Jackson高并发情况下，产生阻塞

情况：
在高并发情况下，查看线程栈信息，有大量的线程BLOCKED。

从线程栈得知，线程栈中出现了阻塞，锁在了com.fasterxml.jackson.databind.ser.SerializerCache.untypedValueSerializer(SerializerCache.java:74)上。

 "catalina-exec-1453" #1525 daemon prio=5 os_prio=0 tid=0x00007f1010098800 nid=0x2675 waiting for monitor entry [0x00007f0fd74dc000]
 java.lang.Thread.State: BLOCKED (on object monitor)
 at com.fasterxml.jackson.databind.ser.SerializerCache.untypedValueSerializer(SerializerCache.java:74)
  - waiting to lock <0x000000072ac28ed8> (a com.fasterxml.jackson.databind.ser.SerializerCache)
 at com.fasterxml.jackson.databind.SerializerProvider._findExplicitUntypedSerializer(SerializerProvider.java:1087)
 at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.hasSerializerFor(DefaultSerializerProvider.java:392)
 at com.fasterxml.jackson.databind.ObjectMapper.canSerialize(ObjectMapper.java:2412)
 at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.canWrite(AbstractJackson2HttpMessageConverter.java:165)
 at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:157)
 at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:101)
 at org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.handleReturnValue(RequestResponseBodyMethodProcessor.java:199)
 at org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:71)
 at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:128)
 at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod(RequestMappingHandlerAdapter.java:781)
 at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:721)
 at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:83)
 at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:943)
 at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:877)
 at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:966)
 at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:857)
 at javax.servlet.http.HttpServlet.service(HttpServlet.java:624)
 at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:842)
 at javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
 at org.springframework.web.filter.HttpPutFormContentFilter.doFilterInternal(HttpPutFormContentFilter.java:87)
 at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
 at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
 at com.qianye.zhaimi.master.filter.ExceptionFilter.doFilter(ExceptionFilter.java:53)
 at org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:344)
 at org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:261)
 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
 at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:88)
 at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
 at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
 at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
 at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:505)
 at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
 at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
 at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:956)
 at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
 at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:423)
 at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1079)
 at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:625)
 at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1760)
 at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1719)
  - locked <0x0000000733f4d960> (a org.apache.tomcat.util.net.NioChannel)
 at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
 at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
 at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
 at java.lang.Thread.run(Thread.java:745)

分析：

深入分析源代码，从代码中查看得知序列化方法中采用了synchronized关键字，导致BLOCKED。

 /**
  * Method that checks if the shared (and hence, synchronized) lookup Map might have
  * untyped serializer for given type.
  */
 public JsonSerializer<Object> untypedValueSerializer(Class<?> type)
 {
     synchronized (this) {
         return _sharedMap.get(new TypeKey(type, false));
     }
 }