Add Servlet 3.1 methods to mock request

Issue: SPR-11492

spring-test/src/main/java/org/springframework/mock/web/MockHttpServletRequest.java

@@ -348,6 +348,10 @@ public class MockHttpServletRequest implements HttpServletRequest {
 		return (this.content != null ? this.content.length : -1);
 	}
 
+	public long getContentLengthLong() {
+		return getContentLength();
+	}
+
 	public void setContentType(String contentType) {
 		this.contentType = contentType;
 		if (contentType != null) {
@@ -999,6 +1003,20 @@ public class MockHttpServletRequest implements HttpServletRequest {
 		return getSession(true);
 	}
 
+	/**
+	 * The implementation of this (Servlet 3.1+) method calls
+	 * {@link MockHttpSession#changeSessionId()} if the session is a mock session.
+	 * Otherwise it simply returns the current session id.
+	 * @since 4.0.3
+	 */
+	public String changeSessionId() {
+		Assert.isTrue(this.session != null, "The request does not have a session");
+		if (this.session instanceof MockHttpSession) {
+			return ((MockHttpSession) session).changeSessionId();
+		}
+		return this.session.getId();
+	}
+
 	public void setRequestedSessionIdValid(boolean requestedSessionIdValid) {
 		this.requestedSessionIdValid = requestedSessionIdValid;
 	}

spring-test/src/main/java/org/springframework/mock/web/MockHttpSession.java

@@ -53,7 +53,7 @@ public class MockHttpSession implements HttpSession {
 
 	private static int nextId = 1;
 
-	private final String id;
+	private String id;
 
 	private final long creationTime = System.currentTimeMillis();
 
@@ -110,6 +110,16 @@ public class MockHttpSession implements HttpSession {
 		return this.id;
 	}
 
+	/**
+	 * As of Servlet 3.1 the id of a session can be changed.
+	 * @return the new session id.
+	 * @since 4.0.3
+	 */
+	public String changeSessionId() {
+		this.id = Integer.toString(nextId++);
+		return this.id;
+	}
+
 	public void access() {
 		this.lastAccessedTime = System.currentTimeMillis();
 		this.isNew = false;