Using TreeSet causes serialization exception

• Problem Description: sort the second level classifications under the first level classification according to the order field (Integer type). At that time, a TreeSet was made and a user-defined Comparator was passed. It was OK to run locally, and the sorting was arranged. It also passed the test environment. As a result, once the thing came online, it reported a serialization exception, and the online environment was cached. Check the data and say that TreeSet has a bug that can't be serialized. It's immediately depressed.
• The exception information is as follows
  
• Scene description
  The JavaBean s corresponding to the following requirements are simulated as follows:

public class ProductCategory implements Serializable {
    private String id;
    private String name;
    private Integer order;
    private Set<ProductCategory> children = new HashSet<>();
}

Omit getter,setter methods, hibernate for online environment, and previous JavaBean s, which have been used for a long time and dare not be changed. The subcategory corresponds to a HashSet(), which is unordered. Now we need to sort the children subcategory. The simulation test is as follows:

	@Test
    public void contextLoads() {
        ProductCategory productCategory = new ProductCategory();
		// Create 4 subclass objects
        ProductCategory children1 = new ProductCategory();
        ProductCategory children2 = new ProductCategory();
        ProductCategory children3 = new ProductCategory();
        ProductCategory children4 = new ProductCategory();
		// ID
        children1.setId(UUID.randomUUID().toString());
        children2.setId(UUID.randomUUID().toString());
        children3.setId(UUID.randomUUID().toString());
        children4.setId(UUID.randomUUID().toString());
		// name
        children1.setName(UUID.randomUUID().toString());
        children2.setName(UUID.randomUUID().toString());
        children3.setName(UUID.randomUUID().toString());
        children4.setName(UUID.randomUUID().toString());
		// order sort field
        children1.setOrder(10);
        children2.setOrder(8);
        children3.setOrder(9);
        children4.setOrder(12);
        
		// Pass a TreeSet to replace the HashSet, and customize a Comparator to compare through the order field (for self Baidu, which is not familiar with the Comparator) 
        productCategory1.setChildren(new TreeSet<>(Comparator.comparingInt(o -> o.getOrder() == null ? 0 : o.getOrder())));
        // All added to the HashSet
        productCategory.getChildren().add(children1);
        productCategory.getChildren().add(children2);
        productCategory.getChildren().add(children3);
        productCategory.getChildren().add(children4);
		////////////////////////////////////////
		// Here, the children have been arranged according to the order (8, 9, 10, 12), so the printed results will not be put. You can debug and view them by yourself.
		////////////////////////////////////////
		// Simulate online cache and save a copy of redis (the result is reported as java.io.NotSerializableException in this step)
        redisTemplate.opsForValue().set("productcategory", productCategory.getChildren());
    }

• Solution

		// Still use the original HashSet
		productCategory2.getChildren().add(children1);
        productCategory2.getChildren().add(children2);
        productCategory2.getChildren().add(children3);
        productCategory2.getChildren().add(children4);
		
		// Convert HastSet to List
        List<ProductCategory> list = new ArrayList<>(productCategory1.getChildren());
        // sort
        Collections.sort(list, (Comparator.comparingInt(o -> (o.getOrder() == null ? 0 : o.getOrder()))));
        // Turn it into a LinkedHashSet and plug it back to ensure order.
        productCategory1.setChildren(new LinkedHashSet<>(list));
        // Simulate the online cache and save a copy of redis (it can also be serialized normally here)
        redisTemplate.opsForValue().set("productcategory", productCategory.getChildren());

The default sorting of hashsets is based on HashCode. Without affecting the business, you can also override the hashCode() method or implement the Comparable interface to override the compareTo method.