Strange eclipse problems in Linux

Install JDK in windows and configure environment variables. After eclipce is downloaded and decompressed, eclipse will be able to create a Java project normally, but it will be free of charge in Linux, although environment variables are configured, eclipse does not have the default JDK. What's more strange is that it does not recognize the JDK directory when you configure the JDK by yourself, for example:

Target is not a JDK root. system library was not found. Clearly, the JDK directory is selected.

Solution: remove the JRE directory under the/usr/studybin/jdk1.7.0 _ 25 directory and move it to the directory where studybin is located. The problem is solved.

But why? Please kindly advise.