[Core] Look up docstring converter by type as fallback

Fixes: #2458

Author: mpkorstanje

CHANGELOG.md

@@ -16,6 +16,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ### Removed
 
 ### Fixed
+* [Core] Look up docstring converter by type as fallback ([#2459](https://github.com/cucumber/cucumber-jvm/pull/2459) M.P. Korstanje)
 
 ## [7.2.1] (2022-01-04)
 

core/src/main/java/io/cucumber/core/stepexpression/DocStringArgument.java

@@ -2,15 +2,17 @@
 
 import io.cucumber.docstring.DocString;
 
+import static java.util.Objects.requireNonNull;
+
 public final class DocStringArgument implements Argument {
 
     private final DocStringTransformer<?> docStringType;
     private final String content;
     private final String contentType;
 
     DocStringArgument(DocStringTransformer<?> docStringType, String content, String contentType) {
-        this.docStringType = docStringType;
-        this.content = content;
+        this.docStringType = requireNonNull(docStringType);
+        this.content = requireNonNull(content);
         this.contentType = contentType;
     }
 

core/src/main/java/io/cucumber/core/stepexpression/StepExpression.java

@@ -6,16 +6,18 @@
 import java.util.ArrayList;
 import java.util.List;
 
+import static java.util.Objects.requireNonNull;
+
 public final class StepExpression {
 
     private final Expression expression;
     private final DocStringTransformer<?> docStringType;
     private final RawTableTransformer<?> tableType;
 
     StepExpression(Expression expression, DocStringTransformer<?> docStringType, RawTableTransformer<?> tableType) {
-        this.expression = expression;
-        this.docStringType = docStringType;
-        this.tableType = tableType;
+        this.expression = requireNonNull(expression);
+        this.docStringType = requireNonNull(docStringType);
+        this.tableType = requireNonNull(tableType);
     }
 
     public Class<? extends Expression> getExpressionType() {

docstring/src/main/java/io/cucumber/docstring/DocStringTypeRegistry.java

@@ -51,15 +51,16 @@ private static String emptyToAnonymous(String contentType) {
     }
 
     List<DocStringType> lookup(String contentType, Type type) {
-        if (contentType == null) {
-            return lookUpByType(type);
+        DocStringType docStringType = lookupByContentTypeAndType(orDefault(contentType), type);
+        if (docStringType != null) {
+            return Collections.singletonList(docStringType);
         }
 
-        DocStringType docStringType = lookupByContentTypeAndType(contentType, type);
-        if (docStringType == null) {
-            return Collections.emptyList();
-        }
-        return Collections.singletonList(docStringType);
+        return lookUpByType(type);
+    }
+
+    private String orDefault(String contentType) {
+        return contentType == null ? DEFAULT_CONTENT_TYPE : contentType;
     }
 
     private List<DocStringType> lookUpByType(Type type) {

docstring/src/main/java/io/cucumber/docstring/DocStringTypeRegistryDocStringConverter.java

@@ -39,10 +39,19 @@ public <T> T convert(DocString docString, Type targetType) {
                 targetType.getTypeName()));
         }
         if (docStringTypes.size() > 1) {
+            List<String> suggestedContentTypes = suggestedContentTypes(docStringTypes);
+            if (docString.getContentType() == null) {
+                throw new CucumberDocStringException(format(
+                    "Multiple converters found for type %s, add one of the following content types to your docstring %s",
+                    targetType.getTypeName(),
+                    suggestedContentTypes));
+            }
             throw new CucumberDocStringException(format(
-                "Multiple converters found for type %s, add one of the following content types to your docstring %s",
+                "Multiple converters found for type %s, and the content type '%s' did not match any of the registered types %s. Change the content type of the docstring or register a docstring type for '%s'",
                 targetType.getTypeName(),
-                suggestedContentTypes(docStringTypes)));
+                docString.getContentType(),
+                suggestedContentTypes,
+                docString.getContentType()));
         }
 
         return (T) docStringTypes.get(0).transform(docString.getContent());

docstring/src/test/java/io/cucumber/docstring/DocStringTypeRegistryDocStringConverterTest.java

@@ -60,6 +60,57 @@ void target_type_to_string_is_predefined() {
         assertThat(converted, is("hello world"));
     }
 
+    @Test
+    void default_converter_is_used_if_registered_converter_does_not_match_type() {
+        registry.defineDocStringType(new DocStringType(
+            JsonNode.class,
+            "json",
+            (String s) -> new ObjectMapper().readTree(s)));
+
+        DocString docString = DocString.create(
+            "hello world", "json");
+
+        String converted = converter.convert(docString, String.class);
+        assertThat(converted, is("hello world"));
+    }
+
+    @Test
+    void default_converter_never_conflicts_with_registered_converter_for_string() {
+        registry.defineDocStringType(new DocStringType(
+            String.class,
+            "text",
+            (String s) -> s));
+
+        DocString docString = DocString.create("hello world");
+
+        String converted = converter.convert(docString, String.class);
+        assertThat(converted, is("hello world"));
+    }
+
+    @Test
+    void throws_if_converter_type_conflicts_with_type() {
+        registry.defineDocStringType(new DocStringType(
+            JsonNode.class,
+            "json",
+            (String s) -> new ObjectMapper().readTree(s)));
+
+        registry.defineDocStringType(new DocStringType(
+            String.class,
+            "text",
+            (String s) -> s));
+
+        DocString docString = DocString.create("hello world", "json");
+
+        CucumberDocStringException exception = assertThrows(
+            CucumberDocStringException.class,
+            () -> converter.convert(docString, String.class));
+
+        assertThat(exception.getMessage(),
+            is("Multiple converters found for type java.lang.String, and the content type 'json' " +
+                    "did not match any of the registered types [text]. Change the content type of the docstring " +
+                    "or register a docstring type for 'json'"));
+    }
+
     @Test
     void converts_doc_string_to_doc_string() {
         DocString docString = DocString.create(
@@ -146,11 +197,6 @@ void throws_when_multiple_convertors_available() {
             "xml",
             (String s) -> new ObjectMapper().readTree(s)));
 
-        registry.defineDocStringType(new DocStringType(
-            JsonNode.class,
-            "",
-            (String s) -> new ObjectMapper().readTree(s)));
-
         DocString docString = DocString.create(
             "{\"hello\":\"world\"}");