Build fails when annotation processor list is empty (but present) (#1077)

Fixes #892

Author: wilx

src/main/java/org/apache/maven/plugin/compiler/AbstractCompilerMojo.java

@@ -884,7 +884,7 @@ private void executeReal() throws MojoExecutionException, CompilationFailureExce
 
         compilerConfiguration.setSourceLocations(compileSourceRoots);
 
-        compilerConfiguration.setAnnotationProcessors(annotationProcessors);
+        compilerConfiguration.setAnnotationProcessors(normalizeAnnotationProcessors(annotationProcessors));
 
         compilerConfiguration.setProcessorPathEntries(resolveProcessorPathEntries());
 
@@ -1631,6 +1631,21 @@ private static List<String> removeEmptyCompileSourceRoots(List<String> compileSo
         return newCompileSourceRootsList;
     }
 
+    /**
+     * Removes blank annotation processor names.
+     *
+     * @param processors the configured annotation processor names, or {@code null}.
+     * @return the non-blank annotation processor names, or {@code null} if none remain.
+     */
+    static String[] normalizeAnnotationProcessors(String[] processors) {
+        if (processors != null) {
+            processors = Arrays.stream(processors)
+                    .filter(processor -> !StringUtils.isBlank(processor))
+                    .toArray(String[]::new);
+        }
+        return processors == null || processors.length == 0 ? null : processors;
+    }
+
     /**
      * We just compare the timestamps of all local dependency files (inter-module dependency classpath) and the own
      * generated classes and if we got a file which is &gt;= the build-started timestamp, then we caught a file which

src/test/java/org/apache/maven/plugin/compiler/CompilerMojoTest.java

@@ -35,6 +35,7 @@
 import static org.apache.maven.api.plugin.testing.MojoExtension.setVariableValueToObject;
 import static org.apache.maven.plugin.compiler.MojoTestUtils.getMockMavenProject;
 import static org.apache.maven.plugin.compiler.MojoTestUtils.getMockMavenSession;
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
@@ -80,6 +81,27 @@ void testCompilerBasic(CompilerMojo compilerMojo) throws Exception {
         assertTrue(testClass::exists);
     }
 
+    @Test
+    @InjectMojo(goal = COMPILE, pom = "classpath:/unit/compiler-empty-annotation-processors-test/plugin-config.xml")
+    void testCompilerEmptyAnnotationProcessors(CompilerMojo compilerMojo) throws Exception {
+        setUpCompilerMojoTestEnv(compilerMojo);
+
+        compilerMojo.execute();
+
+        File testClass = new File(compilerMojo.getOutputDirectory(), "TestCompile.class");
+        assertTrue(testClass::exists);
+    }
+
+    @Test
+    void testNormalizeAnnotationProcessors() {
+        assertNull(AbstractCompilerMojo.normalizeAnnotationProcessors(null));
+        assertNull(AbstractCompilerMojo.normalizeAnnotationProcessors(new String[] {"", " "}));
+        assertArrayEquals(
+                new String[] {"com.example.First", "com.example.Second"},
+                AbstractCompilerMojo.normalizeAnnotationProcessors(
+                        new String[] {"", "com.example.First", " ", "com.example.Second"}));
+    }
+
     @Test
     @InjectMojo(goal = COMPILE, pom = "classpath:/unit/compiler-basic-sourcetarget/plugin-config.xml")
     void testCompilerBasicSourceTarget(CompilerMojo compilerMojo) throws Exception {

src/test/resources/unit/compiler-empty-annotation-processors-test/plugin-config.xml

@@ -0,0 +1,38 @@
+<!--
+  ~ 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.
+  -->
+
+<project>
+  <build>
+    <plugins>
+      <plugin>
+        <artifactId>maven-compiler-plugin</artifactId>
+        <configuration>
+          <compileSourceRoots>
+            <compileSourceRoot>${basedir}/target/test-classes/unit/compiler-empty-annotation-processors-test/src/main/java</compileSourceRoot>
+          </compileSourceRoots>
+          <annotationProcessors>
+          </annotationProcessors>
+          <compilerId>javac</compilerId>
+          <outputDirectory>${basedir}/target/test/unit/compiler-empty-annotation-processors-test/target/classes</outputDirectory>
+          <buildDirectory>${basedir}/target/test/unit/compiler-empty-annotation-processors-test/target</buildDirectory>
+        </configuration>
+      </plugin>
+    </plugins>
+  </build>
+</project>

src/test/resources/unit/compiler-empty-annotation-processors-test/src/main/java/TestCompile.java

@@ -0,0 +1,19 @@
+/*
+ * 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.
+ */
+public class TestCompile {}