com.nytimes.android.external.cache.CacheLoader Maven / Gradle / Ivy

Go to download
Show more of this group Show more artifacts with this name
Show all versions of cache Show documentation
Store
There is a newer version: 3.0.0-alpha
package com.nytimes.android.external.cache;


import java.io.Serializable;
import java.util.Map;

import javax.annotation.Nonnull;
import javax.annotation.Nullable;

public abstract class CacheLoader {
  /**
   * Constructor for use by subclasses.
   */
  protected CacheLoader() {}

  /**
   * Computes or retrieves the value corresponding to {@code key}.
   *
   * @param key the non-null key whose value should be loaded
   * @return the value associated with {@code key}; must not be null
   * @throws Exception if unable to load the result
   * @throws InterruptedException if this method is interrupted. {@code InterruptedException} is
   *     treated like any other {@code Exception} in all respects except that, when it is caught,
   *     the thread's interrupt status is set
   */
  @Nullable
  public abstract V load(K key) throws Exception;


  @Nullable
  public ListenableFuture reload(@Nonnull K key, @Nonnull V oldValue) throws Exception {
    Preconditions.checkNotNull(key);
    Preconditions.checkNotNull(oldValue);
    return Futures.immediateFuture(load(key));
  }


  @Nonnull
  public Map loadAll(Iterable keys) throws Exception {
    // This will be caught by getAll(), causing it to fall back to multiple calls to
    // LoadingCache.get
    throw new UnsupportedLoadingOperationException();
  }

  /**
   * Returns a cache loader based on an existing function instance. Note that there's no need
   * to create a new function just to pass it in here; just subclass {@code CacheLoader} and
   * implement {@link #load load} instead.
   *
   * @param function the function to be used for loading values; must never return {@code null}
   * @return a cache loader that loads values by passing each key to {@code function}
   */
  @Nonnull
  public static  CacheLoader from(@Nonnull Function function) {
    return new FunctionToCacheLoader<>(function);
  }

  private static final class FunctionToCacheLoader
      extends CacheLoader implements Serializable {
    private final Function computingFunction;

    public FunctionToCacheLoader(@Nonnull Function computingFunction) {
      this.computingFunction = Preconditions.checkNotNull(computingFunction);
    }

    @Nullable
    @Override
    public V load(@Nonnull K key) {
      return computingFunction.apply(Preconditions.checkNotNull(key));
    }

    private static final long serialVersionUID = 0;
  }

  /**
   * Returns a cache loader based on an existing supplier instance. Note that there's no need
   * to create a new supplier just to pass it in here; just subclass {@code CacheLoader} and
   * implement {@link #load load} instead.
   *
   * @param supplier the supplier to be used for loading values; must never return {@code null}
   * @return a cache loader that loads values by calling {@link Supplier#get}, irrespective of the
   *     key
   */
  @Nonnull
  public static  CacheLoader from(@Nonnull Supplier supplier) {
    return new SupplierToCacheLoader<>(supplier);
  }


  private static final class SupplierToCacheLoader
      extends CacheLoader implements Serializable {
    private final Supplier computingSupplier;

    public SupplierToCacheLoader(@Nonnull Supplier computingSupplier) {
      this.computingSupplier = Preconditions.checkNotNull(computingSupplier);
    }

    @Nonnull
    @Override
    public V load(@Nonnull Object key) {
      Preconditions.checkNotNull(key);
      return computingSupplier.get();
    }

    private static final long serialVersionUID = 0;
  }

  /**
   * Exception thrown by {@code loadAll()} to indicate that it is not supported.
   *
   * @since 19.0
   */
  public static final class UnsupportedLoadingOperationException
      extends UnsupportedOperationException {
    // Package-private because this should only be thrown by loadAll() when it is not overridden.
    // Cache implementors may want to catch it but should not need to be able to throw it.
    UnsupportedLoadingOperationException() {}
  }

  /**
   * Thrown to indicate that an invalid response was returned from a call to {@link CacheLoader}.
   *
   * @since 11.0
   */
  public static final class InvalidCacheLoadException extends RuntimeException {
    public InvalidCacheLoadException(String message) {
      super(message);
    }
  }
}