rgoers/src/site/xdoc/manual/messages.xml - logging-log4j1 - Git at Google

 <?xml version="1.0"?>
 <!--
     Licensed to the Apache Software Foundation (ASF) under one or more
     contributor license agreements.  See the NOTICE file distributed with
     this work for additional information regarding copyright ownership.
     The ASF licenses this file to You under the Apache License, Version 2.0
     (the "License"); you may not use this file except in compliance with
     the License.  You may obtain a copy of the License at

          http://www.apache.org/licenses/LICENSE-2.0

     Unless required by applicable law or agreed to in writing, software
     distributed under the License is distributed on an "AS IS" BASIS,
     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     See the License for the specific language governing permissions and
     limitations under the License.
 -->

 <document>
   <properties>
     <title>Log4J 2.0 API Messages</title>
     <author email="rgoers@apache.org">Ralph Goers</author>
   </properties>

   <body>
     <section name="Log4j 2.0 API">
       <a name="Messages"/>
       <subsection name="Messages">
         <p>
           Although Log4j 2 provides Logger methods that accept Strings and Objects, all of these are ulitmately
           captured in Message objects that are then associated with the log event. Applications are free to
           construct Messages of their own and pass them to the Logger. Although it may seem more expensive than
           passing the message format and parameters directly to the event, testing has shown that with modern
           JVMs the cost of creating and destroying events is minor, especially when complex tasks are encapsulated
           in the Message instead of the application. In addition, when using the methods that accept Strings and
           parameters, the underlying Message object will only be created if any configured global filters
           or the Logger's log level allow the message to be processed.
         </p>
         <p>
           Consider an application that has a Map object containing {"Name" = "John Doe", "Address" = "123 Main
           St.",
           "Phone" = "(999) 555-1212"} and a User object that has a getId method that returns "jdoe". The developer
           would like to add an informational message that returns "User John Doe has logged in using id jdoe". The
           way this could be accomplished is by doing:
         </p>
         <pre>
           logger.info("User {} has logged in using id {}, map.get("Name"), user.getId());
         </pre>
         <p>
           While there is nothing inherently wrong with this, as the complexity of the objects and desired output
           increases this technique becomes harder to use. As an alternative, using Messages allows:
         </p>
         <pre>
           logger.info(new LoggedInMessage(map, user));
         </pre>
         <p>
           In this alternative the formatting is delegated to the LoggedInMessage object's getFormattedMessage
           method.
           Although in this alternative a new object is created, none of the methods on the objects passed to the
           LoggedInMessage are invoked until the LoggedInMessage is formatted. This is especially useful when an
           Object's toString method does not produce the information you would like to appear in the log.
         </p>
         <p>
           Another advantage to Messages is that they simplify writing Layouts. In other logging frameworks the
           Layout must loop through the parameters individually and determine what to do based on what objects
           are encountered. With Messages the Layout has the option of delegating the formatting to the Message or
           performing its formatting based on the type of Message encountered.
         </p>
         <p>
           Borrowing from the earlier example illustrating Markers to identify SQL statements being logged, Messages
           can also be leveraged. First, the Message is defined.
         </p>
         <source><![CDATA[

   public class SQLMessage implements Message {
       public enum SQLType {
           UPDATE,
           QUERY
       };

       prviate final SQLType type;
       private final String table;
       private final Map<String, String> cols;

       public SQLMessage(SQLType type, String table) {
           this(type, table, null);
       }

       public SQLMessage(SQLType type, String table, Map<String, String> cols) {
           this.type = type;
           this.table = table;
           this.cols = cols;
       }

       public String getFormattedMessage() {
           switch (type) {
               case UPDATE:
                 return createUpdateString();
                 break;
               case QUERY:
                 return createQueryString();
                 break;
               default;
           }
       }

       public String getMessageFormat() {
           return type + " " + table;
       }

       public Object getParameters() {
           return cols;
       }

       private String createUpdateString() {
       }

       private String createQueryString() {
       }

       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();
       }
   }]]></source>
       <p>
         Next we can use the message in our application.
       </p>
         <source><![CDATA[    import org.apache.logging.log4j.Logger;
     import org.apache.logging.log4j.LogManager;
     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", SQL_MARKER);
         private static final Marker QUERY_MARKER = MarkerManager.getMarker("SQL_QUERY", SQL_MARKER);

         public String doQuery(String table) {
             logger.entry(param);

