---
title: "Declarative Services in depth"
chapter: "06"
---

# Declarative Services in depth

Declarative Services (DS) is the default component model for modern OSGi.

## Component lifecycle

SCR—the Service Component Runtime—reads component descriptions, waits for
required references and configuration, creates the instance, injects
references, calls activation, publishes services, and later deactivates it.

```java
@Component(
  service = Reconciler.class,
  configurationPid = "com.acme.reconciler"
)
public final class DefaultReconciler implements Reconciler {
  private Ledger ledger;

  @Reference
  void bindLedger(Ledger ledger) { this.ledger = ledger; }

  @Activate
  void activate(ReconcilerConfig config) { /* validate */ }
}
```

Use constructor injection for mandatory unary references when your DS level and
tooling support it. Use bind/unbind methods when dynamic replacement is a real
requirement.

## Cardinality

`1..1` is one mandatory service; `0..1` optional; `1..n` at least one;
`0..n` any number. Multiple cardinality can inject a collection, map, tuple, or
component service objects depending on the field option.

## Immediate and delayed

A delayed component activates on first service use, reducing startup work.
Immediate components activate as soon as satisfied. Components without a
service are normally immediate. Avoid making everything eager.

## Configuration policy

`OPTIONAL`, `REQUIRE`, and `IGNORE` decide whether configuration is necessary.
Typed configuration annotations turn string dictionaries into a checked
contract. Validate invariants during activation so invalid instances never
publish.

## Factory components

A factory configuration can create multiple independently configured component
instances—for example one connector per charging-network tenant.

## Tooling

bnd generates descriptors and requirements from DS annotations. At runtime,
inspect `scr:list`, `scr:info`, satisfied references, component failure reasons,
and configuration PIDs before editing code.

## Feynman check

A bundle can be ACTIVE while a DS component is unsatisfied. Explain this by
separating bundle lifecycle from component prerequisites.