Allow custom builder to be used in SpringBootServletInitializer

Previously, to use a custom SpringApplicationBuilder subclass in
SpringBootServletInitializer, it was necessary to either override
createRootApplicationContext or configure and duplicate the logic
that configures the builder with initializers, the context class, etc.

This commit introduces a new method, createSpringApplicationBuilder,
that can be overridden to use a custom builder without having to
duplicate any configuration logic.

Closes gh-2694

spring-boot/src/main/java/org/springframework/boot/context/web/SpringBootServletInitializer.java

@@ -83,7 +83,7 @@ public abstract class SpringBootServletInitializer implements WebApplicationInit
 
 	protected WebApplicationContext createRootApplicationContext(
 			ServletContext servletContext) {
-		SpringApplicationBuilder builder = new SpringApplicationBuilder();
+		SpringApplicationBuilder builder = createSpringApplicationBuilder();
 		ApplicationContext parent = getExistingRootWebApplicationContext(servletContext);
 		if (parent != null) {
 			this.logger.info("Root context already created (using as parent).");
@@ -108,6 +108,18 @@ public abstract class SpringBootServletInitializer implements WebApplicationInit
 		return run(application);
 	}
 
+	/**
+	 * Returns the {@code SpringApplicationBuilder} that is used to configure and create
+	 * the {@link SpringApplication}. The default implementation returns a new
+	 * {@code SpringApplicationBuilder} in its default state.
+	 *
+	 * @return the {@code SpringApplicationBuilder}.
+	 * @since 1.3.0
+	 */
+	protected SpringApplicationBuilder createSpringApplicationBuilder() {
+		return new SpringApplicationBuilder();
+	}
+
 	/**
 	 * Called to run a fully configured {@link SpringApplication}.
 	 * @param application the application to run

spring-boot/src/test/java/org/springframework/boot/context/web/SpringBootServletInitializerTests.java

@@ -33,6 +33,7 @@ import org.springframework.mock.web.MockServletContext;
 import org.springframework.web.context.WebApplicationContext;
 
 import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.is;
 import static org.junit.Assert.assertThat;
 
 /**
@@ -72,6 +73,13 @@ public class SpringBootServletInitializerTests {
 				equalToSet(Config.class, ErrorPageFilter.class));
 	}
 
+	@Test
+	public void applicationBuilderCanBeCustomized() throws Exception {
+		CustomSpringBootServletInitializer servletInitializer = new CustomSpringBootServletInitializer();
+		servletInitializer.createRootApplicationContext(this.servletContext);
+		assertThat(servletInitializer.applicationBuilder.built, is(true));
+	}
+
 	private Matcher<? super Set<Object>> equalToSet(Object... items) {
 		Set<Object> set = new LinkedHashSet<Object>();
 		Collections.addAll(set, items);
@@ -88,6 +96,22 @@ public class SpringBootServletInitializerTests {
 
 	}
 
+	private class CustomSpringBootServletInitializer extends
+			MockSpringBootServletInitializer {
+
+		private final CustomSpringApplicationBuilder applicationBuilder = new CustomSpringApplicationBuilder();
+
+		@Override
+		protected SpringApplicationBuilder createSpringApplicationBuilder() {
+			return this.applicationBuilder;
+		}
+
+		@Override
+		protected SpringApplicationBuilder configure(SpringApplicationBuilder application) {
+			return application.sources(Config.class);
+		}
+	}
+
 	@Configuration
 	public class WithConfigurationAnnotation extends MockSpringBootServletInitializer {
 	}
@@ -106,4 +130,16 @@ public class SpringBootServletInitializerTests {
 
 	}
 
+	private static class CustomSpringApplicationBuilder extends SpringApplicationBuilder {
+
+		private boolean built;
+
+		@Override
+		public SpringApplication build() {
+			this.built = true;
+			return super.build();
+		}
+
+	}
+
 }