Support evaluation of expressions for component, environment and custom fields

The evaluation uses system properties and environment variables to eval expressions
in the form ${key} and ${key:defaultValue}.

This also defaults the component in a K8s environment using the hostname.

Author: rob-bygrave

src/main/java/io/avaje/logback/encoder/Eval.java

@@ -0,0 +1,78 @@
+package io.avaje.logback.encoder;
+
+/**
+ * Helper to evaluate expressions like {@code "${my.property}"}, {@code "${MY_PROPERTY:someDefaultValue}"} etc.
+ */
+final class Eval {
+
+  /**
+   * Return the default component value using environment variables.
+   * <p>
+   * For K8s this derives the component name from the HOSTNAME.
+   */
+  static String defaultComponent() {
+    String component = System.getenv("COMPONENT");
+    if (component != null) {
+      return component;
+    }
+    if (System.getenv("KUBERNETES_PORT") != null) {
+      // in k8s we can default off the hostname
+      return k8sComponent(System.getenv("HOSTNAME"));
+    }
+    return null;
+  }
+
+  static String k8sComponent(String hostname) {
+    if (hostname == null) {
+      return null;
+    }
+    int p0 = hostname.lastIndexOf('-');
+    if (p0 > 1) {
+      int p1 = hostname.lastIndexOf('-', p0 - 1);
+      if (p1 > 0) {
+        return hostname.substring(0, p1);
+      }
+    }
+    return null;
+  }
+
+  /**
+   * Evaluate the expression and otherwise return the original value.
+   * <p>
+   * Expressions are in the form {@code ${key:defaultValue}}
+   * <p>
+   * Examples:
+   * <pre>{@code
+   *
+   *  ${APP_ENV:localDev}
+   *  ${system.name:unknown}
+   *  ${MY_COMPONENT:myDefaultValue}
+   *
+   * }</pre>
+   */
+  static String eval(String value) {
+    if (value == null || !value.startsWith("${") || !value.endsWith("}")) {
+      return value;
+    }
+    String raw = value.substring(2, value.length() - 1);
+    String[] split = raw.split(":", 2);
+    String key = split[0];
+    String val = System.getProperty(key);
+    if (val != null) {
+      return val;
+    }
+    val = System.getenv(key);
+    if (val != null) {
+      return val;
+    }
+    val = System.getProperty(toSystemPropertyKey(key));
+    if (val != null) {
+      return val;
+    }
+    return split.length == 2 ? split[1] : value;
+  }
+
+  static String toSystemPropertyKey(String key) {
+    return key.replace('_', '.').toLowerCase();
+  }
+}

src/main/java/io/avaje/logback/encoder/JsonEncoder.java

@@ -37,7 +37,7 @@ public final class JsonEncoder extends EncoderBase<ILoggingEvent> {
   public JsonEncoder() {
     this.json = JsonStream.builder().build();
     this.properties = json.properties("component", "env", "timestamp", "level", "logger", "message", "thread", "stackhash", "stacktrace");
-    this.component = System.getenv("COMPONENT");
+    this.component = Eval.defaultComponent();
     this.environment = System.getenv("ENVIRONMENT");
     this.stackHasher = new StackHasher(StackElementFilter.builder().allFilters().build());
   }
@@ -131,11 +131,11 @@ public void setIncludeStackHash(boolean includeStackHash) {
   }
 
   public void setComponent(String component) {
-    this.component = component;
+    this.component = Eval.eval(component);
   }
 
   public void setEnvironment(String environment) {
-    this.environment = environment;
+    this.environment = Eval.eval(environment);
   }
 
   public void setThrowableConverter(ThrowableHandlingConverter throwableConverter) {
@@ -147,7 +147,12 @@ public void setCustomFields(String customFields) {
       return;
     }
     var mapper = SimpleMapper.builder().jsonStream(json).build();
-    mapper.map().fromJson(customFields).forEach((k, v) -> customFieldsMap.put(k, mapper.toJson(v)));
+    mapper.map().fromJson(customFields).forEach((key, value) -> {
+      if (value instanceof String) {
+        value = Eval.eval((String) value);
+      }
+      customFieldsMap.put(key, mapper.toJson(value));
+    });
   }
 