             logger.debug(QUERY_MARKER, new SQLMessage(SQLMessage.SQLType.QUERY, table);

             return logger.exit();
         }

         public String doUpdate(String table, Map<String, String> params) {
             logger.entry(param);

             logger.debug(UPDATE_MARKER, new SQLMessage(SQLMessage.SQLType.UPDATE, table, parmas);

             return logger.exit();
         }
     }]]></source>
         <p>
           Notice that in contrast to the prior version of this example, the logger.debug in doUpdate no longer
           needs to be wrapped in an isDebugEnabled call as creation of the SQLMessage is on the same order of
           magnitude of performing that check. Furthermore, all the formatting of the SQL columns is now hidden
           in the SQLMessage instead of having to take place in the business logic. Finally, if desired, Filters
           and/or Layouts can be written to take special action when an SQLMessage is encountered.
         </p>
       <a name="FormattedMessage"/>
       <h4>FormattedMessage</h4>
         <p>
           A FormattedMessage will have setFormat and getFormat methods. The setFormat method may be called by a
           Layout to provide advice on how the Message should be formatted. If the Message doesn't recognize the
           format name it will simply format the data using its default format. An example of this is the
           StructuredDataMessage which accepts a format String of "XML" which will cause it to format the event data
           as XML instead of the RFC 5424 format.
         </p>
       <a name="LocalizedMessage"/>
       <h4>LocalizedMessage</h4>
         <p>
           <a href="../log4j2-api/apidocs/org/apache/logging/log4j/message/LocalizedMessage.html">LocalizedMessage</a>
           is provided primarily to provide compatibility with Log4j 1.x. Generally,
           the best approach to localization is to have the client UI render the events in the client's locale.
         </p>
         <p>
           LocalizedMessage extends a ParameterizedMessage by incorporating a ResourceBundle and allowing
           the message pattern parameter to be the key to the message pattern in the bundle. If no bundle is specified,
           LocalizedMessage will attempt to locate a bundle with the name of the Logger used to log the event. The
           parameters to the Message will be incorporated into the Message whereever the "{}" placeholders occur.
         </p>
       <a name="LoggerNameAwareMessage"/>
       <h4>LoggerNameAwareMessage</h4>
         <p>
           LoggerNameAwareMessage is an interface with a setLoggerName method. This method will be called during
           event construction so that the Message has the name of the Logger used to log the event when the
           message is being formatted.
         </p>
       <a name="MapMessage"/>
       <h4>MapMessage</h4>
         <p>
           A MapMessage contains a Map of String keys and values. MapMessage implements FormattedMessage and accepts
           a format specifier of "XML", in which case the Map will be formatted as XML. Otherwise, the Map will be
           formatted as "key1=value1 key2=value2...".
         </p>
       <a name="ObjectMessage"/>
       <h4>ObjectMessage</h4>
         <p>
           Formats an Object by calling its toString method.
         </p>
       <a name="ParameterizedMessage"/>
       <h4>ParameterizedMessage</h4>
         <p>
           <a href="../log4j2-api/apidocs/org/apache/logging/log4j/message/ParameterizedMessage.html">ParameterizedMessage</a>
           handles messages that contain "{}" in the format to represent replaceable tokens and the replacement
           parameters.
         </p>
       <a name="SimpleMessage"/>
       <h4>SimpleMessage</h4>
         <p>
           SimpleMessage contains a String that requires no formatting.
         </p>
       <a name="StructuredDataMessage"/>
       <h4>StructuredDataMessage</h4>
         <p>
           <a href="../log4j2-api/apidocs/org/apache/logging/log4j/message/StructuredDataMessage.html">StructuredDataMessage</a>
           allows applications to add items to a Map as well as set the id to allow a message to be formatted as a
           Structured Data element in accordance with <a href="http://tools.ietf.org/html/rfc5424">RFC 5424</a>.
         </p>
       <a name="ThreadDumpMessage"/>
       <h4>ThreadDumpMessage</h4>
         <p>
           A ThreadDumpMessage, if logged, will generate stack traces for all threads. If running on Java 6+ the
           stack traces will include any locks that are held.
         </p>
       <a name="TimestampMessage"/>
       <h4>TimestampMessage</h4>
         <p>
           A TimestampMessage will provide a getTimestamp method that is called during event construction. The
           timestamp in the Message will be used in lieu of the current timestamp.
         </p>
       </subsection>
     </section>
   </body>
 </document>
	<?xml version="1.0"?>
	<!--
	Licensed to the Apache Software Foundation (ASF) under one or more
	contributor license agreements. See the NOTICE file distributed with
	this work for additional information regarding copyright ownership.
	The ASF licenses this file to You under the Apache License, Version 2.0
	(the "License"); you may not use this file except in compliance with
	the License. You may obtain a copy of the License at

