() != null) {
+            writeListOrRef(gen, provider,
+                    "functions",
+                    workflow.getFunctions().getFunctionDefs(),
+                    workflow.getFunctions().getRefValue());
+        }
+        if (workflow.getRetries() != null) {
+            writeListOrRef(gen, provider,
+                    "retries",
+                    workflow.getRetries().getRetryDefs(),
+                    workflow.getRetries().getRefValue());
+        }
+        if (workflow.getErrors() != null) {
+            writeListOrRef(gen, provider,
+                    "errors",
+                    workflow.getErrors().getErrorDefs(),
+                    workflow.getErrors().getRefValue());
+        }
+        if (workflow.getSecrets() != null) {
+            writeListOrRef(gen, provider,
+                    "secrets",
+                    workflow.getSecrets().getSecretDefs(),
+                    workflow.getSecrets().getRefValue());
+        }
+
+        // --- Always-array fields ---
+        if (workflow.getStates() != null && !workflow.getStates().isEmpty()) {
+            gen.writeArrayFieldStart("states");
+            for (State state : workflow.getStates()) {
+                gen.writeObject(state);
+            }
+            gen.writeEndArray();
+        }
+        if (workflow.getExtensions() != null && !workflow.getExtensions().isEmpty()) {
+            gen.writeArrayFieldStart("extensions");
+            for (Extension ext : workflow.getExtensions()) {
+                gen.writeObject(ext);
+            }
+            gen.writeEndArray();
+        }
+
+        gen.writeEndObject();
     }
-    return new String(hexChars);
-  }
 }
diff --git a/api/src/main/java/io/serverlessworkflow/api/workflow/Auth.java b/api/src/main/java/io/serverlessworkflow/api/workflow/Auth.java
index 280053fa..e7828370 100644
--- a/api/src/main/java/io/serverlessworkflow/api/workflow/Auth.java
+++ b/api/src/main/java/io/serverlessworkflow/api/workflow/Auth.java
@@ -16,42 +16,48 @@
 
 package io.serverlessworkflow.api.workflow;
 
-import io.serverlessworkflow.api.auth.AuthDefinition;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 
-public class Auth {
-  private String refValue;
-  private List<AuthDefinition> authDefs;
-
-  public Auth() {}
-
-  public Auth(AuthDefinition authDef) {
-    this.authDefs = new ArrayList<>();
-    this.authDefs.add(authDef);
-  }
-
-  public Auth(List<AuthDefinition> authDefs) {
-    this.authDefs = authDefs;
-  }
-
-  public Auth(String refValue) {
-    this.refValue = refValue;
-  }
-
-  public String getRefValue() {
-    return refValue;
-  }
-
-  public void setRefValue(String refValue) {
-    this.refValue = refValue;
-  }
-
-  public List<AuthDefinition> getAuthDefs() {
-    return authDefs;
-  }
+import io.serverlessworkflow.api.auth.AuthDefinition;
 
-  public void setAuthDefs(List<AuthDefinition> authDefs) {
-    this.authDefs = authDefs;
-  }
+public class Auth {
+    private String refValue;
+    private List<AuthDefinition> authDefs;
+
+    public Auth() {
+    }
+
+    public Auth(AuthDefinition authDef) {
+        this.authDefs = new ArrayList<>();
+        this.authDefs.add(authDef);
+    }
+
+    public Auth(List<AuthDefinition> authDefs) {
+        this.authDefs = authDefs;
+    }
+
+    public Auth(String refValue) {
+        this.refValue = refValue;
+    }
+
+    public String getRefValue() {
+        return refValue;
+    }
+
+    public void setRefValue(String refValue) {
+        this.refValue = refValue;
+    }
+
+    public List<AuthDefinition> getAuthDefs() {
+        if (authDefs == null) {
+            return Collections.emptyList();
+        }
+        return authDefs;
+    }
+
+    public void setAuthDefs(List<AuthDefinition> authDefs) {
+        this.authDefs = authDefs;
+    }
 }
diff --git a/api/src/main/java/io/serverlessworkflow/api/workflow/BaseWorkflow.java b/api/src/main/java/io/serverlessworkflow/api/workflow/BaseWorkflow.java
index 61692caf..e3fc3d55 100644
--- a/api/src/main/java/io/serverlessworkflow/api/workflow/BaseWorkflow.java
+++ b/api/src/main/java/io/serverlessworkflow/api/workflow/BaseWorkflow.java
@@ -15,6 +15,9 @@
  */
 package io.serverlessworkflow.api.workflow;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 import com.fasterxml.jackson.core.JsonProcessingException;
 import com.fasterxml.jackson.databind.JsonNode;
 import com.fasterxml.jackson.dataformat.yaml.YAMLFactory;
@@ -23,53 +26,53 @@
 import io.serverlessworkflow.api.Workflow;
 import io.serverlessworkflow.api.mapper.JsonObjectMapper;
 import io.serverlessworkflow.api.mapper.YamlObjectMapper;
-import org.slf4j.Logger;
-import org.slf4j.LoggerFactory;
 
