logback-android/src/main/java/ch/qos/logback/core/property/ResourceExistsPropertyDefiner.java
public String getPropertyValue() {
    if (OptionHelper.isEmpty(resourceStr)) {
      addError("The \"resource\" property must be set.");
      return null;
    }

    URL resourceURL = Loader.getResourceBySelfClassLoader(resourceStr);
    return booleanAsStr(resourceURL != null);
  }
public String getPropertyValue() {
    if (OptionHelper.isEmpty(resourceStr)) {
      addError("The \"resource\" property must be set.");
      return null;
    }

    URL resourceURL = Loader.getResourceBySelfClassLoader(resourceStr);
    return booleanAsStr(resourceURL != null);
  }
Returns the string "true" if the {@link #setResource(String) resource} specified by the
user is available on the class path, "false" otherwise.

@return "true"|"false" depending on the availability of resource on the classpath