gauge
gauge是测量值，数值可以上下变化。
（ps. 对比counter，通常用于自增场景）
/**
* A gauge tracks a value that may go up or down. The value that is published for gauges is
* an instantaneous sample of the gauge at publishing time.
*
*/
public interface Gauge extends Meter {
class Builder<T> {
private final String name;
private final ToDoubleFunction<T> f;
private Tags tags = Tags.empty();
// 默认是弱引用方式，可能导致value()返回Double.NAN
private boolean strongReference = false;
@Nullable
private Meter.Id syntheticAssociation = null;
// 被关联的对象
@Nullable
private final T obj;
@Nullable
private String description;
@Nullable
private String baseUnit;
public Gauge register(MeterRegistry registry) {
return registry.gauge(new Meter.Id(name, tags, baseUnit, description, Type.GAUGE, syntheticAssociation), obj,
strongReference ? new StrongReferenceGaugeFunction<>(obj, f) : f);
}
}
}
@FunctionalInterface
public interface ToDoubleFunction<T> {
double applyAsDouble(T value);
}
gauge会关联真实对象，并且把从真实对象获取计数值，抽象为ToDoubleFunction。
DefaultGauge是基本实现，用WeakReference关联真实的对象。
使用例子：
// 参数：
// - metric name
// - 要关联的真实对象
// - ToDoubleFunction 的实现
Queue<Message> receivedMessages = registry.gauge("unprocessed.messages", new ConcurrentLinkedQueue<>(), ConcurrentLinkedQueue::size);
注册gauge直接返回了底层关联的真实对象，这和其他的metrics类型不一样。
// maintain a reference to myGauge
AtomicInteger myGauge = registry.gauge("numberGauge", new AtomicInteger(0));
// ... elsewhere you can update the value it holds using the object reference
myGauge.set(27);
myGauge.set(11);
反复手动更新gauge的值是无意义的。只有在发布时刻（publish time）的实例值才有意义。
场景问题： 返回NAN
gauge使用过程中，常见问题是gauge测量值返回NAN。
DefaultGauge使用WeakReference关联gauge和真实对象。如果对象被回收，那么就回返回NAN。之所以使用WeakReference，是为了避免影响gc。
public class DefaultGauge<T> extends AbstractMeter implements Gauge {
private final WeakReference<T> ref;
private final ToDoubleFunction<T> value;
public DefaultGauge(Meter.Id id, @Nullable T obj, ToDoubleFunction<T> value) {
super(id);
this.ref = new WeakReference<>(obj);
this.value = value;
}
public double value() {
T obj = ref.get();
if (obj != null) {
try {
return value.applyAsDouble(ref.get());
}
catch (Throwable ex) {
logger.log("Failed to apply the value function for the gauge '" + getId().getName() + "'.", ex);
}
}
return Double.NaN;
}
}
micormeter也提供了强引用的方式关联gauge和真实对象的方式。
class StrongReferenceGaugeFunction<T> implements ToDoubleFunction<T> {
/**
* Holding a reference to obj inside of this function effectively prevents it from being
* garbage collected. Implementors of {@link Gauge} can then assume that they should hold
* {@code obj} as a weak reference.
* <p>
* If obj is {@code null} initially then this gauge will not be reported.
*/
@Nullable
@SuppressWarnings("FieldCanBeLocal")
private final T obj;
private final ToDoubleFunction<T> f;
StrongReferenceGaugeFunction(@Nullable T obj, ToDoubleFunction<T> f) {
this.obj = obj;
this.f = f;
}
@Override
public double applyAsDouble(T value) {
return f.applyAsDouble(value);
}
}