   public void setTimestampPattern(String pattern) {

src/test/java/io/avaje/logback/encoder/EvalTest.java

@@ -0,0 +1,56 @@
+package io.avaje.logback.encoder;
+
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class EvalTest {
+
+  @BeforeAll
+  static void beforeAll() {
+    System.setProperty("my.property", "x42");
+  }
+
+  @Test
+  void eval_basic() {
+    assertThat(Eval.eval("${my.property}")).isEqualTo("x42");
+  }
+
+  @Test
+  void eval_defaultValue() {
+    assertThat(Eval.eval("${my.property:someDefaultValue}")).isEqualTo("x42");
+    assertThat(Eval.eval("${my.other:someDefaultValue}")).isEqualTo("someDefaultValue");
+    assertThat(Eval.eval("${my.other}")).isEqualTo("${my.other}");
+  }
+
+  @Test
+  void eval_nonMatchingEnds() {
+    assertThat(Eval.eval("my.property")).isEqualTo("my.property");
+    assertThat(Eval.eval("${my.property")).isEqualTo("${my.property");
+    assertThat(Eval.eval("my.property}")).isEqualTo("my.property}");
+  }
+
+  @Test
+  void toSystemPropertyKey() {
+    assertThat(Eval.toSystemPropertyKey("FOO")).isEqualTo("foo");
+    assertThat(Eval.toSystemPropertyKey("MY_FOO")).isEqualTo("my.foo");
+    assertThat(Eval.toSystemPropertyKey("A_MY_FOO")).isEqualTo("a.my.foo");
+    assertThat(Eval.toSystemPropertyKey("my.foo")).isEqualTo("my.foo");
+  }
+
+  @Test
+  void k8sComponent_expected() {
+    assertThat(Eval.k8sComponent("some-x-y")).isEqualTo("some");
+    assertThat(Eval.k8sComponent("my-some-x-y")).isEqualTo("my-some");
+    assertThat(Eval.k8sComponent("foo-bar-some-x-y")).isEqualTo("foo-bar-some");
+  }
+
+  @Test
+  void k8sComponent_unexpected() {
+    assertThat(Eval.k8sComponent(null)).isNull();
+    assertThat(Eval.k8sComponent("")).isNull();
+    assertThat(Eval.k8sComponent("some")).isNull();
+    assertThat(Eval.k8sComponent("some-x")).isNull();
+  }
+}

src/test/java/io/avaje/logback/encoder/JsonEncoderTest.java

@@ -68,17 +68,33 @@ void encode_component() {
     }
 
     @Test
-    void throwable_usingDefault() {
+    void customFieldsEval() {
+        System.setProperty("some.custom.property", "Hi!");
         JsonEncoder encoder = new JsonEncoder();
+        encoder.setCustomFields("{\"my-custom\":\"${some.custom.property}\", \"other\": \"myLiteral\", \"more\": 12}");
         encoder.start();
 
         byte[] bytes = encoder.encode(createLogEvent(createThrowable()));
         SimpleMapper simpleMapper = SimpleMapper.builder().build();
         Map<String, Object> asMap = simpleMapper.map().fromJson(bytes);
 
-        assertThat((String)asMap.get("stacktrace")).startsWith("java.lang.NullPointerException: ");
+        assertThat((String)asMap.get("my-custom")).isEqualTo("Hi!");
+        assertThat((String)asMap.get("other")).isEqualTo("myLiteral");
+        assertThat((Long)asMap.get("more")).isEqualTo(12L);
     }
 
+  @Test
+  void throwable_usingDefault() {
+    JsonEncoder encoder = new JsonEncoder();
+    encoder.start();
+
+    byte[] bytes = encoder.encode(createLogEvent(createThrowable()));
+    SimpleMapper simpleMapper = SimpleMapper.builder().build();
+    Map<String, Object> asMap = simpleMapper.map().fromJson(bytes);
+
+    assertThat((String)asMap.get("stacktrace")).startsWith("java.lang.NullPointerException: ");
+  }
+
     @Test
     void throwable_usingConverter() {
         final TrimPackageAbbreviator trimPackages = new TrimPackageAbbreviator();
@@ -120,7 +136,7 @@ void throwable_usingConverter_includeStackHash() {
         Map<String, Object> asMap = simpleMapper.map().fromJson(bytes);
 
         assertThat((String)asMap.get("stacktrace")).startsWith("j.l.NullPointerException: ");
-        assertThat((String)asMap.get("stackhash")).isEqualTo("2a4a23a6");
+        assertThat((String)asMap.get("stackhash")).isEqualTo("cd6925a6");
     }
 
     @Test