GH-464: Fix backOff API in UniformRandomBackOffPolicy

Fixes: #464
Issue link: #464

When `maxBackOffPeriod` is less than `minBackOffPeriod`, delta is taken
taken as zero in `UniformRandomBackOffPolicy` `backOff()` method.

* Update tests to use `DummySleeper`
* Extract `UniformRandomBackOffPolicy` `backOffSuppliers` into local variables
* Modify `UniformRandomBackOffPolicy` tests

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

@@ -134,8 +134,8 @@ public long getMaxBackOffPeriod() {
 	protected void doBackOff() throws BackOffInterruptedException {
 		try {
 			Long min = this.minBackOffPeriod.get();
-			long delta = this.maxBackOffPeriod.get() == this.minBackOffPeriod.get() ? 0
-					: this.random.nextInt((int) (this.maxBackOffPeriod.get() - min));
+			Long max = this.maxBackOffPeriod.get();
+			long delta = max <= min ? 0 : this.random.nextInt((int) (max - min));
 			this.sleeper.sleep(min + delta);
 		}
 		catch (InterruptedException e) {

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

@@ -36,10 +36,18 @@ public void testSetSleeper() {
 		int maxBackOff = 10000;
 		backOffPolicy.setMinBackOffPeriod(minBackOff);
 		backOffPolicy.setMaxBackOffPeriod(maxBackOff);
-		UniformRandomBackOffPolicy withSleeper = backOffPolicy.withSleeper(new DummySleeper());
+
+		DummySleeper dummySleeper = new DummySleeper();
+		UniformRandomBackOffPolicy withSleeper = backOffPolicy.withSleeper(dummySleeper);
 
 		assertThat(withSleeper.getMinBackOffPeriod()).isEqualTo(minBackOff);
 		assertThat(withSleeper.getMaxBackOffPeriod()).isEqualTo(maxBackOff);
+
+		assertThat(dummySleeper.getBackOffs()).isEmpty();
+		withSleeper.backOff(null);
+
+		assertThat(dummySleeper.getBackOffs()).hasSize(1);
+		assertThat(dummySleeper.getBackOffs()[0]).isLessThan(maxBackOff);
 	}
 
 	@Test
@@ -60,4 +68,20 @@ public void sleep(long backOffPeriod) throws InterruptedException {
 		assertThat(Thread.interrupted()).isTrue();
 	}
 
+	@Test
+	public void testMaxBackOffLessThanMinBackOff() {
+		UniformRandomBackOffPolicy backOffPolicy = new UniformRandomBackOffPolicy();
+		int minBackOff = 1000;
+		int maxBackOff = 10;
+		backOffPolicy.setMinBackOffPeriod(minBackOff);
+		backOffPolicy.setMaxBackOffPeriod(maxBackOff);
+
+		DummySleeper dummySleeper = new DummySleeper();
+		UniformRandomBackOffPolicy withSleeper = backOffPolicy.withSleeper(dummySleeper);
+		assertThat(dummySleeper.getBackOffs()).isEmpty();
+		withSleeper.backOff(null);
+		assertThat(dummySleeper.getBackOffs()).hasSize(1);
+		assertThat(dummySleeper.getBackOffs()[0]).isEqualTo(minBackOff);
+	}
+
 }