Use readNBytes in StringHttpMessageConverter when contentLength is av…

…ailable

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

Closes gh-30942

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\"}";