NDC (Apache Log4j 1.2.17 API)
Date: 2012-05-06

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## org.apache.log4j
Class NDC

    java.lang.Object
      org.apache.log4j.NDC

    public class NDC

extends Object

The NDC class implements nested diagnostic contexts as defined by Neil Harrison in the article "Patterns for Logging Diagnostic Messages" part of the book "Pattern Languages of Program Design 3" edited by Martin et al.

A Nested Diagnostic Context, or NDC in short, is an instrument to distinguish interleaved log output from different sources. Log output is typically interleaved when a server handles multiple clients near-simultaneously.

Interleaved log output can still be meaningful if each log entry from different contexts had a distinctive stamp. This is where NDCs come into play.

Note that NDCs are managed on a per thread basis. NDC operations such as `push`, `pop()`, `clear()`, `getDepth()` and `setMaxDepth(int)` affect the NDC of the current thread only. NDCs of other threads remain unaffected.

For example, a servlet can build a per client request NDC consisting the clients host name and other information contained in the the request. Cookies are another source of distinctive information. To build an NDC one uses the `push` operation. Simply put,

- Contexts can be nested.

- When entering a context, call `NDC.push`. As a side effect, if there is no nested diagnostic context for the current thread, this method will create it.

- When leaving a context, call `NDC.pop`.

- When exiting a thread make sure to call `NDC.remove()`.

There is no penalty for forgetting to match each `push` operation with a corresponding `pop`, except the obvious mismatch between the real application context and the context set in the NDC.

If configured to do so, `PatternLayout` and `TTCCLayout` instances automatically retrieve the nested diagnostic context for the current thread without any user intervention. Hence, even if a servlet is serving multiple clients simultaneously, the logs emanating from the same code (belonging to the same category) can still be distinguished because each client request will have a different NDC tag.

Heavy duty systems should call the `remove()` method when leaving the run method of a thread. This ensures that the memory used by the thread can be freed by the Java garbage collector. There is a mechanism to lazily remove references to dead threads. In practice, this means that you can be a little sloppy and sometimes forget to call `remove()` before exiting a thread.

A thread may inherit the nested diagnostic context of another (possibly parent) thread using the `inherit` method. A thread may obtain a copy of its NDC with the `cloneStack` method and pass the reference to any other thread, in particular to a child.

Since:
  0.7.0
Author:
  Ceki Gülcü

Method Summary
`static void` | `clear()` Clear any nested diagnostic information if any.
`static Stack` | `cloneStack()` Clone the diagnostic context for the current thread.
`static String` | `get()` Never use this method directly, use the `LoggingEvent.getNDC()` method instead.
`static int` | `getDepth()` Get the current nesting depth of this diagnostic context.
`static void` | `inherit(Stack stack)` Inherit the diagnostic context of another thread.
`static String` | `peek()` Looks at the last diagnostic context at the top of this NDC without removing it.
`static String` | `pop()` Clients should call this method before leaving a diagnostic context.
`static void` | `push(String message)` Push new diagnostic context information for the current thread.
`static void` | `remove()` Remove the diagnostic context for this thread.
`static void` | `setMaxDepth(int maxDepth)` Set maximum depth of this diagnostic context.

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Method Detail

### clear

    public static void clear()

  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(int)` method with a zero `maxDepth` argument.

  Since:
    0.8.4c

### cloneStack

    public static Stack cloneStack()

  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.

  Returns:
    Stack A clone of the current thread's diagnostic context.

### inherit

    public static void inherit(Stack 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?

  Parameters:
    `stack` - The diagnostic context of the parent thread.

### get

    public static String get()

  Never use this method directly, use the `LoggingEvent.getNDC()` method instead.

### getDepth

    public static int getDepth()

  Get the current nesting depth of this diagnostic context.

  Since:
    0.7.5
  See Also:
    `setMaxDepth(int)`

### pop

    public static String pop()

  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.

  Returns:
    String The innermost diagnostic context.

### peek

    public static String peek()

  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.

  Returns:
    String The innermost diagnostic context.

### push

    public static void push(String message)

  Push new diagnostic context information for the current thread.

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

  Parameters:
    `message` - The new diagnostic context information.

### remove

    public static void remove()

  Remove the diagnostic context for this thread.

  Each thread that created a diagnostic context by calling `push(java.lang.String)` 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.

### setMaxDepth

    public static void setMaxDepth(int 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.

  Since:
    0.7.5
  See Also:
    `getDepth()`

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