MustBeClosedChecker
This method returns a resource which must be managed carefully, not just left for garbage collection. If it is a constant that will persist for the lifetime of your program, move it to a private static final field. Otherwise, you should use it in a try-with-resources.

Alternate names: MustBeClosed

The problem

Some methods and constructors require that the returned resource is closed. This includes user methods/constructors annotated with @MustBeClosed and some additional well-known APIs. (Some further well-known APIs are covered by a different check, StreamResourceLeak.)

This is enforced by checking that invocations occur within the resource variable initializer of a try-with-resources statement:

try (AutoCloseable resource = createTheResource()) {
  doSomething(resource);
}

or the return statement of another method annotated with @MustBeClosed:

@MustBeClosed
AutoCloseable createMyResource() {
  return createTheResource();
}

To support legacy code, the following pattern is also supported:

AutoCloseable resource = createTheResource();
try {
  doSomething(resource);
} finally {
  resource.close();
}

Lambdas and method references that return such a resource are only allowed if the functional interface method they implement is also annotated with @MustBeClosed, which requires callers of that method to close the resource. For example, if the MyResource constructor is annotated with @MustBeClosed:

interface ResourceFactory {
  @MustBeClosed
  AutoCloseable create();
}

// OK: callers of ResourceFactory.create() must close the resource.
ResourceFactory factory = MyResource::new;

// Error: callers of Supplier.get() are not required to close the resource.
Supplier<AutoCloseable> supplier = MyResource::new;

Suppression

Suppress false positives by adding the suppression annotation @SuppressWarnings("MustBeClosedChecker") to the enclosing element.