WebSphere 7 HTTPSession实现是否违反了J2EE规范？

Question

WebSphere 7 HTTPSession实现是否违反了J2EE规范？

Pau*_*aft 12 java websphere deadlock java-ee

最近看到一个问题,所有200个Web容器线程都挂起了,这意味着没有任何服务可以为传入的请求提供服务,因此应用程序冻结了.

这是一个简单的Web应用程序和JMeter测试,我认为这证明了这个问题的原因.Web应用程序由两个类组成,即以下servlet:

public class SessionTestServlet extends HttpServlet {

    protected static final String SESSION_KEY = "session_key";

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // set data on session so the listener is invoked
        String sessionData = new String("Session data");
        request.getSession().setAttribute(SESSION_KEY, sessionData);
        PrintWriter writer = response.getWriter();                           
        writer.println("<html><body>OK</body></html>");                      
        writer.flush();
        writer.close();
    }
}

Run Code Online (Sandbox Code Playgroud)

以及HttpSessionListener和HTTPSessionAttributeListener的以下实现:

public class SessionTestListener implements 
        HttpSessionListener, HttpSessionAttributeListener {

    private static final ConcurrentMap<String, HttpSession> allSessions 
        = new ConcurrentHashMap<String, HttpSession>(); 

    public void attributeRemoved(HttpSessionBindingEvent hsbe) {}

    public void attributeAdded(HttpSessionBindingEvent hsbe) {
        System.out.println("Attribute added, " + hsbe.getName() 
            + "=" + hsbe.getValue());

        int count = 0;
        for (HttpSession session : allSessions.values()) {
            if (session.getAttribute(SessionTestServlet.SESSION_KEY) != null) {
                count++;
            }
        }
        System.out.println(count + " of " + allSessions.size() 
            + " sessions have attribute set.");
    }

    public void attributeReplaced(HttpSessionBindingEvent hsbe) {}

    public void sessionCreated(HttpSessionEvent hse) {
        allSessions.put(hse.getSession().getId(), session);                              
    }

    public void sessionDestroyed(HttpSessionEvent hse) {
        allSessions.remove(hse.getSession().getId());
    }                
}

Run Code Online (Sandbox Code Playgroud)

JMeter测试每秒都有100个请求命中servlet:

