fix spring-doc issue alibaba#387.

extension/src/main/java/com/alibaba/fastjson2/support/spring/http/converter/FastJsonHttpMessageConverter.java

@@ -117,11 +117,18 @@ private Object readType(Type type, HttpInputMessage inputMessage) {
     protected void writeInternal(Object object, HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException {
         try (ByteArrayOutputStream baos = new ByteArrayOutputStream()) {
             HttpHeaders headers = outputMessage.getHeaders();
-
-            int len = JSON.writeTo(baos, object, config.getDateFormat(), config.getWriterFilters(), config.getWriterFeatures());
+            int contentLength;
+
+            if (object instanceof String && JSON.isValidObject((String) object)) {
+                byte[] strBytes = ((String) object).getBytes(config.getCharset());
+                contentLength = strBytes.length;
+                baos.write(strBytes, 0, strBytes.length);
+            } else {
+                contentLength = JSON.writeTo(baos, object, config.getDateFormat(), config.getWriterFilters(), config.getWriterFeatures());
+            }
 
             if (headers.getContentLength() < 0 && config.isWriteContentLength()) {
-                headers.setContentLength(len);
+                headers.setContentLength(contentLength);
             }
 
             baos.writeTo(outputMessage.getBody());

extension/src/test/java/com/alibaba/fastjson2/springdoc/OpenApiJsonWriterTest.java

@@ -1,27 +1,82 @@
 package com.alibaba.fastjson2.springdoc;
 
-import com.alibaba.fastjson2.JSON;
-import com.alibaba.fastjson2.JSONFactory;
-import com.alibaba.fastjson2.JSONWriter;
-import com.alibaba.fastjson2.support.springdoc.OpenApiJsonWriter;
+import com.alibaba.fastjson2.support.spring.http.converter.FastJsonHttpMessageConverter;
+import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.context.annotation.ComponentScan;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.core.Ordered;
+import org.springframework.core.annotation.Order;
+import org.springframework.http.MediaType;
+import org.springframework.http.converter.HttpMessageConverter;
+import org.springframework.test.context.ContextConfiguration;
+import org.springframework.test.context.junit.jupiter.SpringExtension;
+import org.springframework.test.context.web.WebAppConfiguration;
+import org.springframework.test.web.servlet.MockMvc;
+import org.springframework.test.web.servlet.setup.MockMvcBuilders;
+import org.springframework.web.bind.annotation.PostMapping;
+import org.springframework.web.bind.annotation.RequestBody;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RestController;
+import org.springframework.web.context.WebApplicationContext;
+import org.springframework.web.filter.CharacterEncodingFilter;
+import org.springframework.web.servlet.config.annotation.EnableWebMvc;
+import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
 
-import static org.junit.jupiter.api.Assertions.assertEquals;
+import java.util.List;
 
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.post;
+import static org.springframework.test.web.servlet.result.MockMvcResultHandlers.print;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
+
+@ExtendWith(SpringExtension.class)
+@WebAppConfiguration
+@ContextConfiguration
 public class OpenApiJsonWriterTest {
-    String jsonStr = "{\"openapi\":\"3.0.1\",\"info\":{\"title\":\"OpenAPI definition\",\"version\":\"v0\"},\"servers\":[{\"url\":\"http://localhost:8099\",\"description\":\"Generated server url\"}],\"paths\":{},\"components\":{}}";
+    @Autowired
+    private WebApplicationContext wac;
 
-    @Test
-    public void test() {
-        OpenApiJsonWriter writer = OpenApiJsonWriter.INSTANCE;
-        writer.write(JSONWriter.of(), null);
-        writer.write(JSONWriter.of(), jsonStr);
+    private MockMvc mockMvc;
+
+    private static String openapi = "{\"openapi\":\"3.0.1\",\"info\":{\"title\":\"OpenAPI definition\",\"version\":\"v0\"},\"servers\":[{\"url\":\"http://localhost:8099\",\"description\":\"Generated server url\"}],\"paths\":{},\"components\":{}}";
+
+    @BeforeEach
+    public void setup() {
+        this.mockMvc = MockMvcBuilders.webAppContextSetup(this.wac) //
+                .addFilter(new CharacterEncodingFilter("UTF-8", true)) // 设置服务器端返回的字符集为：UTF-8
+                .build();
     }
 
     @Test
-    public void test1() {
-        JSONFactory.getDefaultObjectWriterProvider().register(String.class, OpenApiJsonWriter.INSTANCE);
-        assertEquals(jsonStr, JSON.toJSONString(jsonStr));
-        JSONFactory.getDefaultObjectWriterProvider().unregister(String.class, OpenApiJsonWriter.INSTANCE);
+    public void test() throws Exception {
+        mockMvc.perform(
+                (post("/test").characterEncoding("UTF-8")
+                        .contentType(MediaType.APPLICATION_JSON_VALUE)
+                        .content(openapi)
+                )).andExpect(status().isOk()).andDo(print());
+    }
+
+    @RestController
+    @RequestMapping
+    public static class TestController {
+        @PostMapping(path = "/test", consumes = MediaType.APPLICATION_JSON_VALUE, produces = MediaType.APPLICATION_JSON_VALUE)
+        public String test(@RequestBody String openapi) {
+            return openapi;
+        }
+    }
+
+    @ComponentScan(basePackages = "com.alibaba.fastjson2.springdoc")
+    @Configuration
+    @Order(Ordered.LOWEST_PRECEDENCE + 1)
+    @EnableWebMvc
+    public static class WebMvcConfig
+            implements WebMvcConfigurer {
+        @Override
+        public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
+            FastJsonHttpMessageConverter converter = new FastJsonHttpMessageConverter();
+            converters.add(converter);
+        }
     }
 }