How do I set the default Java installation/runtime (Windows)?

This is a bit of a pain on Windows. Here's what I do.

Install latest Sun JDK, e.g. 6u11, in path like c:\install\jdk\sun\6u11, then let the installer install public JRE in the default place (c:\program files\blah). This will setup your default JRE for the majority of things.

Install older JDKs as necessary, like 5u18 in c:\install\jdk\sun\5u18, but don't install the public JREs.

When in development, I have a little batch file that I use to setup a command prompt for each JDK version. Essentially just set JAVA_HOME=c:\jdk\sun\JDK_DESIRED and then set PATH=%JAVA_HOME%\bin;%PATH%. This will put the desired JDK first in the path and any secondary tools like Ant or Maven can use the JAVA_HOME variable.

The path is important because most public JRE installs put a linked executable at c:\WINDOWS\System32\java.exe, which usually overrides most other settings.

I just had that problem (Java 1.8 vs. Java 9 on Windows 7) and my findings are:

short version

default seems to be (because of Path entry)

c:\ProgramData\Oracle\Java\javapath\java -version

select the version you want (test, use tab completing in cmd, not sure what those numbers represent), I had 2 options, see longer version for details

c:\ProgramData\Oracle\Java\javapath_target_[tab]

remove junction/link and link to your version (the one ending with 181743567 in my case for Java 8)

rmdir javapath
mklink /D javapath javapath_target_181743567

longer version:

Reinstall Java 1.8 after Java 9 didn't work. The sequence of installations was jdk1.8.0_74, jdk-9.0.4 and attempt to make Java 8 default with jdk1.8.0_162...

After jdk1.8.0_162 installation I still have

java -version
java version "9.0.4"
Java(TM) SE Runtime Environment (build 9.0.4+11)
Java HotSpot(TM) 64-Bit Server VM (build 9.0.4+11, mixed mode)

What I see in path is

Path=...;C:\ProgramData\Oracle\Java\javapath;...

So I checked what is that and I found it is a junction (link)

c:\ProgramData\Oracle\Java>dir
 Volume in drive C is OSDisk
 Volume Serial Number is DA2F-C2CC

 Directory of c:\ProgramData\Oracle\Java

2018-02-07  17:06    <DIR>          .
2018-02-07  17:06    <DIR>          ..
2018-02-08  17:08    <DIR>          .oracle_jre_usage
2017-08-22  11:04    <DIR>          installcache
2018-02-08  17:08    <DIR>          installcache_x64
2018-02-07  17:06    <JUNCTION>     javapath [C:\ProgramData\Oracle\Java\javapath_target_185258831]
2018-02-07  17:06    <DIR>          javapath_target_181743567
2018-02-07  17:06    <DIR>          javapath_target_185258831

Those hashes doesn't ring a bell, but when I checked

c:\ProgramData\Oracle\Java\javapath_target_181743567>.\java -version
java version "1.8.0_162"
Java(TM) SE Runtime Environment (build 1.8.0_162-b12)
Java HotSpot(TM) 64-Bit Server VM (build 25.162-b12, mixed mode)

c:\ProgramData\Oracle\Java\javapath_target_185258831>.\java -version
java version "9.0.4"
Java(TM) SE Runtime Environment (build 9.0.4+11)
Java HotSpot(TM) 64-Bit Server VM (build 9.0.4+11, mixed mode)

so to make Java 8 default again I had to delete the link as described here

rmdir javapath

and recreate with Java I wanted

mklink /D javapath javapath_target_181743567

tested:

c:\ProgramData\Oracle\Java>java -version
java version "1.8.0_162"
Java(TM) SE Runtime Environment (build 1.8.0_162-b12)
Java HotSpot(TM) 64-Bit Server VM (build 25.162-b12, mixed mode)

** update (Java 10) **

With Java 10 it is similar, only javapath is in c:\Program Files (x86)\Common Files\Oracle\Java\ which is strange as I installed 64-bit IMHO

.\java -version
java version "10.0.2" 2018-07-17
Java(TM) SE Runtime Environment 18.3 (build 10.0.2+13)
Java HotSpot(TM) 64-Bit Server VM 18.3 (build 10.0.2+13, mixed mode)

I have several JDK (1.4, 1.5, 1.6) installed in C:\Java with their JREs. Then I let Sun update the public JRE in C:\Program Files\Java.
Lately there is an improvement, installing in jre6. Previously, there was a different folder per new version (1.5.0_4, 1.5.0_5, etc.), which was taking lot of space

I simply install all the versions of JDK I need and the latest installed becomes default, so I just reinstall the one I want to be default if necessary.

an alterable way to run an .jar app is create an .bat cmd for it. for example, you have jre10 and jre8 installed on your pc,and jre10 is your default jre. but your jar is specified to work with jre8,following cmd will work:

"C:\Program Files\Java\jre1.8.0_181\bin\java.exe" -jar JabRef-4.3.1.jar

Need to remove C:\Program Files (x86)\Common Files\Oracle\Java\javapath from environment and replace by JAVA_HOME which is works fine for me

Stacked by this issue and have resolved it in 2020, in Windows 10. I'm using Java 8 RE and 14.1 JDK and it worked well until Eclipse upgrade to version 2020-09. After that I can't run Eclipse because it needed to use Java 11 or newer and it found only 8 version. It was because of order of environment variables of "Path":

I suppose C:\Program Files (x86)\Common Files\Oracle\Java\javapath is path to link to installed JRE exe files (in my case Java 8) and the issue was resolved by move down this link after %JAVA_HOME%, what leads to Java 14.1/bin folder.

It seems that order of environment variables affects order of searched folders while executable file is requested. Thanks for your comment or better explanation.