<?xml version="1.0" encoding="UTF-8"?>
<jmeterTestPlan version="1.2" properties="2.1">
  <hashTree>
    <TestPlan guiclass="TestPlanGui" testclass="TestPlan" testname="Test Plan" enabled="true">
      <stringProp name="TestPlan.comments"></stringProp>
      <boolProp name="TestPlan.functional_mode">false</boolProp>
      <boolProp name="TestPlan.serialize_threadgroups">false</boolProp>
      <elementProp name="TestPlan.user_defined_variables" elementType="Arguments" guiclass="ArgumentsPanel" testclass="Arguments" testname="User Defined Variables" enabled="true">
        <collectionProp name="Arguments.arguments"/>
      </elementProp>
      <stringProp name="TestPlan.user_define_classpath"></stringProp>
    </TestPlan>
    <hashTree>
      <ThreadGroup guiclass="ThreadGroupGui" testclass="ThreadGroup" testname="Thread Group" enabled="true">
        <stringProp name="ThreadGroup.on_sample_error">continue</stringProp>
        <elementProp name="ThreadGroup.main_controller" elementType="LoopController" guiclass="LoopControlPanel" testclass="LoopController" testname="Loop Controller" enabled="true">
          <boolProp name="LoopController.continue_forever">false</boolProp>
          <intProp name="LoopController.loops">-1</intProp>
        </elementProp>
        <stringProp name="ThreadGroup.num_threads">100</stringProp>
        <stringProp name="ThreadGroup.ramp_time">1</stringProp>
        <longProp name="ThreadGroup.start_time">1327193422000</longProp>
        <longProp name="ThreadGroup.end_time">1327193422000</longProp>
        <boolProp name="ThreadGroup.scheduler">false</boolProp>
        <stringProp name="ThreadGroup.duration"></stringProp>
        <stringProp name="ThreadGroup.delay"></stringProp>
      </ThreadGroup>
      <hashTree>
        <HTTPSamplerProxy guiclass="HttpTestSampleGui" testclass="HTTPSamplerProxy" testname="HTTP Request" enabled="true">
          <elementProp name="HTTPsampler.Arguments" elementType="Arguments" guiclass="HTTPArgumentsPanel" testclass="Arguments" testname="User Defined Variables" enabled="true">
            <collectionProp name="Arguments.arguments"/>
          </elementProp>
          <stringProp name="HTTPSampler.domain">localhost</stringProp>
          <stringProp name="HTTPSampler.port">9080</stringProp>
          <stringProp name="HTTPSampler.connect_timeout"></stringProp>
          <stringProp name="HTTPSampler.response_timeout"></stringProp>
          <stringProp name="HTTPSampler.protocol">http</stringProp>
          <stringProp name="HTTPSampler.contentEncoding"></stringProp>
          <stringProp name="HTTPSampler.path">/SESSION_TESTWeb/SessionTestServlet</stringProp>
          <stringProp name="HTTPSampler.method">GET</stringProp>
          <boolProp name="HTTPSampler.follow_redirects">true</boolProp>
          <boolProp name="HTTPSampler.auto_redirects">false</boolProp>
          <boolProp name="HTTPSampler.use_keepalive">true</boolProp>
          <boolProp name="HTTPSampler.DO_MULTIPART_POST">false</boolProp>
          <boolProp name="HTTPSampler.monitor">false</boolProp>
          <stringProp name="HTTPSampler.embedded_url_re"></stringProp>
        </HTTPSamplerProxy>
        <hashTree>
          <ConstantTimer guiclass="ConstantTimerGui" testclass="ConstantTimer" testname="Constant Timer" enabled="true">
            <stringProp name="ConstantTimer.delay">1000</stringProp>
          </ConstantTimer>
          <hashTree/>
        </hashTree>
      </hashTree>
      <ResultCollector guiclass="ViewResultsFullVisualizer" testclass="ResultCollector" testname="View Results Tree" enabled="true">
        <boolProp name="ResultCollector.error_logging">false</boolProp>
        <objProp>
          <name>saveConfig</name>
          <value class="SampleSaveConfiguration">
            <time>true</time>
            <latency>true</latency>
            <timestamp>true</timestamp>
            <success>true</success>
            <label>true</label>
            <code>true</code>
            <message>true</message>
            <threadName>true</threadName>
            <dataType>true</dataType>
            <encoding>false</encoding>
            <assertions>true</assertions>
            <subresults>true</subresults>
            <responseData>false</responseData>
            <samplerData>false</samplerData>
            <xml>true</xml>
            <fieldNames>false</fieldNames>
            <responseHeaders>false</responseHeaders>
            <requestHeaders>false</requestHeaders>
            <responseDataOnError>false</responseDataOnError>
            <saveAssertionResultsFailureMessage>false</saveAssertionResultsFailureMessage>
            <assertionsResultsToSave>0</assertionsResultsToSave>
            <bytes>true</bytes>
          </value>
        </objProp>
        <stringProp name="filename"></stringProp>
      </ResultCollector>
      <hashTree/>
      <ResultCollector guiclass="SummaryReport" testclass="ResultCollector" testname="Summary Report" enabled="true">
        <boolProp name="ResultCollector.error_logging">false</boolProp>
        <objProp>
          <name>saveConfig</name>
          <value class="SampleSaveConfiguration">
            <time>true</time>
            <latency>true</latency>
            <timestamp>true</timestamp>
            <success>true</success>
            <label>true</label>
            <code>true</code>
            <message>true</message>
            <threadName>true</threadName>
            <dataType>true</dataType>
            <encoding>false</encoding>
            <assertions>true</assertions>
            <subresults>true</subresults>
            <responseData>false</responseData>
            <samplerData>false</samplerData>
            <xml>true</xml>
            <fieldNames>false</fieldNames>
            <responseHeaders>false</responseHeaders>
            <requestHeaders>false</requestHeaders>
            <responseDataOnError>false</responseDataOnError>
            <saveAssertionResultsFailureMessage>false</saveAssertionResultsFailureMessage>
            <assertionsResultsToSave>0</assertionsResultsToSave>
            <bytes>true</bytes>
          </value>
        </objProp>
        <stringProp name="filename"></stringProp>
      </ResultCollector>
      <hashTree/>
    </hashTree>
  </hashTree>
</jmeterTestPlan>

