Add AsyncLoader to load and update value periodically (#5590)

Fixes #5506.

### Motivation:
`AsyncLoader`  can be useful in the following situations.

- When it is necessary to periodically read and update information from
a file such as resolv.conf .
- When data is not valid after a certain period of time, such as an
OAuth 2.0 access token.

We already have an implementation for that on
[AbstractOAuth2AuthorizationGrant.java](https://github.com/line/armeria/compare/main...injae-kim:async-loader?expand=1#diff-866e837e1f45041da819b2fed992548e97c9b2a8a72071e68c374f9d4678e53a).
However, I hope to generalize it and add new features to use it in
various cases.

### Modifications:

- Add `AsyncLoader` to load and update value periodically

### Result:

- Closes #5506.
- Now user can load, cache, update value periodically by using
`AsyncLoader`

---------

Co-authored-by: Ikhun Um <[email protected]>

core/src/main/java/com/linecorp/armeria/common/util/AsyncLoader.java

@@ -0,0 +1,81 @@
+/*
+ * Copyright 2024 LINE Corporation
+ *
+ * LINE Corporation licenses this file to you under the Apache License,
+ * version 2.0 (the "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at:
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+
+package com.linecorp.armeria.common.util;
+
+import java.util.concurrent.CompletableFuture;
+import java.util.function.Function;
+
+import com.linecorp.armeria.common.annotation.Nullable;
+import com.linecorp.armeria.common.annotation.UnstableApi;
+
+/**
+ * A loader which atomically loads, caches and updates value.
+ *
+ * <p>Example usage:
+ * <pre>{@code
+ * WebClient client = WebClient.of("https://example.com");
+ * Function<String, CompletableFuture<String>> loader = cache -> {
+ *     // Fetch new data from the remote server.
+ *     ResponseEntity<String> response =
+ *         client.prepare().get("/api/v1/items").asString().execute();
+ *     return response.thenApply(res -> res.content());
+ * };
+ *
+ * AsyncLoader<String> asyncLoader =
+ *     AsyncLoader
+ *         .builder(loader)
+ *         // Expire the loaded value after 60 seconds.
+ *         .expireAfterLoad(Duration.ofSeconds(60))
+ *         .build();
+ *
+ * // Fetch the value. This will call the loader function because the cache is empty.
+ * String value1 = asyncLoader.load().join();
+ * System.out.println("Loaded value: " + value1);
+ *
+ * // This will return the cached value because it's not expired yet.
+ * String value2 = asyncLoader.load().join();
+ * assert value1 == value2;
+ *
+ * // Wait for more than 60 seconds so that the cache is expired.
+ * Thread.sleep(61000);
+ *
+ * // Fetch the value again. This will call the loader function because the cache has expired.
+ * String value3 = asyncLoader.load().join();
+ * assert value1 != value3;
+ * }</pre>
+ */
+@FunctionalInterface
+@UnstableApi
+public interface AsyncLoader<T> {
+
+    /**
+     * Returns a newly created {@link AsyncLoaderBuilder} with the specified loader.
+     *
+     * @param loader function to load value. {@code T} is the previously cached value.
+     */
+    static <T> AsyncLoaderBuilder<T> builder(
+            Function<@Nullable ? super T, ? extends CompletableFuture<? extends T>> loader) {
+        //noinspection unchecked
+        return new AsyncLoaderBuilder<>((Function<T, CompletableFuture<T>>) loader);
+    }
+
+    /**
+     * Returns a {@link CompletableFuture} which will be completed with the loaded value.
+     * A new value is fetched by the loader only if nothing is cached or the cache value has expired.
+     */
+    CompletableFuture<T> load();
+}

