Mayor Adams has known for a year and a half that his mismanagement of the migrant crisis would force New York City to cut billions in spending — yet last week, his first move was to go full apocalypse, canceling police-academy classes and thus endangering his only real campaign promise, to improve public safety.

If Adams really wants to protect the NYPD — and libraries too! — there are other places he can cut.

The mayor projects canceling police-academy classes will save $42 million this year, $289 million the next year and $229 million for each of the following two years.

If the mayor doesn’t want to hold up these police-academy classes, he can find that money elsewhere.

The city’s Independent Budget Office has two ideas.

To save $253 million a year, the city could halve its reimbursement to its public-sector retirees for their Medicare Part B (for outpatient care) premiums, which are about $170 a month per retiree.

As the IBO points out, “The majority of other public-sector employers (including the federal government) do not offer any level of Medicare Part B reimbursement as part of retiree fringe benefit packages.”

In the private sector, retirees pay their Medicare premiums out of their Social Security or pension income.

Plus, the city — with City Council legislation — could do this without having to open up any union contracts.

Were the city to require current employees to pay a portion of their health-care premiums, as is near-universal in the private sector, the city could save $584 million a year — but that would require negotiations with unions (so not great that Adams just agreed to union contracts with no such concessions).

Maybe the mayor doesn’t want to rile up workers or retirees.

Fine: Then cancel the four-borough jails plan, which is going to cost us at least $312 million a year in debt costs starting in a couple of years, even before its $8.9 billion price tag grows.

The four-borough jail plan is the first big infrastructure project in decades that will harm, not help, the neighborhoods it affects.

Spend a fraction of the money to fix up Rikers Island instead of putting jails in residential neighborhoods.

What about bloat? The city’s Education Department is famous for that.

When former Mayor Bill de Blasio took over in 2014, the department had 23,237 non-teaching “civilian” staff. Now it has 25,973, or 12% more, despite the fact school enrollment has shrunk.

Adams proposes to cut this only by 373 people. Bringing it back to 2014 levels would save an additional $236.3 million annually, almost enough to cover the police-class cuts.

You don’t need to save hundreds of millions to save libraries. Adams’s proposed cuts to branches are just $21 million annually — easy to find elsewhere instead.

New York City has a commission on human rights, largely to investigate employment and housing discrimination; it spends $14.1 million a year. But the state has an identical office. Let the state do it.

Thinking longer-term? New York City has one entirely useless office, with no power and no authority: the public advocate.

Reforming city government — requiring a public vote — to eliminate it would save another $5 million a year, helping rescue the libraries.

Which would you rather have, libraries or an elected official whose only job is to complain?

As Adams said Monday, “It is unbelievable to me that I have to hold up a police class.”

Well, yes, it is unbelievable.

Nicole Gelinas is a contributing editor to the Manhattan Institute’s City Journal.