Check that cli `jar` command only writes `.jar`s

Update `JarCommand` to check that the file extension of the output is
`.jar`.

Fixes gh-581

spring-boot-cli/src/main/java/org/springframework/boot/cli/command/jar/JarCommand.java

@@ -110,6 +110,8 @@ public class JarCommand extends OptionParsingCommand {
 					"The name of the resulting jar and at least one source file must be specified");
 
 			File output = new File((String) nonOptionArguments.remove(0));
+			Assert.isTrue(output.getName().toLowerCase().endsWith(".jar"), "The output '"
+					+ output + "' is not a JAR file.");
 			deleteIfExists(output);
 
 			GroovyCompiler compiler = createCompiler(options);

spring-boot-cli/src/test/java/org/springframework/boot/cli/CliTester.java

@@ -35,6 +35,7 @@ import org.junit.runners.model.Statement;
 import org.springframework.boot.cli.command.AbstractCommand;
 import org.springframework.boot.cli.command.OptionParsingCommand;
 import org.springframework.boot.cli.command.grab.GrabCommand;
+import org.springframework.boot.cli.command.jar.JarCommand;
 import org.springframework.boot.cli.command.run.RunCommand;
 import org.springframework.boot.cli.command.test.TestCommand;
 import org.springframework.boot.cli.util.OutputCapture;
@@ -82,6 +83,12 @@ public class CliTester implements TestRule {
 		return getOutput();
 	}
 
+	public String jar(String... args) throws Exception {
+		Future<JarCommand> future = submitCommand(new JarCommand(), args);
+		this.commands.add(future.get(this.timeout, TimeUnit.MILLISECONDS));
+		return getOutput();
+	}
+
 	private <T extends OptionParsingCommand> Future<T> submitCommand(final T command,
 			String... args) {
 		final String[] sources = getSources(args);

spring-boot-cli/src/test/java/org/springframework/boot/cli/ReproIntegrationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2013 the original author or authors.
+ * Copyright 2012-2014 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.
@@ -18,6 +18,7 @@ package org.springframework.boot.cli;
 
 import org.junit.Rule;
 import org.junit.Test;
+import org.junit.rules.ExpectedException;
 
 import static org.hamcrest.Matchers.containsString;
 import static org.junit.Assert.assertThat;
@@ -32,6 +33,9 @@ public class ReproIntegrationTests {
 	@Rule
 	public CliTester cli = new CliTester("src/test/resources/repro-samples/");
 
+	@Rule
+	public ExpectedException thrown = ExpectedException.none();
+
 	@Test
 	public void grabAntBuilder() throws Exception {
 		this.cli.run("grab-ant-builder.groovy");
@@ -54,4 +58,10 @@ public class ReproIntegrationTests {
 				containsString("{\"message\":\"Hello World\"}"));
 	}
 
+	@Test
+	public void jarFileExtensionNeeded() throws Exception {
+		this.thrown.expect(IllegalStateException.class);
+		this.thrown.expectMessage("is not a JAR file");
+		this.cli.jar("secure.groovy", "crsh.groovy");
+	}
 }