-/** Base Workflow provides some extra functionality for the Workflow types */
+/**
+ * Base Workflow provides some extra functionality for the Workflow types
+ */
 public class BaseWorkflow {
 
-  private static JsonObjectMapper jsonObjectMapper = new JsonObjectMapper();
-  private static YamlObjectMapper yamlObjectMapper = new YamlObjectMapper();
+    private static JsonObjectMapper jsonObjectMapper = new JsonObjectMapper();
+    private static YamlObjectMapper yamlObjectMapper = new YamlObjectMapper();
 
-  private static Logger logger = LoggerFactory.getLogger(BaseWorkflow.class);
+    private static Logger logger = LoggerFactory.getLogger(BaseWorkflow.class);
 
-  public static Workflow fromSource(String source) {
-    // try it as json markup first, if fails try yaml
-    try {
-      return jsonObjectMapper.readValue(source, Workflow.class);
-    } catch (Exception e) {
-      logger.info("Unable to convert as json markup, trying as yaml");
-      try {
-        return yamlObjectMapper.readValue(source, Workflow.class);
-      } catch (Exception ee) {
-        throw new IllegalArgumentException(
-            "Could not convert markup to Workflow: " + ee.getMessage());
-      }
+    public static Workflow fromSource(String source) {
+        // try it as json markup first, if fails try yaml
+        try {
+            return jsonObjectMapper.readValue(source, Workflow.class);
+        } catch (Exception e) {
+            logger.info("Unable to convert as json markup, trying as yaml");
+            try {
+                return yamlObjectMapper.readValue(source, Workflow.class);
+            } catch (Exception ee) {
+                throw new IllegalArgumentException(
+                        "Could not convert markup to Workflow: " + ee.getMessage());
+            }
+        }
     }
-  }
 
-  public static String toJson(Workflow workflow) {
-    try {
-      return jsonObjectMapper.writeValueAsString(workflow);
-    } catch (JsonProcessingException e) {
-      logger.error("Error mapping to json: " + e.getMessage());
-      return null;
+    public static String toJson(Workflow workflow) {
+        try {
+            return jsonObjectMapper.writeValueAsString(workflow);
+        } catch (JsonProcessingException e) {
+            logger.error("Error mapping to json: {}", e.getMessage());
+            throw new IllegalArgumentException("Could not convert workflow to json: " + e.getMessage());
+        }
     }
-  }
 
