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。

 1 "catalina-exec-1453" #1525 daemon prio=5 os_prio=0 tid=0x00007f1010098800 nid=0x2675 waiting for monitor entry [0x00007f0fd74dc000]
 2 java.lang.Thread.State: BLOCKED (on object monitor)
 3 at com.fasterxml.jackson.databind.ser.SerializerCache.untypedValueSerializer(SerializerCache.java:74)
 4  - waiting to lock <0x000000072ac28ed8> (a com.fasterxml.jackson.databind.ser.SerializerCache)
 5 at com.fasterxml.jackson.databind.SerializerProvider._findExplicitUntypedSerializer(SerializerProvider.java:1087)
 6 at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.hasSerializerFor(DefaultSerializerProvider.java:392)
 7 at com.fasterxml.jackson.databind.ObjectMapper.canSerialize(ObjectMapper.java:2412)
 8 at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.canWrite(AbstractJackson2HttpMessageConverter.java:165)
 9 at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:157)
10 at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:101)
11 at org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.handleReturnValue(RequestResponseBodyMethodProcessor.java:199)
12 at org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:71)
13 at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:128)
14 at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod(RequestMappingHandlerAdapter.java:781)
15 at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:721)
16 at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:83)
17 at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:943)
18 at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:877)
19 at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:966)
20 at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:857)
21 at javax.servlet.http.HttpServlet.service(HttpServlet.java:624)
22 at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:842)
23 at javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
24 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
25 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
26 at org.springframework.web.filter.HttpPutFormContentFilter.doFilterInternal(HttpPutFormContentFilter.java:87)
27 at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
28 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
29 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
30 at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
31 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
32 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
33 at com.qianye.zhaimi.master.filter.ExceptionFilter.doFilter(ExceptionFilter.java:53)
34 at org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:344)
35 at org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:261)
36 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
37 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
38 at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:88)
39 at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
40 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
41 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
42 at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
43 at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
44 at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:505)
45 at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
46 at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
47 at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:956)
48 at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
49 at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:423)
50 at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1079)
51 at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:625)
52 at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1760)
53 at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1719)
54  - locked <0x0000000733f4d960> (a org.apache.tomcat.util.net.NioChannel)
55 at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
56 at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
57 at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
58 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。

 1 /**
 2  * Method that checks if the shared (and hence, synchronized) lookup Map might have
 3  * untyped serializer for given type.
 4  */
 5 public JsonSerializer<Object> untypedValueSerializer(Class<?> type)
 6 {
 7     synchronized (this) {
 8         return _sharedMap.get(new TypeKey(type, false));
 9     }
10 }