[Index](../README.md) · [Relaxed binding →](02-relaxed-binding.md)

# 1. Two mechanisms, not two styles

`@Value` and `@ConfigurationProperties` are often presented as a matter of taste. They are not.
They are two different mechanisms that happen to read from the same `Environment`, and almost
every difference in behaviour follows from that.

## `@Value` is placeholder resolution

`@Value("${demo.mail.host}")` is a string. Spring's `PropertySourcesPlaceholderConfigurer`
resolves it against the environment during bean post-processing, converts the result to the
field's type, and assigns it. There is no model of what a property *is* — only a name, a
lookup and a conversion.

Consequences:

- The property name lives in a string literal. Renaming a property is a text search.
- One field, one lookup. There is no way to express "these six properties belong together".
- Defaulting is `:` inside the placeholder, and nothing else.
- There is no validation step, because there is no object to validate.
- SpEL is available: `#{...}` can compute rather than look up.

## `@ConfigurationProperties` is binding

`Binder` walks a target type, works out which properties it needs, asks the
`ConfigurationPropertySource`s for each one, converts, and constructs the object. The target
type is the model.

Consequences:

- The property names are derived from the type. Renaming a component renames the property.
- Nested types, lists and maps bind, because the binder knows the shape it is filling.
- Defaults are `@DefaultValue`, applied per component during construction.
- The bound object can be validated as an object, once, at startup.
- No SpEL. The binder maps values; it does not compute them.

## The one that surprises people

Because both read from the same environment, the *relaxed* rules ought to differ — and in the
Spring Framework alone they do. Inside Spring Boot they largely do not, because Boot attaches
an extra property source that gives placeholder resolution the binder's name matching.
[Chapter 2](02-relaxed-binding.md) has the measured matrix.

## Which to use

Use `@ConfigurationProperties` for anything that is a group of settings — which is most
things. Reach for `@Value` when you need a single value in a place where a type would be
ceremony, or when you need SpEL. [Chapter 6](06-when-value-still-wins.md) is the honest list.