AutoCloseable "resource leak" warning for factory created instances?

Question

These "resource leak" warnings I'm getting in Eclipse for AutoCloseables seem to be a life-saver.

However, how do I get them to work for factory created instances?

For example (a works, but b doesn't):

public static void main(String[] args) {
    // a) This emits a warning
    new AutoCloseable() {
        @Override
        public void close() throws Exception {}
    };

    // b) But this doesn't!
    newResource();
}

public static AutoCloseable newResource() {
    return new AutoCloseable() {
        @Override
        public void close() throws Exception {}
    };
}

Is there an annotation I can stick on newResource() or something I can do to let the compiler (or is it Eclipse?) know of the ownership change?

Heighttopaper

Intellij can generally warn if some return values are disregarded. This could help as you would than have to assign the value locally. – Bergren

Try this: https://mcmap.net/q/438582/-using-annotation-to-ensure-that-value-returned-by-method-is-not-discarded – Bergren

Answer 1

The Neon Eclipse documentation on "resource leak" detection explains what is going on; see "avoiding resource leaks". It states:

Ownership / responsibility

The above diagnostics basically assume that a method that creates an instance of a resource type is also responsible for closing this resource. However, some resources will be shared among several methods. Here the analysis makes the following assumptions:

1. If a method returns a resource to its caller, it is not responsible for closing; no problem is reported.
2. If a resource is stored in a field, no single method is considered as responsible for closing; no problem is reported.
3. If a method obtains a resource via a method call rather than by a new expression, it may or may not be responsible; any problems are only flagged as potential resource leaks.
4. If a resource is passed as an argument in a method call or constructor call, the current method may or may not be responsible; any problems are only flagged as potential resource leaks.

Point #1 explains why there is no "resource leak" warning for the return statement in the newResource method.

Point #3 explains why there is no "resource leak" warning for the newResource() call. At best, it would be a "potential resource leak" warning. Either you have those warnings disabled, or the previous warning is inhibiting it.

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Q: Is there an annotation to tell Eclipse about transfer of resource ownership?

A: The Neon Eclipse documentation doesn't mention any such annotation. (And it does go into detail about the annotations for null checking!)

Wheaten