Java – Ant is using the JRE instead of JDK, although the environment variables are set

1. In Eclipse click Run → External Tools → External Tools Configurations.
2. Click the JRE tab.
3. Click the Installed JREs... button.
4. Click the Add button.
   (Select Standard VM, where applicable.)
5. Click the Directory button.
6. Browse to your JDK version (not JRE) of your installed Java
   (e.g. C:\Program Files\Java\jdk1.7.0_04).
7. Click Finish and OK.
8. Select the JDK at Separate JRE and click Close.
9. Re-run your Ant script — have fun!

This worked in a particular scenario I encountered.

The JRE is the Java Runtime Environment. It is a package of everything necessary to run a compiled Java program, including the Java Virtual Machine (JVM), the Java Class Library, the java command, and other infrastructure. However, it cannot be used to create new programs.

The JDK is the Java Development Kit, the full-featured SDK for Java. It has everything the JRE has, but also the compiler (javac) and tools (like javadoc and jdb). It is capable of creating and compiling programs.

Usually, if you only care about running Java programs on computer you will only install the JRE. It's all you need. On the other hand, if you are planning to do some Java programming, you need to install the JDK instead.

Sometimes, even if you are not planning to do any Java development on a computer, you still need the JDK installed. For example, if you are deploying a web application with JSP, you are technically just running Java programs inside the application server. Why would you need the JDK then? Because the application server will convert JSP into Java servlets and needs to use the JDK to compile the servlets. I am sure that there are more examples.