我正在使用Java EntityManager在WSO2 AM中使用数据库，并且应用程序正在使用多个线程，有时当应用程序有很多请求时应用程序挂起。
线程转储如下，
Thread 23217: (state = BLOCKED)
- java.util.HashSet.add(java.lang.Object) @bci=8, line=217 (Compiled frame; information may be imprecise)
- java.util.AbstractCollection.addAll(java.util.Collection) @bci=29, line=342 (Compiled frame)
- java.util.LinkedHashSet.<init>(java.util.Collection) @bci=22, line=169 (Interpreted frame)
- org.eclipse.persistence.internal.jpa.metamodel.ManagedTypeImpl.getAttributes() @bci=13, line=160 (Interpreted frame)
- org.eclipse.persistence.internal.jpa.metamodel.IdentifiableTypeImpl.initializeIdAttributes() @bci=12, line=95 (Compiled frame)
- org.eclipse.persistence.internal.jpa.metamodel.MetamodelImpl.initialize(java.lang.ClassLoader) @bci=251, line=493 (Compiled frame)
- org.eclipse.persistence.internal.jpa.EntityManagerSetupImpl.getMetamodel(java.lang.ClassLoader) @bci=25, line=3476 (Interpreted frame)
- org.eclipse.persistence.internal.jpa.EntityManagerSetupImpl.deploy(java.lang.ClassLoader, java.util.Map) @bci=955, line=754 (Interpreted frame)
- org.eclipse.persistence.internal.jpa.EntityManagerFactoryDelegate.getAbstractSession() @bci=54, line=205 (Interpreted frame)
- org.eclipse.persistence.internal.jpa.EntityManagerFactoryDelegate.createEntityManagerImpl(java.util.Map, javax.persistence.SynchronizationType) @bci=5, line=305 (Interpreted frame)
- org.eclipse.persistence.internal.jpa.EntityManagerFactoryImpl.createEntityManagerImpl(java.util.Map, javax.persistence.SynchronizationType) @bci=90, line=337 (Interpreted frame)
- org.eclipse.persistence.internal.jpa.EntityManagerFactoryImpl.createEntityManager() @bci=3, line=303 (Interpreted frame)
- com.abc.example.dao.GenericDAO.<init>(com.abc.example..utils.DBProperties) @bci=111, line=35 (Interpreted frame)
- com.abc.example.dao.NewDAO.<init>(com.abc.example.utils.DBProperties) @bci=2, line=21 (Interpreted frame)
- com.abc.example.services.MyService.<init>(java.lang.String) @bci=56, line=19 (Interpreted frame)
- com.abc.example.services.InitService.<init>(com.abc.example.ws.GetRequest) @bci=3, line=17 (Interpreted frame)
- com.abc.example.ws.MySOAPImpl.getRequest(com.abc.example.ws.GetRequest) @bci=36, line=107 (Interpreted frame)
- sun.reflect.GeneratedMethodAccessor127.invoke(java.lang.Object, java.lang.Object[]) @bci=40 (Interpreted frame)
- sun.reflect.DelegatingMethodAccessorImpl.invoke(java.lang.Object, java.lang.Object[]) @bci=6, line=43 (Compiled frame)
- java.lang.reflect.Method.invoke(java.lang.Object, java.lang.Object[]) @bci=57, line=606 (Compiled frame)
- org.apache.cxf.service.invoker.AbstractInvoker.performInvocation(org.apache.cxf.message.Exchange, java.lang.Object, java.lang.reflect.Method, java.lang.Object[]) @bci=57, line=181 (Interpreted frame)
- org.apache.cxf.service.invoker.AbstractInvoker.invoke(org.apache.cxf.message.Exchange, java.lang.Object, java.lang.reflect.Method, java.util.List) @bci=26, line=97 (Interpreted frame)
- org.apache.cxf.jaxws.AbstractJAXWSMethodInvoker.invoke(org.apache.cxf.message.Exchange, java.lang.Object, java.lang.reflect.Method, java.util.List) @bci=132, line=232 (Interpreted frame)
- org.apache.cxf.jaxws.JAXWSMethodInvoker.invoke(org.apache.cxf.message.Exchange, java.lang.Object, java.lang.reflect.Method, java.util.List) @bci=76, line=69 (Interpreted frame)
- org.apache.cxf.service.invoker.AbstractInvoker.invoke(org.apache.cxf.message.Exchange, java.lang.Object) @bci=215, line=75 (Interpreted frame)
- org.apache.cxf.interceptor.ServiceInvokerInterceptor$1.run() @bci=26, line=59 (Interpreted frame)
