It seems like this should be a simple task, with the options in the Preferences menu for different JREs and the ability to set different compiler and build paths per project. However, it also seems to simply not work.
For example, I have my JAVA_HOME set to a jre for Java 1.6. It's still not clear to me how Eclipse uses this, but it appears to be defaulting to this and not taking the project overrides. I have also installed Java 1.5, and added a JRE for this in eclipse in the Java->Installed JREs section.
In my project, I've set the compiler compliance level to 1.5. In the build path for the project, I've added the System Library for the Java 1.5 JRE. However, I'm getting compile errors for a class that implements PreparedStatement for not implementing abstract methods that only exist in Java 1.6 PreparedStatement. Specifically, the methods
setAsciiStream(int, InputStream, long) and
setAsciiStream(int, InputStream)
Strangely enough, it worked when we were compiling it against Java 1.4, which it was originally written for. We added the JREs for Java 1.4 and referenced that system library in the project, and set the project's compiler level to 1.4, and it works fine. But when I do the same changes to try to point to Java 1.5, it instead uses 1.6.
Any ideas why?
First off, are you setting your desired JRE or your desired JDK?
Even if your Eclipse is set up properly, there might be a wacky project-specific setting somewhere. You can open up a context menu on a given Java project in the Project Explorer and select Properties > Java Compiler to check on that.
If none of that helps, leave a comment and I'll take another look.
From the menu bar: Project -> Properties -> Java Compiler
Enable project specific settings (checked) Uncheck "use Compliance from execution environment '.... Select the desired "compiler compliance level"
That will allow you to compile "1.5" code using a "1.6" JDK.
If you want to acutally use a 1.5 JDK to produce "1.5" compliant code, then install a suitable 1.5 JDK and tell eclipse where it is installed via:
Window -> preferences -> Installed JREs
And then go back to your project
Project -> properties -> Java Build Path -> libraries
remove the 1.6 system libaries, and: add library... -> JRE System LIbrary -> Alternate JRE -> The JRE you want.
Verify that the correct JRE is on the project's build path, save everything, and enjoy!
Just to clarify, do you have JAVA_HOME set as a system variable or set in Eclipse classpath variables? I'm pretty sure (but not totally sure!) that the system variable is used by the command line compiler (and Ant), but that Eclipse modifies this accroding to the JDK used
Eclipse uses it's own internal compiler that can compile to several Java versions.
From Eclipse Help > Java development user guide > Concepts > Java Builder
The Java builder builds Java programs using its own compiler (the Eclipse Compiler for Java) that implements the Java Language Specification.
For Eclipse Mars.1 Release (4.5.1), this can target 1.3 to 1.8 inclusive.
When you configure a project:
[project-name] > Properties > Java Compiler > Compiler compliance level
This configures the Eclipse Java compiler to compile code to the specified Java version, typically 1.8 today.
Host environment variables, eg JAVA_HOME etc, are not used.
The Oracle/Sun JDK compiler is not used.
Source: Stackoverflow.com