[Java] hot reload Yaml

高可用條件之一是能夠 hot reload (熱重載)。簡單的做法是每次使用的時候就是讀檔，使用 Properties 或是 snakeyaml。另一種方法是使用 commons-configuration2 套件提供的功能：

public static ReloadingFileBasedConfigurationBuilder<YAMLConfiguration> getBuilder(File file) {
   Parameters params = new Parameters();
   ReloadingFileBasedConfigurationBuilder<YAMLConfiguration> builder =
      new ReloadingFileBasedConfigurationBuilder<>(YAMLConfiguration.class)
         .configure(params.fileBased().setFile(file));

   // 每次讀取時檢查
   builder.addEventListener(ConfigurationBuilderEvent.CONFIGURATION_REQUEST,
      event -> builder.getReloadingController().checkForReloading(null));

   // 每分鐘檢查
   PeriodicReloadingTrigger trigger = new PeriodicReloadingTrigger(builder.getReloadingController(),
      null, 1, TimeUnit.MINUTES);
   trigger.start();

   return builder;
}

@Test
public void testHotReload() throws Exception {
   File tmpFile = File.createTempFile("test", ".yml");
   tmpFile.deleteOnExit();

   String raw1 = "sample:\n    property: test-1\n";
   Files.write(tmpFile.toPath(), raw1.getBytes());

   ImmutableConfiguration config1 = getBuilder(tmpFile).getConfiguration();
   Assert.assertEquals("test before hot reload", "test-1", config1.getString("sample.property"));

   // NOTE. File#setLastModified() only guarantees that the file modification time is accurate to the second, so
   // the modification must be delayed by 1 second.
   Thread.sleep(1000);
   String raw2 = "sample:\n    property: test-2\n";
   Files.write(tmpFile.toPath(), raw2.getBytes());

   ImmutableConfiguration config2 = getBuilder(tmpFile).getConfiguration();
   Assert.assertEquals("test after hot reload", "test-2", config2.getString("sample.property"));
}

參考資料：

• Apache Commons Configuration: Automatic Reloading of Configuration Sources