Being a library HttpClient is not to dictate which logging framework the user has to use. Therefore HttpClient utilizes the logging interface provided by the Commons Logging package. Commons Logging provides a simple and generalized log interface to various logging packages. By using Commons Logging, HttpClient can be configured for a variety of different logging behaviours. That means the user will have to make a choice which logging framework to use. By default Commons Logging supports the following logging frameworks:
SimpleLog ( internal to Commons Logging)
By implementing some simple interfaces Commons Logging can be extended to support basically any other custom logging framework. Commons Logging tries to automatically discover the logging framework to use. If it fails to select the expected one, you must configure Commons Logging by hand. Please refer to the Commons Logging documentation for more information.
HttpClient performs three different kinds of logging: the standard context logging used within each class, HTTP header logging and full wire logging.
Context logging contains information about the internal operation of HttpClient as it performs HTTP requests. Each class has its own log named according to the class's fully qualified name. For example the class DefaultHttpClient has a log named org.apache.http.impl.client.DefaultHttpClient. Since all classes follow this convention it is possible to configure context logging for all classes using the single log named org.apache.http.impl.client.
The wire log is used to log all data transmitted to and from servers when executing HTTP requests. The wire log uses the org.apache.http.wire logging category. This log should only be enabled to debug problems, as it will produce an extremely large amount of log data.
Because the content of HTTP requests is usually less important for debugging than the HTTP headers, the org.apache.http.headers logging category for capturing HTTP headers only.
Commons Logging can delegate to a variety of loggers for processing the actual output. Below are configuration examples for Commons Logging, Log4j and java.util.logging.
Commons Logging comes with a basic logger called SimpleLog. This logger writes all logged messages to System.err. The following examples show how to configure Commons Logging via system properties to use SimpleLog. It is strongly recommended configuring Commons Logging system properties through JVM process arguments at the start up.
Enable header wire + context logging - <>
-Dorg.apache.commons.logging.Log=org.apache.commons.logging.impl.SimpleLog -Dorg.apache.commons.logging.simplelog.showdatetime=true -Dorg.apache.commons.logging.simplelog.log.org.apache.http=DEBUG -Dorg.apache.commons.logging.simplelog.log.org.apache.http.wire=ERROR
Enable full wire + context logging
-Dorg.apache.commons.logging.Log=org.apache.commons.logging.impl.SimpleLog -Dorg.apache.commons.logging.simplelog.showdatetime=true -Dorg.apache.commons.logging.simplelog.log.org.apache.http=DEBUG
Enable context logging for connection management
-Dorg.apache.commons.logging.Log=org.apache.commons.logging.impl.SimpleLog -Dorg.apache.commons.logging.simplelog.showdatetime=true -Dorg.apache.commons.logging.simplelog.log.org.apache.http.impl.conn=DEBUG
Enable context logging for connection management / request execution
-Dorg.apache.commons.logging.Log=org.apache.commons.logging.impl.SimpleLog -Dorg.apache.commons.logging.simplelog.showdatetime=true -Dorg.apache.commons.logging.simplelog.log.org.apache.http.impl.conn=DEBUG -Dorg.apache.commons.logging.simplelog.log.org.apache.http.impl.client=DEBUG -Dorg.apache.commons.logging.simplelog.log.org.apache.http.client=DEBUG
The simplest way to configure Log4j is via a log4j.properties file. Log4j will automatically read and configure itself using a file named log4j.properties when it's present at the root of the application classpath. Below are some Log4j configuration examples.
Note: Log4j is not included in the HttpClient distribution.
