Ignore interface/class without default constructs fields in SB config

extensions/spring-boot-properties/deployment/src/main/java/io/quarkus/spring/boot/properties/deployment/ClassConfigurationPropertiesUtil.java

@@ -309,13 +309,7 @@ private ResultHandle populateConfigObject(ClassLoader classLoader, ClassInfo con
                                 break;
                             }
                         }
-                    } else {
-                        if (!fieldTypeClassInfo.hasNoArgsConstructor()) {
-                            throw new IllegalArgumentException(
-                                    "Nested configuration class '" + fieldTypeClassInfo
-                                            + "' must contain a no-args constructor ");
-                        }
-
+                    } else if (fieldTypeClassInfo.hasNoArgsConstructor()) {
                         if (!Modifier.isPublic(fieldTypeClassInfo.flags())) {
                             throw new IllegalArgumentException(
                                     "Nested configuration class '" + fieldTypeClassInfo + "' must be public ");
@@ -325,6 +319,10 @@ private ResultHandle populateConfigObject(ClassLoader classLoader, ClassInfo con
                                 getFullConfigName(prefixStr, namingStrategy, field), namingStrategy, failOnMismatchingMember,
                                 null, methodCreator);
                         createWriteValue(methodCreator, configObject, field, setter, useFieldAccess, nestedConfigObject);
+                    } else {
+                        LOGGER.warn("Nested configuration class '" + fieldTypeClassInfo
+                                + "' declared in '" + currentClassInHierarchy.name() + "." + field.name() + "' is either an "
+                                + "interface or does not have a non-args constructor, so this field will not be initialized");
                     }
                 } else {
                     String fullConfigName = getFullConfigName(prefixStr, namingStrategy, field);

integration-tests/spring-boot-properties/src/main/java/io/quarkus/it/spring/boot/ClassProperties.java

@@ -9,6 +9,8 @@ public final class ClassProperties {
 
     private AnotherClass anotherClass;
 
+    private Interface anInterface;
+
     public String getValue() {
         return value;
     }
@@ -24,4 +26,12 @@ public AnotherClass getAnotherClass() {
     public void setAnotherClass(AnotherClass anotherClass) {
         this.anotherClass = anotherClass;
     }
+
+    public Interface getAnInterface() {
+        return anInterface;
+    }
+
+    public void setAnInterface(Interface anInterface) {
+        this.anInterface = anInterface;
+    }
 }

integration-tests/spring-boot-properties/src/main/java/io/quarkus/it/spring/boot/ClassPropertiesResource.java

@@ -21,4 +21,10 @@ public String getValue() {
     public boolean isAnotherClassValue() {
         return properties.getAnotherClass().isValue();
     }
+
+    @Path("/interface")
+    @GET
+    public boolean getUrlFromClassWithInterface() {
+        return properties.getAnInterface() == null;
+    }
 }

integration-tests/spring-boot-properties/src/main/java/io/quarkus/it/spring/boot/Interface.java

@@ -0,0 +1,5 @@
+package io.quarkus.it.spring.boot;
+
+public interface Interface {
+    String getUrl();
+}

integration-tests/spring-boot-properties/src/main/resources/application.properties

@@ -2,4 +2,4 @@ cl.value=class-value
 cl.another-class.value=true
 bean.value=1
 bean.inner-class.value=inner-class-value
-interface.value=interface-value
+interface.value=interface-value

integration-tests/spring-boot-properties/src/test/java/io/quarkus/it/spring/boot/ClassPropertiesTest.java

@@ -24,4 +24,11 @@ void shouldHaveAnotherClassValue() {
                 .then()
                 .body(is(equalTo("true")));
     }
+
+    @Test
+    void shouldInterfaceNotBePopulated() {
+        when().get("/class/interface")
+                .then()
+                .body(is(equalTo("true")));
+    }
 }