如何转义JAXB生成的XML字符串双引号并解决RestTemplate提交报错

问题场景 #

我需要构建XML并通过RestTemplate的POST请求提交，实现代码如下：

final RestTemplate restTemplate1 = new RestTemplate();
List<HttpMessageConverter<?>> messageConverters = new ArrayList<>();
MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter();
converter.setSupportedMediaTypes(Collections.singletonList(MediaType.APPLICATION_XML));
messageConverters.add(converter);
restTemplate1.setMessageConverters(messageConverters);

HttpHeaders headers = new HttpHeaders();
headers.setBasicAuth("okmAdmin", "admin");
headers.setContentType(MediaType.APPLICATION_XML);

JAXBContext context = null;
context = JAXBContext.newInstance(Folder.class);
Marshaller mar = null;
mar = context.createMarshaller();
mar.setProperty("jaxb.encoding", "Unicode");
mar.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.FALSE);

//Print XML String to Console
StringWriter sw = new StringWriter();

Folder folder = new Folder();
folder.setPath("/okm:root/test4");

//Write XML to StringWriter
mar.marshal(folder, sw);

String xmlContent = sw.toString();

System.out.println(xmlContent);

HttpEntity<String> request = new HttpEntity<String>(xmlContent, headers);

ResponseEntity<Folder> response = restTemplate1.postForEntity("http://localhost:8080/OpenKM/services/rest/folder/create", request, Folder.class);

生成的XML内容：

<?xml version="1.0" encoding="Unicode" standalone="yes"?><folder><path>/okm:root/test4</path><permissions>0</permissions><subscribed>false</subscribed><hasChildren>false</hasChildren><style>0</style></folder>

提交时出现错误：

2022-12-05 16:50:21.121 ERROR 15764 --- [nio-8082-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.springframework.web.client.HttpClientErrorException$BadRequest: 400 : "JAXBException occurred : Unexpected character '"' (code 34) in prolog; expected '<'<EOL> at [row,col {unknown-source}]: [1,1]. "] with root cause

错误原因 #

不是XML本身的双引号需要转义，而是实现逻辑冲突：
手动用JAXB把对象转成XML字符串后，又配置了MappingJackson2HttpMessageConverter处理XML。Jackson会把传入的XML字符串当成普通字符串处理，自动给整个XML内容包裹双引号，导致服务器收到的请求体开头是"<?xml...，解析时触发"prolog里意外的双引号"错误。

解决方案 #

方案1：使用JAXB消息转换器，自动处理XML序列化（推荐） #

直接用Spring提供的Jaxb2RootElementHttpMessageConverter，无需手动转XML，让框架自动完成对象到XML的转换：

final RestTemplate restTemplate1 = new RestTemplate();
List<HttpMessageConverter<?>> messageConverters = new ArrayList<>();
// 替换为JAXB专用的XML消息转换器
Jaxb2RootElementHttpMessageConverter jaxbConverter = new Jaxb2RootElementHttpMessageConverter();
jaxbConverter.setSupportedMediaTypes(Collections.singletonList(MediaType.APPLICATION_XML));
messageConverters.add(jaxbConverter);
restTemplate1.setMessageConverters(messageConverters);

HttpHeaders headers = new HttpHeaders();
headers.setBasicAuth("okmAdmin", "admin");
headers.setContentType(MediaType.APPLICATION_XML);

Folder folder = new Folder();
folder.setPath("/okm:root/test4");

// 直接传入Folder对象，框架自动序列化XML
HttpEntity<Folder> request = new HttpEntity<>(folder, headers);
ResponseEntity<Folder> response = restTemplate1.postForEntity(
    "http://localhost:8080/OpenKM/services/rest/folder/create", 
    request, 
    Folder.class
);

方案2：保留手动XML转换，使用字符串消息转换器 #

如果必须手动生成XML字符串，就改用StringHttpMessageConverter，让框架直接发送原始字符串，不做额外处理：

final RestTemplate restTemplate1 = new RestTemplate();
List<HttpMessageConverter<?>> messageConverters = new ArrayList<>();
// 使用字符串转换器，直接发送XML原始内容
StringHttpMessageConverter stringConverter = new StringHttpMessageConverter(Charset.forName("Unicode"));
stringConverter.setSupportedMediaTypes(Collections.singletonList(MediaType.APPLICATION_XML));
messageConverters.add(stringConverter);
restTemplate1.setMessageConverters(messageConverters);

HttpHeaders headers = new HttpHeaders();
headers.setBasicAuth("okmAdmin", "admin");
headers.setContentType(MediaType.APPLICATION_XML);

JAXBContext context = JAXBContext.newInstance(Folder.class);
Marshaller mar = context.createMarshaller();
mar.setProperty("jaxb.encoding", "Unicode");
mar.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.FALSE);

StringWriter sw = new StringWriter();
Folder folder = new Folder();
folder.setPath("/okm:root/test4");
mar.marshal(folder, sw);
String xmlContent = sw.toString();

HttpEntity<String> request = new HttpEntity<>(xmlContent, headers);
ResponseEntity<Folder> response = restTemplate1.postForEntity(
    "http://localhost:8080/OpenKM/services/rest/folder/create", 
    request, 
    Folder.class
);

内容的提问来源于stack exchange，提问作者user1999453