I am quite confused. Currently I am testing my spring application using
org.springframework.boot
Great answer by Ivo, which really should be getting more votes, but here's some added information. If you check Maven for information on the dependencies, Java Hamcrest 2.0.0.0 and Hamcrest JUnit 2.0.0.0, there are some usage links. At the time I write this, it's 74 and 68 respectively, and no major projects from a brief browsing.
The Hamcrest project has just released 2.1, and they reworked the package structure after thorough consideration. Make sure you follow the upgrade procedure. The artifact java-hamcrest 2.0.0.0 is considered a failed attempt now.
EDIT: After several years the answer is to use the latest Hamcrest 2 version (2.2 from 17th October 2019). For additional details also refer to @dschulten's answer.
Following is my original answer which I leave as context to understand the problem and confusion around Hamcrest versions 1.3 and 2.0.0.0 back in the day.
Based on Hamcrest Github
and JUnit Lambda (Junit 5)
My take on it
=> In this situation, I expect it is prudent for the hamcrest guys to wait for / coordinate with the JUnit 5 project before moving forward substantially.
As @heenenee mentioned, the hamcrest guys became busy with other stuff and so not much is happening with the project at this point.
To answer
Can somebody explain me the connection between them? Because it seems that hamcrest-junit 2.0 has everything build in from hamcrest 1.3
... but I do not feel really comfortable. Because I can just find people writing about 1.3 and can not really find the use of hamcrest-junit 2.0.
At the moment, other than for your case there is not much incentive to move to hamcrest-junit 2.0. Once Junit 5 releases I expect that there will be more incentive to move forward again.