Document how to use Tomcat 7 or Jetty 8 with Gradle

Previously, the documentation only provided examples of the required
configuration for Maven users. This commit adds equivalent configuration
snippets for those using Gradle. It also removes the recommendation to
override the version of the Servlet API as this is unnecessary. The pom
files for the Jetty 8 and Tomcat 7 samples have also been updated
accordingly.

Closes gh-2346

spring-boot-docs/src/main/asciidoc/howto.adoc

@@ -604,8 +604,8 @@ Example in Gradle:
 	}
 
 	dependencies {
-		compile("org.springframework.boot:spring-boot-starter-web:{spring-boot-version}")
-		compile("org.springframework.boot:spring-boot-starter-undertow:{spring-boot-version}")
+		compile 'org.springframework.boot:spring-boot-starter-web:{spring-boot-version}")
+		compile 'org.springframework.boot:spring-boot-starter-undertow:{spring-boot-version}")
 		// ...
 	}
 ----
@@ -653,16 +653,20 @@ add a listener to the `Builder`:
 === Use Tomcat 7
 Tomcat 7 works with Spring Boot, but the default is to use Tomcat 8. If you cannot use
 Tomcat 8 (for example, because you are using Java 1.6) you will need to change your
-classpath to reference Tomcat 7 and Servlet API 3.0.
+classpath to reference Tomcat 7 .
 
-If you are using the starter poms and parent you can just change the version properties,
-e.g. for a simple webapp or service:
+
+
+==== Use Tomcat 7 with Maven
+[[howto-use-tomcat-7-maven]]
+
+If you are using the starter poms and parent you can just change the Tomcat version
+property, e.g. for a simple webapp or service:
 
 [source,xml,indent=0,subs="verbatim,quotes,attributes"]
 ----
 	<properties>
 		<tomcat.version>7.0.56</tomcat.version>
-		<servlet-api.version>3.0.1</servlet-api.version>
 	</properties>
 	<dependencies>
 		...
@@ -676,12 +680,42 @@ e.g. for a simple webapp or service:
 
 
 
+==== Use Tomcat 7 with Gradle
+[[howto-use-tomcat-7-gradle]]
+
+You can use a resolution strategy to change the versions of the Tomcat dependencies,
+e.g. for a simple webapp or service:
+
+[source,groovy,indent=0,subs="verbatim,quotes,attributes"]
+----
+	configurations.all {
+		resolutionStrategy {
+			eachDependency {
+				if (it.requested.group == 'org.apache.tomcat.embed') {
+					it.useVersion '7.0.56'
+				}
+			}
+		}
+	}
+
+	dependencies {
+		compile 'org.springframework.boot:spring-boot-starter-web'
+	}
+----
+
+
+
 [[howto-use-jetty-8]]
 === Use Jetty 8
 Jetty 8 works with Spring Boot, but the default is to use Jetty 9. If you cannot use
 Jetty 9 (for example, because you are using Java 1.6) you will need to change your
-classpath to reference Jetty 8 and Servlet API 3.0. You will also need to exclude
-Jetty's WebSocket-related dependencies.
+classpath to reference Jetty 8. You will also need to exclude Jetty's WebSocket-related
+dependencies.
+
+
+
+[[howto-use-jetty-8-maven]]
+==== Use Jetty 8 with Maven
 
 If you are using the starter poms and parent you can just add the Jetty starter with
 the required WebSocket exclusion and change the version properties, e.g. for a simple
@@ -692,7 +726,6 @@ webapp or service:
 	<properties>
 		<jetty.version>8.1.15.v20140411</jetty.version>
 		<jetty-jsp.version>2.2.0.v201112011158</jetty-jsp.version>
-		<servlet-api.version>3.0.1</servlet-api.version>
 	</properties>
 	<dependencies>
 		<dependency>
@@ -720,6 +753,36 @@ webapp or service:
 
 
 
+[[howto-use-jetty-8-gradle]]
+==== Use Jetty 8 with Gradle
+
+You can use a resolution strategy to change the version of the Jetty dependencies, e.g.
+for a simple webapp or service:
+
+[source,groovy,indent=0,subs="verbatim,quotes,attributes"]
+----
+	configurations.all {
+		resolutionStrategy {
+			eachDependency {
+				if (it.requested.group == 'org.eclipse.jetty') {
+					it.useVersion '8.1.15.v20140411'
+				}
+			}
+		}
+	}
+
+	dependencies {
+		compile ('org.springframework.boot:spring-boot-starter-web') {
+			exclude group: 'org.springframework.boot', module: 'spring-boot-starter-tomcat'
+		}
+		compile ('org.springframework.boot:spring-boot-starter-jetty') {
+			exclude group: 'org.eclipse.jetty.websocket'
+		}
+	}
+----
+
+
+
 [[howto-create-websocket-endpoints-using-serverendpoint]]
 === Create WebSocket endpoints using @ServerEndpoint
 If you want to use `@ServerEndpoint` in a Spring Boot application that used an embedded

spring-boot-samples/spring-boot-sample-jetty8/pom.xml

@@ -19,7 +19,6 @@
 		<main.basedir>${basedir}/../..</main.basedir>
 		<jetty.version>8.1.15.v20140411</jetty.version>
 		<jetty-jsp.version>2.2.0.v201112011158</jetty-jsp.version>
-		<servlet-api.version>3.0.1</servlet-api.version>
 	</properties>
 	<dependencies>
 		<dependency>

spring-boot-samples/spring-boot-sample-tomcat7-jsp/pom.xml

@@ -20,7 +20,6 @@
 		<main.basedir>${basedir}/../..</main.basedir>
 		<m2eclipse.wtp.contextRoot>/</m2eclipse.wtp.contextRoot>
 		<tomcat.version>7.0.56</tomcat.version>
-		<servlet-api.version>3.0.1</servlet-api.version>
 		<java.version>1.7</java.version>
 	</properties>
 	<dependencies>