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| |
| <section> |
| <h2><a name="Log4j_2_API"></a>Log4j 2 API</h2> |
| <section> |
| <h3><a name="Markers"></a>Markers</h3> |
| <a name="Markers"></a> |
| |
| <p> |
| One of the primary purpose of a logging framework is to provide the means to generate debugging and |
| diagnostic information only when it is needed, and to allow filtering of that information so that it |
| does not overwhelm the system or the individuals who need to make use of it. As an example, an |
| application desires to log its entry, exit and other operations separately from SQL statements |
| being executed, and wishes to be able to log queries separate from updates. One way to accomplish |
| this is shown below: |
| </p> |
| |
| <div> |
| <pre class="prettyprint linenums"> |
| import org.apache.logging.log4j.Logger; |
| import org.apache.logging.log4j.LogManager; |
| import org.apache.logging.log4j.MarkerManager; |
| import java.util.Map; |
| |
| public class MyApp { |
| |
| private Logger logger = LogManager.getLogger(MyApp.class.getName()); |
| private static final Marker SQL_MARKER = MarkerManager.getMarker("SQL"); |
| private static final Marker UPDATE_MARKER = MarkerManager.getMarker("SQL_UPDATE").setParents(SQL_MARKER); |
| private static final Marker QUERY_MARKER = MarkerManager.getMarker("SQL_QUERY").setParents(SQL_MARKER); |
| |
| public String doQuery(String table) { |
| logger.traceEntry(); |
| |
| logger.debug(QUERY_MARKER, "SELECT * FROM {}", table); |
| |
| String result = ... |
| |
| return logger.traceExit(result); |
| } |
| |
| public String doUpdate(String table, Map<String, String> params) { |
| logger.traceEntry(); |
| |
| if (logger.isDebugEnabled()) { |
| logger.debug(UPDATE_MARKER, "UPDATE {} SET {}", table, formatCols()); |
| } |
| |
| String result = ... |
| |
| return logger.traceExit(result); |
| } |
| |
| private String formatCols(Map<String, String> cols) { |
| StringBuilder sb = new StringBuilder(); |
| boolean first = true; |
| for (Map.Entry<String, String> entry : cols.entrySet()) { |
| if (!first) { |
| sb.append(", "); |
| } |
| sb.append(entry.getKey()).append("=").append(entry.getValue()); |
| first = false; |
| } |
| return sb.toString(); |
| } |
| }</pre></div> |
| |
| <p> |
| In the example above it is now possible to add MarkerFilters to only allow SQL update operations |
| to be logged, all SQL updates to be logged or to log everything in MyApp. |
| </p> |
| |
| <p> |
| Some important rules about Markers must be considered when using them. |
| </p> |
| |
| <ol style="list-style-type: decimal"> |
| |
| <li>Markers must be unique. They are permanently registered by name so care should be taken |
| to ensure that Markers used in your application are distinct from those in the application's |
| dependencies, unless that is what is desired.</li> |
| |
| <li>Parent Markers can be added or removed dynamically. However, this is fairly expensive to do. |
| Instead, it is recommended that the parents be identified when obtaining the Marker the first time |
| as shown in the examples above. Specifically, the set method replaces all the markers in |
| a single operation while add and remove act on only a single Marker at a time.</li> |
| |
| <li>Evaluating Markers with multiple ancestors is much more expensive than Markers with no parents. |
| For example, in one set of tests to evaluate whether a Marker matched its grandparent took 3 |
| times longer than evaluating the Marker itself. Even then though, evaluating Markers is |
| inexpensive compared to resolving the callers class name or line number.</li> |
| </ol> |
| </section> |
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