---
title: "Beans, lifecycle, and scopes"
chapter: "03"
---

# Beans, lifecycle, and scopes

The container creates a bean, injects dependencies, applies processors, calls
initialization callbacks, exposes it for use, and invokes destruction callbacks
when the context closes.

## Lifecycle tools

- Constructor: establish valid required state.
- `@PostConstruct`: initialize after dependencies exist.
- `@PreDestroy`: release managed resources.
- `BeanPostProcessor`: change or wrap many beans as infrastructure.
- `SmartLifecycle`: coordinate start/stop for active components.

Keep constructors fast and avoid remote calls during startup when possible.
Startup should fail clearly for invalid configuration but not depend on a
temporary network response unless absolutely required.

## Scopes

| Scope | Meaning |
|---|---|
| Singleton | One bean instance per application context; default |
| Prototype | New instance each time requested |
| Request | One instance per HTTP request |
| Session | One instance per HTTP session |
| Application | One per servlet application |

A singleton may serve many threads. Mutable fields can create races. Prefer
stateless singleton services and keep request data in method variables.

## Proxies

Spring may wrap a bean with a proxy to add transactions, security, caching, or
AOP behavior. Calls entering through the proxy receive that behavior.
Self-invocation—one method calling another on `this`—can bypass proxy advice.

## Feynman check

A proxy is a receptionist standing before the real worker. If the worker calls
their own second task internally, the call never passes the receptionist.