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| <div class="section"><h2>Log4j 2 API<a name="Log4j_2_API"></a></h2> |
| <div class="section"><h3>Thread Context<a name="Thread_Context"></a></h3> |
| <div class="section"><h4>Introduction<a name="Introduction"></a></h4> |
| <p>Log4j introduced the concept of the Mapped Diagnostic Context or MDC. It has been documented and |
| discussed in numerous places including |
| <a class="externalLink" href="http://veerasundar.com/blog/2009/10/log4j-mdc-mapped-diagnostic-context-what-and-why/">Log4j MDC: What and Why</a> and |
| <a class="externalLink" href="http://blog.f12.no/wp/2004/12/09/log4j-and-the-mapped-diagnostic-context/">Log4j and the Mappend Diagnostic Context</a>. |
| In addition, Log4j 1.x provides support for a Nested Diagnostic Context or NDC. It too has been documented |
| and discussed in various places such as |
| <a class="externalLink" href="http://lstierneyltd.com/blog/development/log4j-nested-diagnostic-contexts-ndc/">Log4j NDC</a>. |
| SLF4J/Logback followed with its own implementation of the MDC, which is documented very well at |
| <a class="externalLink" href="http://logback.qos.ch/manual/mdc.html">Mapped Diagnostic Context</a>. |
| </p> |
| <p>Log4j 2 continues with the idea of the MDC and the NDC but merges them into a single Thread Context. |
| The Thread Context Map is the equivalent of the MDC and the Thread Context Stack is the equivalent of the |
| NDC. Although these are frequently used for purposes other than diagnosing problems, they are still |
| frequently referred to as the MDC and NDC in Log4j 2 since they are already well known by those acronyms. |
| </p> |
| </div><div class="section"><h4>Fish Tagging<a name="Fish_Tagging"></a></h4> |
| <p>Most real-world systems have to deal with multiple clients simultaneously. In a typical multithreaded |
| implementation of such asystem, different threads will handle different clients. Logging is |
| especially well suited to trace and debug complex distributed applications. A common approach to |
| differentiate the logging output of one client from another is to instantiate a new separate logger for |
| each client. This promotes the proliferation of loggers and increases the management overhead of logging. |
| </p> |
| <p>A lighter technique is to uniquely stamp each log request initiated from the same client interaction. |
| Neil Harrison described this method in the book "Patterns for Logging Diagnostic Messages," in <i>Pattern |
| Languages of Program Design 3</i>, edited by R. Martin, D. Riehle, and F. Buschmann |
| (Addison-Wesley, 1997). Just as a fish can be tagged and have its movement tracked, stamping log |
| events with a common tag or set of data elements allows the complete flow of a transaction or a request |
| to be tracked. We call this <i>Fish Tagging</i>. |
| </p> |
| <p>Log4j provides two mechanisms for performing Fish Tagging; the Thread Context Map and the Thread |
| Context Stack. The Thread Context Map allows any number of items to be added and be identified |
| using key/value pairs. The Thread Context Stack allows one or more items to be pushed on the |
| Stack and then be identified by their order in the Stack or by the data itself. Since key/value |
| pairs are more flexible, the Thread Context Map is recommended when data items may be added during |
| the processing of the request or when there are more than one or two items. |
| </p> |
| <p>To uniquely stamp each request using the Thread Context Stack, the user pushes contextual information |
| on to the Stack. |
| </p> |
| <div><pre> |
| ThreadContext.push(UUID.randomUUID().toString()); // Add the fishtag; |
| |
| logger.debug("Message 1"); |
| . |
| . |
| . |
| logger.debug("Message 2"); |
| . |
| . |
| ThreadContext.pop();</pre></div> |
| <p> |
| The alternative to the Thread Context Stack is the Thread Context Map. In this case, attributes |
| associated with the request being processed are adding at the beginning and removed at the end |
| as follows: |
| </p> |
| <div><pre> |
| ThreadContext.put("id", UUID.randomUUID().toString(); // Add the fishtag; |
| ThreadContext.put("ipAddress", request.getRemoteAddr()); |
| ThreadContext.put("loginId", session.getAttribute("loginId")); |
| ThreadContext.put("hostName", request.getServerName()); |
| . |
| logger.debug("Message 1"); |
| . |
| . |
| logger.debug("Message 2"); |
| . |
| . |
| ThreadContext.clear();</pre></div> |
| <p>The Stack and the Map are managed per thread and is based on |
| <a class="externalLink" href="http://docs.oracle.com/javase/6/docs/api/java/lang/InheritableThreadLocal.html">InheritableThreadLocal</a>. |
| Thus, in many cases the contents of the Stack and Map will be passed to child threads. However, as |
| discussed in the |
| <a class="externalLink" href="http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/Executors.html#privilegedThreadFactory()">Executors</a> |
| class and in other cases where thread pooling is utilized, the ThreadContext may not always be |
| automatically passed to worker threads. In those cases the pooling mechanism should provide a means for |
| doing so. The getContext() and cloneStack() methods can be used to obtain copies of the Map and Stack |
| respectively. |
| </p> |
| <p> |
| Note that all methods of the |
| <a href="../log4j2-api/apidocs/org/apache/logging/log4j/ThreadContext.html">ThreadContext</a> |
| class are static. |
| </p> |
| </div><div class="section"><h4>Including the ThreadContext when writing logs<a name="Including_the_ThreadContext_when_writing_logs"></a></h4> |
| <p> |
| The PatternLayout provides mechanisms to print the contents of the ThreadContext Map and Stack. Using |
| %X by itself will cause the full contents of the Map to be included while %X{key} will cause the value |
| of the specified key to be included. %x will include the full contents of the Stack. |
| </p> |
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