- java.util.concurrent.Executors$RunnableAdapter.call() @bci=4, line=471 (Interpreted frame)
- java.util.concurrent.FutureTask.run() @bci=42, line=262 (Interpreted frame)
- org.apache.cxf.interceptor.ServiceInvokerInterceptor$2.run() @bci=46, line=126 (Interpreted frame)
- org.apache.cxf.workqueue.SynchronousExecutor.execute(java.lang.Runnable) @bci=1, line=37 (Interpreted frame)
- org.apache.cxf.interceptor.ServiceInvokerInterceptor.handleMessage(org.apache.cxf.message.Message) @bci=165, line=131 (Interpreted frame)
- org.apache.cxf.phase.PhaseInterceptorChain.doIntercept(org.apache.cxf.message.Message) @bci=137, line=307 (Compiled frame)
- org.apache.cxf.transport.ChainInitiationObserver.onMessage(org.apache.cxf.message.Message) @bci=416, line=121 (Interpreted frame)
- org.apache.cxf.transport.http.AbstractHTTPDestination.invoke(javax.servlet.ServletConfig, javax.servlet.ServletContext, javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse) @bci=102, line=251 (Interpreted frame)
- org.apache.cxf.transport.servlet.ServletController.invokeDestination(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse, org.apache.cxf.transport.http.AbstractHTTPDestination) @bci=55, line=234 (Interpreted frame)
- org.apache.cxf.transport.servlet.ServletController.invoke(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse, boolean) @bci=319, line=208 (Interpreted frame)
- org.apache.cxf.transport.servlet.ServletController.invoke(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse) @bci=4, line=160 (Interpreted frame)
- org.apache.cxf.transport.servlet.CXFNonSpringServlet.invoke(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse) @bci=42, line=171 (Interpreted frame)
- org.apache.cxf.transport.servlet.AbstractHTTPServlet.handleRequest(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse) @bci=161, line=293 (Interpreted frame)
- org.apache.cxf.transport.servlet.AbstractHTTPServlet.doPost(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse) @bci=3, line=212 (Interpreted frame)
- javax.servlet.http.HttpServlet.service(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse) @bci=131, line=755 (Interpreted frame)
- org.apache.cxf.transport.servlet.AbstractHTTPServlet.service(javax.servlet.ServletRequest, javax.servlet.ServletResponse) @bci=51, line=268 (Interpreted frame)
- org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(javax.servlet.ServletRequest, javax.servlet.ServletResponse) @bci=446, line=305 (Interpreted frame)
- org.apache.catalina.core.ApplicationFilterChain.doFilter(javax.servlet.ServletRequest, javax.servlet.ServletResponse) @bci=101, line=210 (Interpreted frame)
- org.apache.catalina.core.StandardWrapperValve.invoke(org.apache.catalina.connector.Request, org.apache.catalina.connector.Response) @bci=783, line=222 (Interpreted frame)
- org.apache.catalina.core.StandardContextValve.invoke(org.apache.catalina.connector.Request, org.apache.catalina.connector.Response) @bci=166, line=123 (Interpreted frame)
- org.apache.catalina.authenticator.AuthenticatorBase.invoke(org.apache.catalina.connector.Request, org.apache.catalina.connector.Response) @bci=363, line=472 (Interpreted frame)
- org.apache.catalina.core.StandardHostValve.invoke(org.apache.catalina.connector.Request, org.apache.catalina.connector.Response) @bci=148, line=171 (Interpreted frame)
- org.apache.catalina.valves.ErrorReportValve.invoke(org.apache.catalina.connector.Request, org.apache.catalina.connector.Response) @bci=6, line=99 (Interpreted frame)
