How do I measure JVM startup time?

Question

I have an application written in java and I want to know how much time it takes before reaching static void int main(String args) and what it is doing at that stage how can I achieve that?

I am aware that microsoft have a tool called MPGO (Manager profile guided optimisation) is there an equivalent for Java?

Answer 1

A simple way to measure start-up time from inside Java application:

import java.lang.management.ManagementFactory;

public class Test {

    public static void main(String[] args) {
        long currentTime = System.currentTimeMillis();
        long vmStartTime = ManagementFactory.getRuntimeMXBean().getStartTime();
        System.out.println(currentTime - vmStartTime);
    }
}

You may use JVMTI agent for tracing VM events like class loading, garbage collection, method compilation etc. Here is a simple agent I've made - vmtrace (and the compiled dll for Windows).

Run java -agentpath:path\to\vmtrace.dll Main, and the event trace will be printed to stderr:

[0.00000] VMTrace started
[0.00182] Dynamic code generated: flush_icache_stub
[0.00187] Dynamic code generated: get_cpu_info_stub
[0.00519] Dynamic code generated: getCPUIDNameInfo_stub
[0.00524] Dynamic code generated: forward exception
[0.00526] Dynamic code generated: call_stub
...
[0.01182] Loading class: java/lang/Object
[0.01198] Loading class: java/lang/String
[0.01206] Loading class: java/io/Serializable
...
[0.05620] VM initialized
[0.05664] Class prepared: java/lang/invoke/MethodHandle
[0.05672] Loading class: java/lang/invoke/MethodHandleImpl
[0.05732] Class prepared: java/lang/invoke/MethodHandleImpl
[0.05738] Loading class: java/lang/invoke/MethodHandleImpl$1
[0.05743] Class prepared: java/lang/invoke/MethodHandleImpl$1
[0.05755] Loading class: java/lang/invoke/MethodHandleImpl$2
[0.05759] Loading class: java/util/function/Function
[0.05768] Class prepared: java/util/function/Function
...