在IntelliJ IDEA中可以将文件夹标记为各种root文件夹，大致如下图所示：
这些root文件夹有何区别呢？
1. Source roots (or source folders)
By assigning a folder to this category, you tell IntelliJ IDEA that this folder and its subfolders contain source code that should be compiled as part of the build process.
通过将文件夹加入这种类别，来告诉IntelliJ IDEA，这个文件夹和它的子文件夹中包含源码，在构建工程时，需要作为一部分被编译进去。
2. Test source roots (or test source folders; shown as rootTest)
These roots are similar to source roots but are for code intended for testing (e.g. for unit tests). Test source folders let you keep the code related to testing separate from the production code.
Compilation results for sources and test sources, normally, are placed into different folders.
这个类型的文件夹也用于存放源码，不过是测试的源码（比如单元测试）。test source 文件夹可以帮助你将测试代码和产品代码分离开。
3. Resource roots
are for resource files used in your application (images, various configuration XML and properties files, etc.).
During the build process, all the contents of the resource folders are copied to the output folder as is.
Similarly to sources, you can specify that your resources are generated. You can also specify which folder within the output folder your resources should be copied to.
该类文件夹用于存放你的应用中需要用到的资源文件（如：图片、xml或者properties配置文件等）。
在构建过程中，resources文件夹中的内容均会按照原文件的样子被复制到输出文件夹。和sources文件夹一样，你可以定制你的resources文件夹的结构。你也可以指定你的resources文件夹中的文件被拷贝到输出文件的哪个文件夹中。
ps：默认情况下，工程编译后，resources中的文件和文件夹会被放置在和源码编译后相同的文件夹中，所以如果在源码中以相对路径读取resources中的配置文件时，可认为src中的java文件夹和resources为同一个根目录。
4. Test resource roots
(or test resource folders; shown as rootTestResourceIJ; available only in Java modules) are for resource files associated with your test sources. In all other respects, these folders are similar to resource folders.
（只有在java模式中可以使用）用于存放测试源码中关联的资源文件。除此之外，和resource文件夹没有区别。
**5. Excluded roots (shown as rootExcluded) are ones that IntelliJ IDEA
“almost ignores”.**
Very limited coding assistance is provided for files in excluded folders. Classes contained in excluded folders don’t appear in code completion suggestion lists, references to such classes are shown in the editor as unresolved. When searching, IntelliJ IDEA doesn’t look in excluded folders, etc.
另外补充一下IntelliJ IDEA中的各种文件夹图标：
