Roughly speaking:
getClass().getResource("/")
~= Thread.currentThread().getContextClassLoader().getResource(".")
Suppose your project structure is like the following:
+-- src
¦ +-- main
¦ +-- test
¦ +-- test
¦ +-- java
¦ ¦ +-- com
¦ ¦ +-- github
¦ ¦ +-- xyz
¦ ¦ +-- proj
¦ ¦ +-- MainTest.java
¦ ¦ +-- TestBase.java
¦ +-- resources
¦ +-- abcd.txt
+-- target
+-- test-classes
+-- com
+-- abcd.txt
// in MainClass.java
this.getClass.getResource("/") -> "~/proj_dir/target/test-classes/"
this.getClass.getResource(".") -> "~/proj_dir/target/test-classes/com/github/xyz/proj/"
Thread.currentThread().getContextClassLoader().getResources(".") -> "~/proj_dir/target/test-classes/"
Thread.currentThread().getContextClassLoader().getResources("/") -> null