-  public static String toYaml(Workflow workflow) {
-    try {
-      String jsonString = jsonObjectMapper.writeValueAsString(workflow);
-      JsonNode jsonNode = jsonObjectMapper.readTree(jsonString);
-      YAMLFactory yamlFactory =
-          new YAMLFactory()
-              .disable(YAMLGenerator.Feature.MINIMIZE_QUOTES)
-              .disable(YAMLGenerator.Feature.WRITE_DOC_START_MARKER);
-      return new YAMLMapper(yamlFactory).writeValueAsString(jsonNode);
-    } catch (Exception e) {
-      logger.error("Error mapping to yaml: " + e.getMessage());
-      return null;
+    public static String toYaml(Workflow workflow) {
+        try {
+            String jsonString = jsonObjectMapper.writeValueAsString(workflow);
+            JsonNode jsonNode = jsonObjectMapper.readTree(jsonString);
+            YAMLFactory yamlFactory =
+                    new YAMLFactory()
+                            .disable(YAMLGenerator.Feature.MINIMIZE_QUOTES)
+                            .disable(YAMLGenerator.Feature.WRITE_DOC_START_MARKER);
+            return new YAMLMapper(yamlFactory).writeValueAsString(jsonNode);
+        } catch (Exception e) {
+            logger.error("Error mapping to yaml: {}", e.getMessage());
+            throw new IllegalArgumentException("Could not convert workflow to yaml: " + e.getMessage());
+        }
     }
-  }
 }
diff --git a/api/src/main/java/io/serverlessworkflow/api/workflow/Errors.java b/api/src/main/java/io/serverlessworkflow/api/workflow/Errors.java
index 8431b94a..9e474139 100644
--- a/api/src/main/java/io/serverlessworkflow/api/workflow/Errors.java
+++ b/api/src/main/java/io/serverlessworkflow/api/workflow/Errors.java
@@ -15,36 +15,42 @@
  */
 package io.serverlessworkflow.api.workflow;
 
-import io.serverlessworkflow.api.error.ErrorDefinition;
+import java.util.Collections;
 import java.util.List;
 
-public class Errors {
-  private String refValue;
-  private List<ErrorDefinition> errorDefs;
-
-  public Errors() {}
-
-  public Errors(List<ErrorDefinition> errorDefs) {
-    this.errorDefs = errorDefs;
-  }
-
-  public Errors(String refValue) {
-    this.refValue = refValue;
-  }
-
-  public String getRefValue() {
-    return refValue;
-  }
-
-  public void setRefValue(String refValue) {
-    this.refValue = refValue;
-  }
-
-  public List<ErrorDefinition> getErrorDefs() {
-    return errorDefs;
-  }
+import io.serverlessworkflow.api.error.ErrorDefinition;
 
-  public void setErrorDefs(List<ErrorDefinition> errorDefs) {
-    this.errorDefs = errorDefs;
-  }
+public class Errors {
+    private String refValue;
+    private List<ErrorDefinition> errorDefs;
+
+    public Errors() {
+    }
+
+    public Errors(List<ErrorDefinition> errorDefs) {
+        this.errorDefs = errorDefs;
+    }
+
+    public Errors(String refValue) {
+        this.refValue = refValue;
+    }
+
+    public String getRefValue() {
+        return refValue;
+    }
+
+    public void setRefValue(String refValue) {
+        this.refValue = refValue;
+    }
+
+    public List<ErrorDefinition> getErrorDefs() {
+        if (errorDefs == null) {
+            return Collections.emptyList();
+        }
+        return errorDefs;
+    }
+
+    public void setErrorDefs(List<ErrorDefinition> errorDefs) {
+        this.errorDefs = errorDefs;
+    }
 }
diff --git a/api/src/main/java/io/serverlessworkflow/api/workflow/Events.java b/api/src/main/java/io/serverlessworkflow/api/workflow/Events.java
index 24080e51..59af2c3f 100644
--- a/api/src/main/java/io/serverlessworkflow/api/workflow/Events.java
+++ b/api/src/main/java/io/serverlessworkflow/api/workflow/Events.java
@@ -15,36 +15,42 @@
  */
 package io.serverlessworkflow.api.workflow;
 
-import io.serverlessworkflow.api.events.EventDefinition;
+import java.util.Collections;
 import java.util.List;
 
-public class Events {
-  private String refValue;
-  private List<EventDefinition> eventDefs;
-
-  public Events() {}
-
-  public Events(List<EventDefinition> eventDefs) {
-    this.eventDefs = eventDefs;
-  }
-
-  public Events(String refValue) {
-    this.refValue = refValue;
-  }
-
-  public String getRefValue() {
-    return refValue;
-  }
-
-  public void setRefValue(String refValue) {
-    this.refValue = refValue;
-  }
-
-  public List<EventDefinition> getEventDefs() {
-    return eventDefs;
-  }
+import io.serverlessworkflow.api.events.EventDefinition;
 
