logback-android/src/main/java/ch/qos/logback/classic/util/CopyOnInheritThreadLocal.java
@Override
  protected HashMap<String, String> childValue(
      HashMap<String, String> parentValue) {
    if (parentValue == null) {
      return null;
    } else {
      return new HashMap<String, String>(parentValue);
    }
  }
@Override
  protected HashMap<String, String> childValue(
      HashMap<String, String> parentValue) {
    if (parentValue == null) {
      return null;
    } else {
      return new HashMap<String, String>(parentValue);
    }
  }
Child threads should get a copy of the parent's hashmap.