chore: introduce selfie for unit testing

Author: simschla

build-logic/src/main/groovy/buildlogic.java-common-conventions.gradle

@@ -8,6 +8,7 @@ plugins {
 	// Apply the java Plugin to add support for Java.
 	id 'java'
 	id 'buildlogic.spotless-java-conventions'
+	id 'buildlogic.java-selfie-tests-conventions'
 }
 
 repositories {

build-logic/src/main/groovy/buildlogic.java-selfie-tests-conventions.gradle

@@ -0,0 +1,11 @@
+plugins {
+	id 'java'
+}
+
+tasks.withType(Test).configureEach {
+	it.environment project.properties.subMap(["selfie"]) // optional, see "Overwrite everything" below
+	it.inputs.files(fileTree("src/test") {
+		// optional, improves up-to-date checking
+		include "**/*.ss"
+	})
+}

gradle/libs.versions.toml

@@ -6,6 +6,7 @@ durian = "1.2.0"
 junit = "5.8.1"
 native-include-googleJavaFormat = "1.24.0"
 picocli = "4.7.6"
+selfie = "2.5.1"
 slf4j = "2.0.17"
 spotless-lib = "3.1.0"
 
@@ -21,6 +22,7 @@ junit-jupiter-engine = { module = "org.junit.jupiter:junit-jupiter-engine", vers
 native-include-googleJavaFormat = { module = "com.google.googlejavaformat:google-java-format", version.ref = "native-include-googleJavaFormat" }
 picocli = { module = "info.picocli:picocli", version.ref = "picocli" }
 picocli-codegen = { module = "info.picocli:picocli-codegen", version.ref = "picocli" }
+selfie = { module = "com.diffplug.selfie:selfie-runner-junit5", version.ref = "selfie" }
 slf4j-api = { module = "org.slf4j:slf4j-api", version.ref = "slf4j" }
 slf4j-binding = { module = "org.slf4j:slf4j-jdk14", version.ref = "slf4j" }
 spotless-lib = { module = "com.diffplug.spotless:spotless-lib", version.ref = "spotless-lib" }
@@ -32,4 +34,4 @@ native-includes = [
     "native-include-googleJavaFormat"
 ]
 spotless-libs = ["spotless-lib", "spotless-lib-extra"]
-test-libs = ["assertj-core", "junit-jupiter-api"]
+test-libs = ["assertj-core", "junit-jupiter-api", "selfie"]

testlib/src/main/java/com/diffplug/spotless/ResourceHarness.java

@@ -34,6 +34,7 @@
 
 import org.assertj.core.api.AbstractCharSequenceAssert;
 import org.assertj.core.util.CheckReturnValue;
+import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.io.TempDir;
 
 import com.diffplug.common.base.Errors;
@@ -52,6 +53,13 @@ public class ResourceHarness {
     @TempDir
     File folderDontUseDirectly;
 
+    SelfieExpectations selfieExpectations;
+
+    @BeforeEach
+    void initSelfie() {
+        selfieExpectations = SelfieExpectations.create(rootFolder());
+    }
+
     /** Returns the root folder (canonicalized to fix OS X issue) */
     public File rootFolder() {
         return Errors.rethrow().get(() -> folderDontUseDirectly.getCanonicalFile());
@@ -71,6 +79,10 @@ public File newFolder(String subpath) throws IOException {
         return targetDir;
     }
 
+    public SelfieExpectations selfie() {
+        return selfieExpectations;
+    }
+
     /**
      * List the resources found in the specified path (directory) on the classpath
      * @param path - absolute path using unix-style file separators (if it is relative, it will be made absolute before class path scanning)

testlib/src/main/java/com/diffplug/spotless/SelfieExpectations.java

@@ -0,0 +1,58 @@
+/*
+ * Copyright 2025 DiffPlug
+ *
+ * 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.
+ */
+package com.diffplug.spotless;
+
+import java.io.File;
+import java.nio.file.Files;
+import java.util.Objects;
+
+import org.jetbrains.annotations.NotNull;
+
+import com.diffplug.selfie.Camera;
+import com.diffplug.selfie.Selfie;
+import com.diffplug.selfie.Snapshot;
+import com.diffplug.selfie.StringSelfie;
+
+public class SelfieExpectations {
+
+    @NotNull private final File rootFolder;
+
+    private SelfieExpectations(@NotNull File rootFolder) {
+        this.rootFolder = Objects.requireNonNull(rootFolder);
+    }
+
+    public static SelfieExpectations create(@NotNull File rootFolder) {
+        Objects.requireNonNull(rootFolder);
+        if (!rootFolder.isDirectory()) {
+            throw new IllegalArgumentException("Root folder must be a directory: " + rootFolder);
+        }
+        return new SelfieExpectations(rootFolder);
+    }
+
+    private static final Camera<Resource> RESOURCE_CAMERA = (Resource resource) -> {
+        File file = new File(resource.rootFolder(), resource.resourcePath());
+        if (!file.exists()) {
+            throw new IllegalArgumentException("Resource not found: " + file);
+        }
+        return Snapshot.of(ThrowingEx.get(() -> Files.readString(file.toPath())));
+    };
+
+    record Resource(File rootFolder, String resourcePath) {}
+
+    public StringSelfie expectResource(String resourcePath) {
+        return Selfie.expectSelfie(new Resource(rootFolder, resourcePath), RESOURCE_CAMERA);
+    }
+}

testlib/src/main/java/selfie/SelfieSettings.java

@@ -0,0 +1,34 @@
+/*
+ * Copyright 2025 DiffPlug
+ *
+ * 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.
+ */
+package selfie;
+
+import java.io.File;
+
+import org.jetbrains.annotations.NotNull;
+
+import com.diffplug.selfie.junit5.SelfieSettingsAPI;
+
+public class SelfieSettings extends SelfieSettingsAPI {
+
+    @Override
+    public @NotNull File getRootFolder() {
+        File f = new File(System.getProperty("user.dir"), "src/test/resources");
+        if (f.exists()) {
+            return f;
+        }
+        return super.getRootFolder(); // fallback to default
+    }
+}