logger.info("User {} has logged in using id {}", map.get("Name"), user.getId());
Log4j 2 API
Messages
Although Log4j 2 provides Logger methods that accept Strings and Objects, all of these are ultimately 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.
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:
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:
logger.info(new LoggedInMessage(map, user));
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.
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.
Borrowing from the earlier example illustrating Markers to identify SQL statements being logged, Messages can also be leveraged. First, the Message is defined.
public class SQLMessage implements Message {
public enum SQLType {
UPDATE,
QUERY
};
private 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();
}
}
Next we can use the message in our application.
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();
}
}
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.
FormattedMessage
The message pattern passed to a
FormattedMessage
is first checked to see if it is a valid java.text.MessageFormat
pattern. If it is, a MessageFormatMessage
is used to format it. If not
it is next checked to see if it contains any tokens that are valid
format specifiers for String.format()
. If so, a StringFormattedMessage
is used to format it. Finally, if the pattern doesn’t match either of
those then a ParameterizedMessage
is used to format it.
LocalizedMessage
LocalizedMessage
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.
LocalizedMessage
incorporates a ResourceBundle
and allows 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 message retrieved
from the bundle will be formatted using a FormattedMessage.
LoggerNameAwareMessage
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.
MapMessage
A MapMessage
contains a Map of String keys and values. MapMessage
implements FormattedMessage
and accepts format specifiers of "XML",
"JSON" or "JAVA", in which case the Map will be formatted as XML, JSON
or as documented by
java.util.AbstractMap.toString()
.
Otherwise, the Map will be formatted as "key1=value1 key2=value2 …"
.
Some Appenders make special use of MapMessage
objects:
-
When a JMS Appender is configured with a no layout, it converts a Log4j
MapMessage
to a JMSjavax.jms.MapMessage
. -
When a JDBC Appender is configured with no layout, it converts a Log4j
MapMessage
to values in a SQL INSERT statement. -
When a MongoDB4 Appender is configured with no layout, it converts a Log4j
MapMessage
to fields in a MongoDB object.
MessageFormatMessage
MessageFormatMessage
handles messages that use a
conversion
format. While this Message
has more flexibility than
ParameterizedMessage
, it is also about two times slower.
MultiformatMessage
A MultiformatMessage
will have a getFormats method and a
getFormattedMessage
method that accepts and array of format Strings. The
getFormats
method may be called by a Layout to provide it information on
what formatting options the Message supports. The Layout may then call
getFormattedMessage
with one or more for the formats. If the Message
doesn’t recognize the format name it will simply format the data using
its default format. An example of this is 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.
ObjectMessage
Formats an Object
by calling its toString
method. Since Log4j 2.6,
Layouts trying to be low-garbage or garbage-free will call the
formatTo(StringBuilder)
method instead.
ParameterizedMessage
ParameterizedMessage
handles messages that contain "\{}" in the format to represent
replaceable tokens and the replacement parameters.
ReusableObjectMessage
In garbage-free mode, this message is used to pass logged Objects to the Layout and Appenders. Functionally equivalent to ObjectMessage.
ReusableParameterizedMessage
In garbage-free mode, this message is used to handle messages that contain "\{}" in the format to represent replaceable tokens and the replacement parameters. Functionally equivalent to ParameterizedMessage.
ReusableSimpleMessage
In garbage-free mode, this message is used to pass logged `String`s and `CharSequence`s to the Layout and Appenders. Functionally equivalent to SimpleMessage.
SimpleMessage
SimpleMessage
contains a String
or CharSequence
that requires no
formatting.
StringFormattedMessage
StringFormattedMessage
handles messages that use a
conversion
format that is compliant with
java.lang.String.format().
While this Message has more flexibility than ParameterizedMessage
, it is
also 5 to 10 times slower.
StructuredDataMessage
StructuredDataMessage
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 RFC 5424.
ThreadDumpMessage
A ThreadDumpMessage, if logged, will generate stack traces for all threads. The stack traces will include any locks that are held.
TimestampMessage
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.