[bs-55] Add strategy for setting system exit code

* Added ExitCodeGenerator and SpringApplication.exit
convenience method
* User can add bean of type ExitCodeGenerator or supply
one in the call to exit()

[Fixes #48475971]

spring-bootstrap/src/test/java/org/springframework/bootstrap/SpringApplicationTests.java

@@ -51,6 +51,7 @@ import org.springframework.web.context.support.StaticWebApplicationContext;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.instanceOf;
 import static org.hamcrest.Matchers.sameInstance;
+import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertThat;
 import static org.junit.Assert.assertTrue;
@@ -279,6 +280,29 @@ public class SpringApplicationTests {
 		assertNotNull(this.context);
 	}
 
+	@Test
+	public void exit() throws Exception {
+		SpringApplication application = new SpringApplication(ExampleConfig.class);
+		application.setWebEnvironment(false);
+		ApplicationContext context = application.run();
+		assertNotNull(context);
+		assertEquals(0, SpringApplication.exit(context));
+	}
+
+	@Test
+	public void exitWithExplicitCOde() throws Exception {
+		SpringApplication application = new SpringApplication(ExampleConfig.class);
+		application.setWebEnvironment(false);
+		ApplicationContext context = application.run();
+		assertNotNull(context);
+		assertEquals(2, SpringApplication.exit(context, new ExitCodeGenerator() {
+			@Override
+			public int getExitCode() {
+				return 2;
+			}
+		}));
+	}
+
 	@Test
 	public void defaultCommandLineArgs() throws Exception {
 		SpringApplication application = new SpringApplication(ExampleConfig.class);