This compilation unit is not on the build path of a Java project 解决办法​

 

搜索发现，大致是因为项目文件缺失。

解决办法：找到项目根目录下的.project文件，修改，加入jdt支持，我的项目修改后如下所示↓

 

<?xml version="1.0" encoding="UTF-8"?>
<projectDescription>
    <name>V4</name>
    <comment></comment>
    <projects>
    </projects>
    <buildSpec>
        <buildCommand>
            <name>org.eclipse.jdt.core.javabuilder</name>
            <arguments>
            </arguments>
        </buildCommand>
    </buildSpec>
    <natures>
        <nature>org.eclipse.jdt.core.javanature</nature>
        <nature>org.apache.ivyde.eclipse.ivynature</nature>
    </natures>
</projectDescription>