FileClassPathEntry does not exist throws IllegalStateException

Closes gh-84

Author: rwinch

spring-boot-testjars/src/main/java/org/springframework/experimental/boot/server/exec/FileClasspathEntry.java

@@ -33,7 +33,11 @@ public FileClasspathEntry(File file) {
 	}
 
 	public List<String> resolve() {
-		return Arrays.asList(this.file.getAbsolutePath());
+		String absolutePath = this.file.getAbsolutePath();
+		if (!this.file.exists()) {
+			throw new IllegalStateException("Could not find file to add to the classpath '" + absolutePath + "'");
+		}
+		return Arrays.asList(absolutePath);
 	}
 
 }

spring-boot-testjars/src/test/java/org/springframework/experimental/boot/server/exec/FileClasspathEntryTests.java

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2012-2023 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 org.springframework.experimental.boot.server.exec;
+
+import java.io.File;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
+
+class FileClasspathEntryTests {
+
+	@Test
+	void resolveWhenDoesNotExist() {
+		File doesNotExist = new File("does-not-exist");
+		FileClasspathEntry notFoundEntry = new FileClasspathEntry(doesNotExist);
+		assertThatIllegalStateException().isThrownBy(notFoundEntry::resolve)
+				.withMessageContaining("Could not find file to add to the classpath");
+	}
+
+}