Maybe I should have used a Lock here

build.log

build	[pool-4-thread-9] [] DEBUG c.p.f.s.x.r.ChangeAwareDynamicProperty:41 - Handling non-existent version 10.0-MULTI_THREADED
build	[pool-4-thread-7] [] DEBUG c.p.f.s.x.r.ChangeAwareDynamicProperty:41 - Handling non-existent version 10.0-MULTI_THREADED

ChangeAwareDynamicProperty.java

public class ChangeAwareDynamicProperty extends DynamicProperty {

    // current value of property. May be null, if the value has never been fetched
    private volatile String currentVersion;

    private final ConcurrentMap<String, Long> changedVersion = new ConcurrentHashMap<String, Long>();

    @Override
    public String toString() {
        final String version = super.toString();
        if (this.currentVersion == null) {
            this.currentVersion = version;
        } else if (!this.currentVersion.equals(version)) {  // change of version in the underlaying source detected
            final Long alreadyExisting = this.changedVersion.putIfAbsent(version, System.currentTimeMillis()); // another thread might have already picked up that new version
            if (alreadyExisting == null) {
                System.out.println(String.format("Handling non-existent version %s", version));
                if (!wasVersionHandledBefore(version)) {   // if this is a brand new version
                    try {
                        doCostlyOperationThatInvolvesNetwork();
                        this.currentVersion = version;
                    } catch (Exception e) {
                        throw new IllegalStateException(String.format("Unable to introduce new version %s using DynamicProperty. Do it manually.", version), e);
                    }
                }
                this.changedVersion.remove(version);
            }
        }
        return this.currentVersion;
    }

    protected boolean wasVersionHandledBefore(final String version) {
        // some check here
    }

    protected void doCostlyOperationThatInvolvesNetwork() {
        // may take a few seconds
    }
}

FixedChangeAwareDynamicProperty.java

public class ChangeAwareDynamicProperty extends DynamicProperty {

    // current value of property. May be null, if the value has never been fetched
    private volatile String currentVersion;

    private final ConcurrentMap<String, Long> changedVersion = new ConcurrentHashMap<String, Long>();

    @Override
    public String toString() {
        final String version = super.toString();
        if (this.currentVersion == null) {
            this.currentVersion = version;
        } else if (!this.currentVersion.equals(version)) {  // change of version in the underlaying source detected
            final Long alreadyExisting = this.changedVersion.putIfAbsent(version, System.currentTimeMillis()); // another thread might have already picked up that new version
            if (alreadyExisting == null) {
                System.out.println(String.format("Handling non-existent version %s", version));
                if (!wasVersionHandledBefore(version)) {   // if this is a brand new version
                    try {
                        doCostlyOperationThatInvolvesNetwork();
                        this.changedVersion.remove(this.currentVersion); // cleanup previous value
                        this.currentVersion = version;
                    } catch (Exception e) {
                        throw new IllegalStateException(String.format("Unable to introduce new version %s using DynamicProperty. Do it manually.", version), e);
                    }
                }
            }
        }
        return this.currentVersion;
    }

    protected boolean wasVersionHandledBefore(final String version) {
        // some check here
    }

    protected void doCostlyOperationThatInvolvesNetwork() {
        // may take a few seconds
    }
}