	http://www.apache.org/licenses/LICENSE-2.0

	Unless required by applicable law or agreed to in writing, software
	distributed under the License is distributed on an "AS IS" BASIS,
	WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	See the License for the specific language governing permissions and
	limitations under the License.
	-->

	<document>
	<properties>
	<title>Log4J 2.0 API Messages</title>
	<author email="rgoers@apache.org">Ralph Goers</author>
	</properties>

	<body>
	<section name="Log4j 2.0 API">
	<a name="Messages"/>
	<subsection name="Messages">
	<p>
	Although Log4j 2 provides Logger methods that accept Strings and Objects, all of these are ulitmately
	captured in Message objects that are then associated with the log event. Applications are free to
	construct Messages of their own and pass them to the Logger. Although it may seem more expensive than
	passing the message format and parameters directly to the event, testing has shown that with modern
	JVMs the cost of creating and destroying events is minor, especially when complex tasks are encapsulated
	in the Message instead of the application. In addition, when using the methods that accept Strings and
	parameters, the underlying Message object will only be created if any configured global filters
	or the Logger's log level allow the message to be processed.
	</p>
	<p>
	Consider an application that has a Map object containing {"Name" = "John Doe", "Address" = "123 Main
	St.",
	"Phone" = "(999) 555-1212"} and a User object that has a getId method that returns "jdoe". The developer
	would like to add an informational message that returns "User John Doe has logged in using id jdoe". The
	way this could be accomplished is by doing:
	</p>
	<pre>
	logger.info("User {} has logged in using id {}, map.get("Name"), user.getId());
	</pre>
	<p>
	While there is nothing inherently wrong with this, as the complexity of the objects and desired output
	increases this technique becomes harder to use. As an alternative, using Messages allows:
	</p>
	<pre>
	logger.info(new LoggedInMessage(map, user));
	</pre>
	<p>
	In this alternative the formatting is delegated to the LoggedInMessage object's getFormattedMessage
	method.
	Although in this alternative a new object is created, none of the methods on the objects passed to the
	LoggedInMessage are invoked until the LoggedInMessage is formatted. This is especially useful when an
	Object's toString method does not produce the information you would like to appear in the log.
	</p>
	<p>
	Another advantage to Messages is that they simplify writing Layouts. In other logging frameworks the
	Layout must loop through the parameters individually and determine what to do based on what objects
	are encountered. With Messages the Layout has the option of delegating the formatting to the Message or
	performing its formatting based on the type of Message encountered.
	</p>
	<p>
	Borrowing from the earlier example illustrating Markers to identify SQL statements being logged, Messages
	can also be leveraged. First, the Message is defined.
	</p>
	<source><![CDATA[

	public class SQLMessage implements Message {
	public enum SQLType {
	UPDATE,
	QUERY
	};

	prviate final SQLType type;
	private final String table;
	private final Map<String, String> cols;

	public SQLMessage(SQLType type, String table) {
	this(type, table, null);
	}

	public SQLMessage(SQLType type, String table, Map<String, String> cols) {
	this.type = type;
	this.table = table;
	this.cols = cols;
	}

	public String getFormattedMessage() {
	switch (type) {
	case UPDATE:
	return createUpdateString();
	break;
	case QUERY:
	return createQueryString();
	break;
	default;
	}
	}

	public String getMessageFormat() {
	return type + " " + table;
	}

	public Object getParameters() {
	return cols;
	}

	private String createUpdateString() {
	}

	private String createQueryString() {
	}

	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();
	}
	}]]></source>
	<p>
	Next we can use the message in our application.
	</p>
	<source><![CDATA[ import org.apache.logging.log4j.Logger;
	import org.apache.logging.log4j.LogManager;
	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", SQL_MARKER);
	private static final Marker QUERY_MARKER = MarkerManager.getMarker("SQL_QUERY", SQL_MARKER);

	public String doQuery(String table) {
	logger.entry(param);

