GC 和 static 最终字段

Question

We are working on an payment application based on spring-boot 2.5.6. The application is hosted on docker and launched on Java 14. We created an abstract class named GeneralEnum which is @Embeddable and contains two variables int type and String value . All enumeration classes extend this base class and define constants inside it.

This decision is made to customize enumeration used in whole layer of application. You can see one example in below:

@Embeddable
public class ShapeEnum extends GeneralEnum {
   public static final ShapeEnum CIRCLE = new ShapeEnum(0, 'Circle');
   public static final ShapeEnum RECTANGLE = new ShapeEnum(1, 'Rectangle');

   public ShapeEnum(int type, String value) {
       super(type, value);
   }
}

This class is used in entity layer as embedded variable as follow:

@Entity
public class Shape {
    @Embedded
    @AttributeOverride(name = "type", column = @Column(name = "shape_type"))
    private ShapeEnum shapeType;
}

The system worked perfectly, but something wrong happens and the value field of ShapeEnum.RECTANGLE changed to Circle and type is changed to 0 (.,.) and after that system would fail (dumping RAM shown the change of value). As I tracked, the problem is occurred after running GC and when the ShapeEnum is not used for a while.

Would anyone make an idea what happens and how tackle this problem? For now, restarting docker container causes the app will work properly.

Thanks in advance.

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The dump for another class is (TicketStatus class) shown below. The highlighted part is changed without reason. The first and second constants' value and type are the same. I am sure that setValue and setType won't call in the source-code.

Answer 1

OK, so first I will say what the problem is not.

This problem is not caused by the garbage collector. The GC doesn't change the values of variables. Not static variables, and not instance variables. So it is not the GC that is causing the type field of some ShapeEnum that denotes a rectangle ( 1 ) to change to a circle ( 0 ).

---

You say:

As I tracked, the problem is occurred after running GC and when the ShapeEnum is not used for a while.

That statement is not conclusive evidence. What you are actually (probably) saying is that you observed an object with type of 0 when it should have been 1 ... at some time after a GC run. You (probably) didn't observe the change actually happening, and you (probably) can't be sure that the value was correct immediately before the GC ran. In fact, you (probably) can't even be sure that the value was wrong immediately after the GC ran.

(If you do have clear evidence rather than supposition, please add it to the question.)

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So what actually is causing this change?

At this stage we can only guess. My guess would be that:

• some part of your application is calling the setValue method when it shouldn't, or

• you have multiple objects that represent the "enums" for (say) rectangle.

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It strikes me that your implementation of these ersatz enums is fragile. They should be (at least loosely) immutable. You probably alter the GeneralEnum class to remove the setters, and make the type and value fields final .

If the objects are immutable, you won't have to worry about something changing them. There would be other ways to get a ShapeEnum with type == 1 and value.equals("Circle") , but you wouldn't be able to break CIRCLE or RECTANGLE .