找不到javac编译器com.sun.tools.javac.Main不在classpath上

Eclipse is an IDE and as such, it has (at least) two Java versions: The one which it uses itself to run (JAVA_HOME) and a JVM which it uses to run your application. The two don't need to be the same.

So to fix your problem, you need to look into Eclipse's preferences, specifically Installed JREs which gives you a list of Java VMs which Eclipse will use to run Java code from projects. My guess is that there will be several entries there and the default will be a JRE instead of a JDK.

Make sure you have a JDK in the list and then go to your project. In the project, you can select which Java VM to use under Java Build Path -> Libraries.

[EDIT] Look closely at the last screenshot: You've configured Eclipse to use C:\Program Files\Java\jdk1.8.0_31\jre which means you've pointed it at the JRE inside of the JDK. Use C:\Program Files\Java\jdk1.8.0_31 instead (without the \jre at the end).

[编辑2]如果删除jre文件夹，Java将停止工作。任何JDK也包含一个JRE。JRE包含rt.jarwith String.class等。Java编译器tools.jar位于JDK中。

如果在Eclipse中重新创建JRE条目无济于事，则必须手动将其添加到类路径中。

您可以使用变量（“添加变量...”）来确保当您切换到新的/不同的JRE时Eclipse更新了路径。尝试JAVA_HOME扩展lib/tools.jar