Add maxAttempts to ExponentialBackOff

If you wish to stop after a certain number of attempts with an
`ExponentialBackOff` you have to calculate the `maxElapsedTime`
corresponding to the number of attempts.

Add a new property to make it more convenient to stop after a
certain number of attempts.

See gh-27071

spring-core/src/main/java/org/springframework/util/backoff/ExponentialBackOff.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -47,9 +47,11 @@ import org.springframework.util.Assert;
  * <p>Note that the default max elapsed time is {@link Long#MAX_VALUE}. Use
  * {@link #setMaxElapsedTime(long)} to limit the maximum length of time
  * that an instance should accumulate before returning
- * {@link BackOffExecution#STOP}.
+ * {@link BackOffExecution#STOP}. Or use {@link #setMaxAttempts} to limit
+ * the number of attempts.
  *
  * @author Stephane Nicoll
+ * @author Gary Russell
  * @since 4.1
  */
 public class ExponentialBackOff implements BackOff {
@@ -74,6 +76,10 @@ public class ExponentialBackOff implements BackOff {
 	 */
 	public static final long DEFAULT_MAX_ELAPSED_TIME = Long.MAX_VALUE;
 
+	/**
+	 * The default maximum attempts.
+	 */
+	public static final int DEFAULT_MAX_ATTEMPTS = Integer.MAX_VALUE;
 
 	private long initialInterval = DEFAULT_INITIAL_INTERVAL;
 
@@ -83,6 +89,8 @@ public class ExponentialBackOff implements BackOff {
 
 	private long maxElapsedTime = DEFAULT_MAX_ELAPSED_TIME;
 
+	private int maxAttempts = DEFAULT_MAX_ATTEMPTS;
+
 
 	/**
 	 * Create an instance with the default settings.
@@ -90,6 +98,7 @@ public class ExponentialBackOff implements BackOff {
 	 * @see #DEFAULT_MULTIPLIER
 	 * @see #DEFAULT_MAX_INTERVAL
 	 * @see #DEFAULT_MAX_ELAPSED_TIME
+	 * @see #DEFAULT_MAX_ATTEMPTS
 	 */
 	public ExponentialBackOff() {
 	}
@@ -152,6 +161,8 @@ public class ExponentialBackOff implements BackOff {
 	/**
 	 * The maximum elapsed time in milliseconds after which a call to
 	 * {@link BackOffExecution#nextBackOff()} returns {@link BackOffExecution#STOP}.
+	 * @param maxElapsedTime the maxElapsedTime.
+	 * @see #setMaxAttempts(int)
 	 */
 	public void setMaxElapsedTime(long maxElapsedTime) {
 		this.maxElapsedTime = maxElapsedTime;
@@ -160,11 +171,35 @@ public class ExponentialBackOff implements BackOff {
 	/**
 	 * Return the maximum elapsed time in milliseconds after which a call to
 	 * {@link BackOffExecution#nextBackOff()} returns {@link BackOffExecution#STOP}.
+	 * @return the maxElapsedTime.
+	 * @see #getMaxAttempts()
 	 */
 	public long getMaxElapsedTime() {
 		return this.maxElapsedTime;
 	}
 
+	/**
+	 * The maximum number of attempts after which a call to
+	 * {@link BackOffExecution#nextBackOff()} returns {@link BackOffExecution#STOP}.
+	 * @param maxAttempts the maxAttempts.
+	 * @since 5.3.8
+	 * @see #setMaxElapsedTime(long)
+	 */
+	public void setMaxAttempts(int maxAttempts) {
+		this.maxAttempts = maxAttempts;
+	}
+
+	/**
+	 * Return the maximum number of attempts after which a call to
+	 * {@link BackOffExecution#nextBackOff()} returns {@link BackOffExecution#STOP}.
+	 * @return the maxAttempts.
+	 * @since 5.3.8
+	 * @see #getMaxElapsedTime()
+	 */
+	public int getMaxAttempts() {
+		return this.maxAttempts;
+	}
+
 	@Override
 	public BackOffExecution start() {
 		return new ExponentialBackOffExecution();
@@ -182,14 +217,17 @@ public class ExponentialBackOff implements BackOff {
 
 		private long currentElapsedTime = 0;
 
+		private int attempts;
+
 		@Override
 		public long nextBackOff() {
-			if (this.currentElapsedTime >= maxElapsedTime) {
+			if (this.currentElapsedTime >= maxElapsedTime || this.attempts >= maxAttempts) {
 				return STOP;
 			}
 
 			long nextInterval = computeNextInterval();
 			this.currentElapsedTime += nextInterval;
+			this.attempts++;
 			return nextInterval;
 		}
 

spring-core/src/test/java/org/springframework/util/ExponentialBackOffTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,6 +16,10 @@
 
 package org.springframework.util;
 
+import java.util.ArrayList;
+import java.util.List;
+import java.util.stream.IntStream;
+
 import org.junit.jupiter.api.Test;
 
 import org.springframework.util.backoff.BackOffExecution;
@@ -131,4 +135,17 @@ class ExponentialBackOffTests {
 		assertThat(execution.toString()).isEqualTo("ExponentialBackOff{currentInterval=4000ms, multiplier=2.0}");
 	}
 
+	@Test
+	void maxAttempts() {
+		ExponentialBackOff bo = new ExponentialBackOff();
+		bo.setInitialInterval(1_000L);
+		bo.setMultiplier(2.0);
+		bo.setMaxInterval(10_000L);
+		bo.setMaxAttempts(6);
+		List<Long> delays = new ArrayList<>();
+		BackOffExecution boEx = bo.start();
+		IntStream.range(0, 7).forEach(i -> delays.add(boEx.nextBackOff()));
+		assertThat(delays).containsExactly(1_000L, 2_000L, 4_000L, 8_000L, 10_000L, 10_000L, -1L);
+	}
+
 }