jersery集成jackson实现restful api,由于jdk版本不一致导致的坑

问题背景

项目中使用jersey+jackson实现 restful api,返回信息格式为json,开发测试环境都是OK的，然鹅在线上当访问时一直报500，错误信息如下：

com.sun.jersey.spi.container.ContainerResponse write
: The registered message body writers compatible with the MIME media type are:
application/json ->com.sun.jersey.json.impl.provider.entity.JSONJAXBElementProvider$Appcom.sun.jersey.json.impl.provider.entity.JSONArrayProvider$Appcom.sun.jersey.json.impl.provider.entity.JSONObjectProvider$Appcom.sun.jersey.json.impl.provider.entity.JSONRootElementProvider$Appcom.sun.jersey.json.impl.provider.entity.JSONListElementProvider$Appcom.sun.jersey.core.impl.provider.entity.FormProvidercom.sun.jersey.core.impl.provider.entity.StringProvidercom.sun.jersey.core.impl.provider.entity.ByteArrayProvidercom.sun.jersey.core.impl.provider.entity.FileProvidercom.sun.jersey.core.impl.provider.entity.InputStreamProvidercom.sun.jersey.core.impl.provider.entity.DataSourceProvidercom.sun.jersey.core.impl.provider.entity.XMLJAXBElementProvider$Generalcom.sun.jersey.core.impl.provider.entity.ReaderProvidercom.sun.jersey.core.impl.provider.entity.DocumentProvidercom.sun.jersey.core.impl.provider.entity.StreamingOutputProvidercom.sun.jersey.core.impl.provider.entity.SourceProvider$SourceWritercom.sun.jersey.json.impl.provider.entity.JSONJAXBElementProvider$Generalcom.sun.jersey.json.impl.provider.entity.JSONArrayProvider$Generalcom.sun.jersey.json.impl.provider.entity.JSONObjectProvider$Generalcom.sun.jersey.json.impl.provider.entity.JSONWithPaddingProvidercom.sun.jersey.server.impl.template.ViewableMessageBodyWritercom.sun.jersey.core.impl.provider.entity.XMLRootElementProvider$Generalcom.sun.jersey.core.impl.provider.entity.XMLListElementProvider$Generalcom.sun.jersey.json.impl.provider.entity.JSONRootElementProvider$Generalcom.sun.jersey.json.impl.provider.entity.JSONListElementProvider$GeneralMapped exception to response: 500 (Internal Server Error)
javax.ws.rs.WebApplicationException: com.sun.jersey.api.MessageException: A message body writer for Java class com.account.manage.common.AccountInfoRes
ponse, and Java type class com.account.manage.common.AccountInfoResponse, and MIME media type application/json was not foundat com.sun.jersey.spi.container.ContainerResponse.write(ContainerResponse.java:285)at com.sun.jersey.server.impl.application.WebApplicationImpl._handleRequest(WebApplicationImpl.java:1479)at com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:1391)at com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:1381)at com.sun.jersey.spi.container.servlet.WebComponent.service(WebComponent.java:416)at com.sun.jersey.spi.container.servlet.ServletContainer.service(ServletContainer.java:538)at com.sun.jersey.spi.container.servlet.ServletContainer.service(ServletContainer.java:716)at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:85)at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)

问题分析

开发测试与线上环境唯一不同的就是jdk版本，线上是jdk1.6,本地及测试时jdk7及以上，猜测是不是由于版本不一致导致的问题，有了猜测，还需要进一步的去验证猜测是不是正确：

在开发DEA中切切换项目的jdk的版本，当设置为1.6进行测试时，果然不出所料报了和线上一样的错误，这就进一步验证了确实是因为版本不一致导致的，但我们不能止步于此，需要更深入的了解到，到底是在哪里出错的，这个只能读源码了；
找到报错的位置，很明显通过一层层调用，最后是在调用com.sun.jersey.spi.container.ContainerResponse.write(ContainerResponse.java:285)这个方法时里面报错，打开源代码很快能够定位好发生异常的代码处

 final MessageBodyWriter p = getMessageBodyWorkers().getMessageBodyWriter(entity.getClass(), entityType,annotations, contentType);if (p == null) {String message = "A message body writer for Java class " + entity.getClass().getName() +", and Java type " + entityType +", and MIME media type " + contentType + " was not found";LOGGER.severe(message);Map<MediaType, List<MessageBodyWriter>> m = getMessageBodyWorkers().getWriters(contentType);LOGGER.severe("The registered message body writers compatible with the MIME media type are:\n" +getMessageBodyWorkers().writersToString(m));if (request.getMethod().equals("HEAD")) {isCommitted = true;responseWriter.writeStatusAndHeaders(-1, this);responseWriter.finish();return;} else {throw new WebApplicationException(new MessageException(message), 500);}}

很显然是在获取MessageBodyWriter时获取为null,进入_getMessageBodyWriter方法，通过debug,能够知道该方法是通过mediaType获取对应的MessageBodyWriter的provider,在此处获取不到，那么MessageBodyWriter是什么呢，通过看源码它是一个接口，jersey所有的写response的操作都直接或间接的实现它，与之相对应的是MessageBodyReader接口；
那MessageBodyWriter和MessageBodyReader是什么时候加载的呢，对于这种通过spring实现的web项目，当时是在应用启动的时候加载与初始化了应用所需的所有资源，启动debug,在jerseyserver启动实现类WebApplicationImpl的初始化方法_initiate（）

