Question

How can I calculate the maximum “maximum heap size” avalable for java application

• in Windows using cmd commands?
• in Linux using sh commands?

I want to set -Xmx value as large as possible.

I want to be sure that java will not complain that -Xmx value is too large.

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From a linked answer to another question I learned the following values for 32-bit Java:

OS: Windows XP SP2, JVM: Sun 1.6.0_02, Max heap size: 1470 MB
OS: Windows XP SP2, JVM: IBM 1.5, Max heap size: 1810 MB
OS: Windows Server 2003 SE, JVM: IBM 1.5, Max heap size: 1850 MB
OS: Linux 2.6, JVM: IBM 1.5, Max heap size: 2750 MB 

How do I automatically detect which OS and which Java is installed?
OK, I can default to the minimum value – 1470 MB.
But how can I be sure that in newer versions of Java this value will not shrink?

The question also remains open for 64-bit Java.

Answer 1

Try to run java with 2 gigabytes or memory, decrease the amount, repeat until java runs.

Linux:

#!/bin/sh
let mem=2000
while [ 1 ]
do
    java "-Xmx"$mem"m" -version > /dev/null 2>&1
    if [ $? -eq 0 ]
    then
        break
    fi
    let mem=$mem-100
    if [ $mem -lt 100 ]
    then
        #// something went wrong with our test
        let mem=700
        break
    fi
done
echo $mem 

Windows:

@echo off
setlocal enabledelayedexpansion
set mem=2000
:decmem
set /a mem=mem-100
if !mem! == 0 (
    rem // something went wrong with our test
    set mem=700
    goto start
)
java -Xmx!mem!m -version > nul 2> nul
if errorlevel 1 goto decmem
:start
echo !mem!