Use readNBytes in StringHttpMessageConverter when contentLength is available

When the content length is known, use readNBytes on the InputStream in
StringHttpMessageConverter, which avoids some extra copying and allocations.

Closes gh-30942

Author: kilink

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

@@ -93,7 +93,14 @@ public boolean supports(Class<?> clazz) {
 	@Override
 	protected String readInternal(Class<? extends String> clazz, HttpInputMessage inputMessage) throws IOException {
 		Charset charset = getContentTypeCharset(inputMessage.getHeaders().getContentType());
-		return StreamUtils.copyToString(inputMessage.getBody(), charset);
+		long length = inputMessage.getHeaders().getContentLength();
+		final byte[] bytes;
+		if (length >= 0 && length <= Integer.MAX_VALUE) {
+			bytes = inputMessage.getBody().readNBytes((int) length);
+		} else {
+			bytes = inputMessage.getBody().readAllBytes();
+		}
+		return new String(bytes, charset);
 	}
 
 	@Override

spring-web/src/test/java/org/springframework/http/converter/StringHttpMessageConverterTests.java

@@ -70,6 +70,17 @@ public void read() throws IOException {
 		assertThat(result).as("Invalid result").isEqualTo(body);
 	}
 
+	@Test
+	public void readWithContentLengthHeader() throws IOException {
+		String body = "Hello World";
+		MockHttpInputMessage inputMessage = new MockHttpInputMessage(body.getBytes(StandardCharsets.UTF_8));
+		inputMessage.getHeaders().setContentLength(body.length());
+		inputMessage.getHeaders().setContentType(TEXT_PLAIN_UTF_8);
+		String result = this.converter.read(String.class, inputMessage);
+
+		assertThat(result).as("Invalid result").isEqualTo(body);
+	}
+
 	@Test // gh-24123
 	public void readJson() throws IOException {
 		String body = "{\"result\":\"\u0414\u0410\"}";