@Target (ANNOTATION_TYPE ) @Retention (RUNTIME ) public @interface ContentType

元注解，指定一个注解表示一种内容类型，例如时间跨度或频率。

以下示例显示了如何创建和使用温度内容类型。

首先使用ContentType注解声明一个温度注解：

@MetadataDefinition
@ContentType
@Name("com.example.Temperature")
@Label("Temperature")
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Temperature {
  public final static String KELVIN = "KELVIN";
  public final static String CELSIUS = "CELSIUS";
  public final static String FAHRENEHIT = "FAHRENHEIT";

  String value() default CELSIUS;
}

然后声明一个事件，注释一个字段并提交数据：

@Name("com.example.CPU")
@Label("CPU")
@Category({ "Hardware", "CPU" })
@Period("1 s")
@StackTrace(false)
static public class CPUEvent extends Event {
  @Label("ID")
  String id;

  @Temperature(Temperature.KELVIN)
  @Label("Temperature")
  float temperature;
}

public static void main(String... args) throws InterruptedException {
  FlightRecorder.addPeriodicEvent(CPUEvent.class, () -> {
    for (var cpu : listCPUs()) {
      CPUEvent event = new CPUEvent();
      event.id = cpu.id();
      event.temperature = cpu.temperature(); // in Kelvin
      event.commit();
    }
  });
  Thread.sleep(10_000);
}

最后，在显示事件数据时检查注解：

void printTemperaturesInCelsius(Path file) throws IOException {
  for (RecordedEvent event : RecordingFile.readAllEvents(file)) {
    for (ValueDescriptor field : event.getEventType().getFields()) {
      for (AnnotationElement ae : field.getAnnotationElements()) {
        ContentType type = ae.getAnnotation(ContentType.class);
        if (type != null) {
          if (ae.getTypeName().equals("com.example.Temperature")) {
            double value = event.getDouble(field.getName());
            String unit = (String) ae.getValue("value");
            double celsius = switch (unit) {
              case "CELSIUS" -> value;
              case "KELVIN" -> value - 273.15;
              case "FAHRENHEIT" -> (value - 32) / 1.8;
              default -> throw new IllegalStateException("Unknown temperature unit '" + unit + "'");
            };
            System.out.println(celsius + " C");
          } else {
            System.err.println("Can't format content type " + ae.getTypeName() + " for field " + field.getName());
          }
        }
      }
    }
  }
}

自从：: 9

注解 ContentType