allow subclassing of SpringApplicationBuilder

spring-boot/src/main/java/org/springframework/boot/builder/SpringApplicationBuilder.java

@@ -80,10 +80,21 @@ public class SpringApplicationBuilder {
 	private boolean registerShutdownHookApplied;
 
 	public SpringApplicationBuilder(Object... sources) {
-		this.application = new SpringApplication(sources);
+		this.application = createSpringApplication(sources);
 	}
 
-	/**
+    /**
+     * Creates a new {@link org.springframework.boot.SpringApplication} instances from the given sources. Subclasses may
+     * override in order to provide a custom subclass of {@link org.springframework.boot.SpringApplication}
+     *
+     * @param sources The sources
+     * @return The {@link org.springframework.boot.SpringApplication} instance
+     */
+    protected SpringApplication createSpringApplication(Object... sources) {
+        return new SpringApplication(sources);
+    }
+
+    /**
 	 * Accessor for the current application context.
 	 * @return the current application context (or null if not yet running)
 	 */