Avoid closing Jackson JsonGenerator for error cases

Prior to this commit, a change introduced in gh-25910 would close the
`JsonGenerator` after it's been used for JSON serialization. This would
not only close it and recycle resources, but also flush the underlyning
buffer to the output.
In a case where the JSON serialization process would throw an exception,
the buffer would be still flushed to the response output. Before the
change introduced in gh-25910, the response body could be still empty at
that point and error handling could write an error body instead.

This commits only closes the `JsonGenerator` when serialization has been
successful.

Note that we're changing this in the spirit of backwards compatibility
in the 5.2.x line, but change this won't be merged forward on the 5.3.x
line, for several reasons:

* this behavior is not consistent. If the JSON output exceeds a
certain size, or if Jackson has been configured to flush after each
write, the response output might still contain an incomplete JSON
payload (just like before this change)

* this behavior is not consistent with the WebFlux and Messaging codecs,
which are flushing or closing the generator

* not closing the generator for error cases prevents resources from
being recycled as expected by Jackson

Fixes gh-26246

Author: bclozel

spring-web/src/main/java/org/springframework/http/converter/json/AbstractJackson2HttpMessageConverter.java

@@ -311,7 +311,8 @@ protected void writeInternal(Object object, @Nullable Type type, HttpOutputMessa
 		JsonEncoding encoding = getJsonEncoding(contentType);
 
 		OutputStream outputStream = StreamUtils.nonClosing(outputMessage.getBody());
-		try (JsonGenerator generator = this.objectMapper.getFactory().createGenerator(outputStream, encoding)) {
+		JsonGenerator generator = this.objectMapper.getFactory().createGenerator(outputStream, encoding);
+		try {
 			writePrefix(generator, object);
 
 			Object value = object;
@@ -346,6 +347,7 @@ protected void writeInternal(Object object, @Nullable Type type, HttpOutputMessa
 
 			writeSuffix(generator, object);
 			generator.flush();
+			generator.close();
 		}
 		catch (InvalidDefinitionException ex) {
 			throw new HttpMessageConversionException("Type definition error: " + ex.getType(), ex);

spring-web/src/test/java/org/springframework/http/converter/json/MappingJackson2HttpMessageConverterTests.java

@@ -16,11 +16,13 @@
 
 package org.springframework.http.converter.json;
 
+import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.lang.reflect.Type;
 import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -135,13 +137,7 @@ public void readUntyped() throws IOException {
 	@Test
 	public void write() throws IOException {
 		MockHttpOutputMessage outputMessage = new MockHttpOutputMessage();
-		MyBean body = new MyBean();
-		body.setString("Foo");
-		body.setNumber(42);
-		body.setFraction(42F);
-		body.setArray(new String[] {"Foo", "Bar"});
-		body.setBool(true);
-		body.setBytes(new byte[] {0x1, 0x2});
+		MyBean body = createSampleBean();
 		converter.write(body, null, outputMessage);
 		String result = outputMessage.getBodyAsString(StandardCharsets.UTF_8);
 		assertThat(result.contains("\"string\":\"Foo\"")).isTrue();
@@ -157,13 +153,7 @@ public void write() throws IOException {
 	@Test
 	public void writeWithBaseType() throws IOException {
 		MockHttpOutputMessage outputMessage = new MockHttpOutputMessage();
-		MyBean body = new MyBean();
-		body.setString("Foo");
-		body.setNumber(42);
-		body.setFraction(42F);
-		body.setArray(new String[] {"Foo", "Bar"});
-		body.setBool(true);
-		body.setBytes(new byte[] {0x1, 0x2});
+		MyBean body = createSampleBean();
 		converter.write(body, MyBase.class, null, outputMessage);
 		String result = outputMessage.getBodyAsString(StandardCharsets.UTF_8);
 		assertThat(result.contains("\"string\":\"Foo\"")).isTrue();
@@ -194,6 +184,16 @@ public void readInvalidJson() throws IOException {
 				converter.read(MyBean.class, inputMessage));
 	}
 
+	@Test // See gh-26246
+	public void writeInvalidJson() throws IOException {
+		MockHttpOutputMessage outputMessage = new MockHttpOutputMessage();
+		MyBean bean = createSampleBean();
+		List<Object> body = Arrays.asList(bean, new ByteArrayOutputStream());
+		assertThatExceptionOfType(HttpMessageConversionException.class)
+				.isThrownBy(() -> converter.write(body, null, outputMessage));
+		assertThat(outputMessage.getBodyAsString(StandardCharsets.UTF_8)).isEmpty();
+	}
+
 	@Test
 	public void readValidJsonWithUnknownProperty() throws IOException {
 		String body = "{\"string\":\"string\",\"unknownProperty\":\"value\"}";
@@ -492,6 +492,17 @@ public void writeAscii() throws Exception {
 		assertThat(outputMessage.getHeaders().getContentType()).as("Invalid content-type").isEqualTo(contentType);
 	}
 
+	private MyBean createSampleBean() {
+		MyBean body = new MyBean();
+		body.setString("Foo");
+		body.setNumber(42);
+		body.setFraction(42F);
+		body.setArray(new String[] {"Foo", "Bar"});
+		body.setBool(true);
+		body.setBytes(new byte[] {0x1, 0x2});
+		return body;
+	}
+
 
 	interface MyInterface {