Use DataBuffer::toString instead of CharBuffer

This commit ensures that converting DataBuffers to a String does not
use an expensive ByteBuffer allocation.

Closes gh-29889

Author: xavier-b

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

@@ -16,7 +16,6 @@
 
 package org.springframework.core.codec;
 
-import java.nio.CharBuffer;
 import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
@@ -189,9 +188,8 @@ public String decode(DataBuffer dataBuffer, ResolvableType elementType,
 			@Nullable MimeType mimeType, @Nullable Map<String, Object> hints) {
 
 		Charset charset = getCharset(mimeType);
-		CharBuffer charBuffer = charset.decode(dataBuffer.toByteBuffer());
+		String value = dataBuffer.toString(charset);
 		DataBufferUtils.release(dataBuffer);
-		String value = charBuffer.toString();
 		LogFormatUtils.traceDebug(logger, traceOn -> {
 			String formatted = LogFormatUtils.formatValue(value, !traceOn);
 			return Hints.getLogPrefix(hints) + "Decoded " + formatted;

spring-web/src/main/java/org/springframework/http/codec/FormHttpMessageReader.java

@@ -17,7 +17,6 @@
 package org.springframework.http.codec;
 
 import java.net.URLDecoder;
-import java.nio.CharBuffer;
 import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
 import java.util.Collections;
@@ -129,8 +128,7 @@ public Mono<MultiValueMap<String, String>> readMono(ResolvableType elementType,
 
 		return DataBufferUtils.join(message.getBody(), this.maxInMemorySize)
 				.map(buffer -> {
-					CharBuffer charBuffer = charset.decode(buffer.toByteBuffer());
-					String body = charBuffer.toString();
+					String body = buffer.toString(charset);
 					DataBufferUtils.release(buffer);
 					MultiValueMap<String, String> formData = parseFormData(charset, body);
 					logFormData(formData, hints);