Default value must be manually handled

client/src/main/java/org/mvndaemon/mvnd/client/DaemonParameters.java

@@ -491,16 +491,21 @@ private static List<CoreExtension> readCoreExtensionsDescriptor(Path multiModule
 
     private static List<CoreExtension> filterCoreExtensions(List<CoreExtension> coreExtensions) {
         Set<String> exclusions = new HashSet<>();
-        String exclusionsString =
-                systemProperty(Environment.MVND_CORE_EXTENSIONS_EXCLUDE).asString();
+        String exclusionsString = systemProperty(Environment.MVND_CORE_EXTENSIONS_EXCLUDE)
+                .orDefault()
+                .asString();
         if (exclusionsString != null) {
             exclusions.addAll(Arrays.stream(exclusionsString.split(","))
                     .filter(e -> e != null && !e.trim().isEmpty())
                     .collect(Collectors.toList()));
         }
-        return coreExtensions.stream()
-                .filter(e -> !exclusions.contains(e.getGroupId() + ":" + e.getArtifactId()))
-                .collect(Collectors.toList());
+        if (!exclusions.isEmpty()) {
+            return coreExtensions.stream()
+                    .filter(e -> !exclusions.contains(e.getGroupId() + ":" + e.getArtifactId()))
+                    .collect(Collectors.toList());
+        } else {
+            return coreExtensions;
+        }
     }
 
     private static Properties loadProperties(Path path) {

integration-tests/src/test/java/org/mvndaemon/mvnd/it/MavenConfIgnoreExtDefNativeIT.java

@@ -0,0 +1,24 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.mvndaemon.mvnd.it;
+
+import org.mvndaemon.mvnd.junit.MvndNativeTest;
+
+@MvndNativeTest(projectDir = "src/test/projects/maven-conf-ignore-ext-def")
+class MavenConfIgnoreExtDefNativeIT extends MavenConfNativeIT {}

integration-tests/src/test/java/org/mvndaemon/mvnd/it/MavenConfIgnoreExtDefTest.java

@@ -0,0 +1,27 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.mvndaemon.mvnd.it;
+
+import org.junit.jupiter.api.condition.DisabledOnOs;
+import org.junit.jupiter.api.condition.OS;
+import org.mvndaemon.mvnd.junit.MvndTest;
+
+@DisabledOnOs(OS.LINUX)
+@MvndTest(projectDir = "src/test/projects/maven-conf-ignore-ext-def")
+class MavenConfIgnoreExtDefTest extends MavenConfIgnoreExtDefNativeIT {}

integration-tests/src/test/java/org/mvndaemon/mvnd/it/MavenConfIgnoreExtNativeIT.java

@@ -0,0 +1,34 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.mvndaemon.mvnd.it;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import org.mvndaemon.mvnd.junit.MvndNativeTest;
+
+@MvndNativeTest(projectDir = "src/test/projects/maven-conf-ignore-ext")
+class MavenConfIgnoreExtNativeIT extends MavenConfNativeIT {
+    @Override
+    protected List<String> mvndParams() {
+        ArrayList<String> result = new ArrayList<>(super.mvndParams());
+        result.add("-Dmvnd.coreExtensionsExclude=foo:bar");
+        return result;
+    }
+}

integration-tests/src/test/java/org/mvndaemon/mvnd/it/MavenConfIgnoreExtTest.java