-  public void setEventDefs(List<EventDefinition> eventDefs) {
-    this.eventDefs = eventDefs;
-  }
+public class Events {
+    private String refValue;
+    private List<EventDefinition> eventDefs;
+
+    public Events() {
+    }
+
+    public Events(List<EventDefinition> eventDefs) {
+        this.eventDefs = eventDefs;
+    }
+
+    public Events(String refValue) {
+        this.refValue = refValue;
+    }
+
+    public String getRefValue() {
+        return refValue;
+    }
+
+    public void setRefValue(String refValue) {
+        this.refValue = refValue;
+    }
+
+    public List<EventDefinition> getEventDefs() {
+        if (eventDefs == null) {
+            return Collections.emptyList();
+        }
+        return eventDefs;
+    }
+
+    public void setEventDefs(List<EventDefinition> eventDefs) {
+        this.eventDefs = eventDefs;
+    }
 }
diff --git a/api/src/main/java/io/serverlessworkflow/api/workflow/Functions.java b/api/src/main/java/io/serverlessworkflow/api/workflow/Functions.java
index f269bc08..6e927f9b 100644
--- a/api/src/main/java/io/serverlessworkflow/api/workflow/Functions.java
+++ b/api/src/main/java/io/serverlessworkflow/api/workflow/Functions.java
@@ -15,36 +15,42 @@
  */
 package io.serverlessworkflow.api.workflow;
 
-import io.serverlessworkflow.api.functions.FunctionDefinition;
+import java.util.Collections;
 import java.util.List;
 
-public class Functions {
-  private String refValue;
-  private List<FunctionDefinition> functionDefs;
-
-  public Functions() {}
-
-  public Functions(List<FunctionDefinition> functionDefs) {
-    this.functionDefs = functionDefs;
-  }
-
-  public Functions(String refValue) {
-    this.refValue = refValue;
-  }
-
-  public String getRefValue() {
-    return refValue;
-  }
-
-  public void setRefValue(String refValue) {
-    this.refValue = refValue;
-  }
-
-  public List<FunctionDefinition> getFunctionDefs() {
-    return functionDefs;
-  }
+import io.serverlessworkflow.api.functions.FunctionDefinition;
 
-  public void setFunctionDefs(List<FunctionDefinition> functionDefs) {
-    this.functionDefs = functionDefs;
-  }
+public class Functions {
+    private String refValue;
+    private List<FunctionDefinition> functionDefs;
+
+    public Functions() {
+    }
+
+    public Functions(List<FunctionDefinition> functionDefs) {
+        this.functionDefs = functionDefs;
+    }
+
+    public Functions(String refValue) {
+        this.refValue = refValue;
+    }
+
+    public String getRefValue() {
+        return refValue;
+    }
+
+    public void setRefValue(String refValue) {
+        this.refValue = refValue;
+    }
+
+    public List<FunctionDefinition> getFunctionDefs() {
+        if (functionDefs == null) {
+            return Collections.emptyList();
+        }
+        return functionDefs;
+    }
+
+    public void setFunctionDefs(List<FunctionDefinition> functionDefs) {
+        this.functionDefs = functionDefs;
+    }
 }
diff --git a/api/src/main/java/io/serverlessworkflow/api/workflow/Retries.java b/api/src/main/java/io/serverlessworkflow/api/workflow/Retries.java
index af1ae1e0..fa539ac0 100644
--- a/api/src/main/java/io/serverlessworkflow/api/workflow/Retries.java
+++ b/api/src/main/java/io/serverlessworkflow/api/workflow/Retries.java
@@ -15,36 +15,42 @@
  */
 package io.serverlessworkflow.api.workflow;
 
-import io.serverlessworkflow.api.retry.RetryDefinition;
+import java.util.Collections;
 import java.util.List;
 
