Handle an empty query parameter with @Default

core/src/main/java/com/linecorp/armeria/internal/server/annotation/AnnotatedValueResolver.java

@@ -753,12 +753,17 @@ private static AnnotatedValueResolver ofInjectableTypes0(String name, AnnotatedE
 
                 // Do not convert value here because the element type is String.
                 if (values != null && !values.isEmpty()) {
-                    if (queryDelimiter != null && values.size() == 1) {
+                    if (values.size() == 1) {
                         final String first = values.get(0);
-                        Splitter.on(queryDelimiter)
-                                .splitToStream(first)
-                                .map(resolver::convert)
-                                .forEach(resolvedValues::add);
+                        if (first.isEmpty()) {
+                            return resolvedValues;
+                        }
+                        if (queryDelimiter != null) {
+                            Splitter.on(queryDelimiter)
+                                    .splitToStream(first)
+                                    .map(resolver::convert)
+                                    .forEach(resolvedValues::add);
+                        }
                     } else {
                         values.stream().map(resolver::convert).forEach(resolvedValues::add);
                     }
@@ -1026,6 +1031,17 @@ private Object convert(@Nullable String value) {
         if (value == null) {
             return defaultOrException();
         }
+        if (value.isEmpty()) {
+            logger.warn("Parameter is present but the value is missing: " + httpElementName);
+            if (String.class.isAssignableFrom(elementType)) {
+                return value;
+            }
+            if (elementType.isPrimitive()) {
+                throw new IllegalArgumentException("Cannot set null to primitive type parameter: " +
+                                                   httpElementName);
+            }
+            return null;
+        }
         return convert(value, elementType, enumConverter);
     }
 

core/src/main/java/com/linecorp/armeria/server/annotation/Default.java

@@ -22,6 +22,8 @@
 import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;
+import java.util.List;
+import java.util.Set;
 
 /**
  * Specifies the default value of an optional parameter.
@@ -35,7 +37,19 @@
      * When {@link Default} annotation exists but {@link Default#value()} is not specified, {@code null}
      * value would be set if the parameter is not present in the request.
      *
-     * {@link Default} annotation is not allowed for a path variable. If a user uses {@link Default}
+     * <p>When {@link Default} annotation exists and {@link Default#value()} is not specified, but parameter is
+     * present in the request without the value e.g., {@code /foo?bar=}, then actions below will be processed
+     * depending on the type of the parameter.
+     * <table>
+     * <caption>Actions</caption>
+     * <tr><th>Type</th><th>Action</th></tr>
+     * <tr><td>Primitive types</td><td>An {@link IllegalArgumentException} will be thrown</td></tr>
+     * <tr><td>{@link String}</td><td>An empty string {@code ""} will be set</td></tr>
+     * <tr><td>{@link List} or {@link Set}</td><td>An empty collection will be set</td></tr>
+     * <tr><td>Other</td><td>The {@code null} value will be set</td></tr>
+     * </table>
+     *
+     * <p>{@link Default} annotation is not allowed for a path variable. If a user uses {@link Default}
      * annotation on a path variable, {@link IllegalArgumentException} would be raised.
      */
     String value() default UNSPECIFIED;