core/src/main/java/com/linecorp/armeria/common/util/AsyncLoaderBuilder.java

@@ -0,0 +1,112 @@
+/*
+ * Copyright 2024 LINE Corporation
+ *
+ * LINE Corporation licenses this file to you under the Apache License,
+ * version 2.0 (the "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at:
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+
+package com.linecorp.armeria.common.util;
+
+import static com.google.common.base.Preconditions.checkState;
+import static java.util.Objects.requireNonNull;
+
+import java.time.Duration;
+import java.util.concurrent.CompletableFuture;
+import java.util.function.BiFunction;
+import java.util.function.Function;
+import java.util.function.Predicate;
+
+import com.linecorp.armeria.common.annotation.Nullable;
+import com.linecorp.armeria.common.annotation.UnstableApi;
+
+/**
+ * A builder for creating a new {@link AsyncLoader}.
+ */
+@UnstableApi
+public final class AsyncLoaderBuilder<T> {
+
+    private final Function<@Nullable T, CompletableFuture<T>> loader;
+    @Nullable
+    private Duration expireAfterLoad;
+    @Nullable
+    private Predicate<? super T> expireIf;
+    @Nullable
+    private Predicate<? super T> refreshIf;
+    @Nullable
+    private BiFunction<? super Throwable, ? super @Nullable T,
+            ? extends @Nullable CompletableFuture<T>> exceptionHandler;
+
+    AsyncLoaderBuilder(Function<@Nullable T, CompletableFuture<T>> loader) {
+        requireNonNull(loader, "loader");
+        this.loader = loader;
+    }
+
+    /**
+     * Expires the loaded value after the given duration since it was loaded.
+     * New value will be loaded by the loader function on next {@link AsyncLoader#load()}.
+     */
+    public AsyncLoaderBuilder<T> expireAfterLoad(Duration expireAfterLoad) {
+        requireNonNull(expireAfterLoad, "expireAfterLoad");
+        checkState(!expireAfterLoad.isNegative(), "expireAfterLoad: %s (expected: >= 0)", expireAfterLoad);
+        this.expireAfterLoad = expireAfterLoad;
+        return this;
+    }
+
+    /**
+     * Expires the loaded value after the given milliseconds since it was loaded.
+     * New value will be loaded by the loader function on next {@link AsyncLoader#load()}.
+     */
+    public AsyncLoaderBuilder<T> expireAfterLoadMillis(long expireAfterLoadMillis) {
+        checkState(expireAfterLoadMillis >= 0,
+                   "expireAfterLoadMillis: %s (expected: >= 0)", expireAfterLoadMillis);
+        expireAfterLoad = Duration.ofMillis(expireAfterLoadMillis);
+        return this;
+    }
+
+    /**
+     * Expires the loaded value if the predicate matches.
+     * New value will be loaded by the loader function on next {@link AsyncLoader#load()}.
+     */
+    public AsyncLoaderBuilder<T> expireIf(Predicate<? super T> expireIf) {
+        requireNonNull(expireIf, "expireIf");
+        this.expireIf = expireIf;
+        return this;
+    }
+
+    /**
+     * Asynchronously refreshes the loaded value which has not yet expired if the predicate matches.
+     * This pre-fetch strategy can remove an additional loading time on a cache miss.
+     */
+    public AsyncLoaderBuilder<T> refreshIf(Predicate<? super T> refreshIf) {
+        requireNonNull(refreshIf, "refreshIf");
+        this.refreshIf = refreshIf;
+        return this;
+    }
+
+    /**
+     * Handles the exception thrown by the loader function.
+     * If the exception handler returns {@code null}, {@link AsyncLoader#load()} completes exceptionally.
+     */
+    public AsyncLoaderBuilder<T> exceptionHandler(BiFunction<? super Throwable, ? super @Nullable T,
+            ? extends @Nullable CompletableFuture<T>> exceptionHandler) {
+        requireNonNull(exceptionHandler, "exceptionHandler");
+        this.exceptionHandler = exceptionHandler;
+        return this;
+    }
+
+    /**
+     * Returns a newly created {@link AsyncLoader} with the entries in this builder.
+     */
+    public AsyncLoader<T> build() {
+        return new DefaultAsyncLoader<>(loader, expireAfterLoad, expireIf, refreshIf, exceptionHandler);
+    }
+}