- org.wso2.carbon.tomcat.ext.valves.CompositeValve.continueInvocation(org.apache.catalina.connector.Request, org.apache.catalina.connector.Response) @bci=190, line=178 (Interpreted frame)
- org.wso2.carbon.tomcat.ext.valves.CarbonTomcatValve$1.invoke(org.apache.catalina.connector.Request, org.apache.catalina.connector.Response, org.wso2.carbon.tomcat.ext.valves.CompositeValve) @bci=3, line=47 (Interpreted frame)
- org.wso2.carbon.webapp.mgt.TenantLazyLoaderValve.invoke(org.apache.catalina.connector.Request, org.apache.catalina.connector.Response, org.wso2.carbon.tomcat.ext.valves.CompositeValve) @bci=89, line=56 (Interpreted frame)
- org.wso2.carbon.tomcat.ext.valves.TomcatValveContainer.invokeValves(org.apache.catalina.connector.Request, org.apache.catalina.connector.Response, org.wso2.carbon.tomcat.ext.valves.CompositeValve) @bci=26, line=47 (Interpreted frame)
- org.wso2.carbon.tomcat.ext.valves.CompositeValve.invoke(org.apache.catalina.connector.Request, org.apache.catalina.connector.Response) @bci=345, line=141 (Interpreted frame)
- org.wso2.carbon.tomcat.ext.valves.CarbonStuckThreadDetectionValve.invoke(org.apache.catalina.connector.Request, org.apache.catalina.connector.Response) @bci=102, line=156 (Interpreted frame)
- org.wso2.carbon.ui.valve.XSSValve.invoke(org.apache.catalina.connector.Request, org.apache.catalina.connector.Response) @bci=116, line=118 (Interpreted frame)
- org.wso2.carbon.ui.valve.CSRFValve.invoke(org.apache.catalina.connector.Request, org.apache.catalina.connector.Response) @bci=17, line=72 (Interpreted frame)
- org.apache.catalina.valves.AccessLogValve.invoke(org.apache.catalina.connector.Request, org.apache.catalina.connector.Response) @bci=6, line=936 (Interpreted frame)
- org.wso2.carbon.tomcat.ext.valves.CarbonContextCreatorValve.invoke(org.apache.catalina.connector.Request, org.apache.catalina.connector.Response) @bci=11, line=52 (Interpreted frame)
- org.apache.catalina.core.StandardEngineValve.invoke(org.apache.catalina.connector.Request, org.apache.catalina.connector.Response) @bci=71, line=118 (Interpreted frame)
- org.apache.catalina.connector.CoyoteAdapter.service(org.apache.coyote.Request, org.apache.coyote.Response) @bci=196, line=407 (Interpreted frame)
- org.apache.coyote.http11.AbstractHttp11Processor.process(org.apache.tomcat.util.net.SocketWrapper) @bci=537, line=1004 (Compiled frame)
- org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(org.apache.tomcat.util.net.SocketWrapper, org.apache.tomcat.util.net.SocketStatus) @bci=158, line=589 (Interpreted frame)
- org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run() @bci=129, line=1653 (Interpreted frame)
- java.util.concurrent.ThreadPoolExecutor.runWorker(java.util.concurrent.ThreadPoolExecutor$Worker) @bci=95, line=1145 (Compiled frame)
- java.util.concurrent.ThreadPoolExecutor$Worker.run() @bci=5, line=615 (Interpreted frame)
- java.lang.Thread.run() @bci=11, line=745 (Interpreted frame)
即使HashSet不是线程安全的，许多线程（大约100个）也会被阻塞。并且所有线程在线程转储中都有相同的堆栈跟踪。服务器卡住了。在那段时间里，有许多请求进入服务器。对于每个请求，都会创建一个EntityManager。
修改 GenericDAO实现是
Map<String, String> properties = new HashMap<String, String>();
properties.put("javax.persistence.jdbc.driver", "com.mysql.jdbc.Driver");
properties.put("javax.persistence.jdbc.url", "jdbc:mysql://"+dbProperties.getHost()+":"+dbProperties.getPort()+"/"+dbProperties.getDbName());
properties.put("javax.persistence.jdbc.user", dbProperties.getUser());
properties.put("javax.persistence.jdbc.password", dbProperties.getPassword());
entityManager = Persistence.createEntityManagerFactory("TestDAL", properties).createEntityManager();