Run Code Online (Sandbox Code Playgroud)

当针对部署在WebSphere 7上的测试Web应用程序运行此测试时,应用程序会快速停止响应,并且核心转储显示以下内容:

1LKDEADLOCK    Deadlock detected !!!
NULL           ---------------------
NULL           
2LKDEADLOCKTHR  Thread "WebContainer : 2" (0x000000000225C600)
3LKDEADLOCKWTR    is waiting for:
4LKDEADLOCKMON      sys_mon_t:0x00000000151938C0 infl_mon_t: 0x0000000015193930:
4LKDEADLOCKOBJ      com/ibm/ws/session/store/memory/MemorySession@00000000A38EA0C8/00000000A38EA0D4: 
3LKDEADLOCKOWN    which is owned by:
2LKDEADLOCKTHR  Thread "WebContainer : 1" (0x00000000021FB500)
3LKDEADLOCKWTR    which is waiting for:
4LKDEADLOCKMON      sys_mon_t:0x0000000015193820 infl_mon_t: 0x0000000015193890:
4LKDEADLOCKOBJ      com/ibm/ws/session/store/memory/MemorySession@00000000A14E22C0/00000000A14E22CC: 
3LKDEADLOCKOWN    which is owned by:
2LKDEADLOCKTHR  Thread "WebContainer : 2" (0x000000000225C600)
NULL

Run Code Online (Sandbox Code Playgroud)

当执行servlet的doGet()方法的线程(T1)在HttpSession实现(S1)的实例上调用setAttribute()时,它会锁定在S1的监视器上.在持有该锁的同时,它进入侦听器的attributeAdded()方法内的allSessions的迭代,并调用getAttribute().它看起来像在getAttribute()里面,WebSphere锁定了该实例的监视器(可能是因为它设置了lastUpdateTime字段？).因此,T1将依次锁定S1,S2,S3,S4,S5的监视器......同时在servlet中的setAttribute()调用中保持对S1的锁定.

因此,如果同时另一个线程(T2)锁定在servlet中的另一个会话(S2)的监视器上,然后进入addAttribute()中的循环,则S1和S2监视器上的线程死锁.

我一直无法在J2EE规范中找到任何明确的内容,但Servlet 2.4规范的这一部分暗示容器不应该在HttpSession实现的实例上进行同步:

SRV.7.7.1线程问题

执行请求线程的多个servlet可以同时具有对单个会话对象的活动访问权.开发人员负责根据需要同步对会话资源的访问.

当我们对它运行测试时,JBoss没有显示任何死锁.所以我的问题是:

我的理解是否正确？
如果是这样,这是否是WebSphere中的J2EE规范的错误或违反？
如果没有,并且它是开发人员应该了解和编写的有效行为,这种行为是否记录在任何地方？

谢谢

Answer 1

Pau*_*aft 4

Servlet 2.5 MR6 包含对问题中引用的 Servlet 规范部分的澄清：

澄清 SRV 7.7.1“线程问题”（第 33 期）

更改当前的段落

“执行请求线程的多个 servlet 可能会同时主动访问单个会话对象。开发人员有责任适当同步对会话资源的访问。”

读书

“执行请求线程的多个 servlet 可能会同时主动访问同一个会话对象。容器必须确保以线程安全的方式执行表示会话属性的内部数据结构的操作。开发人员有责任对属性对象本身。这将保护 HttpSession 对象内的属性集合免受并发访问，从而消除应用程序导致该集合损坏的机会。”

这在 Servlet 3.0 MR1 中仍然存在，并且使 WAS 的行为看起来更加合理。但是，我认为 * set *Attribute 可能会同步，但 * get *Attribute 不会同步。

所以我认为答案是：

根据 2.5 MR6 中的说明，WAS符合Servlet 规范
该规范留下了误解的空间
WAS 对其同步的热情超出了规范的合理预期，并且 AFAIK 这种行为在任何地方都没有明确记录

（附带说明一下，更改测试用例以便listener.attributeAdded() 调用setAttribute 而不是getAttribute 不会导致JBoss 4 或5 上的死锁。）

归档时间：	13 年，11 月前
查看次数：	3213 次
最近记录：	13 年，11 月前