public static  void clear() 
{
    Stack stack = getCurrentStack();    
    if(stack != null) 
      stack.setSize(0);
}

Clear any nested diagnostic information if any. This method is useful in cases where the same thread can be potentially used over and over in different unrelated contexts.

This method is equivalent to calling the #setMaxDepth method with a zero maxDepth argument.

 public static Stack cloneStack() 
{
    Stack stack = getCurrentStack();
    if(stack == null)
      return null;
    else {
      return (Stack) stack.clone();
    }
}

Clone the diagnostic context for the current thread.

Internally a diagnostic context is represented as a stack. A given thread can supply the stack (i.e. diagnostic context) to a child thread so that the child can inherit the parent thread's diagnostic context.

The child thread uses the inherit method to inherit the parent's diagnostic context.

 public static String get() 
{
    Stack s = getCurrentStack();
    if(s != null && !s.isEmpty()) 
      return ((DiagnosticContext) s.peek()).fullMessage;
    else
      return null;
}

Never use this method directly, use the org.apache.log4j.spi.LoggingEvent#getNDC method instead.

 public static int getDepth() 
{
    Stack stack = getCurrentStack();          
    if(stack == null)
      return 0;
    else
      return stack.size();
}

Get the current nesting depth of this diagnostic context.

 public static  void inherit(Stack stack) 
{
    if(stack != null)
      ht.put(Thread.currentThread(), stack);
}

Inherit the diagnostic context of another thread.

The parent thread can obtain a reference to its diagnostic context using the #cloneStack method. It should communicate this information to its child so that it may inherit the parent's diagnostic context.

The parent's diagnostic context is cloned before being inherited. In other words, once inherited, the two diagnostic contexts can be managed independently.

In java, a child thread cannot obtain a reference to its parent, unless it is directly handed the reference. Consequently, there is no client-transparent way of inheriting diagnostic contexts. Do you know any solution to this problem?

 public static String peek() 
{
    Stack stack = getCurrentStack();
    if(stack != null && !stack.isEmpty())
      return ((DiagnosticContext) stack.peek()).message;
    else
      return "";
}

Looks at the last diagnostic context at the top of this NDC without removing it.

The returned value is the value that was pushed last. If no context is available, then the empty string "" is returned.

 public static String pop() 
{
    Stack stack = getCurrentStack();
    if(stack != null && !stack.isEmpty()) 
      return ((DiagnosticContext) stack.pop()).message;
    else
      return "";
}

Clients should call this method before leaving a diagnostic context.

The returned value is the value that was pushed last. If no context is available, then the empty string "" is returned.

 public static  void push(String message) 
{
    Stack stack = getCurrentStack();
    if(stack == null) {
      DiagnosticContext dc = new DiagnosticContext(message, null);      
      stack = new Stack();
      Thread key = Thread.currentThread();
      ht.put(key, stack);
      stack.push(dc);
    } else if (stack.isEmpty()) {
      DiagnosticContext dc = new DiagnosticContext(message, null);            
      stack.push(dc);
    } else {
      DiagnosticContext parent = (DiagnosticContext) stack.peek();
      stack.push(new DiagnosticContext(message, parent));
    }
}

Push new diagnostic context information for the current thread.

The contents of the message parameter is determined solely by the client.

 public static  void remove() 
{
    ht.remove(Thread.currentThread());
    // Lazily remove dead-thread references in ht.
    lazyRemove();
}

Remove the diagnostic context for this thread.

Each thread that created a diagnostic context by calling #push should call this method before exiting. Otherwise, the memory used by the thread cannot be reclaimed by the VM.

As this is such an important problem in heavy duty systems and because it is difficult to always guarantee that the remove method is called before exiting a thread, this method has been augmented to lazily remove references to dead threads. In practice, this means that you can be a little sloppy and occasionally forget to call #remove before exiting a thread. However, you must call remove sometime. If you never call it, then your application is sure to run out of memory.

 public static  void setMaxDepth(int maxDepth) 
{
    Stack stack = getCurrentStack();    
    if(stack != null && maxDepth <  stack.size()) 
      stack.setSize(maxDepth);
}

Set maximum depth of this diagnostic context. If the current depth is smaller or equal to maxDepth, then no action is taken.

This method is a convenient alternative to multiple #pop calls. Moreover, it is often the case that at the end of complex call sequences, the depth of the NDC is unpredictable. The setMaxDepth method circumvents this problem.

For example, the combination

void foo() {
  int depth = NDC.getDepth();

  ... complex sequence of calls

  NDC.setMaxDepth(depth);
}

ensures that between the entry and exit of foo the depth of the diagnostic stack is conserved.