It is pretty safe and standard. If the security of http://repo1.maven.org/maven2/ is ever compromised, there will be a great deal of resonance in Java developers. I have never heard that this site has been hacked.
However, you are not limited to the default repository. You can set up your own repository using Nexus , Artifactory and install secure artifacts for them. You can also block remote repositories using the Nexus / Artifactory setting. Although, I never had to do this. But look here , perhaps it is possible.
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