Improve null-safety of loader/spring-boot-loader-tools

See gh-46926

loader/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/AbstractJarWriter.java

@@ -224,6 +224,7 @@ public abstract class AbstractJarWriter implements LoaderClassesWriter {
 	@Override
 	public void writeLoaderClasses(String loaderJarResourceName) throws IOException {
 		URL loaderJar = getClass().getClassLoader().getResource(loaderJarResourceName);
+		Assert.state(loaderJar != null, "Unable to load resource '%s'".formatted(loaderJarResourceName));
 		try (JarInputStream inputStream = new JarInputStream(new BufferedInputStream(loaderJar.openStream()))) {
 			JarEntry entry;
 			while ((entry = inputStream.getNextJarEntry()) != null) {

loader/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/DefaultLaunchScript.java

@@ -31,6 +31,8 @@ import java.util.regex.Pattern;
 
 import org.jspecify.annotations.Nullable;
 
+import org.springframework.util.Assert;
+
 /**
  * Default implementation of {@link LaunchScript}. Provides the default Spring Boot launch
  * script or can load a specific script File. Also support mustache style template
@@ -63,7 +65,9 @@ public class DefaultLaunchScript implements LaunchScript {
 
 	private String loadContent(@Nullable File file) throws IOException {
 		if (file == null) {
-			return loadContent(getClass().getResourceAsStream("launch.script"));
+			InputStream stream = getClass().getResourceAsStream("launch.script");
+			Assert.state(stream != null, "Unable to load resource 'launch.script'");
+			return loadContent(stream);
 		}
 		return loadContent(new FileInputStream(file));
 	}
@@ -85,7 +89,7 @@ public class DefaultLaunchScript implements LaunchScript {
 		outputStream.flush();
 	}
 
-	private String expandPlaceholders(@Nullable String content, Map<?, ?> properties) throws IOException {
+	private String expandPlaceholders(String content, @Nullable Map<?, ?> properties) throws IOException {
 		StringBuilder expanded = new StringBuilder();
 		Matcher matcher = PLACEHOLDER_PATTERN.matcher(content);
 		while (matcher.find()) {

loader/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/FileUtils.java

@@ -25,6 +25,8 @@ import java.util.jar.Manifest;
 
 import org.jspecify.annotations.Nullable;
 
+import org.springframework.util.Assert;
+
 /**
  * Utilities for manipulating files and directories in Spring Boot tooling.
  *
@@ -43,7 +45,9 @@ public abstract class FileUtils {
 	 */
 	public static void removeDuplicatesFromOutputDirectory(File outputDirectory, File originDirectory) {
 		if (originDirectory.isDirectory()) {
-			for (String name : originDirectory.list()) {
+			String[] files = originDirectory.list();
+			Assert.state(files != null, "'files' must not be null");
+			for (String name : files) {
 				File targetFile = new File(outputDirectory, name);
 				if (targetFile.exists() && targetFile.canWrite()) {
 					if (!targetFile.isDirectory()) {

loader/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/Library.java

@@ -109,6 +109,7 @@ public class Library {
 	 * @throws IOException on error
 	 */
 	InputStream openStream() throws IOException {
+		Assert.state(this.file != null, "'file' must not be null");
 		return new FileInputStream(this.file);
 	}
 

loader/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/MainClassFinder.java

@@ -154,7 +154,10 @@ public abstract class MainClassFinder {
 		return null;
 	}
 
-	private static void pushAllSorted(Deque<File> stack, File[] files) {
+	private static void pushAllSorted(Deque<File> stack, File @Nullable [] files) {
+		if (files == null) {
+			return;
+		}
 		Arrays.sort(files, Comparator.comparing(File::getName));
 		for (File file : files) {
 			stack.push(file);

loader/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/SizeCalculatingEntryWriter.java

@@ -115,17 +115,18 @@ final class SizeCalculatingEntryWriter implements EntryWriter {
 		}
 
 		private OutputStream convertToFileOutputStream(ByteArrayOutputStream byteArrayOutputStream) throws IOException {
-			initializeTempFile();
+			File tempFile = initializeTempFile();
-			FileOutputStream fileOutputStream = new FileOutputStream(this.tempFile);
+			FileOutputStream fileOutputStream = new FileOutputStream(tempFile);
 			StreamUtils.copy(byteArrayOutputStream.toByteArray(), fileOutputStream);
 			return fileOutputStream;
 		}
 
-		private void initializeTempFile() throws IOException {
+		private File initializeTempFile() throws IOException {
 			if (this.tempFile == null) {
 				this.tempFile = File.createTempFile("springboot-", "-entrycontent");
 				this.tempFile.deleteOnExit();
 			}
+			return this.tempFile;
 		}
 
 		@Override