Avoid reader on empty content to be shared by multiple requests

This commit avoids several instances of MockHttpServletRequest to
have a common reader for empty content as closing it will have an
unwanted side effect on the others.

Closes gh-32848

Author: snicoll

Diff for: spring-test/src/main/java/org/springframework/mock/web/MockHttpServletRequest.java

@@ -100,9 +100,6 @@ public class MockHttpServletRequest implements HttpServletRequest {
 
 	private static final TimeZone GMT = TimeZone.getTimeZone("GMT");
 
-	private static final BufferedReader EMPTY_BUFFERED_READER =
-			new BufferedReader(new StringReader(""));
-
 	/**
 	 * Date formats as specified in the HTTP RFC.
 	 * @see <a href="https://tools.ietf.org/html/rfc7231#section-7.1.1.1">Section 7.1.1.1 of RFC 7231</a>
@@ -736,7 +733,7 @@ else if (this.inputStream != null) {
 			this.reader = new BufferedReader(sourceReader);
 		}
 		else {
-			this.reader = EMPTY_BUFFERED_READER;
+			this.reader = new BufferedReader(new StringReader(""));
 		}
 		return this.reader;
 	}

Diff for: spring-test/src/test/java/org/springframework/mock/web/MockHttpServletRequestTests.java

@@ -93,6 +93,17 @@ void readEmptyInputStreamWorksAcrossRequests() throws IOException {
 		secondRequest.getInputStream().close();
 	}
 
+	@Test // gh-32820
+	void readEmptyReaderWorksAcrossRequests() throws IOException {
+		MockHttpServletRequest firstRequest = new MockHttpServletRequest();
+		firstRequest.getReader().read(new char[256]);
+		firstRequest.getReader().close();
+
+		MockHttpServletRequest secondRequest = new MockHttpServletRequest();
+		secondRequest.getReader().read(new char[256]);
+		secondRequest.getReader().close();
+	}
+
 	@Test
 	void setContentAndGetReader() throws IOException {
 		byte[] bytes = "body".getBytes(Charset.defaultCharset());