Fixes #289.

docs/changelog.md

@@ -1,5 +1,9 @@
 # Changelog
 
+## (unreleased to Maven Central)
+
+- Fixes https://github.com/structurizr/dsl/issues/289 (Cannot invoke "Object.equals(Object)" because "r" is null).
+
 ## 1.30.2 (20th June 2023)
 
 - Fixes https://github.com/structurizr/dsl/issues/242 (deploymentViews hierarchical identifiers issue).

src/main/java/com/structurizr/dsl/AbstractRelationshipParser.java

@@ -25,6 +25,12 @@ protected Relationship createRelationship(Element sourceElement, String descript
             throw new RuntimeException("A relationship between \"" + sourceElement.getCanonicalName() + "\" and \"" + destinationElement.getCanonicalName() + "\" is not permitted");
         }
 
+        if (relationship == null) {
+            if (sourceElement.hasEfferentRelationshipWith(destinationElement, description) || sourceElement.hasEfferentRelationshipWith(destinationElement)) {
+                throw new RuntimeException("A relationship between \"" + sourceElement.getCanonicalName() + "\" and \"" + destinationElement.getCanonicalName() + "\" already exists");
+            }
+        }
+
         return relationship;
     }
 

src/test/dsl/relationship-already-exists.dsl

@@ -0,0 +1,13 @@
+workspace {
+
+  model {
+        a = softwareSystem "A"
+        b = softwareSystem "B" {
+            c = container "C"
+        }
+
+        c -> a
+        b -> a
+  }
+
+}

src/test/java/com/structurizr/dsl/DslTests.java

@@ -1085,4 +1085,15 @@ void test_MultiLineWithError() {
         }
     }
 
+    @Test
+    void test_RelationshipAlreadyExists() throws Exception {
+        try {
+            StructurizrDslParser parser = new StructurizrDslParser();
+            parser.parse(new File("src/test/dsl/relationship-already-exists.dsl"));
+            fail();
+        } catch (StructurizrDslParserException e) {
+            assertEquals("A relationship between \"SoftwareSystem://B\" and \"SoftwareSystem://A\" already exists at line 10: b -> a", e.getMessage());
+        }
+    }
+
 }