Add option to decode from a DataBuffer

See gh-22782

spring-core/src/main/java/org/springframework/core/codec/AbstractDataBufferDecoder.java

@@ -45,6 +45,7 @@ import org.springframework.util.MimeType;
  * @since 5.0
  * @param <T> the element type
  */
+@SuppressWarnings("deprecation")
 public abstract class AbstractDataBufferDecoder<T> extends AbstractDecoder<T> {
 
 
@@ -70,8 +71,14 @@ public abstract class AbstractDataBufferDecoder<T> extends AbstractDecoder<T> {
 
 	/**
 	 * How to decode a {@code DataBuffer} to the target element type.
+	 * @deprecated as of 5.2, please implement
+	 * {@link #decode(DataBuffer, ResolvableType, MimeType, Map)} instead
 	 */
-	protected abstract T decodeDataBuffer(DataBuffer buffer, ResolvableType elementType,
-			@Nullable MimeType mimeType, @Nullable Map<String, Object> hints);
+	@Deprecated
+	protected T decodeDataBuffer(DataBuffer buffer, ResolvableType elementType,
+			@Nullable MimeType mimeType, @Nullable Map<String, Object> hints) {
+
+		return decode(buffer, elementType, mimeType, hints);
+	}
 
 }

spring-core/src/main/java/org/springframework/core/codec/ByteArrayDecoder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 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.
@@ -45,7 +45,7 @@ public class ByteArrayDecoder extends AbstractDataBufferDecoder<byte[]> {
 	}
 
 	@Override
-	protected byte[] decodeDataBuffer(DataBuffer dataBuffer, ResolvableType elementType,
+	public byte[] decode(DataBuffer dataBuffer, ResolvableType elementType,
 			@Nullable MimeType mimeType, @Nullable Map<String, Object> hints) {
 
 		byte[] result = new byte[dataBuffer.readableByteCount()];

spring-core/src/main/java/org/springframework/core/codec/ByteBufferDecoder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 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.
@@ -48,7 +48,7 @@ public class ByteBufferDecoder extends AbstractDataBufferDecoder<ByteBuffer> {
 	}
 
 	@Override
-	protected ByteBuffer decodeDataBuffer(DataBuffer dataBuffer, ResolvableType elementType,
+	public ByteBuffer decode(DataBuffer dataBuffer, ResolvableType elementType,
 			@Nullable MimeType mimeType, @Nullable Map<String, Object> hints) {
 
 		int byteCount = dataBuffer.readableByteCount();

spring-core/src/main/java/org/springframework/core/codec/DataBufferDecoder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 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.
@@ -64,7 +64,7 @@ public class DataBufferDecoder extends AbstractDataBufferDecoder<DataBuffer> {
 	}
 
 	@Override
-	protected DataBuffer decodeDataBuffer(DataBuffer buffer, ResolvableType elementType,
+	public DataBuffer decode(DataBuffer buffer, ResolvableType elementType,
 			@Nullable MimeType mimeType, @Nullable Map<String, Object> hints) {
 
 		if (logger.isDebugEnabled()) {

spring-core/src/main/java/org/springframework/core/codec/Decoder.java

@@ -22,10 +22,12 @@ import java.util.Map;
 import org.reactivestreams.Publisher;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
+import reactor.core.publisher.MonoProcessor;
 
 import org.springframework.core.ResolvableType;
 import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.lang.Nullable;
+import org.springframework.util.Assert;
 import org.springframework.util.MimeType;
 
 /**
@@ -75,6 +77,33 @@ public interface Decoder<T> {
 	Mono<T> decodeToMono(Publisher<DataBuffer> inputStream, ResolvableType elementType,
 			@Nullable MimeType mimeType, @Nullable Map<String, Object> hints);
 
+	/**
+	 * Decode a data buffer to an Object of type T. This is useful when the input
+	 * stream consists of discrete messages (or events) and the content for each
+	 * can be decoded on its own.
+	 * @param buffer the {@code DataBuffer} to decode
+	 * @param targetType the expected output type
+	 * @param mimeType the MIME type associated with the data
+	 * @param hints additional information about how to do encode
+	 * @return the decoded value, possibly {@code null}
+	 * @since 5.2
+	 */
+	@SuppressWarnings("ConstantConditions")
+	default T decode(DataBuffer buffer, ResolvableType targetType,
+			@Nullable MimeType mimeType, @Nullable Map<String, Object> hints) throws DecodingException {
+
+		MonoProcessor<T> processor = MonoProcessor.create();
+		decodeToMono(Mono.just(buffer), targetType, mimeType, hints).subscribeWith(processor);
+
+		Assert.state(processor.isTerminated(), "DataBuffer decoding should have completed.");
+		Throwable ex = processor.getError();
+		if (ex != null) {
+			throw (ex instanceof CodecException ? (CodecException) ex :
+					new DecodingException("Failed to decode: " + ex.getMessage(), ex));
+		}
+		return processor.peek();
+	}
+
 	/**
 	 * Return the list of MIME types this decoder supports.
 	 */

spring-core/src/main/java/org/springframework/core/codec/ResourceDecoder.java

@@ -64,7 +64,7 @@ public class ResourceDecoder extends AbstractDataBufferDecoder<Resource> {
 	}
 
 	@Override
-	protected Resource decodeDataBuffer(DataBuffer dataBuffer, ResolvableType elementType,
+	public Resource decode(DataBuffer dataBuffer, ResolvableType elementType,
 			@Nullable MimeType mimeType, @Nullable Map<String, Object> hints) {
 
 		byte[] bytes = new byte[dataBuffer.readableByteCount()];

spring-core/src/main/java/org/springframework/core/codec/StringDecoder.java

@@ -202,7 +202,7 @@ public final class StringDecoder extends AbstractDataBufferDecoder<String> {
 	}
 
 	@Override
-	protected String decodeDataBuffer(DataBuffer dataBuffer, ResolvableType elementType,
+	public String decode(DataBuffer dataBuffer, ResolvableType elementType,
 			@Nullable MimeType mimeType, @Nullable Map<String, Object> hints) {
 
 		Charset charset = getCharset(mimeType);