Improve decodeToMono support

This commit ensures uniform support for decodeToMono across the various
byte and String related decoders.

Issue: SPR-16253

spring-core/src/test/java/org/springframework/core/codec/ByteArrayDecoderTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -21,6 +21,7 @@ import java.util.Collections;
 import org.junit.Test;
 import org.reactivestreams.Publisher;
 import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
 import reactor.test.StepVerifier;
 
 import org.springframework.core.ResolvableType;
@@ -39,6 +40,7 @@ public class ByteArrayDecoderTests extends AbstractDataBufferAllocatingTestCase
 
 	private final ByteArrayDecoder decoder = new ByteArrayDecoder();
 
+
 	@Test
 	public void canDecode() {
 		assertTrue(this.decoder.canDecode(ResolvableType.forClass(byte[].class),
@@ -65,4 +67,19 @@ public class ByteArrayDecoderTests extends AbstractDataBufferAllocatingTestCase
 				.verify();
 	}
 
+	@Test
+	public void decodeToMono() {
+		DataBuffer fooBuffer = stringBuffer("foo");
+		DataBuffer barBuffer = stringBuffer("bar");
+		Flux<DataBuffer> source = Flux.just(fooBuffer, barBuffer);
+		Mono<byte[]> output = this.decoder.decodeToMono(source,
+				ResolvableType.forClassWithGenerics(Publisher.class, byte[].class),
+				null, Collections.emptyMap());
+
+		StepVerifier.create(output)
+				.consumeNextWith(bytes -> assertArrayEquals("foobar".getBytes(), bytes))
+				.expectComplete()
+				.verify();
+	}
+
 }

spring-core/src/test/java/org/springframework/core/codec/ByteBufferDecoderTests.java

@@ -22,6 +22,7 @@ import java.util.Collections;
 import org.junit.Test;
 import org.reactivestreams.Publisher;
 import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
 import reactor.test.StepVerifier;
 
 import org.springframework.core.ResolvableType;
@@ -63,4 +64,19 @@ public class ByteBufferDecoderTests extends AbstractDataBufferAllocatingTestCase
 				.expectComplete()
 				.verify();
 	}
+
+	@Test
+	public void decodeToMono() {
+		DataBuffer fooBuffer = stringBuffer("foo");
+		DataBuffer barBuffer = stringBuffer("bar");
+		Flux<DataBuffer> source = Flux.just(fooBuffer, barBuffer);
+		Mono<ByteBuffer> output = this.decoder.decodeToMono(source,
+				ResolvableType.forClassWithGenerics(Publisher.class, ByteBuffer.class),
+				null, Collections.emptyMap());
+
+		StepVerifier.create(output)
+				.expectNext(ByteBuffer.wrap("foobar".getBytes()))
+				.expectComplete()
+				.verify();
+	}
 }

spring-core/src/test/java/org/springframework/core/codec/DataBufferDecoderTests.java

@@ -17,15 +17,19 @@
 package org.springframework.core.codec;
 
 import java.nio.ByteBuffer;
+import java.nio.charset.StandardCharsets;
+import java.time.Duration;
 import java.util.Collections;
 
 import org.junit.Test;
 import org.reactivestreams.Publisher;
 import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
 
 import org.springframework.core.ResolvableType;
 import org.springframework.core.io.buffer.AbstractDataBufferAllocatingTestCase;
 import org.springframework.core.io.buffer.DataBuffer;
+import org.springframework.core.io.buffer.support.DataBufferTestUtils;
 import org.springframework.util.MimeTypeUtils;
 
 import static org.junit.Assert.*;
@@ -53,11 +57,25 @@ public class DataBufferDecoderTests extends AbstractDataBufferAllocatingTestCase
 		DataBuffer barBuffer = stringBuffer("bar");
 		Flux<DataBuffer> source = Flux.just(fooBuffer, barBuffer);
 		Flux<DataBuffer> output = this.decoder.decode(source,
-				ResolvableType.forClassWithGenerics(Publisher.class, ByteBuffer.class),
+				ResolvableType.forClassWithGenerics(Publisher.class, DataBuffer.class),
 				null, Collections.emptyMap());
 
 		assertSame(source, output);
 
 		release(fooBuffer, barBuffer);
 	}
-}
+
+	@Test
+	public void decodeToMono() {
+		DataBuffer fooBuffer = stringBuffer("foo");
+		DataBuffer barBuffer = stringBuffer("bar");
+		Flux<DataBuffer> source = Flux.just(fooBuffer, barBuffer);
+		Mono<DataBuffer> output = this.decoder.decodeToMono(source,
+				ResolvableType.forClassWithGenerics(Publisher.class, DataBuffer.class),
+				null, Collections.emptyMap());
+
+		DataBuffer outputBuffer = output.block(Duration.ofSeconds(5));
+		assertEquals("foobar", DataBufferTestUtils.dumpString(outputBuffer, StandardCharsets.UTF_8));
+
+		release(outputBuffer);
+	}}