Fix MockHttpServletRequest InputStream for empty body

Prior to this commit, a recent change applied in gh-29125 changed the
behavior of `MockHttpServletRequest` instances. In case of an empty
request body, the returned `InputStream` would be static and could not
be reused across requests.
This could result in `java.io.IOException: Stream closed` exceptions if
a previous request was read.

This commit ensures that a new instance of an empty stream is returned
for each request instance.

Fixes gh-29901

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -101,9 +101,6 @@ public class MockHttpServletRequest implements HttpServletRequest {
 
 	private static final TimeZone GMT = TimeZone.getTimeZone("GMT");
 
-	private static final ServletInputStream EMPTY_SERVLET_INPUT_STREAM =
-			new DelegatingServletInputStream(InputStream.nullInputStream());
-
 	private static final BufferedReader EMPTY_BUFFERED_READER =
 			new BufferedReader(new StringReader(""));
 
@@ -513,12 +510,12 @@ public class MockHttpServletRequest implements HttpServletRequest {
 		}
 		else if (this.reader != null) {
 			throw new IllegalStateException(
-					"Cannot call getInputStream() after getReader() has already been called for the current request")			;
+					"Cannot call getInputStream() after getReader() has already been called for the current request");
 		}
 
 		this.inputStream = (this.content != null ?
 				new DelegatingServletInputStream(new ByteArrayInputStream(this.content)) :
-				EMPTY_SERVLET_INPUT_STREAM);
+				new DelegatingServletInputStream(InputStream.nullInputStream()));
 		return this.inputStream;
 	}
 

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

@@ -82,6 +82,17 @@ class MockHttpServletRequestTests {
 		assertThat(StreamUtils.copyToString(request.getInputStream(), Charset.defaultCharset())).isEqualTo("body");
 	}
 
+	@Test
+	void readEmptyInputStreamWorksAcrossRequests() throws IOException {
+		MockHttpServletRequest firstRequest = new MockHttpServletRequest();
+		firstRequest.getInputStream().readAllBytes();
+		firstRequest.getInputStream().close();
+
+		MockHttpServletRequest secondRequest = new MockHttpServletRequest();
+		secondRequest.getInputStream().readAllBytes();
+		secondRequest.getInputStream().close();
+	}
+
 	@Test
 	void setContentAndGetReader() throws IOException {
 		byte[] bytes = "body".getBytes(Charset.defaultCharset());