It depends on what you are using it for, “fast” is relative. Java can be fast for applications where performance and scalability are not a primary features. If performance and scalability are core objectives, even performance-engineered Java isn’t really competitive with a systems language. You can bend Java to make it perform better than most people believe, especially today, but the gap is still pretty large in practice.
I wrote performance-engineered Java for years. Even getting it to within 2x worse than performance-engineered C++ took heroic efforts and ugly Java code.
I have yet to run a Java program that I haven't had to later kill due to RAM exhaustion. I don't know why. Yeah an Integer takes 160 bits and that's without the JVM overhead, but still. Somehow it feels like Java uses even more memory than Python. Logically you'd point the finger at whoever wrote the software rather than the language/runtime itself, but somehow it's always Java. It's like the Prius of languages.
Ok, just glanced at my corp workstation and some Java build analysis server is using 25GB RES, 50GB VIRT when I have no builds going. The hell is it doing.
GC usually only runs when the process wants to allocate an object but there's no space left on the heap. It's entirely possible that it did a bunch of work previously which created a bunch of garbage now waiting to be cleaned up. See the G1PeriodicGCInterval flag to enable idle collections (assuming G1).
Java is also fairly greedy with memory by default. It likes to grow the heap and then hold onto that memory unless 70% of the heap is free after a collection. The ratios used to grow and shrink the heap can be tuned with MinHeapFreeRatio and MaxHeapFreeRatio.
Before I even went on my rant, I was guessing there's just some confusing default like this but there's also some historical reason why it's like that.
> Ok, just glanced at my corp workstation and some Java build analysis server is using 25GB RES, 50GB VIRT when I have no builds going. The hell is it doing.
Allocating a heap of the size it was configured to use, probably.
Different processor and JVM. My understanding is that early versions of the Java card runtime didn't even support garbage collection. It was a very different environment to program, even if the language was "Java".
Java is fast for long-running server processes. Even HFT shops competing for milliseconds use it. But yeah every user-facing interactive Java application manages to feel clunky.
Learned from an NYC exchange 10 years ago that Java can be written so as to not use garbage collection. Fast and no pause for GC.
1. Resource and reuse objects that otherwise are garbage collected. Use `new` sparingly.
2. Avoid Java idioms that create garbage, e.g. for (String s : strings) {...},
substitute with (int i = 0, strings_len = strings.length(), i < strings_len) {
String s = strings[i]; ...}
I wrote performance-engineered Java for years. Even getting it to within 2x worse than performance-engineered C++ took heroic efforts and ugly Java code.