What causes "Not implemented yet - decide how to handle" in hibernate/hibernate-orm?

Edit the fetch-profile definition to state the timing explicitly: fetch = FetchType.EAGER (or LAZY) on every @FetchOverrides entry / element.

hibernate/hibernate-orm · error · UnsupportedOperationException

Not implemented yet - decide how to handle

Error message

Not implemented yet - decide how to handle

What it means

FetchTiming.forType maps a JPA FetchType to Hibernate's internal fetch timing: EAGER to IMMEDIATE, LAZY to DELAYED. FetchType.DEFAULT - the value present when a fetch type is omitted - has no mapping, so the switch throws UnsupportedOperationException('Not implemented yet - decide how to handle'). The single production caller is FetchProfileHelper.createFetchProfile, which runs while the SessionFactory is built, so a fetch-profile definition whose fetch override still carries DEFAULT fails at startup.

Source

Thrown at hibernate-core/src/main/java/org/hibernate/engine/FetchTiming.java:29

 *
 * @author Steve Ebersole
 * @see FetchStyle
 */
public enum FetchTiming {
	/**
	 * Perform fetching immediately.  Also called eager fetching
	 */
	IMMEDIATE,
	/**
	 * Performing fetching later, when needed.  Also called lazy fetching.
	 */
	DELAYED;

	public static FetchTiming forType(FetchType type) {
		return switch (type) {
			case EAGER -> IMMEDIATE;
			case LAZY -> DELAYED;
			case DEFAULT -> throw new UnsupportedOperationException( "Not implemented yet - decide how to handle" );
		};
	}
}

View on GitHub (pinned to fad1729dce)

Solutions

Edit the fetch-profile definition to state the timing explicitly: fetch = FetchType.EAGER (or LAZY) on every @FetchOverrides entry / <fetch> element.
Remove the incomplete fetch override entirely if default fetching is what you want - profiles exist to override, so an entry with DEFAULT is meaningless anyway.
If your own code calls FetchTiming.forType, resolve DEFAULT to EAGER or LAZY before calling it.
Upgrade Hibernate - placeholder semantics for DEFAULT inside fetch profiles may be resolved in later 6.x releases.

Example fix

// before - throws at SessionFactory bootstrap
@FetchProfile(name = "order-with-items")
@FetchOverrides({
    @FetchOverride(entity = Order.class, association = "items",
                   fetch = FetchType.DEFAULT)  // DEFAULT has no FetchTiming
})
// after
@FetchProfile(name = "order-with-items")
@FetchOverrides({
    @FetchOverride(entity = Order.class, association = "items",
                   fetch = FetchType.LAZY)
})

Defensive patterns

Strategy: validation

Validate before calling

// Scan fetch-profile definitions at build time so DEFAULT never reaches the bootstrap
for (var profile : metadata.getFetchProfiles()) {
    for (var fetch : profile.getFetches()) {
        if (fetch.getType() == FetchType.DEFAULT) {
            throw new IllegalStateException("Fetch profile '" + profile.getName()
                + "' must declare EAGER or LAZY, found DEFAULT on " + fetch.getAssociation());
        }
    }
}

Prevention

Always write an explicit fetch = FetchType.EAGER/LAZY in @FetchOverride entries.
If you call FetchTiming.forType yourself, resolve DEFAULT (e.g. to EAGER) before the call.
Fail fast in CI: build the SessionFactory in a smoke test so mapping errors surface before deployment.

When it happens

Trigger: Defining a fetch profile (@FetchProfile with @FetchOverrides, or an XML fetch-profile) whose fetch override does not resolve to EAGER or LAZY - e.g. explicitly setting fetch = FetchType.DEFAULT or a mapping path that leaves the default in place. FetchProfileHelper calls FetchTiming.forType(mappingFetch.getType()) during factory init and the DEFAULT branch throws. Custom code calling FetchTiming.forType(FetchType.DEFAULT) directly hits the same wall.

Common situations: Migrating Hibernate 5 fetch-profile XML/annotations to Hibernate 6 where DEFAULT previously slipped through; generated mapping code that copies FetchType.DEFAULT from a defaulted annotation field; copy-pasted fetch-profile snippets missing the fetch attribute on some drivers of the annotation.

Related errors

AI-assisted analysis of hibernate/hibernate-orm@fad1729dce (2026-08-22). Data as JSON: /api/errors/4192cc13cd8ba7ac. Report an issue: GitHub.