Calculating capacity before allocation.

This commit optimizes the `CharSequenceEncoder` to allocate `DataBuffer`
instances with a predicted capacity.

Issue: SPR-17558

Author: poutsma

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

@@ -20,6 +20,8 @@
 import java.nio.charset.CoderMalfunctionError;
 import java.nio.charset.StandardCharsets;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
 
 import org.reactivestreams.Publisher;
 import reactor.core.publisher.Flux;
@@ -49,6 +51,9 @@ public final class CharSequenceEncoder extends AbstractEncoder<CharSequence> {
 	 */
 	public static final Charset DEFAULT_CHARSET = StandardCharsets.UTF_8;
 
+	private final ConcurrentMap<Charset, Float> charsetToMaxBytesPerChar =
+			new ConcurrentHashMap<>(3);
+
 
 	private CharSequenceEncoder(MimeType... mimeTypes) {
 		super(mimeTypes);
@@ -76,7 +81,8 @@ public Flux<DataBuffer> encode(Publisher<? extends CharSequence> inputStream,
 				});
 			}
 			boolean release = true;
-			DataBuffer dataBuffer = bufferFactory.allocateBuffer();
+			int capacity = calculateCapacity(charSequence, charset);
+			DataBuffer dataBuffer = bufferFactory.allocateBuffer(capacity);
 			try {
 				dataBuffer.write(charSequence, charset);
 				release = false;
@@ -93,6 +99,13 @@ public Flux<DataBuffer> encode(Publisher<? extends CharSequence> inputStream,
 		});
 	}
 
+	private int calculateCapacity(CharSequence sequence, Charset charset) {
+		float maxBytesPerChar = this.charsetToMaxBytesPerChar
+				.computeIfAbsent(charset, cs -> cs.newEncoder().maxBytesPerChar());
+		float maxBytesForSequence = sequence.length() * maxBytesPerChar;
+		return (int) Math.ceil(maxBytesForSequence);
+	}
+
 	private Charset getCharset(@Nullable MimeType mimeType) {
 		if (mimeType != null && mimeType.getCharset() != null) {
 			return mimeType.getCharset();