ConfigLoaderTest.java

package com.nemesis.training;

import org.junit.jupiter.api.Test;

import java.util.Properties;

import static org.junit.jupiter.api.Assertions.*;

class ConfigLoaderTest {

    @Test
    void mustLoadApplicationPropertiesWhenFileExists() {
        // Arrange
        String fileName = "application.properties";

        // Act
        Properties props = ConfigLoader.load(fileName);

        // Assert
        assertFalse(props.isEmpty());
    }

    @Test
    void mustThrowIllegalArgumentExceptionWhenFileDoesNotExist() {
        // Arrange
        String fileName = "foo.properties";

        // Act + Assert
        assertThrows(IllegalArgumentException.class, () -> ConfigLoader.load(fileName));
    }

    @Test
    void mustThrowIllegalStateExceptionWhenFileIsEmpty() {
        // Arrange
        String fileName = "empty.properties";

        // Act + Assert
        assertThrows(IllegalStateException.class, () -> ConfigLoader.load(fileName));
    }

    @Test
    void mustGetPropertiesFromApplicationPropertiesWhenGetPropertyIsCalled() {
        // Arrange
        System.setProperty("config.file", "application.properties");

        // Act
        Properties props = ConfigLoader.loadSystemPropertyAndLoad();

        // Arrange
        assertNotNull(props);
    }
}

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package com.nemesis.training;

import java.io.IOException;
import java.io.InputStream;
import java.io.UncheckedIOException;
import java.util.Properties;

public class ConfigLoader {
  public static Properties load(String fileName) {
    Properties props = new Properties();

    try (InputStream input = ConfigLoader.class.getClassLoader().getResourceAsStream(fileName)) {
      if (input == null) {
        throw new IllegalArgumentException("ERROR: File not found: " + fileName);
      }
      props.load(input);

      if(props.isEmpty()) {
          throw new IllegalStateException("ERROR: Properties file is empty: " + fileName);
      }

    } catch (IOException e) {
      throw new UncheckedIOException("ERROR: Error while loading: " + fileName, e);
    }
    return props;
  }

  public static Properties loadSystemPropertyAndLoad() {
    return load(System.getProperty("config.file", "application.properties"));
  }
}