Use InputStream's contracts instead of StreamUtils

Since Spring Framework 6.0 requires Java 17, we can now use
`InputStream` new contracts when manipulating streams.

Closes gh-27702

spring-core/src/main/java/org/springframework/util/FileCopyUtils.java

@@ -108,12 +108,10 @@ public abstract class FileCopyUtils {
 		Assert.notNull(in, "No InputStream specified");
 		Assert.notNull(out, "No OutputStream specified");
 
-		try {
-			return StreamUtils.copy(in, out);
-		}
-		finally {
-			close(in);
-			close(out);
+		try (in; out) {
+			int count = (int) in.transferTo(out);
+			out.flush();
+			return count;
 		}
 	}
 

spring-core/src/main/java/org/springframework/util/StreamUtils.java

@@ -64,9 +64,7 @@ public abstract class StreamUtils {
 			return EMPTY_CONTENT;
 		}
 
-		ByteArrayOutputStream out = new ByteArrayOutputStream(BUFFER_SIZE);
-		copy(in, out);
-		return out.toByteArray();
+		return in.readAllBytes();
 	}
 
 	/**
@@ -152,15 +150,9 @@ public abstract class StreamUtils {
 		Assert.notNull(in, "No InputStream specified");
 		Assert.notNull(out, "No OutputStream specified");
 
-		int byteCount = 0;
-		byte[] buffer = new byte[BUFFER_SIZE];
-		int bytesRead;
-		while ((bytesRead = in.read(buffer)) != -1) {
-			out.write(buffer, 0, bytesRead);
-			byteCount += bytesRead;
-		}
+		int count = (int) in.transferTo(out);
 		out.flush();
-		return byteCount;
+		return count;
 	}
 
 	/**

spring-web/src/main/java/org/springframework/http/converter/ByteArrayHttpMessageConverter.java

@@ -16,7 +16,6 @@
 
 package org.springframework.http.converter;
 
-import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 
 import org.springframework.http.HttpInputMessage;
@@ -53,11 +52,7 @@ public class ByteArrayHttpMessageConverter extends AbstractHttpMessageConverter<
 
 	@Override
 	public byte[] readInternal(Class<? extends byte[]> clazz, HttpInputMessage inputMessage) throws IOException {
-		long contentLength = inputMessage.getHeaders().getContentLength();
-		ByteArrayOutputStream bos =
-				new ByteArrayOutputStream(contentLength >= 0 ? (int) contentLength : StreamUtils.BUFFER_SIZE);
-		StreamUtils.copy(inputMessage.getBody(), bos);
-		return bos.toByteArray();
+		return inputMessage.getBody().readAllBytes();
 	}
 
 	@Override

spring-web/src/main/java/org/springframework/http/converter/ResourceHttpMessageConverter.java

@@ -19,6 +19,7 @@ package org.springframework.http.converter;
 import java.io.FileNotFoundException;
 import java.io.IOException;
 import java.io.InputStream;
+import java.io.OutputStream;
 
 import org.springframework.core.io.ByteArrayResource;
 import org.springframework.core.io.InputStreamResource;
@@ -144,7 +145,9 @@ public class ResourceHttpMessageConverter extends AbstractHttpMessageConverter<R
 		try {
 			InputStream in = resource.getInputStream();
 			try {
-				StreamUtils.copy(in, outputMessage.getBody());
+				OutputStream out = outputMessage.getBody();
+				in.transferTo(out);
+				out.flush();
 			}
 			catch (NullPointerException ex) {
 				// ignore, see SPR-13620