-public class Retries {
-  private String refValue;
-  private List<RetryDefinition> retryDefs;
-
-  public Retries() {}
-
-  public Retries(List<RetryDefinition> retryDefs) {
-    this.retryDefs = retryDefs;
-  }
-
-  public Retries(String refValue) {
-    this.refValue = refValue;
-  }
-
-  public String getRefValue() {
-    return refValue;
-  }
-
-  public void setRefValue(String refValue) {
-    this.refValue = refValue;
-  }
-
-  public List<RetryDefinition> getRetryDefs() {
-    return retryDefs;
-  }
+import io.serverlessworkflow.api.retry.RetryDefinition;
 
-  public void setRetryDefs(List<RetryDefinition> retryDefs) {
-    this.retryDefs = retryDefs;
-  }
+public class Retries {
+    private String refValue;
+    private List<RetryDefinition> retryDefs;
+
+    public Retries() {
+    }
+
+    public Retries(List<RetryDefinition> retryDefs) {
+        this.retryDefs = retryDefs;
+    }
+
+    public Retries(String refValue) {
+        this.refValue = refValue;
+    }
+
+    public String getRefValue() {
+        return refValue;
+    }
+
+    public void setRefValue(String refValue) {
+        this.refValue = refValue;
+    }
+
+    public List<RetryDefinition> getRetryDefs() {
+        if (retryDefs == null) {
+            return Collections.emptyList();
+        }
+        return retryDefs;
+    }
+
+    public void setRetryDefs(List<RetryDefinition> retryDefs) {
+        this.retryDefs = retryDefs;
+    }
 }
diff --git a/api/src/main/java/io/serverlessworkflow/api/workflow/Secrets.java b/api/src/main/java/io/serverlessworkflow/api/workflow/Secrets.java
index 2dbb6b31..03edc0fd 100644
--- a/api/src/main/java/io/serverlessworkflow/api/workflow/Secrets.java
+++ b/api/src/main/java/io/serverlessworkflow/api/workflow/Secrets.java
@@ -15,35 +15,40 @@
  */
 package io.serverlessworkflow.api.workflow;
 
+import java.util.Collections;
 import java.util.List;
 
 public class Secrets {
-  private String refValue;
-  private List<String> secretDefs;
-
-  public Secrets() {}
-
-  public Secrets(String refValue) {
-    this.refValue = refValue;
-  }
-
-  public Secrets(List<String> secretDefs) {
-    this.secretDefs = secretDefs;
-  }
-
-  public String getRefValue() {
-    return refValue;
-  }
-
-  public void setRefValue(String refValue) {
-    this.refValue = refValue;
-  }
-
-  public List<String> getSecretDefs() {
-    return secretDefs;
-  }
-
-  public void setSecretDefs(List<String> secretDefs) {
-    this.secretDefs = secretDefs;
-  }
+    private String refValue;
+    private List<String> secretDefs;
+
+    public Secrets() {
+    }
+
+    public Secrets(String refValue) {
+        this.refValue = refValue;
+    }
+
+    public Secrets(List<String> secretDefs) {
+        this.secretDefs = secretDefs;
+    }
+
+    public String getRefValue() {
+        return refValue;
+    }
+
+    public void setRefValue(String refValue) {
+        this.refValue = refValue;
+    }
+
+    public List<String> getSecretDefs() {
+        if (secretDefs == null) {
+            return Collections.emptyList();
+        }
+        return secretDefs;
+    }
+
+    public void setSecretDefs(List<String> secretDefs) {
+        this.secretDefs = secretDefs;
+    }
 }
diff --git a/pom.xml b/pom.xml
index d62d5b9d..61a17961 100644
--- a/pom.xml
+++ b/pom.xml
@@ -228,6 +228,15 @@
     </dependencies>
   </dependencyManagement>
 
+  <dependencies>
+    <dependency>
+      <groupId>org.slf4j</groupId>
+      <artifactId>slf4j-nop</artifactId>
+      <version>${version.org.slf4j}</version>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+
   <build>
     <plugins>
       <!-- Set properties containing the scm revision -->