Code review changes:

Argument parsing moved to the main class.
Argument validation now throw exception.
More tests added for config factory.
Created unit tests for JmxScraper class.

jmx-scraper/src/main/java/io/opentelemetry/contrib/jmxscraper/ArgumentsParsingException.java

@@ -0,0 +1,10 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.contrib.jmxscraper;
+
+public class ArgumentsParsingException extends Exception {
+  private static final long serialVersionUID = 0L;
+}

jmx-scraper/src/main/java/io/opentelemetry/contrib/jmxscraper/JmxScraper.java

@@ -9,19 +9,101 @@
 import io.opentelemetry.contrib.jmxscraper.config.JmxScraperConfig;
 import io.opentelemetry.contrib.jmxscraper.config.JmxScraperConfigFactory;
 import io.opentelemetry.contrib.jmxscraper.jmx.JmxClient;
+import java.io.DataInputStream;
+import java.io.IOException;
+import java.io.InputStream;
 import java.net.MalformedURLException;
+import java.nio.file.Files;
+import java.nio.file.Paths;
 import java.util.Arrays;
+import java.util.List;
+import java.util.Properties;
 import java.util.concurrent.Executors;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.TimeUnit;
 import java.util.logging.Logger;
 
 public class JmxScraper {
   private static final Logger logger = Logger.getLogger(JmxScraper.class.getName());
+  private static final int EXECUTOR_TERMINATION_TIMEOUT_MS = 5000;
   private final ScheduledExecutorService exec = Executors.newSingleThreadScheduledExecutor();
   private final JmxScraperConfig config;
 
-  JmxScraper(JmxScraperConfig config) {
+  /**
+   * Main method to create and run a {@link JmxScraper} instance.
+   *
+   * @param args - must be of the form "-config {jmx_config_path,'-'}"
+   */
+  @SuppressWarnings({"SystemOut", "SystemExitOutsideMain"})
+  public static void main(String[] args) {
+    try {
+      JmxScraperConfigFactory factory = new JmxScraperConfigFactory();
+      JmxScraperConfig config = JmxScraper.createConfigFromArgs(Arrays.asList(args), factory);
+
+      JmxScraper jmxScraper = new JmxScraper(config);
+      jmxScraper.start();
+
+      Runtime.getRuntime()
+          .addShutdownHook(
+              new Thread() {
+                @Override
+                public void run() {
+                  jmxScraper.shutdown();
+                }
+              });
+    } catch (ArgumentsParsingException e) {
+      System.err.println(
+          "Usage: java -jar <path_to_jmxscraper.jar> "
+              + "-config <path_to_config.properties or - for stdin>");
+      System.exit(1);
+    } catch (ConfigurationException e) {
+      System.err.println(e.getMessage());
+      System.exit(1);
+    }
+  }
+
+  /**
+   * Create {@link JmxScraperConfig} object basing on command line options
+   *
+   * @param args application commandline arguments
+   */
+  static JmxScraperConfig createConfigFromArgs(List<String> args, JmxScraperConfigFactory factory)
+      throws ArgumentsParsingException, ConfigurationException {
+    if (!args.isEmpty() && (args.size() != 2 || !args.get(0).equalsIgnoreCase("-config"))) {
+      throw new ArgumentsParsingException();
+    }
+
+    Properties loadedProperties = new Properties();
+    if (args.size() == 2) {
+      String path = args.get(1);
+      if (path.trim().equals("-")) {
+        loadPropertiesFromStdin(loadedProperties);
+      } else {
+        loadPropertiesFromPath(loadedProperties, path);
+      }
+    }
+
+    return factory.createConfig(loadedProperties);
+  }
+
+  private static void loadPropertiesFromStdin(Properties props) throws ConfigurationException {
+    try (InputStream is = new DataInputStream(System.in)) {
+      props.load(is);
+    } catch (IOException e) {
+      throw new ConfigurationException("Failed to read config properties from stdin", e);
+    }
+  }
+
+  private static void loadPropertiesFromPath(Properties props, String path)
+      throws ConfigurationException {
+    try (InputStream is = Files.newInputStream(Paths.get(path))) {
+      props.load(is);
+    } catch (IOException e) {
+      throw new ConfigurationException("Failed to read config properties file: '" + path + "'", e);
+    }
+  }
+
+  JmxScraper(JmxScraperConfig config) throws ConfigurationException {
     this.config = config;
 
