Fastest way to compile small Java files

Question

I'm looking to programmatically compile (not through IDE) a few small classes such as:

public class Sum {
  public int sum(int[] nums) {
    int total = 0;
    for (int i = 0; i < nums.length; i++) {
      total += nums[i];
    }
    return total;
  }
}

Running javac Sum.java takes 429ms on macOS (2.3 GHz Intel Core i5) but in a Kubernetes container (t3.xlarge) with 3VCPU, and 6G of RAM, it takes 640ms. What causes this difference?

I tried different java versions and tried using javax.tools.JavaCompiler but with few files, it takes up to 2 seconds to compile 3 small files like these.

What's the best configuration of hardware/software to make a compilation of these small files fastest?

Answer 1

The number I just got empirically is 5ms/file...

If you have a lot of Java files to compile, then maybe you care about this. But if that's the case, then the timing numbers you're talking about are not realistic. I wrote this program to compile your test program 1000 times (I created 1000 java files defining classes Sum0 through Sum999.

import javax.tools.JavaCompiler;
import javax.tools.ToolProvider;

public class T {
    public static void main(String[] args) {
        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        int N = 1000;
        String[] files = new String[N];
        for (int i = 0 ; i < N ; i++) {
            files[i] = String.format("stage/Sum%d.java", i);
        }
        int result = compiler.run(null, null, null, files);
        System.out.println("Compile result code = " + result);
    }
}

I ran it, and here are the timing figures for that run:

> ls stage/*.class | wc -l
ls: stage/*.class: No such file or directory
       0

> time java T
Compile result code = 0

real    0m2.511s
user    0m4.737s
sys 0m0.549s

> ls stage/*.class | wc -l
    1000

So the total runtime was 4.737 seconds / 1000 = .005 seconds = 5 ms /file. All I can think of is that you're compiling files one at a time, in which case all the time is some kind of startup/teardown cost, and who cares about that.

The bottom line is that you can probably compile however many Java files you want in just a few seconds, so stop worrying about this.

This was run on a couple of generations back MacBook Pro.