java 56 lines · 8 steps

Validating JSON payloads against a schema in Java

A reusable validator loads a JSON Schema once, then reports every violation as a flat list of readable messages.

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Intermediate json-schema validation recursion error-handling streams

1package com.example.validation;
2 
3import java.util.List;
4import java.util.stream.Collectors;
5 
6import org.everit.json.schema.Schema;
7import org.everit.json.schema.ValidationException;
8import org.everit.json.schema.loader.SchemaLoader;
9import org.json.JSONObject;
10import org.json.JSONTokener;
11 
12public class PayloadValidator {
13 
14    private final Schema schema;
15 
16    public PayloadValidator(String schemaResourcePath) {
17        try (var stream = getClass().getResourceAsStream(schemaResourcePath)) {
18            if (stream == null) {
19                throw new IllegalStateException("Schema not found: " + schemaResourcePath);
20            }
21            JSONObject rawSchema = new JSONObject(new JSONTokener(stream));
22            this.schema = SchemaLoader.builder()
23                    .schemaJson(rawSchema)
24                    .draftV7Support()
25                    .build()
26                    .load()
27                    .build();
28        } catch (Exception e) {
29            throw new IllegalStateException("Failed to load schema", e);
30        }
31    }
32 
33    public List<String> validate(String json) {
34        JSONObject payload;
35        try {
36            payload = new JSONObject(new JSONTokener(json));
37        } catch (org.json.JSONException e) {
38            return List.of("Malformed JSON: " + e.getMessage());
39        }
40        try {
41            schema.validate(payload);
42            return List.of();
43        } catch (ValidationException e) {
44            return flatten(e);
45        }
46    }
47 
48    private List<String> flatten(ValidationException e) {
49        if (e.getCausingExceptions().isEmpty()) {
50            return List.of(e.getPointerToViolation() + ": " + e.getErrorMessage());
51        }
52        return e.getCausingExceptions().stream()
53                .flatMap(cause -> flatten(cause).stream())
54                .collect(Collectors.toList());
55    }
56}

01 / 01

STEP 01

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Three takeaways

1Compile expensive resources like schemas once in the constructor so each validation call stays cheap.
2Returning an empty-or-populated list of errors is friendlier to callers than throwing on the first failure.
3Recursively walking a nested exception's causes turns a tree of violations into one flat, actionable report.

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