Polish "Check that JUnit 5 test classes are package-private"

 - Clear the new fields each time a new tree is begun. This prevents
   state from one tree affecting the checking of another.
 - Allow abstract test classes to be public

Closes gh-281

spring-javaformat/spring-javaformat-checkstyle/src/main/java/io/spring/javaformat/checkstyle/check/SpringJUnit5Check.java

@@ -97,9 +97,11 @@ public int[] getAcceptableTokens() {
 
 	@Override
 	public void beginTree(DetailAST rootAST) {
+		this.testClass = null;
 		this.imports.clear();
 		this.testMethods.clear();
 		this.lifecycleMethods.clear();
+		this.nestedTestClasses.clear();
 	}
 
 	@Override
@@ -181,7 +183,7 @@ private boolean shouldCheck() {
 	}
 
 	private void check() {
-		if (this.testClass != null) {
+		if (this.testClass != null && !isAbstract(this.testClass)) {
 			checkVisibility(Arrays.asList(this.testClass), "junit5.publicClass", null);
 		}
 		checkVisibility(this.nestedTestClasses, "junit5.publicNestedClass", "junit5.privateNestedClass");
@@ -200,6 +202,11 @@ private void check() {
 		checkVisibility(this.lifecycleMethods, "junit5.lifecyclePublicMethod", "junit5.lifecyclePrivateMethod");
 	}
 
+	private boolean isAbstract(DetailAST ast) {
+		DetailAST modifiers = ast.findFirstToken(TokenTypes.MODIFIERS);
+		return modifiers.findFirstToken(TokenTypes.ABSTRACT) != null;
+	}
+
 	private void checkVisibility(List<DetailAST> asts, String publicMessageKey, String privateMessageKey) {
 		for (DetailAST ast : asts) {
 			DetailAST modifiers = ast.findFirstToken(TokenTypes.MODIFIERS);

spring-javaformat/spring-javaformat-checkstyle/src/test/resources/check/JUnit5PublicAbstractIsValid.txt

@@ -0,0 +1 @@
++0 errors

spring-javaformat/spring-javaformat-checkstyle/src/test/resources/source/JUnit5PublicAbstractIsValid.java

@@ -0,0 +1,32 @@
+/*
+ * Copyright 2017-2024 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
+ *
+ *      https://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.
+ */
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * This is a valid example. We allow abstract test classes to be
+ * public so that classes in other packages can extend them.
+ *
+ * @author Andy Wilkinson
+ */
+public abstract class JUnit5PublicAbstractIsValid {
+
+	@Test
+	void doSomethingWorks() {
+		// test here
+	}
+
+}