Test and document otel.javaagent.configuration-file (#3585)

* test and document configuration file property

* use TemporaryFolder rule, documentation changes after review

docs/agent-config.md

@@ -10,6 +10,7 @@ Please report any bugs or unexpected behavior you find.
 ## Contents
 
 * [SDK Autoconfiguration](#sdk-autoconfiguration)
+* [Configuring the agent](#configuring-the-agent)
 * [Peer service name](#peer-service-name)
 * [DB statement sanitization](#db-statement-sanitization)
 * [Suppressing specific auto-instrumentation](#suppressing-specific-auto-instrumentation)
@@ -35,6 +36,23 @@ Here are some quick links into those docs for the configuration options for spec
 * [Span limits](https://github.com/open-telemetry/opentelemetry-java/tree/main/sdk-extensions/autoconfigure/README.md#span-limits)
 * [Using SPI to further configure the SDK](https://github.com/open-telemetry/opentelemetry-java/tree/main/sdk-extensions/autoconfigure/README.md#customizing-the-opentelemetry-sdk)
 
+## Configuring the agent
+
+The agent can consume configuration from one or more of the following sources (ordered from highest to lowest priority):
+* system properties
+* environment variables
+* the [configuration file](#configuration-file)
+* the [`ConfigPropertySource`](https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/main/javaagent-extension-api/src/main/java/io/opentelemetry/javaagent/extension/config/ConfigPropertySource.java) SPI
+
+### Configuration file
+
+You can provide a path to agent configuration file by setting the corresponding property.
+
+| System property                      | Environment variable                 | Description                                                                      |
+|--------------------------------------|--------------------------------------|----------------------------------------------------------------------------------|
+| `otel.javaagent.configuration-file` | `OTEL_JAVAAGENT_CONFIGURATION_FILE` | Path to valid Java properties file which contains the javaagent configuration.|
+
+
 ## Peer service name
 
 The [peer service name](https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/semantic_conventions/span-general.md#general-remote-service-attributes) is the name of a remote service being connected to. It corresponds to `service.name` in the [Resource](https://github.com/open-telemetry/opentelemetry-specification/tree/master/specification/resource/semantic_conventions#service) for the local service.

javaagent-tooling/src/main/java/io/opentelemetry/javaagent/tooling/config/ConfigInitializer.java

@@ -22,8 +22,9 @@ import org.slf4j.LoggerFactory;
 public final class ConfigInitializer {
   private static final Logger logger = LoggerFactory.getLogger(ConfigInitializer.class);
 
-  private static final String CONFIGURATION_FILE_PROPERTY = "otel.javaagent.configuration-file";
-  private static final String CONFIGURATION_FILE_ENV_VAR = "OTEL_JAVAAGENT_CONFIGURATION_FILE";
+  // visible for testing
+  static final String CONFIGURATION_FILE_PROPERTY = "otel.javaagent.configuration-file";
+  static final String CONFIGURATION_FILE_ENV_VAR = "OTEL_JAVAAGENT_CONFIGURATION_FILE";
 
   public static void initialize() {
     Config.internalInitializeConfig(create(loadSpiConfiguration(), loadConfigurationFile()));
@@ -54,7 +55,8 @@ public final class ConfigInitializer {
    * @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() {
+  // visible for testing
+  static Properties loadConfigurationFile() {
     Properties properties = new Properties();
 
     // Reading from system property first and from env after

javaagent-tooling/src/test/groovy/io/opentelemetry/javaagent/tooling/config/ConfigurationFileTest.groovy

@@ -0,0 +1,88 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.javaagent.tooling.config
+
+import org.junit.Rule
+import org.junit.contrib.java.lang.system.EnvironmentVariables
+import org.junit.contrib.java.lang.system.RestoreSystemProperties
+import org.junit.rules.TemporaryFolder
+import spock.lang.Specification
+
+class ConfigurationFileTest extends Specification {
+
+  @Rule
+  public final RestoreSystemProperties restoreSystemProperties = new RestoreSystemProperties()
+  @Rule
+  public final EnvironmentVariables environmentVariables = new EnvironmentVariables()
+  @Rule
+  public final TemporaryFolder tmpFolder = new TemporaryFolder()
+
+  def "should use env property"() {
+    given:
+    def path = createFile("config", "property1=val-env")
+    environmentVariables.set(ConfigInitializer.CONFIGURATION_FILE_ENV_VAR, path)
+
+    when:
+    def properties = ConfigInitializer.loadConfigurationFile()
+
+    then:
+    properties.get("property1") == "val-env"
+  }
+
+  def "should use system property"() {
+    given:
+    def path = createFile("config", "property1=val-sys")
+    System.setProperty(ConfigInitializer.CONFIGURATION_FILE_PROPERTY, path)
+
+    when:
+    def properties = ConfigInitializer.loadConfigurationFile()
+
+    then:
+    properties.get("property1") == "val-sys"
+  }
+
+  def "system property should take precedence over env property"() {
+    given:
+    def pathEnv = createFile("configEnv", "property1=val-env")
+    def pathSys = createFile("configSys", "property1=val-sys")
+
+    environmentVariables.set(ConfigInitializer.CONFIGURATION_FILE_ENV_VAR, pathEnv)
+    System.setProperty(ConfigInitializer.CONFIGURATION_FILE_PROPERTY, pathSys)
+
+    when:
+    def properties = ConfigInitializer.loadConfigurationFile()
+
+    then:
+    properties.get("property1") == "val-sys"
+  }
+
+
+  def "should return empty properties if file does not exist"() {
+    given:
+    environmentVariables.set(ConfigInitializer.CONFIGURATION_FILE_ENV_VAR, "somePath")
+
+    when:
+    def properties = ConfigInitializer.loadConfigurationFile()
+
+    then:
+    !properties.propertyNames().hasMoreElements()
+  }
+
+  def "should return empty properties if property is not set"() {
+    when:
+    def properties = ConfigInitializer.loadConfigurationFile()
+
+    then:
+    !properties.propertyNames().hasMoreElements()
+  }
+
+  def createFile(String name, String contents){
+    def file = tmpFolder.newFile(name)
+    file.write(contents)
+    return file.getAbsolutePath()
+  }
+
+}