	logger.debug(QUERY_MARKER, new SQLMessage(SQLMessage.SQLType.QUERY, table);

	return logger.exit();
	}

	public String doUpdate(String table, Map<String, String> params) {
	logger.entry(param);

	logger.debug(UPDATE_MARKER, new SQLMessage(SQLMessage.SQLType.UPDATE, table, parmas);

	return logger.exit();
	}
	}]]></source>
	<p>
	Notice that in contrast to the prior version of this example, the logger.debug in doUpdate no longer
	needs to be wrapped in an isDebugEnabled call as creation of the SQLMessage is on the same order of
	magnitude of performing that check. Furthermore, all the formatting of the SQL columns is now hidden
	in the SQLMessage instead of having to take place in the business logic. Finally, if desired, Filters
	and/or Layouts can be written to take special action when an SQLMessage is encountered.
	</p>
	<a name="FormattedMessage"/>
	<h4>FormattedMessage</h4>
	<p>
	A FormattedMessage will have setFormat and getFormat methods. The setFormat method may be called by a
	Layout to provide advice on how the Message should be formatted. If the Message doesn't recognize the
	format name it will simply format the data using its default format. An example of this is the
	StructuredDataMessage which accepts a format String of "XML" which will cause it to format the event data
	as XML instead of the RFC 5424 format.
	</p>
	<a name="LocalizedMessage"/>
	<h4>LocalizedMessage</h4>
	<p>
	<a href="../log4j2-api/apidocs/org/apache/logging/log4j/message/LocalizedMessage.html">LocalizedMessage</a>
	is provided primarily to provide compatibility with Log4j 1.x. Generally,
	the best approach to localization is to have the client UI render the events in the client's locale.
	</p>
	<p>
	LocalizedMessage extends a ParameterizedMessage by incorporating a ResourceBundle and allowing
	the message pattern parameter to be the key to the message pattern in the bundle. If no bundle is specified,
	LocalizedMessage will attempt to locate a bundle with the name of the Logger used to log the event. The
	parameters to the Message will be incorporated into the Message whereever the "{}" placeholders occur.
	</p>
	<a name="LoggerNameAwareMessage"/>
	<h4>LoggerNameAwareMessage</h4>
	<p>
	LoggerNameAwareMessage is an interface with a setLoggerName method. This method will be called during
	event construction so that the Message has the name of the Logger used to log the event when the
	message is being formatted.
	</p>
	<a name="MapMessage"/>
	<h4>MapMessage</h4>
	<p>
	A MapMessage contains a Map of String keys and values. MapMessage implements FormattedMessage and accepts
	a format specifier of "XML", in which case the Map will be formatted as XML. Otherwise, the Map will be
	formatted as "key1=value1 key2=value2...".
	</p>
	<a name="ObjectMessage"/>
	<h4>ObjectMessage</h4>
	<p>
	Formats an Object by calling its toString method.
	</p>
	<a name="ParameterizedMessage"/>
	<h4>ParameterizedMessage</h4>
	<p>
	<a href="../log4j2-api/apidocs/org/apache/logging/log4j/message/ParameterizedMessage.html">ParameterizedMessage</a>
	handles messages that contain "{}" in the format to represent replaceable tokens and the replacement
	parameters.
	</p>
	<a name="SimpleMessage"/>
	<h4>SimpleMessage</h4>
	<p>
	SimpleMessage contains a String that requires no formatting.
	</p>
	<a name="StructuredDataMessage"/>
	<h4>StructuredDataMessage</h4>
	<p>
	<a href="../log4j2-api/apidocs/org/apache/logging/log4j/message/StructuredDataMessage.html">StructuredDataMessage</a>
	allows applications to add items to a Map as well as set the id to allow a message to be formatted as a
	Structured Data element in accordance with <a href="http://tools.ietf.org/html/rfc5424">RFC 5424</a>.
	</p>
	<a name="ThreadDumpMessage"/>
	<h4>ThreadDumpMessage</h4>
	<p>
	A ThreadDumpMessage, if logged, will generate stack traces for all threads. If running on Java 6+ the
	stack traces will include any locks that are held.
	</p>
	<a name="TimestampMessage"/>
	<h4>TimestampMessage</h4>
	<p>
	A TimestampMessage will provide a getTimestamp method that is called during event construction. The
	timestamp in the Message will be used in lieu of the current timestamp.
	</p>
	</subsection>
	</section>
	</body>
	</document>