ADDITIONAL SYSTEM INFORMATION :
Dell
Intel Core i7-6820HQ @2.7Ghz
Microsoft Windows [Version 6.1.7601] / Windows 7 enterprise SP1
Java1.8u181 i586
Java1.8u181 x64

A DESCRIPTION OF THE PROBLEM :
Since JDK-8182548, the "arch" can specify 'in order' which jvm archtecture to select.
Basically i require an x64 VM to run my app using webstart because of memory usage.
With both 1.8u181 i586 and x64 installed (as per JDK-8182548) on windows 7.
My JNLP settings were arch="x86_64" and on a separate testrun arch="amd64"
It never runs 64bit.
Whith arch="x86_64" the trace file states:

Log started: Fri, 24 Aug 2018 16:13:28 +0200
Java Web Start 11.181.2.13
Using JRE version
 1.8.0_181-b13 Java HotSpot(TM) Client VM
java.lang.NullPointerException
at com.sun.jnlp.SingleInstanceServiceImpl.getIDString(Unknown Source)
at com.sun.javaws.Launcher.launch(Unknown Source)
at com.sun.javaws.Main.launchApp(Unknown Source)
at com.sun.javaws.Main.continueInSecureThread(Unknown Source)
at com.sun.javaws.Main.access$000(Unknown Source)
at com.sun.javaws.Main$1.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)

(with arch="x86" the or omitting the arch attribute, the JVM does start, but in the 32bit VM)

REGRESSION : Last worked in version 8u181

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
install both 1.8u181 64 and 32 bit
run jnlp file with one(or multiple) resource sets, where 'arch' is either "x86_64" or "amd64"
the popup [Java...] appears and dissapears after long.
The trace file shows it encountered a nullpointer.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The application running on a JVM x64
ACTUAL -
The launcher terminates with a nullpointer.

---------- BEGIN SOURCE ----------
<?xml version="1.0" encoding="utf-8"?>

<jnlp spec="1.0+" codebase="https://localhost:8443/">
  <information>
    <title>a</title>
    <vendor>b</vendor>
    <homepage href="https://localhost:8443/index.html"/>
    <description>c</description>
    <offline-allowed/>
  </information>
  
  <security>
    <all-permissions/>
  </security>
 
  <resources arch="x86_64">
    <property name="jnlp.versionEnabled" value="true"/>
    <j2se version="1.8*" initial-heap-size="128M" max-heap-size="512M" java-vm-args="-Dsun.java2d.d3d=false"/>
    <property name="java.util.Arrays.useLegacyMergeSort" value="true"/>
    <jar href="https://localhost:8443/lib/a.jar" version="10.03.0000" main="true"/>
    <jar href="https://localhost:8443/lib/b.jar" version="10.03.0000"/>
    <jar href="https://localhost:8443/lib/c.jar" version="10.03.0000"/>
    <jar href="https://localhost:8443/lib/d.jar" version="10.03.0000"/>
  </resources>
 
  <application-desc main-class="x.y.z">
    <argument>-host</argument> <argument>localhost</argument>
    <argument>-port</argument> <argument>8443</argument>
    <argument>-secure</argument> <argument>true</argument>
  </application-desc>
</jnlp>
---------- END SOURCE ----------

FREQUENCY : always