Comment by twic

1 day ago

    $ echo 'class Hello { public static void main(String[] args) { System.out.println("Hello, world"); } }' >Hello.java
    $ javac Hello.java 
    $ java --version
    openjdk 21.0.7 2025-04-15 LTS
    OpenJDK Runtime Environment Zulu21.42+21-CRaC-CA (build 21.0.7+6-LTS)
    OpenJDK 64-Bit Server VM Zulu21.42+21-CRaC-CA (build 21.0.7+6-LTS, mixed mode, sharing)
    $ time java -cp . Hello
    Hello, world
    
    real 0m0.026s
    user 0m0.010s
    sys  0m0.018s

For some reason this machine is still on 21. Anyway, the overhead of the JVM itself is pretty low.

For small tools it's fine:

    $ echo 'class Dig { public static void main(String[] args) throws Exception { var addr = java.net.InetAddress.getByName(args[0]); System.out.println(addr); } }' >Dig.java
    $ javac Dig.java
    $ time java -cp . Dig news.ycombinator.com
    news.ycombinator.com/209.216.230.207
    
    real 0m0.047s
    user 0m0.026s
    sys  0m0.021s

    $ time host news.ycombinator.com
    news.ycombinator.com has address 209.216.230.207
    news.ycombinator.com has IPv6 address 2606:7100:1:67::26
    
    real 0m0.052s
    user 0m0.005s
    sys  0m0.004s

Java startup gets slow when you use some massive framework like Spring. If you need a program to start fast, don't use one!

3 comments

twic

tofflos 19 hours ago

You can also choose to run directly from source with a small penalty at startup and verbosity has decreased for simple use cases. It's still not great for scripting but the situation has been steadily improving over the last couple of years.

  void main() {
    IO.println("Hello world");
  }

  $ time java Hello.java
  Hello world

  real    0m0.662s
  user    0m2.421s
  sys     0m0.198s

  void main(String[] args) throws Exception {
    IO.println(InetAddress.getByName(args[0]));
  }

  $ time java Dig.java news.ycombinator.com
  news.ycombinator.com/209.216.230.207

  real    0m1.154s
  user    0m2.379s
  sys     0m0.222s

elFarto 7 hours ago

You can also use a shebang and mark the file as executable on Linux:

  #!/usr/bin/java --source 25
  
  void main(String[] args) throws Exception {
    var addr = java.net.InetAddress.getByName(args[0]);
    System.out.println(addr);
  }

kodebach 16 hours ago

Spring is slow because it scans the classpath, uses reflection and generates proxy classes all at startup. You can avoid some of that using the Spring AOT feature in newer versions.

Or you can just use a different framework like Micronaut which does none of that and uses an annotation processor at compile time instead. (Similar to Spring AOT, with fewer limitations)