Endless abstraction, loading giant things into RAM, looping over every item in huge arrays. Blocking server threads for minutes at a time. In most languages you would be punished severely with awful performance or OOM. Java chugs along until things get horrifically bad.
> almost completely disregard how that code works on the JVM
As an occasional clueless programmer, I've had occasion to touch systems written in Java. Could you point me at some resources?
Reality is complex, and for non-trivial programs complexity is going to be somewhere.
This weaknes stems from Java being OO. On larger projects you start to have abstractions which tend to be insucfficiently documented or the documentation not being updated with changes. Throw in some helper classes here and there, from a certain point of complexity of the project from an outside perspective the whole thing looks like being obfuscated.
"Running Java" requires detailed system engineering / ops expertise specific to how the JVM works and can be tuned, and all too often, how specific frameworks internally pile their abstractions on top of each other. Problem is, a modern "Java app" is a pile of large frameworks and libraries.
As a system engineer, I HATE Java applications written by clueless programmers. They are difficult to handle and I often have to dig in and fix bad usage of framework features that somehow passed code review by equally clueless "senior" developers. I then get to spend a day to find out why this or that connection pool in there managed by this or that library does not handle its connections correctly, just to find the magic framework level variable that tunes that specific screw. Stuff of nightmares.
Every Java project is also __severely__ underdocumented. That's because even if you write documentation for YOUR code, the framework has books of documentation on ITS specifics, and includes high-level libraries which THEMSELVES have heaps of specific documentation. Climbing the tree until one arrives at actual standard library calls is like climbing into clouds and hoping to see the sun some day.