Add Eval.toEnvPropertyKey()

Supports the `${my.prop}` -> MY_PROP environment variable case

Author: rbygrave

README.md

@@ -89,6 +89,60 @@ logger.timestampPattern=ISO_OFFSET_DATE_TIME
 
 ```
 
+## Structured JSON - logger.format=json
+
+By default, the log format is JSON. Example:
+
+```json
+{
+  "component":"my-application",
+  "env":"dev",
+  "timestamp":"2025-07-14T13:44:44.230959+12:00",
+  "level":"TRACE",
+  "logger":"io.avaje.config",
+  "message":"load from [resource:application-test.properties]",
+  "thread":"main"
+}
+```
+
+#### component
+
+A `component` key value is added if:
+- There is a `logger.component` property set in `avaje-logger.properties`
+- There is a `COMPONENT` environment variable set
+- K8s is detected, it will be derived from the `HOSTNAME` environment variable
+
+This key value is expected to represent the application component that is the source of
+the logs.
+
+Examples:
+```properties
+## a literal value
+logger.component=my-application
+
+## uses system property `SERVICE_NAME` or environment variable `SERVICE_NAME`
+logger.component=${SERVICE_NAME}
+
+## uses system property `service.name` or environment variable `SERVICE_NAME`
+logger.component=${service.name}
+
+```
+
+#### env - Environment
+
+An `env` key value is added automatically if:
+- There is a `logger.environment` property set in `avaje-logger.properties`
+- There is an `ENVIRONMENT` environment variable set
+
+Examples:
+```properties
+## uses system property `app.env` or environment variable `APP_ENV`, defaults to `localdev`
+logger.environment=${app.env:localdev}
+
+## literal value
+logger.environment=DEV
+```
+
 
 ## Dynamic log level configuration
 

avaje-simple-json-logger/src/main/java/io/avaje/simplelogger/encoder/Eval.java

@@ -69,10 +69,18 @@ static String eval(String value) {
     if (val != null) {
       return val;
     }
+    val = System.getenv(toEnvPropertyKey(key));
+    if (val != null) {
+      return val;
+    }
     return split.length == 2 ? split[1] : value;
   }
 
   static String toSystemPropertyKey(String key) {
     return key.replace('_', '.').toLowerCase();
   }
+
+  static String toEnvPropertyKey(String key) {
+    return key.replace('.', '_').toUpperCase();
+  }
 }

avaje-simple-json-logger/src/test/java/io/avaje/simplelogger/encoder/EvalTest.java

@@ -0,0 +1,40 @@
+package io.avaje.simplelogger.encoder;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class EvalTest {
+
+  @Test
+  void k8sComponent() {
+    assertEquals("my-component", Eval.k8sComponent("my-component-part0-part1"));
+  }
+
+  @Test
+  void eval() {
+    String expected = System.getProperty("user.home");
+    assertEquals(expected, Eval.eval("${user.home:someDefault}"));
+    assertEquals(expected, Eval.eval("${user.home}"));
+  }
+
+  @Test
+  void eval_expect_defaultValue() {
+    assertEquals("someDefault", Eval.eval("${user.homeDoesNotExist:someDefault}"));
+  }
+
+  @Test
+  void toSystemPropertyKey() {
+    assertEquals("my.prop", Eval.toSystemPropertyKey("MY_PROP"));
+    assertEquals("my.prop", Eval.toSystemPropertyKey("my_prop"));
+    assertEquals("my.prop", Eval.toSystemPropertyKey("my.prop"));
+
+  }
+
+  @Test
+  void toEnvPropertyKey() {
+    assertEquals("MY_PROP", Eval.toEnvPropertyKey("MY_PROP"));
+    assertEquals("MY_PROP", Eval.toEnvPropertyKey("my_prop"));
+    assertEquals("MY_PROP", Eval.toEnvPropertyKey("my.prop"));
+  }
+}

avaje-simple-logger/src/test/resources/avaje-logger-test.properties

@@ -1,5 +1,8 @@
 ## can set config including log levels here
 
+logger.environment=${env:dev}
+logger.component=${my.prop:my-app}
+
 log.level.org.foo=trace
 log.level.io.avaje.config=trace
 log.level.io.avaje.simplelogger=debug