Question

I’m trying to understand how Java objects behave in a Domino environment and am wondering what happens when you use singletons (a singleton is a Java object that’s constructed in such a way that only one instance will ever exist within the JVM).

Say I have a singleton in database 1, use getInstance() on it. The object is created in the JVM and from there on each time I use the getInstance() method the same object is returned.

Now I copy that class over to database 2 and use getInstance() in that database too. Which object is being returned? The one that I originally created in database 1 or the new one from database 2?

Answer 1

I’ve done a test and as jjtbsomhorst suggests, there indeed will be two separate singletons.

XPage SingletonTest.xsp:

<?xml version="1.0" encoding="UTF-8"?>
<xp:view xmlns:xp="http://www.ibm.com/xsp/core">
    <xp:text escape="true" id="computedField1" value="#{javascript:nl.tjit.SingletonTest.INSTANCE.getWhoami()}"></xp:text>
</xp:view>

Java class:

package nl.tjit;
    public enum SingletonTest {
    INSTANCE;

    private final String whoami = "I am your father";

    public String getWhoami() {
        return whoami;
    }
}

The XPage and Java class copied to another database with a slightly modified whoami string produce a different output. So the singletons are not shared between databases.