tomcat reload时内存泄漏的处理 hot deployment

1、我做的应用是以Spring为系统的基础框架,mysql为后台数据库.在tomcat上发布后,总是不能进行热部署(reload),多次reload后,就会出OutOfMemory PermGen,为此烦恼了很久,总于下定决心找找根源.经过3天的不懈努力,小有成果,记录下来实际上下面的分析都已经没什么用了,如果你使用tomcat6.0.26及以后的版本,我所说的这些情况都已经被处理了,并且比我处理的还要多很多.可以下载tomcat6.0.26的源代码看看WebappClassLoader类的处理就成了.通过分析工具的分析(用了YourKit,以及JDK1.6/bin下的jps/jmap/jhat)

2、,发现有下面几个方面会造成memory leak.1.SystemClassLoader与WebappClassLoader加载的类相互引用,tomcat reload只是卸载WebappClassloader中的class,SystemClassLoader是不会卸载的(否则其他应用也停止了).但是WebappClassloader加载的类被SystemClassLoader引用的化,WebappClassloader中的相关类就不会被JVM进行垃圾收集ThreadLocal.set(Object),如果这个Object是WebappsClassLoader加载的,使用之后没有做Thread

3、Local.set(null)或者ThreadLocal.remove(),就会产生memory leak.由于ThreadLocal实际上操作的是java.lang.Thread类中的ThreadLocalMap,Thread类是由SystemClassLoder加载的.而这个线程实例(main thread)在tomcat reload的时候不会销毁重建,必然就产生了SystemClassLoder中的类引用WebappsClassLoader的类.DriverManager也是由SystemClassLoder载入的,当初始化某个JDBC驱动的时候,会向DriverManager中注册该

4、驱动,通常是*.driver,例如com.mysql.jdbc.Driver这个Driver是通过class.forName()加载的,通常也是加载到WebappClassLoader.这就出现了两个classLoader中的类的交叉引用.导致memory leak.解决办法:写一个ServletContextListener,在contextDestroyed方法中统一删除当前Thread的ThreadLocalMap中的内容.public class ApplicationCleanListener implements ServletContextListener public void

5、 contextInitialized(ServletContextEvent event) public void contextDestroyed(ServletContextEvent event)          /处理ThreadLocal   ThreadLocalCleanUtil.clearThreadLocals();   /*   * 如果数据故驱动是通过应用服务器(tomcat etc.)中配置的<公用>连接池,这里不需

6、要 否则必须卸载Driver   *    * 原因: DriverManager是System classloader加载的, Driver是webappclassloader加载的,   * driver保存在DriverManager中,在reload过程中,由于system   * classloader不会销毁,driverManager就一直保持着对driver的引用,   * driver无法卸载,与driver关联的其他类   * ,例如DataSo

7、urce,jdbcTemplate,dao,service.都无法卸载   */   try     System.out.println("clean jdbc Driver.");    for (Enumeration e = DriverManager.getDrivers(); e      .hasMoreElements();)      Driver driver = (

8、Driver) e.nextElement();     if (driver.getClass().getClassLoader() = getClass()       .getClassLoader()       DriverManager.deregisterDriver(driver);            catch (Exception e

9、)     System.out      .println("Exception cleaning up java.sql.DriverManager's driver: "        + e.getMessage();   /* 这个类根据*/public class ThreadLocalCleanUtil /* 得到当前线程组中的所有线程 description:* * ret

10、urn*/private static Thread getThreads()    ThreadGroup tg = Thread.currentThread().getThreadGroup();   while (tg.getParent() != null)     tg = tg.getParent();      int threadCountGuess = tg.activeCount() + 50;   Thread threads = new Th

11、readthreadCountGuess;   int threadCountActual = tg.enumerate(threads);   while (threadCountActual = threadCountGuess)     threadCountGuess *= 2;    threads = new ThreadthreadCountGuess;    threadCountActual = tg.enumerate(threads);

12、0;     return threads;public static void clearThreadLocals()    ClassLoader classloader = Thread     .currentThread()     .getContextClassLoader();   Thread threads = getThreads();   try     Field thr

13、eadLocalsField = Thread.class      .getDeclaredField("threadLocals");    threadLocalsField.setAccessible(true);    Field inheritableThreadLocalsField = Thread.class      .getDeclaredField("inheritableThrea

14、dLocals");    inheritableThreadLocalsField.setAccessible(true);    Class tlmClass = Class      .forName("java.lang.ThreadLocal$ThreadLocalMap");    Field tableField = tlmClass.getDeclaredField("table");

15、60;   tableField.setAccessible(true);    for (int i = 0; i < threads.length; +i)      if (threadsi = null)      continue;     Object threadLocalMap = threadLocalsField.get(threadsi);    

16、clearThreadLocalMap(threadLocalMap, tableField, classloader);     threadLocalMap = inheritableThreadLocalsField.get(threadsi);     clearThreadLocalMap(threadLocalMap, tableField, classloader);       catch (Exception e)   

17、0; e.printStackTrace();   private static void clearThreadLocalMap(Object map,    Field internalTableField, ClassLoader classloader)    throws NoSuchMethodException, IllegalAccessException,    NoSuchFieldException, InvocationTargetException  

18、;  if (map != null)     Method mapRemove = map.getClass().getDeclaredMethod("remove",      new Class ThreadLocal.class );    mapRemove.setAccessible(true);    Object table = (Object) internalTableField.get(map);

19、0;   int staleEntriesCount = 0;    if (table != null)      for (int j = 0; j < table.length; +j)       if (tablej != null)        boolean remove = false;       Object

20、key = (Reference) tablej).get();       if (key != null)         && (key.getClass().getClassLoader() = classloader)         remove = true;        System.

21、out.println("clean threadLocal key,class="          + key.getClass().getCanonicalName()          + ",value=" + key.toString();           

22、60;  Field valueField = tablej         .getClass()         .getDeclaredField("value");       valueField.setAccessible(true);       Object value =

23、 valueField.get(tablej);       if (value != null)         && (value.getClass().getClassLoader() = classloader)         remove = true;        System.out.

24、println("clean threadLocal value,class="          + value.getClass().getCanonicalName()          + ",value=" + value.toString();           &

25、#160;  if (remove)         if (key = null)         +staleEntriesCount;        else          mapRemove.invoke(map, new Object key );  &#

26、160;                               if (staleEntriesCount > 0)      Method mapRemoveStale = map       .getClass() 

27、0;     .getDeclaredMethod("expungeStaleEntries", new Class0);     mapRemoveStale.setAccessible(true);     mapRemoveStale.invoke(map, new Object0);       2.对于使用mysql JDBC驱动的:mysql JDBC驱动会启动一个Timer Thread,这个

28、线程在reload的时候也是无法自动销毁.因此,需要强制结束这个timer可以在 上面的ApplicationCleanListener中加入如下代码:   try     Class ConnectionImplClass = Thread      .currentThread()      .getContextClassLoader()      .loadClass("com.

29、mysql.jdbc.ConnectionImpl");    if (ConnectionImplClass != null      && ConnectionImplClass.getClassLoader() = getClass()        .getClassLoader()      System.out.println("clean mysql timer.");     Field f = ConnectionImplClass.getDeclaredField("cancelTimer");