Improve TestCompiler and allow lookup based class defines

Update the `TestCompiler` so that classes can be defined using
a `Lookup`. This update allows package-private classes to be
accessed without needing a quite so unusual classloader setup.

The `@CompileWithTargetClassAccess` should be added to any
test that needs to use `Lookup` based defines. The test will
run with a completed forked classloader so not to pollute the
main classloader.

This commit also adds some useful additional APIs.

See gh-28120

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

@@ -18,6 +18,9 @@ package org.springframework.aot.test.generator.compile;
 
 import org.junit.jupiter.api.Test;
 
+import org.springframework.aot.test.generator.file.ResourceFiles;
+import org.springframework.aot.test.generator.file.SourceFiles;
+
 import static org.assertj.core.api.Assertions.assertThat;
 
 
@@ -31,8 +34,8 @@ class CompilationExceptionTests {
 
 	@Test
 	void getMessageReturnsMessage() {
-		CompilationException exception = new CompilationException("message");
-		assertThat(exception).hasMessage("message");
+		CompilationException exception = new CompilationException("message", SourceFiles.none(), ResourceFiles.none());
+		assertThat(exception).hasMessageContaining("message");
 	}
 
 }

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

@@ -26,6 +26,7 @@ import org.springframework.aot.test.generator.file.ResourceFiles;
 import org.springframework.aot.test.generator.file.SourceFile;
 import org.springframework.aot.test.generator.file.SourceFiles;
 import org.springframework.aot.test.generator.file.WritableContent;
+import org.springframework.util.ClassUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
@@ -170,7 +171,8 @@ class TestCompilerTests {
 	}
 
 	@Test
-	void compiledCodeCanAccessExistingPackagePrivateClass() {
+	@CompileWithTargetClassAccess(classNames = "com.example.PackagePrivate")
+	void compiledCodeCanAccessExistingPackagePrivateClassIfAnnotated() throws ClassNotFoundException, LinkageError {
 		SourceFiles sourceFiles = SourceFiles.of(SourceFile.of("""
 				package com.example;
 
@@ -187,6 +189,26 @@ class TestCompilerTests {
 				.isEqualTo("Hello from PackagePrivate"));
 	}
 
+	@Test
+	void compiledCodeCannotAccessExistingPackagePrivateClassIfNotAnnotated() {
+		SourceFiles sourceFiles = SourceFiles.of(SourceFile.of("""
+				package com.example;
+
+				public class Test implements PublicInterface {
+
+					public String perform() {
+						return new PackagePrivate().perform();
+					}
+
+				}
+				"""));
+		assertThatExceptionOfType(IllegalAccessError.class)
+				.isThrownBy(() -> TestCompiler.forSystem().compile(sourceFiles,
+						compiled -> compiled.getInstance(PublicInterface.class, "com.example.Test").perform()))
+				.withMessageContaining(ClassUtils.getShortName(CompileWithTargetClassAccess.class));
+	}
+
+
 	private void assertSuppliesHelloWorld(Compiled compiled) {
 		assertThat(compiled.getInstance(Supplier.class).get()).isEqualTo("Hello World!");
 	}