Jackson encoder skips String.class

Jackson2Encoder explicitly disables String from the supported types
consistent with the same change on the decoder side:

https://github.com/spring-projects/spring-framework/commit/0662dbf0447d8e788575fcf73fafe248b74cbe8d

Issue: SPR-15443

spring-test/src/test/java/org/springframework/test/web/reactive/server/samples/JsonContentTests.java

@@ -15,17 +15,22 @@
  */
 package org.springframework.test.web.reactive.server.samples;
 
+import java.net.URI;
+
 import org.junit.Test;
 import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
 
 import org.springframework.http.MediaType;
+import org.springframework.http.ResponseEntity;
 import org.springframework.test.web.reactive.server.WebTestClient;
 import org.springframework.web.bind.annotation.GetMapping;
-import org.springframework.web.bind.annotation.PathVariable;
+import org.springframework.web.bind.annotation.PostMapping;
+import org.springframework.web.bind.annotation.RequestBody;
 import org.springframework.web.bind.annotation.RestController;
 
 /**
- * Sample tests asserting JSON response content.
+ * Samples of tests with serialized JSON content.
  *
  * @author Rossen Stoyanchev
  */
@@ -55,6 +60,16 @@ public class JsonContentTests {
 				.jsonPath("$[2].name").isEqualTo("John");
 	}
 
+	@Test
+	public void postJsonContent() throws Exception {
+		this.client.post().uri("/persons")
+				.contentType(MediaType.APPLICATION_JSON_UTF8)
+				.body("{\"name\":\"John\"}")
+				.exchange()
+				.expectStatus().isCreated()
+				.expectBody().isEmpty();
+	}
+
 
 	@RestController
 	@SuppressWarnings("unused")
@@ -64,6 +79,11 @@ public class JsonContentTests {
 		Flux<Person> getPersons() {
 			return Flux.just(new Person("Jane"), new Person("Jason"), new Person("John"));
 		}
+
+		@PostMapping
+		ResponseEntity<String> savePerson(@RequestBody Person person) {
+			return ResponseEntity.created(URI.create("/persons/" + person.getName())).build();
+		}
 	}
 
 }

spring-test/src/test/java/org/springframework/test/web/reactive/server/samples/ResponseEntityTests.java

@@ -24,7 +24,6 @@ import java.util.Map;
 
 import org.junit.Test;
 import reactor.core.publisher.Flux;
-import reactor.core.publisher.Mono;
 import reactor.test.StepVerifier;
 
 import org.springframework.http.MediaType;
@@ -124,7 +123,7 @@ public class ResponseEntityTests {
 	@Test
 	public void postEntity() throws Exception {
 		this.client.post().uri("/persons")
-				.body(Mono.just(new Person("John")), Person.class)
+				.body(new Person("John"))
 				.exchange()
 				.expectStatus().isCreated()
 				.expectHeader().valueEquals("location", "/persons/John")

spring-web/src/main/java/org/springframework/http/codec/json/Jackson2JsonDecoder.java

@@ -69,7 +69,7 @@ public class Jackson2JsonDecoder extends Jackson2CodecSupport implements HttpMes
 	@Override
 	public boolean canDecode(ResolvableType elementType, MimeType mimeType) {
 		JavaType javaType = this.objectMapper.getTypeFactory().constructType(elementType.getType());
-		// Skip String (CharSequenceDecoder + "*/*" comes after)
+		// Skip String: CharSequenceDecoder + "*/*" comes after
 		return (!CharSequence.class.isAssignableFrom(elementType.resolve(Object.class)) &&
 				this.objectMapper.canDeserialize(javaType) && supportsMimeType(mimeType));
 	}

spring-web/src/main/java/org/springframework/http/codec/json/Jackson2JsonEncoder.java

@@ -105,7 +105,9 @@ public class Jackson2JsonEncoder extends Jackson2CodecSupport implements HttpMes
 	@Override
 	public boolean canEncode(ResolvableType elementType, MimeType mimeType) {
 		Class<?> clazz = elementType.getRawClass();
-		return (this.objectMapper.canSerialize(clazz) && supportsMimeType(mimeType));
+		// Skip String: StringDecoder + "*/*" comes after
+		return (!String.class.isAssignableFrom(elementType.resolve(Object.class)) &&
+				this.objectMapper.canSerialize(clazz) && supportsMimeType(mimeType));
 	}
 
 	@Override

spring-web/src/test/java/org/springframework/http/codec/json/Jackson2JsonEncoderTests.java

@@ -52,7 +52,13 @@ public class Jackson2JsonEncoderTests extends AbstractDataBufferAllocatingTestCa
 		ResolvableType pojoType = ResolvableType.forClass(Pojo.class);
 		assertTrue(this.encoder.canEncode(pojoType, APPLICATION_JSON));
 		assertTrue(this.encoder.canEncode(pojoType, null));
-		assertFalse(this.encoder.canEncode(pojoType, APPLICATION_XML));
+	}
+
+	@Test
+	public void canNotEncode() {
+		assertFalse(this.encoder.canEncode(ResolvableType.forClass(String.class), null));
+		assertFalse(this.encoder.canEncode(ResolvableType.forClass(Pojo.class), APPLICATION_XML));
+
 		ResolvableType sseType = ResolvableType.forClass(ServerSentEvent.class);
 		assertFalse(this.encoder.canEncode(sseType, APPLICATION_JSON));
 	}

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/ResponseEntityResultHandlerTests.java

@@ -291,7 +291,7 @@ public class ResponseEntityResultHandlerTests {
 		this.resultHandler.handleResult(exchange, result).block(Duration.ofSeconds(5));
 
 		assertEquals(HttpStatus.OK, exchange.getResponse().getStatusCode());
-		assertResponseBody(exchange, "\"body\"");
+		assertResponseBody(exchange, "body");
 	}
 
 	@Test // SPR-14877

spring-webflux/src/test/java/org/springframework/web/reactive/result/view/HttpMessageWriterViewTests.java

@@ -67,11 +67,11 @@ public class HttpMessageWriterViewTests {
 	@Test
 	public void singleMatch() throws Exception {
 		this.view.setModelKeys(Collections.singleton("foo2"));
-		this.model.addAttribute("foo1", "bar1");
-		this.model.addAttribute("foo2", "bar2");
-		this.model.addAttribute("foo3", "bar3");
+		this.model.addAttribute("foo1", Collections.singleton("bar1"));
+		this.model.addAttribute("foo2", Collections.singleton("bar2"));
+		this.model.addAttribute("foo3", Collections.singleton("bar3"));
 
-		assertEquals("\"bar2\"", doRender());
+		assertEquals("[\"bar2\"]", doRender());
 	}
 
 	@Test
@@ -94,11 +94,11 @@ public class HttpMessageWriterViewTests {
 	@Test
 	public void multipleMatches() throws Exception {
 		this.view.setModelKeys(new HashSet<>(Arrays.asList("foo1", "foo2")));
-		this.model.addAttribute("foo1", "bar1");
-		this.model.addAttribute("foo2", "bar2");
-		this.model.addAttribute("foo3", "bar3");
+		this.model.addAttribute("foo1", Collections.singleton("bar1"));
+		this.model.addAttribute("foo2", Collections.singleton("bar2"));
+		this.model.addAttribute("foo3", Collections.singleton("bar3"));
 
-		assertEquals("{\"foo1\":\"bar1\",\"foo2\":\"bar2\"}", doRender());
+		assertEquals("{\"foo1\":[\"bar1\"],\"foo2\":[\"bar2\"]}", doRender());
 	}
 
 	@Test