@@ -0,0 +1,24 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.mvndaemon.mvnd.it;
+
+import org.mvndaemon.mvnd.junit.MvndTest;
+
+@MvndTest(projectDir = "src/test/projects/maven-conf-ignore-ext")
+class MavenConfIgnoreExtTest extends MavenConfIgnoreExtNativeIT {}

integration-tests/src/test/java/org/mvndaemon/mvnd/it/MavenConfNativeIT.java

@@ -21,6 +21,8 @@
 import javax.inject.Inject;
 
 import java.io.IOException;
+import java.util.Arrays;
+import java.util.List;
 
 import org.junit.jupiter.api.Test;
 import org.mvndaemon.mvnd.assertj.TestClientOutput;
@@ -43,17 +45,20 @@ class MavenConfNativeIT {
     void version() throws IOException, InterruptedException {
         final TestClientOutput o = new TestClientOutput();
         // this test also exercise the "-D foo=bar" syntax for defining properties
-        client.execute(
-                        o,
-                        "org.apache.maven.plugins:maven-help-plugin:3.2.0:evaluate",
-                        "-D",
-                        "expression=maven.conf",
-                        "-q",
-                        "-DforceStdout",
-                        "--raw-streams")
-                .assertSuccess();
+        client.execute(o, mvndParams().toArray(new String[0])).assertSuccess();
         String conf = parameters.mvndHome().resolve("mvn").resolve("conf").toString();
         assertTrue(
                 o.getMessages().stream().anyMatch(m -> m.toString().contains(conf)), "Output should contain " + conf);
     }
+
+    protected List<String> mvndParams() {
+        return Arrays.asList(
+                "org.apache.maven.plugins:maven-help-plugin:3.2.0:evaluate",
+                "-D",
+                "expression=maven.conf",
+                "-q",
+                "-DforceStdout",
+                "--raw-streams",
+                "-X");
+    }
 }

integration-tests/src/test/projects/maven-conf-ignore-ext-def/.mvn/extensions.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<extensions>
+    <!-- By def ignored: basically the build should "just work" -->
+    <extension>
+        <groupId>io.takari.maven</groupId>
+        <artifactId>takari-smart-builder</artifactId>
+        <version>0.6.4</version>
+    </extension>
+</extensions>

integration-tests/src/test/projects/maven-conf-ignore-ext-def/pom.xml

@@ -0,0 +1,27 @@
+<!--
+
+    Copyright 2019-2022 the original author or authors.
+
+    Licensed under the Apache License, Version 2.0 (the "License");
+    you may not use this file except in compliance with the License.
+    You may obtain a copy of the License at
+
+         http://www.apache.org/licenses/LICENSE-2.0
+
+    Unless required by applicable law or agreed to in writing, software
+    distributed under the License is distributed on an "AS IS" BASIS,
+    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+    See the License for the specific language governing permissions and
+    limitations under the License.
+
+-->
+<project xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://maven.apache.org/POM/4.0.0"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <groupId>org.mvndaemon.mvnd.test.maven-conf</groupId>
+    <artifactId>maven-conf-ignore-ext-def</artifactId>
+    <version>0.0.1-SNAPSHOT</version>
+    <packaging>pom</packaging>
+
+ </project>

integration-tests/src/test/projects/maven-conf-ignore-ext/.mvn/extensions.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<extensions>
+    <!-- nonexistent: if not ignored, will explode -->
+    <extension>
+        <groupId>foo</groupId>
+        <artifactId>bar</artifactId>
+        <version>1.0.0</version>
+    </extension>
+</extensions>

integration-tests/src/test/projects/maven-conf-ignore-ext/pom.xml

@@ -0,0 +1,27 @@
+<!--
+
+    Copyright 2019-2022 the original author or authors.
+
+    Licensed under the Apache License, Version 2.0 (the "License");
+    you may not use this file except in compliance with the License.
+    You may obtain a copy of the License at
+
+         http://www.apache.org/licenses/LICENSE-2.0
+
+    Unless required by applicable law or agreed to in writing, software
+    distributed under the License is distributed on an "AS IS" BASIS,
+    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+    See the License for the specific language governing permissions and
+    limitations under the License.
+
+-->
+<project xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://maven.apache.org/POM/4.0.0"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <groupId>org.mvndaemon.mvnd.test.maven-conf</groupId>
+    <artifactId>maven-conf-ignore-ext</artifactId>
+    <version>0.0.1-SNAPSHOT</version>
+    <packaging>pom</packaging>
+
+ </project>