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# 3. Seeing precedence instead of reasoning about it

Endpoint: [`PrecedenceEndpoint`](../src/main/java/com/ankurm/profiles/web/PrecedenceEndpoint.java).
Transcript: [`01-precedence.txt`](output/01-precedence.txt).

Set `demo.greeting` from five places at once and ask which won:

```
$ DEMO_GREETING=from-environment-variable \
    java -Ddemo.greeting=from-system-property \
         -jar profiles-and-config-1.0.0.jar --spring.profiles.active=prod \
         --demo.greeting=from-command-line-argument
```

```
"effectiveValue": "from-command-line-argument",
"activeProfiles": ["prod", "prod-db", "prod-metrics"],
"holders": [
  { "rank": 1, "value": "from-command-line-argument",  "source": "commandLineArgs" },
  { "rank": 2, "value": "from-system-property",        "source": "systemProperties" },
  { "rank": 3, "value": "from-environment-variable",   "source": "systemEnvironment" },
  { "rank": 4, "value": "from-application-prod-yaml",  "origin": "application-prod.yaml - 4:13" },
  { "rank": 5, "value": "from-application-yaml",       "origin": "application.yaml - 21:13" }
],
"shadowedCount": 4
```

Five sources hold the property. Four of them lose. Each one that came from a file names its
line.

## The whole implementation

```java
for (ConfigurationPropertySource source : ConfigurationPropertySources.get(environment)) {
    ConfigurationProperty property =
            source.getConfigurationProperty(ConfigurationPropertyName.of(name));
    if (property != null) {
        // rank = position, property.getValue(), property.getOrigin()
    }
}
```

`ConfigurationPropertySources.get(...)` returns the sources in precedence order. Iterate,
collect every hit, and the first is the winner. That is the entire diagnostic.

## Why this beats reading the list

The documented order is correct but abstract. It does not tell you that a `DEMO_GREETING` left
over from a shell three weeks ago is sitting at rank 3, and that is the actual question.

## Alternatives if you would rather not add an endpoint

- Actuator's `/actuator/env` gives the same information with sanitisation, and
  `/actuator/env/{name}` narrows to one property. Prefer it in anything real.
- `logging.level.org.springframework.boot.context.config=TRACE` logs which config data
  resources were loaded and in what order.
- `--debug` does *not* show this. It prints the auto-configuration report.