[Guava]Supplier介绍

介绍

• Memoization适用于没有参数的函数（Supplier）
• 当我们想要执行memoized方法时，我们可以简单地调用返回的Supplier的get方法，根据方法的返回值是否存在于内存中，get方法将返回内存中的值或执行memoized方法并将返回值传递给调用方。
• 在Suppliers类中有两种memoization的方法：memoize和memoizeWithExpiration。

Supplier Memoization Without Eviction

我们可以使用supplier的memoize方法并指定委托的supplier作为方法引用：

Supplier memoizedSupplier = Suppliers.memoize(
  CostlySupplier::generateBigNumber);

由于我们没有指定驱逐政策,一旦调用了get方法，返回的值将在Java应用程序仍在运行时保留在内存中。在初始调用之后进行的任何调用都将返回memoized值。

Supplier Memoization with Eviction by Time-To-Live (TTL)

假设我们只想在备忘录中保留供应商返回的值一段时间。
我们可以使用供应商的memoizeWithExpiration方法，并指定过期时间及其相应的时间单位（例如，秒，分钟），以及委托的supplier：

Supplier memoizedSupplier = Suppliers.memoizeWithExpiration(
  CostlySupplier::generateBigNumber, 5, TimeUnit.SECONDS);

在指定的时间过去（5秒）后，缓存将从内存中退出Supplier的返回值，随后对get方法的任何调用都将重新执行generateBigNumber。

例子

public class CostlySupplier {
    private static BigInteger generateBigNumber() {
        try {
            TimeUnit.SECONDS.sleep(2);
        } catch (InterruptedException e) {}
        return new BigInteger("12345");
    }
}

@Test
public void givenMemoizedSupplier_whenGet_thenSubsequentGetsAreFast() {
    Supplier memoizedSupplier; 
    memoizedSupplier = Suppliers.memoize(CostlySupplier::generateBigNumber);
 
    BigInteger expectedValue = new BigInteger("12345");
    assertSupplierGetExecutionResultAndDuration(
      memoizedSupplier, expectedValue, 2000D);
    assertSupplierGetExecutionResultAndDuration(
      memoizedSupplier, expectedValue, 0D);
    assertSupplierGetExecutionResultAndDuration(
      memoizedSupplier, expectedValue, 0D);
}
 
private  void assertSupplierGetExecutionResultAndDuration(
  Supplier supplier, T expectedValue, double expectedDurationInMs) {
    Instant start = Instant.now();
    T value = supplier.get();
    Long durationInMs = Duration.between(start, Instant.now()).toMillis();
    double marginOfErrorInMs = 100D;
 
    assertThat(value, is(equalTo(expectedValue)));
    assertThat(
      durationInMs.doubleValue(), 
      is(closeTo(expectedDurationInMs, marginOfErrorInMs)));
}

原理

Suppliers#memoize

 public static  Supplier memoize(Supplier delegate) {
    return (delegate instanceof MemoizingSupplier)
        ? delegate
        : new MemoizingSupplier(Preconditions.checkNotNull(delegate));
  }

看下 MemoizingSupplier类

@VisibleForTesting
  static class MemoizingSupplier implements Supplier, Serializable {
    final Supplier delegate;
    transient volatile boolean initialized;
  // 读取和写入保证了读取可见的写入值
  // value不需要使用volatile
    transient T value;

    MemoizingSupplier(Supplier delegate) {
      this.delegate = delegate;
    }

    @Override public T get() {
         // 重点在这里 Double Checked Locking的变体
  
      if (!initialized) {
        synchronized (this) {
          if (!initialized) {
            T t = delegate.get();
            value = t;
            initialized = true;
            return t;
          }
        }
      }
      return value;
    }

    @Override public String toString() {
      return "Suppliers.memoize(" + delegate + ")";
    }

    private static final long serialVersionUID = 0;
  }

Suppliers#memoizeWithExpiration

 public static  Supplier memoizeWithExpiration(
      Supplier delegate, long duration, TimeUnit unit) {
    return new ExpiringMemoizingSupplier(delegate, duration, unit);
  }

@VisibleForTesting static class ExpiringMemoizingSupplier
      implements Supplier, Serializable {
    final Supplier delegate;
    final long durationNanos;
    transient volatile T value;
    // The special value 0 means "not yet initialized".
    transient volatile long expirationNanos;

    ExpiringMemoizingSupplier(
        Supplier delegate, long duration, TimeUnit unit) {
      this.delegate = Preconditions.checkNotNull(delegate);
      this.durationNanos = unit.toNanos(duration);
      Preconditions.checkArgument(duration > 0);
    }

    @Override public T get() {
      // Another variant of Double Checked Locking.
      //
      我们使用两个易失性读取。通过将我们的字段放入持有者类，
    额外的内存消耗和间接比额外的易失性读取更昂贵。
      long nanos = expirationNanos;
      long now = Platform.systemNanoTime();
      if (nanos == 0 || now - nanos >= 0) {
        synchronized (this) {
          if (nanos == expirationNanos) {  // recheck for lost race
            T t = delegate.get();
            value = t;
            nanos = now + durationNanos;
      
            expirationNanos = (nanos == 0) ? 1 : nanos;
            return t;
          }
        }
      }
      return value;
    }

    @Override public String toString() {
      // This is a little strange if the unit the user provided was not NANOS,
      // but we don't want to store the unit just for toString
      return "Suppliers.memoizeWithExpiration(" + delegate + ", " +
          durationNanos + ", NANOS)";
    }

    private static final long serialVersionUID = 0;
  }

翻译：
https://www.baeldung.com/guava-memoizer