GH-126: ExpRandomBackOffPolicy: fix maxInterval

Fixes https://github.com/spring-projects/spring-retry/issues/126

The `ExponentialRandomBackOffPolicy` doesn't take into account a `maxInterval` for the next `sleep`
causing a very long sleep problem eventually

* Fallback to `super.getMaxInterval()` when `next` from random is too big
* Fix JavaDocs for ranges
* Add test for `maxInterval`

src/main/java/org/springframework/retry/backoff/ExponentialRandomBackOffPolicy.java

@@ -37,10 +37,12 @@ import java.util.Random;
  * {@link ExponentialBackOffPolicy} yields: [50, 100, 200, 400, 800]
  *
  * {@link ExponentialRandomBackOffPolicy} may yield [76, 151, 304, 580, 901] or [53, 190,
- * 267, 451, 815]
+ * 267, 451, 815] (random distributed values within the ranges of [50-100, 100-200, 200-400,
+ * 400-800, 800-1600])
  *
  * @author Jon Travis
  * @author Dave Syer
+ * @author Chase Diem
  */
 @SuppressWarnings("serial")
 public class ExponentialRandomBackOffPolicy extends ExponentialBackOffPolicy {
@@ -69,6 +71,9 @@ public class ExponentialRandomBackOffPolicy extends ExponentialBackOffPolicy {
 		public synchronized long getSleepAndIncrement() {
 			long next = super.getSleepAndIncrement();
 			next = (long) (next * (1 + r.nextFloat() * (getMultiplier() - 1)));
+			if (next > super.getMaxInterval()) {
+				next = super.getMaxInterval();
+			}
 			return next;
 		}
 

src/test/java/org/springframework/retry/backoff/ExponentialRandomBackOffPolicyTests.java

@@ -29,6 +29,7 @@ import org.springframework.retry.support.RetrySimulator;
 /**
  * @author Dave Syer
  * @author Jon Travis
+ * @author Chase Diem
  *
  */
 public class ExponentialRandomBackOffPolicyTests {
@@ -68,6 +69,27 @@ public class ExponentialRandomBackOffPolicyTests {
 		}
 	}
 
+	@Test
+	public void testMaxInterval() throws Exception {
+		ExponentialBackOffPolicy backOffPolicy = makeBackoffPolicy();
+		backOffPolicy.setInitialInterval(3000);
+		long maxInterval = backOffPolicy.getMaxInterval();
+
+		RetrySimulator simulator = new RetrySimulator(backOffPolicy, makeRetryPolicy());
+		RetrySimulation simulation = simulator.executeSimulation(1);
+
+		List<Long> sleeps = simulation.getLongestTotalSleepSequence().getSleeps();
+		System.out.println("Single trial of " + backOffPolicy + ": sleeps=" + sleeps);
+		assertEquals(MAX_RETRIES - 1, sleeps.size());
+		long initialInterval = backOffPolicy.getInitialInterval();
+		for (int i = 0; i < sleeps.size(); i++) {
+			long expectedMaxValue = 2 * (long) (initialInterval
+					+ initialInterval * Math.max(1, Math.pow(backOffPolicy.getMultiplier(), i)));
+			assertTrue("Found a sleep [" + sleeps.get(i) + "] which exceeds our max interval value of "
+					+ expectedMaxValue + " at interval " + i, sleeps.get(i) <= maxInterval);
+		}
+	}
+
 	@Test
 	public void testMultiBackOff() throws Exception {
 		ExponentialBackOffPolicy backOffPolicy = makeBackoffPolicy();