Tests for functionality: Config fallback to properties files

dd-trace-api/src/main/java/datadog/trace/api/Config.java

@@ -191,11 +191,15 @@ public class Config {
   @Getter private final List<String> traceExecutors;
 
   @Getter private final boolean traceAnalyticsEnabled;
-  private static final Properties propertiesFromConfigFile = loadConfigurationFile();
+
+  // Values from an optionally provided properties file
+  private static Properties propertiesFromConfigFile;
 
   // Read order: System Properties -> Env Variables, [-> default value]
   // Visible for testing
   Config() {
+    propertiesFromConfigFile = loadConfigurationFile();
+
     runtimeId = UUID.randomUUID().toString();
 
     serviceName = getSettingFromEnvironment(SERVICE_NAME, DEFAULT_SERVICE_NAME);
@@ -868,7 +872,7 @@ public class Config {
    * @return The {@link Properties} object. the returned instance might be empty of file does not
    *     exist or if it is in a wrong format.
    */
-  private static Properties loadConfigurationFile() {
+  private static synchronized Properties loadConfigurationFile() {
     Properties properties = new Properties();
 
     String configurationFilePath =
@@ -892,7 +896,7 @@ public class Config {
       properties.load(fileReader);
     } catch (FileNotFoundException fnf) {
       log.error("Configuration file '{}' not found.", configurationFilePath);
-    } catch (IOException e) {
+    } catch (IOException ioe) {
       log.error(
           "Configuration file '{}' cannot be accessed or correctly parsed.", configurationFilePath);
     }

dd-trace-api/src/test/groovy/datadog/trace/api/ConfigTest.groovy

@@ -8,7 +8,9 @@ import spock.lang.Specification
 import static datadog.trace.api.Config.AGENT_HOST
 import static datadog.trace.api.Config.AGENT_PORT_LEGACY
 import static datadog.trace.api.Config.AGENT_UNIX_DOMAIN_SOCKET
+import static datadog.trace.api.Config.CONFIGURATION_FILE
 import static datadog.trace.api.Config.DEFAULT_JMX_FETCH_STATSD_PORT
+import static datadog.trace.api.Config.DEFAULT_SERVICE_NAME
 import static datadog.trace.api.Config.GLOBAL_TAGS
 import static datadog.trace.api.Config.HEADER_TAGS
 import static datadog.trace.api.Config.HTTP_CLIENT_ERROR_STATUSES
@@ -715,4 +717,50 @@ class ConfigTest extends Specification {
     then:
     config.localRootSpanTags.get('_dd.hostname') == InetAddress.localHost.hostName
   }
+
+  def "verify fallback to properties file"() {
+    setup:
+    System.setProperty(PREFIX + CONFIGURATION_FILE, "src/test/resources/dd-java-tracer.properties")
+
+    when:
+    def config = new Config()
+
+    then:
+    config.serviceName == "set-in-properties"
+  }
+
+  def "verify fallback to properties file has lower priority then system property"() {
+    setup:
+    System.setProperty(PREFIX + CONFIGURATION_FILE, "src/test/resources/dd-java-tracer.properties")
+    System.setProperty(PREFIX + SERVICE_NAME, "set-in-system")
+
+    when:
+    def config = new Config()
+
+    then:
+    config.serviceName == "set-in-system"
+  }
+
+  def "verify fallback to properties file has lower priority then env var"() {
+    setup:
+    System.setProperty(PREFIX + CONFIGURATION_FILE, "src/test/resources/dd-java-tracer.properties")
+    environmentVariables.set("DD_SERVICE_NAME", "set-in-env")
+
+    when:
+    def config = new Config()
+
+    then:
+    config.serviceName == "set-in-env"
+  }
+
+  def "verify fallback to properties file that does not exist does not crash app"() {
+    setup:
+    System.setProperty(PREFIX + CONFIGURATION_FILE, "src/test/resources/do-not-exist.properties")
+
+    when:
+    def config = new Config()
+
+    then:
+    config.serviceName == 'unnamed-java-app'
+  }
 }

dd-trace-api/src/test/resources/dd-java-tracer.properties

@@ -0,0 +1 @@
+dd.service.name=set-in-properties