Check for null context when trying to find an exception mapper

If application context refresh fails, SpringApplication will have a 
null context. Previously, this would result in an NPE when looking for
an ExitCodeExceptionMapper.

This commit updates SpringApplication to gracefully handle the context
being null when looking for an ExitCodeExceptionMapper.

Closes gh-4803

spring-boot/src/main/java/org/springframework/boot/SpringApplication.java

@@ -881,6 +881,9 @@ public class SpringApplication {
 
 	private int getExitCodeFromMappedException(ConfigurableApplicationContext context,
 			Throwable exception) {
+		if (context == null) {
+			return 0;
+		}
 		ExitCodeGenerators generators = new ExitCodeGenerators();
 		Collection<ExitCodeExceptionMapper> beans = context
 				.getBeansOfType(ExitCodeExceptionMapper.class).values();

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

@@ -24,6 +24,8 @@ import java.util.List;
 import java.util.Set;
 import java.util.concurrent.atomic.AtomicReference;
 
+import javax.annotation.PostConstruct;
+
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Rule;
@@ -74,6 +76,7 @@ import static org.hamcrest.Matchers.hasItem;
 import static org.hamcrest.Matchers.instanceOf;
 import static org.hamcrest.Matchers.is;
 import static org.hamcrest.Matchers.isA;
+import static org.hamcrest.Matchers.not;
 import static org.hamcrest.Matchers.sameInstance;
 import static org.hamcrest.Matchers.startsWith;
 import static org.junit.Assert.assertArrayEquals;
@@ -83,6 +86,7 @@ import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertThat;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
+import static org.mockito.Matchers.any;
 import static org.mockito.Matchers.anyObject;
 import static org.mockito.Mockito.atLeastOnce;
 import static org.mockito.Mockito.mock;
@@ -615,6 +619,31 @@ public class SpringApplicationTests {
 		assertThat(listener.getExitCode(), equalTo(11));
 	}
 
+	@Test
+	public void exceptionFromRefreshIsHandledGracefully() throws Exception {
+		final SpringBootExceptionHandler handler = mock(SpringBootExceptionHandler.class);
+		SpringApplication application = new SpringApplication(
+				RefreshFailureConfig.class) {
+
+			@Override
+			SpringBootExceptionHandler getSpringBootExceptionHandler() {
+				return handler;
+			}
+
+		};
+		ExitCodeListener listener = new ExitCodeListener();
+		application.addListeners(listener);
+		application.setWebEnvironment(false);
+		try {
+			application.run();
+			fail("Did not throw");
+		}
+		catch (RuntimeException ex) {
+		}
+		verify(handler).registerLoggedException(any(RefreshFailureException.class));
+		assertThat(this.output.toString(), not(containsString("NullPointerException")));
+	}
+
 	@Test
 	public void defaultCommandLineArgs() throws Exception {
 		SpringApplication application = new SpringApplication(ExampleConfig.class);
@@ -966,6 +995,15 @@ public class SpringApplicationTests {
 
 	}
 
+	@Configuration
+	static class RefreshFailureConfig {
+
+		@PostConstruct
+		public void fail() {
+			throw new RefreshFailureException();
+		}
+	}
+
 	static class ExitStatusException extends RuntimeException
 			implements ExitCodeGenerator {
 
@@ -976,6 +1014,10 @@ public class SpringApplicationTests {
 
 	}
 
+	static class RefreshFailureException extends RuntimeException {
+
+	}
+
 	static class AbstractTestRunner implements ApplicationContextAware, Ordered {
 
 		private final String[] expectedBefore;