CAUSEWAY-3899: adds guard for DomainChangeRecord

Author: danhaywood

api/applib/src/main/java/org/apache/causeway/applib/mixins/system/DomainChangeRecord.java

@@ -21,6 +21,7 @@
 import java.lang.annotation.ElementType;
 import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
+import java.util.Optional;
 import java.util.UUID;
 
 import org.apache.causeway.applib.annotation.DomainObject;
@@ -151,16 +152,18 @@ public String toString() {
     @Retention(RetentionPolicy.RUNTIME)
     @interface TargetLogicalTypeName {
         int MAX_LENGTH = 255;
-        boolean NULLABLE = false;
-        String ALLOWS_NULL = "false";
+        boolean NULLABLE = HasTarget.Target.NULLABLE;
+        String ALLOWS_NULL = HasTarget.Target.ALLOWS_NULL;
     }
 
     /**
      * The logical type name of the domain object being changed.
      */
     @TargetLogicalTypeName
     default String getTargetLogicalTypeName() {
-        return getTarget().logicalTypeName();
+        return Optional.ofNullable(getTarget())
+                .map(Bookmark::logicalTypeName)
+                .orElse(null);
     }
 
     @Property(
@@ -312,4 +315,16 @@ default boolean hidePostValue() {
         return getType() != ChangeType.AUDIT_ENTRY;
     }
 
+    /**
+     * A no-op implementation of {@link DomainChangeRecord} that can be used for testing.
+     */
+    class Empty implements DomainChangeRecord {
+        @Override public ChangeType getType() {return null;}
+        @Override public UUID getInteractionId() {return null;}
+        @Override public String getUsername() {return "";}
+        @Override public java.sql.Timestamp getTimestamp() {return null;}
+        @Override public Bookmark getTarget() {return null;}
+        @Override public String getLogicalMemberIdentifier() {return "";}
+    }
+
 }

api/applib/src/test/java/org/apache/causeway/applib/mixins/system/DomainChangeRecord_Test.java

@@ -0,0 +1,37 @@
+package org.apache.causeway.applib.mixins.system;
+
+import java.util.UUID;
+
+import org.apache.causeway.applib.services.bookmark.Bookmark;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class DomainChangeRecord_Test {
+
+    @Test
+    void when_populated() {
+        final var dcr = new DomainChangeRecord.Empty() {
+            @Override
+            public Bookmark getTarget() {
+                return Bookmark.forLogicalTypeNameAndIdentifier("Customer", "12345");
+            }
+        };
+
+        assertThat(dcr.getTargetLogicalTypeName()).isEqualTo("Customer");
+    }
+
+    @Test
+    void when_not_populated() {
+        final var dcr = new DomainChangeRecord.Empty() {
+            @Override
+            public Bookmark getTarget() {
+                return null;
+            }
+        };
+
+        assertThat(dcr.getTargetLogicalTypeName()).isNull();
+    }
+
+}