stats/bookkeeper-stats-providers/otel-metrics-provider/src/main/java/org/apache/bookkeeper/stats/otel/OtelMetricsProvider.java [125:147]:
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    private static final AtomicLong directMemoryUsage;
    private static final Optional<BufferPoolMXBean> poolMxBeanOp;
    private static final Supplier<Double> getDirectMemoryUsage;

    static {
        if (PlatformDependent.useDirectBufferNoCleaner()) {
            poolMxBeanOp = Optional.empty();
            AtomicLong tmpDirectMemoryUsage = null;
            try {
                Field field = PlatformDependent.class.getDeclaredField("DIRECT_MEMORY_COUNTER");
                field.setAccessible(true);
                tmpDirectMemoryUsage = (AtomicLong) field.get(null);
            } catch (Throwable t) {
                log.warn("Failed to access netty DIRECT_MEMORY_COUNTER field {}", t.getMessage());
            }
            directMemoryUsage = tmpDirectMemoryUsage;
            getDirectMemoryUsage = () -> directMemoryUsage != null ? directMemoryUsage.get() : Double.NaN;
        } else {
            directMemoryUsage = null;
            List<BufferPoolMXBean> platformMXBeans = ManagementFactory.getPlatformMXBeans(BufferPoolMXBean.class);
            poolMxBeanOp = platformMXBeans.stream()
                    .filter(bufferPoolMXBean -> bufferPoolMXBean.getName().equals("direct")).findAny();
            getDirectMemoryUsage = () -> poolMxBeanOp.isPresent() ? poolMxBeanOp.get().getMemoryUsed() : Double.NaN;
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stats/bookkeeper-stats-providers/prometheus-metrics-provider/src/main/java/org/apache/bookkeeper/stats/prometheus/PrometheusMetricsProvider.java [225:247]:
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    private static final AtomicLong directMemoryUsage;
    private static final Optional<BufferPoolMXBean> poolMxBeanOp;
    private static final Supplier<Double> getDirectMemoryUsage;

    static {
        if (PlatformDependent.useDirectBufferNoCleaner()) {
            poolMxBeanOp = Optional.empty();
            AtomicLong tmpDirectMemoryUsage = null;
            try {
                Field field = PlatformDependent.class.getDeclaredField("DIRECT_MEMORY_COUNTER");
                field.setAccessible(true);
                tmpDirectMemoryUsage = (AtomicLong) field.get(null);
            } catch (Throwable t) {
                log.warn("Failed to access netty DIRECT_MEMORY_COUNTER field {}", t.getMessage());
            }
            directMemoryUsage = tmpDirectMemoryUsage;
            getDirectMemoryUsage = () -> directMemoryUsage != null ? directMemoryUsage.get() : Double.NaN;
        } else {
            directMemoryUsage = null;
            List<BufferPoolMXBean> platformMXBeans = ManagementFactory.getPlatformMXBeans(BufferPoolMXBean.class);
            poolMxBeanOp = platformMXBeans.stream()
                    .filter(bufferPoolMXBean -> bufferPoolMXBean.getName().equals("direct")).findAny();
            getDirectMemoryUsage = () -> poolMxBeanOp.isPresent() ? poolMxBeanOp.get().getMemoryUsed() : Double.NaN;
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