Java & Python Debugging Guides

Step-by-step fixes for NullPointerException, Python tracebacks, Spring Boot errors, and more

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Python

fastapi response model validation 500 fix

You deployed your FastAPI endpoint, requests go in, and a mysterious 500 comes back — but your logic looks fine. The culprit is usually a response model mismatch.

Python FastAPI Error WebDev
Java

spring profiles not loading fix

Your application-dev.properties exists, but Spring Boot keeps loading the wrong configuration. Here's why profiles silently fail and how to fix them.

Java Spring Spring Boot Configuration
Python

flask celery task stuck pending fix

You fire off a Celery task and check the result — it's PENDING. Five minutes later, still PENDING. Your worker might be running, or it might not. Here's how to tell.

Python Flask Celery WebDev
Python

fastapi file upload error fix

Your FastAPI file upload is throwing a 422 or crashing silently — here's exactly why and how to fix it.

Python FastAPI FileUpload WebDev
Java

spring resttemplate exception handling guide

Your RestTemplate call throws HttpClientErrorException on a 404 — but you expected a null or an empty response. Here's how RestTemplate error handling actually works.

Java Spring RestTemplate WebDev
Python

flask login current user not working fix

You call login_user() and redirect to a protected page — only to get bounced right back to the login form.

Python Flask Authentication WebDev
Python

fastapi dependency injection errors fix

Your FastAPI Depends() chain compiles fine but breaks at runtime — here's how to find and fix the real culprit.

Python FastAPI Debugging WebDev
Java

spring data jpa query method error fix

You wrote a perfectly reasonable repository method name, Spring Data should understand it — but instead you're staring at a startup failure. Here's why.

Java Spring JPA Spring Boot
Python

flask blueprint template conflict fix

Your Flask app loads the wrong template—or throws TemplateNotFound—the moment you split it into blueprints. Here's how to fix it.

Python Flask WebDev Debugging
Python

fastapi websocket connection drops fix

Your FastAPI WebSocket keeps disconnecting for no obvious reason — here's how to diagnose and fix it.

Python FastAPI WebSocket WebDev
Java

spring security 403 forbidden error fix

Your endpoint exists, your credentials are valid, but Spring Security keeps returning 403. Here's how to figure out why.

Java Spring Security WebDev
Python

flask csrf token missing invalid fix

Your form submits fine locally, then BAD REQUEST in production — it's almost always a CSRF token problem.

Python Flask Security WebDev
Database

How to Use MySQL EXPLAIN to Find Query Bottlenecks

Stop guessing why your MySQL queries are slow. Learn how to use EXPLAIN effectively and let Debugly's free visualizer turn raw output into a color-coded flowchart that pinpoints bottlenecks instantly.

MySQL EXPLAIN Query Optimization Database Debugly
Python

fastapi backgroundtasks failures fix

Your FastAPI background task runs without errors — but nothing actually happens. Here's why, and how to fix it.

Python FastAPI BackgroundTasks WebDev
Python

Fix AttributeError: module has no attribute in Python

Getting 'AttributeError: module X has no attribute Y'? This error has different causes from a regular AttributeError. Learn how to fix it with real examples from NumPy, pandas, and more.

Python AttributeError Debugging Modules Import
Java

spring transactional not working fix

You added @Transactional to your method, but the transaction isn't rolling back or isn't applying at all. Here's what's really happening.

Java Spring Spring Boot Transactions
Python

flask circular import error fix

You split your Flask app into multiple files and suddenly ImportError strikes. Circular imports are the culprit — here's how to fix them.

Python Flask ImportError WebDev
Java

spring jackson serialization error fix

Your Spring REST endpoint works fine until it tries to return data — then boom, a 500 with 'No serializer found for class'. Here's how to diagnose and fix it.

Java Spring Jackson WebDev
Java

spring boot autowired null fix

Your @Autowired field is null when it shouldn't be? Here's why Spring isn't injecting your dependencies.

Java Spring Spring Boot Dependency Injection
Java

spring boot nosuchbeandefinition error fix

Spring can't find your bean? NoSuchBeanDefinitionException means the bean you're trying to inject doesn't exist in the application context.

Java Spring Spring Boot Debugging
Java

spring jpa lazyinitializationexception fix

Getting 'failed to lazily initialize a collection' errors? You're trying to access data after the Hibernate session closed.

Java Spring JPA Hibernate
Python

Python KeyError: How to Fix Dictionary Key Errors

Seeing 'KeyError' in your Python code? Learn how to handle missing dictionary keys with practical examples and proven techniques.

Python KeyError Debugging Error Handling Dictionary
Java

Fix StackOverflowError: Java Complete Guide

Getting StackOverflowError? Learn what causes it, how to debug it, and how to fix it for good. Covers recursion issues, circular references, and practical solutions.

Java StackOverflowError Recursion Debugging
LLM

Toon Format: A New JSON-Compatible Data Format for LLMs

JSON is great for APIs, but it's token-heavy for LLMs. Meet Toon Format (TOON)—a schema-aware, token-efficient alternative that could save you 40% on token costs.

LLM Data Format JSON Optimization Toon
Python

Python TypeError: How to fix the 'NoneType'

Is your Python code blowing up with 'NoneType' object is not subscriptable? Here is a quick guide to fixing the most common TypeErrors in Python.

Python TypeError Debugging Error Handling
Java

Solving ConcurrentModificationException in Java

Blowing up your loops with ConcurrentModificationException? Here is the lowdown on why it happens and the right way to modify collections while iterating.

Java ConcurrentModificationException Collections Debugging
Java

Fix NoSuchElementException: Java Iterator Guide

The complete guide to understanding, debugging, and preventing NoSuchElementException in Java. From iterator patterns to Optional handling with practical examples.

Java NoSuchElementException Iterator Exception Handling
Java

Debug IllegalArgumentException Stack Traces Fast

Learn how to debug IllegalArgumentException efficiently using Debugly's stack trace formatter. Get clear, formatted traces for common Java parameter validation errors.

Java IllegalArgumentException Stack Trace Debugly
Java

IllegalArgumentException: 5 Ways to Fix It

Master the critical java.lang.IllegalArgumentException with our comprehensive guide. Learn what causes parameter validation failures, how to debug them effectively.

Java IllegalArgumentException Input Validation Exception Handling
Java

ClassCastException Fixed: Master Type Casting Error

Master ClassCastException handling with comprehensive coverage of type casting, generics, instanceof checks, and modern Java solutions.

Java ClassCastException Type Safety Exception Handling
Java

Fix ArrayIndexOutOfBounds: Complete Solution

Guide to fixing ArrayIndexOutOfBoundsException in Java. From array indexing basics to advanced prevention strategies with practical examples.

Java ArrayIndexOutOfBoundsException Exception Handling Debugging
Java

Fix NullPointerException: Java NPE Guide

Master the dreaded java.lang.NullPointerException with our comprehensive guide. Learn what causes NPE, how to debug it effectively.

Java NullPointerException Exception Handling Debugging
Java

15+ Java Exceptions Every Developer Must Know [2025]

From NullPointerException to custom exceptions - understand every Java exception you'll encounter. Includes practical examples and debugging strategies.

Java Exception Handling RuntimeException Best Practices