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JRE 8 apps in eclipse workspace is possible by having the jre 8 jdk. It is true that in order to use eclipse 2020-09 you need java 11+ jdk/jre, but making projects that support jre 8 is still possible.Even I still use 2020-06 and before. Here is a method you could use to get JRE 8 implemented back in your project. Although you can change the defaults, this one will help you change the environment back to java 8 for 1 project.
Right click the library named somthing like "JRE system library[JavaSE-11]". Press properties. This is what you will see JRE System Library chooser image
Choose "Alternate JRE" then click "Installed JREs"Step Image
Press "Add" then you will see 3 options. Choose standard VM. Find your java 8 jdk directory. Usually it is something like C:\Program Files\Java\jdk1.8.0_251.
You can configure the default arguments for your jvm runtime. Then simply hit ok,select the jdk-8 library and close it. Then choose alternate jre again and in the drop-down choose jdk-1.8.0 or some name like that. This will only work if you also have java 8 installed alongside java 11 that is configured properly JDK
You can use Java 11 to start the Eclipse 2020-09 and configure a project to compile with Java 8. The problem is if you want a Eclipse plugin that uses CORBA for example. This module was removed from Java 11 earlier. In this case, I recommend to use Eclipse 2020-06.
For Kepler SR2 (4.3.2) a feature patch needs to be installed in order to get JAVA 8 support. Follow these steps:
Eclipse - Help (MENU) > Install New Software...
enter the following URL into the 'Work with' field:
http://download.eclipse.org/eclipse/updates/4.3-P-builds/press 'Enter'
- select category 'Eclipse Java 8 Support (for Kepler SR2)'
- click 'Next'
- click 'Next'
- accept the license
- click 'Finish'
- restart Eclipse when asked
source: link
@Elisabeth
In order to have the desired JRE/ JDK on BuildPath, follow these steps.
- Right click on Project from Package Explorer
- Select BuildPath and then select Configure Builpath
- Select Libraries Tab from the popped up Properties window
- Select the current JRE System Library
- Click Remove button
- Click on Add Library button
- Select JRE System Library
- There you will be able to add your desired JRE/ JDK version using 3 different methods.
If you don't find your desired version of JRE/ JDK there in those 3 options, then you will have to install that first. The following link helps you in detail with screen shots to do the same. Hope it helps.
source: link
As written in https://wiki.eclipse.org/JDT_Core/Java8 :
"Starting with I20140318-0830 all our Luna (4.4) builds contain the Eclipse support for Java 8. For Kepler SR2 (4.3.2) a feature patch is available. "

So right now you need to download a non stable eclipse release.
Go to : http://download.eclipse.org/eclipse/downloads/
There download a release older than or equals to I20140318-0830

Then you choose your version according to your OS :

click on the (http) link, the download will start :

Note that according to eclipse website : Java 8 has not yet landed in our standard download packages. But you can add it to your existing Eclipse Kepler SR2 package. It will be included in the Luna packages starting with M7 on May 9/2014.
so after May 9/2014 getting eclipse working with java 8 will be simpler.