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.

Explained by highlit

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

Walkthrough

Space play ←→ step click any line

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.

Related explainers

java

public final class NaturalOrderComparator implements Comparator<String> {
 
    public static final NaturalOrderComparator INSTANCE = new NaturalOrderComparator();

Natural-order string sorting in Java

comparator natural-sort string-parsing

Intermediate 9 steps

package middleware
 
import (
	"net/http"

Localized validation errors in Gin

middleware internationalization validation

Intermediate 8 steps

javascript

async function uploadInBatches(records, uploadFn, { batchSize = 100, concurrency = 3 } = {}) {
  const batches = [];
  for (let i = 0; i < records.length; i += batchSize) {
    batches.push(records.slice(i, i + batchSize));

Uploading records with bounded concurrency

concurrency worker-pool async-await

Advanced 8 steps

python

from flask import Blueprint, jsonify
from marshmallow import Schema, fields, validate, EXCLUDE
from webargs.flaskparser import use_args

Validating query params in Flask with webargs

validation schema query-building

Intermediate 10 steps

java

public URI buildSearchUri(String query, int page, int size, List<String> tags) {
    UriComponentsBuilder builder = UriComponentsBuilder
            .fromUriString("https://api.example.com")
            .path("/v2/products/search")

Building URIs safely with UriComponentsBuilder in Spring

url-building builder-pattern encoding

Intermediate 5 steps

func (h *ExportHandler) BulkExport(c *gin.Context) {
	projectID := c.Param("projectID")
 
	reports, err := h.reports.ListByProject(c.Request.Context(), projectID)