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How to clean up ThreadLocals

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遇见更好的自我
遇见更好的自我 2020-11-28 23:01

Does any one have an example how to do this? Are they handled by the garbage collector? I\'m using Tomcat 6.

7条回答
  • 庸人自扰
    庸人自扰 (楼主)
    2020-11-28 23:52

    @lyaffe's answer is the best possible for Java 6. There are a few issues that this answer resolves using what is available in Java 8.

    @lyaffe's answer was written for Java 6 before MethodHandle became available. It suffers from performance penalties due to reflection. If used as below, MethodHandle provides zero overhead access to fields and methods.

    @lyaffe's answer also goes through the ThreadLocalMap.table explicitly and is prone to bugs. There is a method ThreadLocalMap.expungeStaleEntries() now available that does the same thing.

    The code below has 3 initialization methods to minimize the cost of invoking expungeStaleEntries().

    private static final MethodHandle        s_getThreadLocals     = initThreadLocals();
private static final MethodHandle        s_expungeStaleEntries = initExpungeStaleEntries();
private static final ThreadLocal s_threadLocals        = ThreadLocal.withInitial(() -> getThreadLocals());

public static void expungeThreadLocalMap()
{
   Object threadLocals;

   threadLocals = s_threadLocals.get();

   try
   {
      s_expungeStaleEntries.invoke(threadLocals);
   }
   catch (Throwable e)
   {
      throw new IllegalStateException(e);
   }
}

private static Object getThreadLocals()
{
   ThreadLocal local;
   Object result;
   Thread thread;

   local = new ThreadLocal<>();

   local.set(local);   // Force ThreadLocal to initialize Thread.threadLocals

   thread = Thread.currentThread();

   try
   {
      result = s_getThreadLocals.invoke(thread);
   }
   catch (Throwable e)
   {
      throw new IllegalStateException(e);
   }

   return(result);
}

private static MethodHandle initThreadLocals()
{
   MethodHandle result;
   Field field;

   try
   {
      field = Thread.class.getDeclaredField("threadLocals");

      field.setAccessible(true);

      result = MethodHandles.
         lookup().
         unreflectGetter(field);

      result = Preconditions.verifyNotNull(result, "result is null");
   }
   catch (NoSuchFieldException | SecurityException | IllegalAccessException e)
   {
      throw new ExceptionInInitializerError(e);
   }

   return(result);
}

private static MethodHandle initExpungeStaleEntries()
{
   MethodHandle result;
   Class clazz;
   Method method;
   Object threadLocals;

   threadLocals = getThreadLocals();
   clazz        = threadLocals.getClass();

   try
   {
      method = clazz.getDeclaredMethod("expungeStaleEntries");

      method.setAccessible(true);

      result = MethodHandles.
         lookup().
         unreflect(method);
   }
   catch (NoSuchMethodException | SecurityException | IllegalAccessException e)
   {
      throw new ExceptionInInitializerError(e);
   }

   return(result);
}

    
    
             
    
                                                        
    
            
            
    
              
                
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