Polish "Add module to support testing of generated code"

See gh-28120

Co-authored-by: Andy Wilkinson <wilkinsona@vmware.com>

spring-core-test/src/test/java/com/example/PackagePrivate.java

@@ -0,0 +1,26 @@
+/*
+ * Copyright 2002-2022 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
+ *
+ *      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.
+ */
+
+package com.example;
+
+
+class PackagePrivate {
+
+	String perform() {
+		return "Hello from PackagePrivate";
+	}
+
+}

spring-core-test/src/test/java/com/example/PublicInterface.java

@@ -0,0 +1,23 @@
+/*
+ * Copyright 2002-2022 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
+ *
+ *      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.
+ */
+
+package com.example;
+
+public interface PublicInterface {
+
+	String perform();
+
+}

spring-core-test/src/test/java/org/springframework/aot/test/generator/compile/TestCompilerTests.java

@@ -18,6 +18,7 @@ package org.springframework.aot.test.generator.compile;
 
 import java.util.function.Supplier;
 
+import com.example.PublicInterface;
 import org.junit.jupiter.api.Test;
 
 import org.springframework.aot.test.generator.file.ResourceFile;
@@ -33,6 +34,7 @@ import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
  * Tests for {@link TestCompiler}.
  *
  * @author Phillip Webb
+ * @author Andy Wilkinson
  */
 class TestCompilerTests {
 
@@ -106,7 +108,7 @@ class TestCompilerTests {
 		assertThatExceptionOfType(CompilationException.class).isThrownBy(
 				() -> TestCompiler.forSystem().withSources(
 						SourceFile.of(HELLO_BAD)).compile(compiled -> {
-						}));
+				}));
 	}
 
 	@Test
@@ -167,6 +169,24 @@ class TestCompilerTests {
 		});
 	}
 
+	@Test
+	void compiledCodeCanAccessExistingPackagePrivateClass() {
+		SourceFiles sourceFiles = SourceFiles.of(SourceFile.of("""
+				package com.example;
+
+				public class Test implements PublicInterface {
+
+					public String perform() {
+						return new PackagePrivate().perform();
+					}
+
+				}
+				"""));
+		TestCompiler.forSystem().compile(sourceFiles, compiled -> assertThat(
+				compiled.getInstance(PublicInterface.class, "com.example.Test").perform())
+				.isEqualTo("Hello from PackagePrivate"));
+	}
+
 	private void assertSuppliesHelloWorld(Compiled compiled) {
 		assertThat(compiled.getInstance(Supplier.class).get()).isEqualTo("Hello World!");
 	}

spring-core-test/src/test/java/org/springframework/aot/test/generator/file/SourceFileTests.java

@@ -22,7 +22,6 @@ import com.thoughtworks.qdox.model.JavaSource;
 import org.junit.jupiter.api.Test;
 
 import static org.assertj.core.api.Assertions.assertThat;
-import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
 import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
 
 /**
@@ -42,13 +41,6 @@ class SourceFileTests {
 			}
 			""";
 
-	@Test
-	void ofWhenContentIsNullThrowsException() {
-		assertThatIllegalArgumentException().isThrownBy(
-				() -> SourceFile.of((WritableContent) null)).withMessage(
-						"'writableContent' must not to be empty");
-	}
-
 	@Test
 	void ofWhenContentIsEmptyThrowsException() {
 		assertThatIllegalStateException().isThrownBy(() -> SourceFile.of("")).withMessage(