Enable header wire + context logging - Best for Debugging
log4j.rootLogger=INFO, stdout log4j.appender.stdout=org.apache.log4j.ConsoleAppender log4j.appender.stdout.layout=org.apache.log4j.PatternLayout log4j.appender.stdout.layout.ConversionPattern=%5p [%c] %m%n log4j.logger.org.apache.http=DEBUG log4j.logger.org.apache.http.wire=ERROR
Enable full wire + context logging
log4j.rootLogger=INFO, stdout log4j.appender.stdout=org.apache.log4j.ConsoleAppender log4j.appender.stdout.layout=org.apache.log4j.PatternLayout log4j.appender.stdout.layout.ConversionPattern=%5p [%c] %m%n log4j.logger.org.apache.http=DEBUG
Enable context logging for connection management
log4j.rootLogger=INFO, stdout log4j.appender.stdout=org.apache.log4j.ConsoleAppender log4j.appender.stdout.layout=org.apache.log4j.PatternLayout log4j.appender.stdout.layout.ConversionPattern=%5p [%c] %m%n log4j.logger.org.apache.http.impl.conn=DEBUG
Enable context logging for connection management / request execution
log4j.rootLogger=INFO, stdout log4j.appender.stdout=org.apache.log4j.ConsoleAppender log4j.appender.stdout.layout=org.apache.log4j.PatternLayout log4j.appender.stdout.layout.ConversionPattern=%5p [%c] %m%n log4j.logger.org.apache.http.impl.conn=DEBUG log4j.logger.org.apache.http.impl.client=DEBUG log4j.logger.org.apache.http.client=DEBUG
Note that the default configuration for Log4J is very inefficient as it causes all the logging information to be generated but not actually sent anywhere. The Log4J manual is the best reference for how to configure Log4J. It is available at http://logging.apache.org/log4j/docs/manual.html .
Since JDK 1.4 there has been a package java.util.logging that provides a logging framework similar to Log4J. By default it reads a config file from $JAVA_HOME/jre/lib/logging.properties which looks like this (comments stripped):
handlers=java.util.logging.ConsoleHandler .level=INFO java.util.logging.FileHandler.pattern = %h/java%u.log java.util.logging.FileHandler.limit = 50000 java.util.logging.FileHandler.count = 1 java.util.logging.FileHandler.formatter = java.util.logging.XMLFormatter java.util.logging.ConsoleHandler.level = INFO java.util.logging.ConsoleHandler.formatter = java.util.logging.SimpleFormatter com.xyz.foo.level = SEVERE
To customize logging a custom logging.properties file should be created in the project directory. The location of this file must be passed to the JVM as a system property. This can be done on the command line like so:
$JAVA_HOME/java -Djava.util.logging.config.file=$HOME/myapp/logging.properties -classpath $HOME/myapp/target/classes com.myapp.Main
Alternatively LogManager#readConfiguration(InputStream) can be used to pass it the desired configuration.
Enable header wire + context logging - Best for Debugging
.level = INFO handlers=java.util.logging.ConsoleHandler java.util.logging.ConsoleHandler.formatter = java.util.logging.SimpleFormatter java.util.logging.ConsoleHandler.level = ALL org.apache.http.level = FINEST org.apache.http.wire.level = SEVERE
Enable full wire + context logging
.level = INFO handlers=java.util.logging.ConsoleHandler java.util.logging.ConsoleHandler.formatter = java.util.logging.SimpleFormatter java.util.logging.ConsoleHandler.level = ALL org.apache.http.level = FINEST
Enable context logging for connection management
.level = INFO handlers=java.util.logging.ConsoleHandler java.util.logging.ConsoleHandler.formatter = java.util.logging.SimpleFormatter java.util.logging.ConsoleHandler.level = ALL org.apache.http.impl.conn.level = FINEST
Enable context logging for connection management / request execution
.level = INFO handlers=java.util.logging.ConsoleHandler java.util.logging.ConsoleHandler.formatter = java.util.logging.SimpleFormatter java.util.logging.ConsoleHandler.level = ALL org.apache.http.impl.conn.level = FINEST org.apache.http.impl.client.level = FINEST org.apache.http.client.level = FINEST
More detailed information is available from the Java Logging documentation.