StackOverflow Error - JPA Entity with HashMap <Entity,Integer>

Question

I want to make a little production calculator. I have a Part A which need 7s and its made out of x Part B which need 3s and y Part C which need 2s. Part C is made out of n Part D. So I have a recursion in Class Part and and its number of usage as "recipe". My Enitity Class

@Entity
@Data
public class ProductEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    @Column(unique=true)
    private String name;
    private double productionCycleTimeInSeconds;
    private int batchPerProductionCycle;

    @Enumerated(EnumType.ORDINAL)
    private FacilityEnum facility;

    @ElementCollection
    @JoinTable(name = "end_product_recipe", joinColumns = @JoinColumn(name = "end_product_id"))
    @MapKeyColumn(name = "resourceId")
    @Column(name = "usage_count")
    private Map<ProductEntity, Integer> recipe;
}

For Testing I wanted to init the table with 3 basics

        ProductEntity ironOre = new ProductEntity();
        ironOre.setFacility(FacilityEnum.MINING_MACHINE);
        ironOre.setName("Iron Ore");
        ironOre.setProductionCycleTimeInSeconds(1.0);
        ironOre.setBatchPerProductionCycle(1);
        productRepository.save(ironOre);

        ProductEntity ironIngot = new ProductEntity();
        ironIngot.setFacility(FacilityEnum.SMELTER);
        ironIngot.setName("Iron Ingot");
        ironIngot.setProductionCycleTimeInSeconds(1.0);
        ironIngot.setBatchPerProductionCycle(1);
        Map<ProductEntity, Integer> recipe = new HashMap<>();
        recipe.put(ironOre, 1);
        ironIngot.setRecipe(recipe);
        productRepository.save(ironIngot);

        ProductEntity gear = new ProductEntity();
        gear.setFacility(FacilityEnum.ASSEMBLING_MACHINE);
        gear.setName("Gear");
        gear.setProductionCycleTimeInSeconds(1.0);
        gear.setBatchPerProductionCycle(1);
        recipe.clear();
        recipe.put(ironIngot, 1);
        gear.setRecipe(recipe);
        productRepository.save(gear);

When Code get to last line (productRepository.save(gear);) i get Error:

java.lang.StackOverflowError
    at java.util.HashMap$HashIterator.<init>(HashMap.java:1427)
    at java.util.HashMap$EntryIterator.<init>(HashMap.java:1477)
    at java.util.HashMap$EntrySet.iterator(HashMap.java:1014)
    at java.util.AbstractMap.hashCode(AbstractMap.java:528)
    at org.hibernate.collection.internal.PersistentMap.hashCode(PersistentMap.java:574)
    at com.hoernerice.dspcalculator.entity.ProductEntity.hashCode(ProductEntity.java:12)
    at java.util.Objects.hashCode(Objects.java:98)
    at java.util.HashMap$Node.hashCode(HashMap.java:297)
    at java.util.AbstractMap.hashCode(AbstractMap.java:530)
    at org.hibernate.collection.internal.PersistentMap.hashCode(PersistentMap.java:574)
    at com.hoernerice.dspcalculator.entity.ProductEntity.hashCode(ProductEntity.java:12)
    at java.util.Objects.hashCode(Objects.java:98)
    at java.util.HashMap$Node.hashCode(HashMap.java:297)
    at java.util.AbstractMap.hashCode(AbstractMap.java:530)
    at org.hibernate.collection.internal.PersistentMap.hashCode(PersistentMap.java:574)
    at com.hoernerice.dspcalculator.entity.ProductEntity.hashCode(ProductEntity.java:12)
    at java.util.Objects.hashCode(Objects.java:98)
    at java.util.HashMap$Node.hashCode(HashMap.java:297)
    at java.util.AbstractMap.hashCode(AbstractMap.java:530)
    at org.hibernate.collection.internal.PersistentMap.hashCode(PersistentMap.java:574)
    at com.hoernerice.dspcalculator.entity.ProductEntity.hashCode(ProductEntity.java:12)
....

it repeats so i guess i have an endless loop. but how? where is my mistake? For the first 2 items i have the rows as accepted in database. in the join table are "just" ids as wanted. So that work as accepted

Solved by replacing recipe.clear() with

recipe = new HashMap<>();

Answer 1

HashMap.hashCode() is the responsible of the overflow.

HashMap extends from AbstractMap, which in his hashCode() method, sums the hash code of each entry:

public int hashCode() {             
    int h = 0;
    Iterator<Entry<K,V>> i = entrySet().iterator();
    while (i.hasNext())
        h += i.next().hashCode();   
    return h;
}

Why the infinite recursive loop?

ironIngot.setRecipe(recipe);

and the following

recipe.put(ironIngot, 1);

Is making the HashMap an entry of itself. So when it calls hashCode() it calls, again, his hashcode method when calculating its entrys' hashcode: next().hashCode(). And again, and again...in an infinite loop.

Simplification of the overflow loop

        MapX
       +--------------+
      /|             /|
     / |            / |
    *--+-----------*  |
    |  |           |  |
    |  |- [MapX]------|-- ----
    |  |           |  |       |
    |  +-----------+--+       |
    | /            | /        |
    |/             |/         |
    *--------------*          |
                       MapX   V
                      +--------------+
                     /|             /|
                    *-+------------* |
                    | |            | |
                    | |- [MapX]----|-|---------    
                    | |            | |        |
                    | +------------+-+        |
                    |/             |/         |
                    *--------------*          |
                                       MapX   V
                                       +--------------+
                                      /|             /|
                                     *-+------------* |
                                     | |            | |
                                     | |- [MapX]--------------      
                                     | |            | |      |
                                     | +------------+-+      |
                                     |/             |/       |
                                     *--------------*        V
                                                     (Road to Overflow Town)

As you commented, creating a new HashMap avoids this scenario, as now there's no map that contains itself as one of its entries, you broke the reference chain.

---

The code that explains this behaviour is this one, which in essence, is making the same as yours:

Map<Object,String> map = new HashMap<>();
map.put(map,"letsLooop");
map.hashCode();

This snippet will also lead to a stackoverflow error, due to the infinite loop in the AbstractMap's hashCode() implementation, which will call its own hashCode method in a deadlocking loop that results in a beautiful StackOverflow.