     try {
@@ -51,28 +133,23 @@ private void start() {
 
   private void shutdown() {
     logger.info("Shutting down JmxScraper and exporting final metrics.");
+    // Prevent new tasks to be submitted
     exec.shutdown();
-  }
-
-  /**
-   * Main method to create and run a {@link JmxScraper} instance.
-   *
-   * @param args - must be of the form "-config {jmx_config_path,'-'}"
-   */
-  public static void main(String[] args) {
-    JmxScraperConfigFactory factory = new JmxScraperConfigFactory();
-    JmxScraperConfig config = factory.createConfigFromArgs(Arrays.asList(args));
-
-    JmxScraper jmxScraper = new JmxScraper(config);
-    jmxScraper.start();
-
-    Runtime.getRuntime()
-        .addShutdownHook(
-            new Thread() {
-              @Override
-              public void run() {
-                jmxScraper.shutdown();
-              }
-            });
+    try {
+      // Wait a while for existing tasks to terminate
+      if (!exec.awaitTermination(EXECUTOR_TERMINATION_TIMEOUT_MS, TimeUnit.MILLISECONDS)) {
+        // Cancel currently executing tasks
+        exec.shutdownNow();
+        // Wait a while for tasks to respond to being cancelled
+        if (!exec.awaitTermination(EXECUTOR_TERMINATION_TIMEOUT_MS, TimeUnit.MILLISECONDS)) {
+          logger.warning("Thread pool did not terminate in time: " + exec);
+        }
+      }
+    } catch (InterruptedException e) {
+      // (Re-)Cancel if current thread also interrupted
+      exec.shutdownNow();
+      // Preserve interrupt status
+      Thread.currentThread().interrupt();
+    }
   }
 }

jmx-scraper/src/main/java/io/opentelemetry/contrib/jmxscraper/config/ConfigurationException.java

@@ -5,7 +5,7 @@
 
 package io.opentelemetry.contrib.jmxscraper.config;
 
-public class ConfigurationException extends RuntimeException {
+public class ConfigurationException extends Exception {
   private static final long serialVersionUID = 0L;
 
   public ConfigurationException(String message, Throwable cause) {

jmx-scraper/src/main/java/io/opentelemetry/contrib/jmxscraper/config/JmxScraperConfigFactory.java

@@ -7,18 +7,12 @@
 
 import static io.opentelemetry.contrib.jmxscraper.util.StringUtils.isBlank;
 
-import java.io.DataInputStream;
-import java.io.IOException;
-import java.io.InputStream;
-import java.nio.file.Files;
-import java.nio.file.Paths;
 import java.util.Arrays;
 import java.util.List;
 import java.util.Locale;
 import java.util.Properties;
 import java.util.stream.Collectors;
 
-@SuppressWarnings({"SystemOut", "SystemExitOutsideMain"})
 public class JmxScraperConfigFactory {
   private static final String PREFIX = "otel.";
   static final String SERVICE_URL = PREFIX + "jmx.service.url";
@@ -64,56 +58,7 @@ public class JmxScraperConfigFactory {
 
   private Properties properties = new Properties();
 
-  /**
-   * Create {@link JmxScraperConfig} object basing on command line options
-   *
-   * @param args application commandline arguments
-   */
-  public JmxScraperConfig createConfigFromArgs(List<String> args) {
-    if (!args.isEmpty() && (args.size() != 2 || !args.get(0).equalsIgnoreCase("-config"))) {
-      System.out.println(
-          "Usage: java io.opentelemetry.contrib.jmxscraper.JmxScraper "
-              + "-config <path_to_config.properties or - for stdin>");
-      System.exit(1);
-    }
-
-    Properties loadedProperties = new Properties();
-    if (args.size() == 2) {
-      String path = args.get(1);
-      if (path.trim().equals("-")) {
-        loadPropertiesFromStdin(loadedProperties);
-      } else {
-        loadPropertiesFromPath(loadedProperties, path);
-      }
-    }
-
-    JmxScraperConfig config = createConfig(loadedProperties);
-    validateConfig(config);
-    populateJmxSystemProperties();
-
-    return config;
-  }
-
-  private static void loadPropertiesFromStdin(Properties props) {
-    try (InputStream is = new DataInputStream(System.in)) {
-      props.load(is);
-    } catch (IOException e) {
-      System.out.println("Failed to read config properties from stdin: " + e.getMessage());
-      System.exit(1);
-    }
-  }
-
-  private static void loadPropertiesFromPath(Properties props, String path) {
-    try (InputStream is = Files.newInputStream(Paths.get(path))) {
-      props.load(is);
-    } catch (IOException e) {
-      System.out.println(
-          "Failed to read config properties file at '" + path + "': " + e.getMessage());
-      System.exit(1);
-    }
-  }
-
-  JmxScraperConfig createConfig(Properties props) {
+  public JmxScraperConfig createConfig(Properties props) throws ConfigurationException {
     properties = new Properties();
     // putAll() instead of using constructor defaults
     // to ensure they will be recorded to underlying map
@@ -148,6 +93,9 @@ JmxScraperConfig createConfig(Properties props) {
 