        // Initiate context resolverscrf.init(providerServices, injectableFactory);// Initiate the exception mappersexceptionFactory.init(providerServices);// Initiate message body readers/writersbodyFactory.init();// Initiate string readersstringReaderFactory.init(providerServices);// Inject on all componentsErrors.setReportMissingDependentFieldOrMethod(true);cpFactory.injectOnAllComponents();cpFactory.injectOnProviderInstances(resourceConfig.getProviderSingletons());

前后代码省略，显然bodyFactory.init() 也就是MessageBodyFactory.init()方法初始化MessageBodyReader和MessageBodyWriter，

 public void init() {initReaders();initWriters();}

此处我们只看initWriters（）方法，initReaders也是同样的逻辑，MessageBodyFactory的属性
Map<MediaType, List> writerProviders，通过map保存MediaType与之对应的MessageBodyWriter

一步步跟踪源代码，最终进入ProviderServicel.getServiceClasses(Class<?> service, Set sp)方法中，service类型就是MessageBodyWriter，获取所有实现MessageBodyWriter的Class:

 Class<?>[] pca = ServiceFinder.find(service, true).toClassArray();

最关键的是ServiceFinder.toClassArray()方法，

  public Class<T>[] toClassArray() throws ServiceConfigurationError {List<Class<T>> result = new ArrayList<Class<T>>();Iterator<Class<T>> i = classIterator();while (i.hasNext())result.add(i.next());return result.toArray((Class<T>[])Array.newInstance(Class.class,result.size()));}

会创建一个Class的Iterator,通过迭代将能够通过反射创建实例的Class放置在result中，i.next()方法其实调用的是ServiceFinder.LazyClassIterator.next()方法，代码如下：

     @SuppressWarnings("unchecked")public Class<T> next() {if (!hasNext()) {throw new NoSuchElementException();}String cn = nextName;nextName = null;try {return (Class<T>)ReflectionHelper.classForNameWithException(cn, loader);} catch (ClassNotFoundException ex) {fail(serviceName,SpiMessages.PROVIDER_NOT_FOUND(cn, service));} catch (NoClassDefFoundError ex) {fail(serviceName,SpiMessages.DEPENDENT_CLASS_OF_PROVIDER_NOT_FOUND(ex.getLocalizedMessage(), cn, service));} catch (ClassFormatError ex) {fail(serviceName,SpiMessages.DEPENDENT_CLASS_OF_PROVIDER_FORMAT_ERROR(ex.getLocalizedMessage(), cn, service));} catch (Exception x) {fail(serviceName,SpiMessages.PROVIDER_CLASS_COULD_NOT_BE_LOADED(cn, service, x.getLocalizedMessage()),x);}return null;    /* This cannot happen */}

在这里当nextName为Jackson的处理类JacksonJsonProvider时，即 return (Class)ReflectionHelper.classForNameWithException(cn, loader) 在这一步不会返回JacksonJsonProvider的Class，导致在下一步没法初始化实例

     for (ProviderClass pc : getServiceClasses(provider)) {Object o = getComponent(pc);if (o != null) {ps.add(provider.cast(o));}}

最终导致在处理业务请求后，jersey通过jackson写响应操作时，获取不到对应的Writer而失败；但是在jdk1.7版本及以上正常的，由于最终调用的jdk的native方法就再没有深入研究为啥会出现
这个情况，总之这是jdk本身的一个坑。

public static Class classForNameWithException(String name, ClassLoader cl)throws ClassNotFoundException {if (cl != null) {try {return Class.forName(name, false, cl);} catch (ClassNotFoundException ex) {}}return Class.forName(name);}private static native Class<?> forName0(String name, boolean initialize,ClassLoader loader,Class<?> caller)throws ClassNotFoundException;

解决方案

为避免jdk版本不同而踩坑，通过采用另一种解决方案

替换jackson依赖

<dependency><groupId>com.sun.jersey</groupId><artifactId>jersey-json</artifactId><version>1.17</version>
</dependency>

web.xml配置

<servlet><servlet-name>jersey-serlvet</servlet-name><servlet-class>com.sun.jersey.spi.spring.container.servlet.SpringServlet</servlet-class><init-param><param-name>jersey.config.server.provider.packages</param-name><param-value>******</param-value></init-param><init-param><param-name>com.sun.jersey.api.json.POJOMappingFeature</param-name><param-value>true</param-value></init-param>

若采用jersey-json，则必须配置com.sun.jersey.api.json.POJOMappingFeature，因为处理jackson的provider为JacksonProviderProxy其中会通过该属性判断json是否与entity映射

 public void setFeaturesAndProperties(FeaturesAndProperties fp) {this.jacksonEntityProviderFeatureSet = fp.getFeature("com.sun.jersey.api.json.POJOMappingFeature");}public boolean isWriteable(Class<?> type, Type genericType, Annotation[] annotations, MediaType mediaType) {return this.jacksonEntityProviderFeatureSet && (this.jaxbProvider.isWriteable(type, genericType, annotations, mediaType) || this.pojoProvider.isWriteable(type, genericType, annotations, mediaType));}

在判断是否可以进行写操作时jacksonEntityProviderFeatureSet必须为ture,否则还是会报500错误，错误信息跟之前一样。