What is premain() and how does it get called?

The premain is a mechanism associated with the java.lang.instrument package, used for loading "Agents" which make byte-code changes in Java programs.

The mechanism is explained in the java.lang.instrument documentation.

The gist of it is that the "agent" is deployed in a jar, and that jar has a special entry in its manifest, that tells the instrumentation package where to look for the premain method. The source you quoted is supposed to be a simple agent.

Minimal runnable example

GitHub upstream: https://github.com/cirosantilli/java-cheat/tree/d73d2786cad458973a6b46bc98b9faabae65f3e1/instrument

META-INF/MANIFEST.MF

Premain-Class: Sizeof

Sizeof.java

import java.lang.instrument.Instrumentation;

final public class Sizeof {
    private static Instrumentation instrumentation;

    public static void premain(String args, Instrumentation inst) {
        instrumentation = inst;
    }

    public static long sizeof(Object o) {
        return instrumentation.getObjectSize(o);
    }
}

Main.java

final public class Main {
    public static void main(String [] args) {
        System.out.println("Object");
        System.out.println(Sizeof.sizeof(new Object()));

        System.out.println("/\"\"");
        System.out.println(Sizeof.sizeof(""));

        System.out.println("/\"abc\"");
        System.out.println(Sizeof.sizeof("abc"));

        System.out.println("int[0]");
        System.out.println(Sizeof.sizeof(new int[0]));

        System.out.println("int[10]");
        System.out.println(Sizeof.sizeof(new int[10]));

        class OneInt {
            public int i;
        }
        System.out.println("OneInt");
        System.out.println(Sizeof.sizeof(new OneInt()));

        class TwoInts {
            public int i;
            public int j;
        }
        System.out.println("TwoInts");
        System.out.println(Sizeof.sizeof(new TwoInts()));

        class IntArray0 {
            int[] i = new int[0];
        }
        System.out.println("IntArray0");
        System.out.println(Sizeof.sizeof(new IntArray0()));

        class IntArray10 {
            int[] i = new int[10];
        }
        System.out.println("IntArray10");
        System.out.println(Sizeof.sizeof(new IntArray10()));
    }
}

Makefile

all:
    javac *.java
    jar -cfm Sizeof.jar META-INF/MANIFEST.MF Sizeof.class
    java -ea -javaagent:Sizeof.jar Main

Sample output:

Object
16
/""
24
/"abc"
24
int[0]
16
int[10]
56
OneInt
16
TwoInts
24
IntArray0
16
IntArray10
16

Tested in Ubuntu 16.10, Java HotSpot 1.8.0_92.