fix: SpotBugsTask was not cacheable

src/functionalTest/groovy/com/github/spotbugs/snom/CacheabilityFunctionalTest.groovy

@@ -0,0 +1,114 @@
+/*
+ * Copyright 2019 SpotBugs team
+ *
+ * <p>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
+ *
+ * <p>http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * <p>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 com.github.spotbugs.snom
+
+import org.gradle.testkit.runner.BuildResult
+import org.gradle.testkit.runner.GradleRunner
+import org.gradle.util.GradleVersion
+import spock.lang.Specification
+
+import java.nio.file.Files
+
+class CacheabilityFunctionalTest extends Specification {
+
+
+    /**
+     * Verifies the cacheability of {@link SpotBugsTask} by invoking the same code
+     * in two different, uniquely-named folders.
+     *
+     * Thanks to the presence of sysprop {@code org.gradle.caching.debug=true}, the
+     * build cache key for the task is echoed to stdout.
+     *
+     * If we compare these keys for both builds and they are equal, we can now trust the
+     * task is cacheable.
+     *
+     */
+    def 'spotbugsMain task is cacheable'() {
+        given:
+        def buildDir1 = Files.createTempDirectory(null).toFile()
+        def buildDir2 = Files.createTempDirectory(null).toFile()
+
+        def version = System.getProperty('snom.test.functional.gradle', GradleVersion.current().version)
+
+        initializeBuildFile(buildDir1)
+        initializeBuildFile(buildDir2)
+
+        when:
+        BuildResult result1 =
+                GradleRunner.create()
+                .withProjectDir(buildDir1)
+                .withArguments(':spotbugsMain')
+                .withPluginClasspath()
+                .forwardOutput()
+                .withGradleVersion(version)
+                .build()
+        def hashKeyLine1 = getHashKeyLine(result1)
+
+        then:
+        hashKeyLine1
+
+        when:
+        BuildResult result2 =
+                GradleRunner.create()
+                .withProjectDir(buildDir2)
+                .withArguments(':spotbugsMain')
+                .withPluginClasspath()
+                .forwardOutput()
+                .withGradleVersion(version)
+                .build()
+        def hashKeyLine2 = getHashKeyLine(result2)
+
+        then:
+        hashKeyLine2
+        hashKeyLine1 == hashKeyLine2
+    }
+
+    private static String getHashKeyLine(BuildResult result) {
+        return result.output.find('Build cache key for task \':spotbugsMain\' is .*')
+    }
+
+    private static void initializeBuildFile(File buildDir) {
+        File buildFile = new File(buildDir, 'build.gradle')
+        File propertiesFile = new File(buildDir, 'gradle.properties')
+
+        buildFile << '''
+            |plugins {
+            |    id 'java'
+            |    id 'com.github.spotbugs'
+            |}
+            |
+            |version = 1.0
+            |
+            |repositories {
+            |    mavenCentral()
+            |}
+            |'''.stripMargin()
+
+        File sourceDir = buildDir.toPath().resolve('src').resolve('main').resolve('java').toFile()
+        sourceDir.mkdirs()
+        File sourceFile = new File(sourceDir, 'Foo.java')
+        sourceFile << '''
+            |public class Foo {
+            |    public static void main(String... args) {
+            |        System.out.println("Hello, SpotBugs!");
+            |    }
+            |}
+            |'''.stripMargin()
+
+        propertiesFile << '''
+            |org.gradle.caching = true
+            |org.gradle.caching.debug = true
+            |'''.stripMargin()
+    }
+}

src/main/groovy/com/github/spotbugs/snom/SpotBugsTask.groovy

@@ -35,6 +35,7 @@ import org.gradle.api.file.FileCollection;
 import org.gradle.api.model.ObjectFactory;
 import org.gradle.api.provider.ListProperty;
 import org.gradle.api.provider.Property;
+import org.gradle.api.tasks.CacheableTask
 import org.gradle.api.tasks.Input;
 import org.gradle.api.tasks.InputFile;
 import org.gradle.api.tasks.InputFiles;
@@ -82,6 +83,8 @@ import javax.inject.Inject
  *
  * <p>See also <a href="https://spotbugs.readthedocs.io/en/stable/running.html">SpotBugs Manual about configuration</a>.</p>
  */
+
+@CacheableTask
 class SpotBugsTask extends DefaultTask implements VerificationTask {
     private static final String FEATURE_FLAG_WORKER_API = "com.github.spotbugs.snom.worker";
     private final Logger log = LoggerFactory.getLogger(SpotBugsTask);
@@ -174,8 +177,12 @@ class SpotBugsTask extends DefaultTask implements VerificationTask {
     /**
      * Property to specify the name of project. Some reporting formats use this property.
      * Default value is {@code "${project.name} (${task.name})"}.
+     * <br>
+     * Note that this property, if treated as a task input, can break cacheability.<br>
+     * As such, it has been marked {@link Internal} to exclude it from task up-to-date and
+     * cacheability checks.
      */
-    @Input
+    @Internal
     @NonNull
     final Property<String> projectName;
     /**