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| |
| <div class="section"> |
| <h2><a name="Log4j_2_API"></a>Log4j 2 API</h2> |
| |
| <div class="section"> |
| <h3><a name="Thread_Context"></a>Thread Context</h3> |
| |
| <div class="section"> |
| <h4><a name="Introduction"></a>Introduction</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 Mapped 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><a name="Fish_Tagging"></a>Fish Tagging</h4> |
| |
| <p>Most real-world systems have to deal with multiple clients simultaneously. In a typical multithreaded |
| implementation of such a system, 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 class="prettyprint linenums"> |
| 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 class="prettyprint linenums"> |
| 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> |
| |
| </div> |
| <div class="section"> |
| <h4><a name="CloseableThreadContext"></a>CloseableThreadContext</h4> |
| |
| <p>When placing items on the stack or map, it's necessary to remove then again when appropriate. To assist with |
| this, the <tt>CloseableThreadContext</tt> implements the <a class="externalLink" href="http://docs.oracle.com/javase/7/docs/api/java/lang/AutoCloseable.html">AutoCloseable |
| interface</a>. This allows items to be pushed to the stack or put in the map, and removed when the <tt>close()</tt> method is called - |
| or automatically as part of a try-with-resources. For example, to temporarily push something on to the stack and then remove it: |
| </p> |
| |
| <div> |
| <pre class="prettyprint linenums"> |
| // Add to the ThreadContext stack for this try block only; |
| try (final CloseableThreadContext.Instance ctc = CloseableThreadContext.push(UUID.randomUUID().toString())) { |
| |
| logger.debug("Message 1"); |
| . |
| . |
| logger.debug("Message 2"); |
| . |
| . |
| }</pre></div> |
| |
| <p> |
| Or, to temporarily put something in the map: |
| </p> |
| |
| <div> |
| <pre class="prettyprint linenums"> |
| // Add to the ThreadContext map for this try block only; |
| try (final CloseableThreadContext.Instance ctc = CloseableThreadContext.put("id", UUID.randomUUID().toString()) |
| .put("loginId", session.getAttribute("loginId"))) { |
| |
| logger.debug("Message 1"); |
| . |
| . |
| logger.debug("Message 2"); |
| . |
| . |
| }</pre></div> |
| |
| If you're using a thread pool, then you can initialise a CloseableThreadContext by using the |
| <tt>putAll(final Map<String, String> values)</tt> and/or |
| <tt>pushAll(List<String> messages)</tt> methods; |
| |
| <div> |
| <pre class="prettyprint linenums"> |
| for( final Session session : sessions ) { |
| try (final CloseableThreadContext.Instance ctc = CloseableThreadContext.put("loginId", session.getAttribute("loginId"))) { |
| logger.debug("Starting background thread for user"); |
| final Map<String, String> values = ThreadContext.getImmutableContext(); |
| final List<String> messages = ThreadContext.getImmutableStack().asList(); |
| executor.submit(new Runnable() { |
| public void run() { |
| try (final CloseableThreadContext.Instance ctc = CloseableThreadContext.putAll(values).pushAll(messages)) { |
| logger.debug("Processing for user started"); |
| . |
| logger.debug("Processing for user completed"); |
| } |
| }); |
| } |
| }</pre></div> |
| </div> |
| <div class="section"> |
| <h4><a name="Implementation_details"></a>Implementation details</h4> |
| |
| <p>The Stack and the Map are managed per thread and are based on |
| <a class="externalLink" href="http://docs.oracle.com/javase/6/docs/api/java/lang/ThreadLocal.html">ThreadLocal</a> |
| by default. The Map can be configured to use an |
| <a class="externalLink" href="http://docs.oracle.com/javase/6/docs/api/java/lang/InheritableThreadLocal.html">InheritableThreadLocal</a> |
| by setting system property <tt>log4j2.isThreadContextMapInheritable</tt> to <tt>true</tt>. |
| When configured this way, the contents of the Map will be passed to child threads. However, as |
| discussed in the |
| <a class="externalLink" href="http://docs.oracle.com/javase/6/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="../log4j-api/apidocs/org/apache/logging/log4j/ThreadContext.html">ThreadContext</a> |
| class are static. |
| </p> |
| </div> |
| <div class="section"> |
| <h4><a name="Including_the_ThreadContext_when_writing_logs"></a>Including the ThreadContext when writing logs</h4> |
| |
| <p> |
| The <a href="../log4j-api/apidocs/org/apache/logging/log4j/core/PatternLayout.html">PatternLayout</a> |
| provides mechanisms to print the contents of the |
| <a href="../log4j-api/apidocs/org/apache/logging/log4j/ThreadContext.html">ThreadContext</a> |
| Map and Stack. |
| </p> |
| |
| <ul> |
| |
| <li> |
| Use <tt>%X</tt> by itself to include the full contents of the Map. |
| </li> |
| |
| <li> |
| Use <tt>%X{key}</tt> to include the specified key. |
| </li> |
| |
| <li> |
| Use <tt>%x</tt> to include the full contents of the <a class="externalLink" href="http://docs.oracle.com/javase/6/docs/api/java/util/Stack.html">Stack</a>. |
| </li> |
| </ul> |
| </div> |
| <div class="section"> |
| <h4><a name="Custom_context_data_injectors_for_non_thread-local_context_data"></a>Custom context data injectors for non thread-local context data</h4> |
| |
| <p> |
| With the ThreadContext logging statements can be tagged so log entries that were related in some way |
| can be linked via these tags. The limitation is that this only works for logging done on the same application thread |
| (or child threads when configured). |
| </p> |
| |
| <p> |
| Some applications have a thread model that delegates work to other threads, and |
| in such models, tagging attributes that are put into a thread-local map in one thread are not visible |
| in the other threads and logging done in the other threads will not show these attributes. |
| </p> |
| |
| <p> |
| Log4j 2.7 adds a flexible mechanism to tag logging statements with context data coming from |
| other sources than the ThreadContext. |
| See the manual page on <a href="extending.html#Custom_ContextDataInjector">extending Log4j</a> for details. |
| </p> |
| </div></div> |
| </div> |
| |
| |
| </td> |
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