Q: AEM 6.1 Downgrade of a Java version and quickstart shows UnsupportedClassVersionError major.minor version 52.0 We were using AEM 6.1 quickstart on Windows machine with Java 1.8.
To be consistent with other teams, we had to downgrade our Java version to 1.7.0_79 , after we uninstalled Java 8 and rest the JAVA_HOME and PATH variable to reflect the new (downgraded) version of Java and ran AEM using "crx-quickstart.jar" file, AEM started properly but the default startup page does not load and shows this exception:
java.lang.UnsupportedClassVersionError: org/apache/jsp/libs/granite/core/components/login/login_jsp : Unsupported major.minor version 52.0

RequestURI=/libs/granite/core/content/login.html

Servlet= /libs/granite/core/components/login/login.jsp
ApacheSling/2.4 (jetty/9.2.9.v20150224, Java HotSpot(TM) 64-Bit Server VM 1.7.0_79, Windows 7 6.1 amd64) 

One of the solution that worked for us is to rename or delete the "crx-quickstart" folder.. and starting AEM, would work as AEM will recompile everything using the correct version of Java and everything works as expected. But is there any other way.. not to rename/delete the crx-quickstart folder?

A: Here's a more detailed explanation of the solution: 
Symptom: After you upgrade or downgrade a version of Java on your machine, and see that pages are not loading on AEM, not even the main login page.
What causes this:
When AEM is starting up, it compiles java/jsp files using the version of Java is installed on the pc. When there is a change in the version of Java, the new files should be compiled by the installed version and all references must be updated but there is an issue on AEM 6.1 (this maybe fixed in Service Packs).
Resolution:
NOTE: Please backup your AEM instance before doing this


*

*Go to localhost:4502/system/console/configMgr

*If the page opens, login as "admin"


*

*Go to Menu > OSGI > Bundles [http://localhost:4502/system/console/bundles]

*Find this "Apache Sling Commons FileSystem ClassLoader (org.apache.sling.commons.fsclassloader)" and note the BundleID (it
is 218 for me)

*STOP AEM

*On Windows Explorer go to this path where your AEM is installed.. $path\aem-61-author\crx-quickstart\launchpad\felix\bundle218\data

*Delete the classes folder under it.

*Restart AEM.. this should fix the issue.


*If the page above does not open,


*

*(assuming the bundle-id is 218) 

*STOP AEM

*On Windows Explorer go to this path where your AEM is installed.. __
$path\aem-61-author\crx-quickstart\launchpad\felix\bundle218\data
__

*Delete the classes folder under it.

*Restart AEM.. this should fix the issue.



A: To avoid downgrading your java-version, one work around is to apply Service Pack 2 when you first time install CQ6.1.
First, download your SP2 from adobe package share or copy from other machine. Then unpack your cq jar file and copy your SP2 zip file to crx-quickstart/install folder then install.
The detail step should be like below 
java -jar cq5-author-<port>.jar -unpack -v
cp SP2.zip crx-quickstart/install
java -jar cq5-author-<port>.jar

Once again, the Service Pack 2 must be applied when first time install.
