Juggling Multiple Versions of Java on OS X Mountain Lion

prior to mountain lion, java was bundled with osx. it seems that during the upgrade, the java 6 version i had on my machine was removed, apparently due to a security issue with the java runtime. this means you're forced to install the latest version, which fixed this security problem.

so i went to /applications/utilities/ open a terminal and executed the following command:
java -version ==> "no java runtime present…" a window prompted asking to install java.click “install” and get the latest version.i installed it but right after i downloaded and installed the jdk se 7 from oracle.

after installation, open the java preferences (launchapad/others ) and you will see:

now i knew i had two versions of java, but which one am i using?

$ java -version
java version "1.6.0_35"
java(tm) se runtime environment (build 1.6.0_35-b10-428-11m3811)
java hotspot(tm) 64-bit server vm (build 20.10-b01-428, mixed mode)

so what if i want to use jdk se 7 from oracle?

then i just had to drag java se 7 in the java preferences window to the first position in the list.

this time:

$ java -version
java version "1.7.0_05"
java(tm) se runtime environment (build 1.7.0_05-b06)
java hotspot(tm) 64-bit server vm (build 23.1-b03, mixed mode)

i said to myself, "let’s find out more out how java is installed on os x." so i dug for more.

there are some very useful commands: whereis and which and ls -l .

whereis java ==> /usr/bin/java
ls -l /usr/bin/java ==> /system/library/frameworks/javavm.framework/versions/current/commands/java

when i saw this i was a little bit curious so i went to list the versions directory:

cd /system/library/frameworks/javavm.framework/versions
ls ==>
1.4 1.5 1.6 a currentjdk
1.4.2 1.5.0 1.6.0 current

now why do i have this old versions of java on my machine? i asked on ask different:
http://apple.stackexchange.com/questions/57986/multiple-java-versions-support-on-os-x-and-java-home-location

$ sw_vers
productname:	mac os x
productversion:	10.8.1
buildversion:	12b19
$ ls -l /system/library/frameworks/javavm.framework/versions
total 64
lrwxr-xr-x  1 root  wheel   10 sep 16 15:55 1.4 -> currentjdk
lrwxr-xr-x  1 root  wheel   10 sep 16 15:55 1.4.2 -> currentjdk
lrwxr-xr-x  1 root  wheel   10 sep 16 15:55 1.5 -> currentjdk
lrwxr-xr-x  1 root  wheel   10 sep 16 15:55 1.5.0 -> currentjdk
lrwxr-xr-x  1 root  wheel   10 sep 16 15:55 1.6 -> currentjdk
lrwxr-xr-x  1 root  wheel   10 sep 16 15:55 1.6.0 -> currentjdk
drwxr-xr-x  7 root  wheel  238 sep 16 16:08 a
lrwxr-xr-x  1 root  wheel    1 sep 16 15:55 current -> a
lrwxr-xr-x  1 root  wheel   59 sep 16 15:55 currentjdk -> /system/library/java/javavirtualmachines/1.6.0.jdk/contents

it seems all the old versions are links to the currentjdk version, which is the apple version, except a and current which is linked to a. i read something about this on this question . for me, a acts like a temp variable. if in java preferences you set this in the first position, java 6 from apple a will have java 6 from apple. if you put on the first position java se 7 from oracle, a will point to this version. current points to a.

/java -version
java version "1.6.0_35"
java(tm) se runtime environment (build 1.6.0_35-b10-428-11m3811)
java hotspot(tm) 64-bit server vm (build 20.10-b01-428, mixed mode)

./java -version
java version "1.7.0_05"
java(tm) se runtime environment (build 1.7.0_05-b06)
java hotspot(tm) 64-bit server vm (build 23.1-b03, mixed mode)

this means that this current directory will point to the first java version found in the java preferences. now take a look at the following:

lrwxr-xr-x  1 root  wheel   59 sep 16 15:55 currentjdk -> /system/library/java/javavirtualmachines/1.6.0.jdk/contents

this means java from apple is actually installed here: "/system/library/java/javavirtualmachines/1.6.0.jdk/"

what about java se 7? i could search the filesystem to see, but i found an easier way:

if, in java preferences, java se 7 is in the first position ==>

$ /usr/libexec/java_home
/library/java/javavirtualmachines/1.7.0.jdk/contents/home

if java se 6 (system) is in the first position ==>

$ /usr/libexec/java_home
/system/library/java/javavirtualmachines/1.6.0.jdk/contents/home

so java on mountain lion (osx) is more likely to be installed in one of these locations:

• /system/library/java/javavirtualmachines
• /library/java/javavirtualmachines
• ~/library/java/javavirtualmachines

what about /system/library/frameworks/javavm.framework/versions? it seems like this is linked to the so-called " java bridge ." here it seems to be the native part of the java on osx installation.