Classloader leaks

原博：http://java.jiderhamn.se/2012/01/29/classloader-leaks-iv-threadlocal-dangers-and-why-threadglobal-may-have-been-a-more-appropriate-name

以下记录，主要是为了以后复习使用。

1.Thread Local 引起的Classloader leaks

ThreadLocal主要是解决在多线程中，实现线程变量安全的方法

ThreadLocal使用后，需要删除，如果没有删除，会导致ThreadLocal无法回收，它内部使用了ThreadLocalMap，这个Map在jvm中是软引用，不一定会被回收(只有在内存紧张的时候会回收)

try {
  threadLocal.set(value);
  ...
}
finally {
  threadLocal.remove();
}

 

2.解决这个问题可以使用Servlet过滤器

public class ThreadLocalLeakPreventionFilter implements javax.servlet.Filter {

  private ThreadLocal[] offendingThreadLocals;

  public void init(FilterConfig filterConfig) throws ServletException {
    List<ThreadLocal> threadLocals = new ArrayList<ThreadLocal>();

    // TODO: Needs error handling!!!
    Class clazz = Class.forName("org.apache.axis.utils.XMLUtils");
    if(clazz != null) {
      final Field threadLocalField = 
          clazz.getDeclaredField("documentBuilder");
      if(threadLocalField != null) {
        threadLocalField.setAccessible(true);
        Object threadLocal = threadLocalField.get(null);
        if(threadLocal instanceof ThreadLocal) {
          threadLocals.add((ThreadLocal)threadLocal);
        }
      }
    }
    
    // TODO: Look up more offenders here
    
    this.offendingThreadLocals = 
        threadLocals.toArray(new ThreadLocal[threadLocals.size()]);
  }

  /** 
   * In the doFilter() method we have a chance to clean up the thread
   * before it is returned to the thread pool 
   */
  public void doFilter(ServletRequest servletRequest, 
                       ServletResponse servletResponse, 
                       FilterChain filterChain) 
      throws IOException, ServletException {
    
    try {
      filterChain.doFilter(servletRequest, servletResponse);
    }
    finally {
      // Clean up ThreadLocals
      for(ThreadLocal offendingThreadLocal : offendingThreadLocals) {
        offendingThreadLocal.remove(); // Remove offender from current thread
      }
    }
  }

  public void destroy() {
    offendingThreadLocals = null; // Make available for Garbage Collector
  }
-->
}