Add DataBufferUtils.compose

Added a utility method that composes data buffers into a single buffer.
Depending on the `DataBuffer` implementation, the returned buffer may be
a single buffer containing all data of the provided buffers, or it may
be a true composite that contains references to the buffers.

Issue: SPR-16365

spring-core/src/main/java/org/springframework/core/io/buffer/DataBufferFactory.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -17,6 +17,7 @@
 package org.springframework.core.io.buffer;
 
 import java.nio.ByteBuffer;
+import java.util.List;
 
 /**
  * A factory for {@link DataBuffer}s, allowing for allocation and wrapping of
@@ -61,4 +62,14 @@ public interface DataBufferFactory {
 	 */
 	DataBuffer wrap(byte[] bytes);
 
+	/**
+	 * Create a composite data buffer from the list of provided data buffers. Depending on the
+	 * implementation, the returned buffer may be a single buffer containing all data of the
+	 * provided buffers, or it may be a true composite that contains references to the buffers.
+	 * <p>Note that the given data buffers do <strong>not</strong> have to be released, as they are
+	 * released as part of the returned composite.
+	 * @param dataBuffers the data buffers to be composed
+	 * @return a buffer that composes {@code dataBuffers} into one
+	 */
+	DataBuffer compose(List<DataBuffer> dataBuffers);
 }

spring-core/src/main/java/org/springframework/core/io/buffer/DataBufferUtils.java

@@ -39,6 +39,7 @@ import org.reactivestreams.Subscription;
 import reactor.core.publisher.BaseSubscriber;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.FluxSink;
+import reactor.core.publisher.Mono;
 import reactor.core.publisher.SynchronousSink;
 
 import org.springframework.core.io.Resource;
@@ -434,6 +435,27 @@ public abstract class DataBufferUtils {
 		return WRITE_AGGREGATOR;
 	}
 
+	/**
+	 * Composes the buffers in the given {@link Publisher} into a single data buffer. Depending on
+	 * the {@code DataBuffer} implementation, the returned buffer may be a single buffer containing
+	 * all data of the provided buffers, or it may be a true composite that contains references to
+	 * the buffers.
+	 * @param publisher the data buffers that are to be composed
+	 * @return the composed data buffer
+	 */
+	public static Mono<DataBuffer> compose(Publisher<DataBuffer> publisher) {
+		Assert.notNull(publisher, "'publisher' must not be null");
+
+		Flux<DataBuffer> source = Flux.from(publisher);
+
+		return source.collectList()
+				.filter(dataBuffers -> !dataBuffers.isEmpty())
+				.map(dataBuffers -> {
+					DataBufferFactory bufferFactory = dataBuffers.get(0).factory();
+					return bufferFactory.compose(dataBuffers);
+				});
+	}
+
 
 	private static class ReadableByteChannelGenerator
 			implements BiFunction<ReadableByteChannel, SynchronousSink<DataBuffer>, ReadableByteChannel> {

spring-core/src/main/java/org/springframework/core/io/buffer/DefaultDataBufferFactory.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -17,6 +17,7 @@
 package org.springframework.core.io.buffer;
 
 import java.nio.ByteBuffer;
+import java.util.List;
 
 import org.springframework.util.Assert;
 
@@ -102,6 +103,23 @@ public class DefaultDataBufferFactory implements DataBufferFactory {
 		return DefaultDataBuffer.fromFilledByteBuffer(this, wrapper);
 	}
 
+	/**
+	 * {@inheritDoc}
+	 * <p>This implementation creates a single {@link DefaultDataBuffer} to contain the data
+	 * in {@code dataBuffers}.
+	 */
+	@Override
+	public DataBuffer compose(List<DataBuffer> dataBuffers) {
+		Assert.notEmpty(dataBuffers, "'dataBuffers' must not be empty");
+
+		int capacity = dataBuffers.stream()
+				.mapToInt(DataBuffer::readableByteCount)
+				.sum();
+		DefaultDataBuffer dataBuffer = allocateBuffer(capacity);
+		return dataBuffers.stream()
+				.reduce(dataBuffer, DataBuffer::write);
+	}
+
 	@Override
 	public String toString() {
 		return "DefaultDataBufferFactory (preferDirect=" + this.preferDirect + ")";

spring-core/src/main/java/org/springframework/core/io/buffer/NettyDataBufferFactory.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -17,9 +17,11 @@
 package org.springframework.core.io.buffer;
 
 import java.nio.ByteBuffer;
+import java.util.List;
 
 import io.netty.buffer.ByteBuf;
 import io.netty.buffer.ByteBufAllocator;
+import io.netty.buffer.CompositeByteBuf;
 import io.netty.buffer.Unpooled;
 
 import org.springframework.util.Assert;
@@ -80,6 +82,22 @@ public class NettyDataBufferFactory implements DataBufferFactory {
 		return new NettyDataBuffer(byteBuf, this);
 	}
 
+	/**
+	 * {@inheritDoc}
+	 * <p>This implementation uses Netty's {@link CompositeByteBuf}.
+	 */
+	@Override
+	public DataBuffer compose(List<DataBuffer> dataBuffers) {
+		Assert.notNull(dataBuffers, "'dataBuffers' must not be null");
+		CompositeByteBuf composite = this.byteBufAllocator.compositeBuffer(dataBuffers.size());
+		for (DataBuffer dataBuffer : dataBuffers) {
+			Assert.isInstanceOf(NettyDataBuffer.class, dataBuffer);
+			NettyDataBuffer nettyDataBuffer = (NettyDataBuffer) dataBuffer;
+			composite.addComponent(true, nettyDataBuffer.getNativeBuffer());
+		}
+		return new NettyDataBuffer(composite, this);
+	}
+
 	/**
 	 * Wrap the given Netty {@link ByteBuf} in a {@code NettyDataBuffer}.
 	 * @param byteBuf the Netty byte buffer to wrap

spring-core/src/test/java/org/springframework/core/io/buffer/DataBufferTests.java

@@ -479,5 +479,18 @@ public class DataBufferTests extends AbstractDataBufferAllocatingTestCase {
 		release(buffer);
 	}
 
+	@Test
+	public void composite() {
+		DataBuffer composite = this.bufferFactory.compose(Arrays.asList(stringBuffer("a"),
+				stringBuffer("b"), stringBuffer("c")));
+		assertEquals(3, composite.readableByteCount());
+		byte[] bytes = new byte[3];
+		composite.read(bytes);
+
+		assertArrayEquals(new byte[] {'a','b','c'}, bytes);
+
+		release(composite);
+	}
+
 
 }

spring-core/src/test/java/org/springframework/core/io/buffer/DataBufferUtilsTests.java

@@ -351,5 +351,19 @@ public class DataBufferUtilsTests extends AbstractDataBufferAllocatingTestCase {
 		};
 	}
 
+	@Test
+	public void compose() {
+		DataBuffer foo = stringBuffer("foo");
+		DataBuffer bar = stringBuffer("bar");
+		DataBuffer baz = stringBuffer("baz");
+		Flux<DataBuffer> flux = Flux.just(foo, bar, baz);
+
+		DataBuffer result = DataBufferUtils.compose(flux).block(Duration.ofSeconds(5));
+
+		assertEquals("foobarbaz", DataBufferTestUtils.dumpString(result, StandardCharsets.UTF_8));
+
+		release(result);
+	}
+
 
 }