Change JDK to another version issue - windows

Question

I have started on a project that specifically says that use 1.6 JDK to prevent form test failings.
I came across a peculiar behavior while I tried to change JDK from 1.7 to 1.6.
Even though I changed environment variables form 1.7 to 1.6 JDK version, When I run java.exe -version or java -version in cmd, it still gives the earlier version. But when I run javac -version it gives the 1.6 version.
What I did so far was,
In User Environment variables,
classpath to C:\Program Files\Java\jdk1.6.0_43(JDK installation path)
Added java_home variable and changes path to C:\Program Files\Java\jdk1.6.0_43(JDK installation path)
Changed path to C:\Program Files\Java\jdk1.6.0_43\bin(Java bin)
In system Environment variables,
Added C:\Program Files\Java\jdk1.6.0_43\bin to CLASSPATH
Created JAVA_HOME Variable and added C:\Program Files\Java\jdk1.6.0_43\bin
There was a variable that is related to Java, called QLJAVA and path was set to C:\Program Files (x86)\Java\jre7\lib\ext\QTJava.zip and I removed that variable as well.
Before I edited the CLASSPATH this is the same path that was there
My issues,
Why it selects 1.7 when used java -version, and why gives different result when ran javac -version. or is there something wrong with what I already have done?
What is the QLJAVA variable and what is the use of it and how did
end up in system variables.?
Why to change to 1.6 is not there any backword compatibility.?

Answer 1

Here are some of the guidelines when setting up environment variables for java.
JAVA_HOME should point to the java installation path
PATH should include %JAVA_HOME%/bin
I usually setup different java version as different environment variables and switch them up by changing JAVA_HOME
eg.
JAVA_6=c:\jdk6
JAVA_7=c:\jdk7
JAVA_HOME=%JAVA_6% (or JAVA_HOME=%JAVA_7%)
PATH=%JAVA_HOME%\bin;%PATH%
Also, some other entries in the path may point to a different version of Java so I usually add the %JAVA_HOME%\bin entry first.
Also, when making changes to environment variables, you should close and open a new cmd instance to have the environment variables reloaded.
Lastly, having spaces on your paths can have an effect. You might need to add quotes to them or use the old variable convention of DOS. eg. PROGRA~1 instead of Program Files.
To answer your issues:
your PATH environment variable points to different paths for java and javac. Just set the %JAVA_HOME%\bin as the first entry to the PATH variable and it should pick up the right version of JAVA you want.
As for QTJAVA, I haven't encountered it yet so I can't give a concrete answer. As for how it got there, well, some Software/Programs modify the environment variables when you install them so they got there that way.
It should be backward compatible or tries to be. I haven't seen the code nor have really tried JDK 7 long enough but according to the issue on the link you gave (https://tickets.openmrs.org/browse/TRUNK-3142), it looks like a JDK 7 issue.
Hope this helps.

Answer 2

The loader java.exe is normally placed in the Windows\System32 directory, though there are also copies in the JDK. The compiler javac.exe is always in the JDK. This is the root of what you're seeing.
There's an excellent chance that a copy java.exe appears in a folder that's earlier in your path than Windows\System32\java.exe.
It's also possible installation of v1.6 is failing to overwrite the old java.exe because you don't have enough permission to write in Windows\System32, though an error should be provided in this case.
The best way I know to figure out what is happening is to install a which.exe command, which you can find in Cygwin or MinGW. When you say which java.exe, you'll see the path to the java.exe v1.7 that is causing the problem. Adjust your path, delete, or rename the file.