     config.registrySsl = Boolean.parseBoolean(properties.getProperty(REGISTRY_SSL));
 
+    validateConfig(config);
+    populateJmxSystemProperties();
+
     return config;
   }
 
@@ -168,7 +116,7 @@ private void populateJmxSystemProperties() {
         });
   }
 
-  private int getProperty(String key, int defaultValue) {
+  private int getProperty(String key, int defaultValue) throws ConfigurationException {
     String propVal = properties.getProperty(key);
     if (propVal == null) {
       return defaultValue;
@@ -191,7 +139,8 @@ private String getAndSetPropertyIfUndefined(String key, String defaultValue) {
     return propVal;
   }
 
-  private int getAndSetPropertyIfUndefined(String key, int defaultValue) {
+  private int getAndSetPropertyIfUndefined(String key, int defaultValue)
+      throws ConfigurationException {
     int propVal = getProperty(key, defaultValue);
     if (propVal == defaultValue) {
       properties.setProperty(key, String.valueOf(defaultValue));
@@ -200,7 +149,7 @@ private int getAndSetPropertyIfUndefined(String key, int defaultValue) {
   }
 
   /** Will determine if parsed config is complete, setting any applicable values and defaults. */
-  void validateConfig(JmxScraperConfig config) {
+  private static void validateConfig(JmxScraperConfig config) throws ConfigurationException {
     if (isBlank(config.serviceUrl)) {
       throw new ConfigurationException(SERVICE_URL + " must be specified.");
     }

jmx-scraper/src/main/java/io/opentelemetry/contrib/jmxscraper/jmx/JmxClient.java

@@ -38,7 +38,6 @@ public class JmxClient {
 
   public JmxClient(JmxScraperConfig config) throws MalformedURLException {
     this.url = new JMXServiceURL(config.getServiceUrl());
-    ;
     this.username = config.getUsername();
     this.password = config.getPassword();
     this.realm = config.getRealm();

jmx-scraper/src/test/java/io/opentelemetry/contrib/jmxscraper/JmxScraperTest.java

@@ -0,0 +1,39 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.contrib.jmxscraper;
+
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+import static org.mockito.Mockito.mock;
+
+import io.opentelemetry.contrib.jmxscraper.config.JmxScraperConfigFactory;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+class JmxScraperTest {
+  @Test
+  void shouldThrowExceptionWhenInvalidCommandLineArgsProvided() {
+    // Given
+    List<String> emptyArgs = Collections.singletonList("-inexistingOption");
+    JmxScraperConfigFactory configFactoryMock = mock(JmxScraperConfigFactory.class);
+
+    // When and Then
+    assertThatThrownBy(() -> JmxScraper.createConfigFromArgs(emptyArgs, configFactoryMock))
+        .isInstanceOf(ArgumentsParsingException.class);
+  }
+
+  @Test
+  void shouldThrowExceptionWhenTooManyCommandLineArgsProvided() {
+    // Given
+    List<String> emptyArgs = Arrays.asList("-config", "path", "-inexistingOption");
+    JmxScraperConfigFactory configFactoryMock = mock(JmxScraperConfigFactory.class);
+
+    // When and Then
+    assertThatThrownBy(() -> JmxScraper.createConfigFromArgs(emptyArgs, configFactoryMock))
+        .isInstanceOf(ArgumentsParsingException.class);
+  }
+}