HttpClient - HttpClient Logging Practices
Author: HttpComponents Team

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## Logging Practices

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:

- Log4J
- java.util.logging
- 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 two different kinds of logging: the standard context logging used within each class, and wire logging.

### Context 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 `HttpClient` has a log named `org.apache.commons.httpclient.HttpClient`. Since all classes follow this convention it is possible to configure context logging for all classes using the single log named `org.apache.commons.httpclient`.

### Wire Logging

The wire log is used to log all data transmitted to and from servers when executing HTTP requests. This log should only be enabled to debug problems, as it will produce an extremely large amount of log data, some of it in binary format.

Because the content of HTTP requests is usually less important for debugging than the HTTP headers, these two types of data have been separated into different wire logs. The content log is `httpclient.wire.content` and the header log is `httpclient.wire.header`.

## Configuration Examples

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 Examples

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`.

Note: The system properties must be set before a reference to any Commons Logging class is made.

Enable header wire + context logging - Best for Debugging

> System.setProperty("org.apache.commons.logging.Log", "org.apache.commons.logging.impl.SimpleLog");
> System.setProperty("org.apache.commons.logging.simplelog.showdatetime", "true");
> System.setProperty("org.apache.commons.logging.simplelog.log.httpclient.wire.header", "debug");
> System.setProperty("org.apache.commons.logging.simplelog.log.org.apache.commons.httpclient", "debug");

Enable full wire(header and content) + context logging

> System.setProperty("org.apache.commons.logging.Log", "org.apache.commons.logging.impl.SimpleLog");
> System.setProperty("org.apache.commons.logging.simplelog.showdatetime", "true");
> System.setProperty("org.apache.commons.logging.simplelog.log.httpclient.wire", "debug");
> System.setProperty("org.apache.commons.logging.simplelog.log.org.apache.commons.httpclient", "debug");

Enable just context logging

> System.setProperty("org.apache.commons.logging.Log", "org.apache.commons.logging.impl.SimpleLog");
> System.setProperty("org.apache.commons.logging.simplelog.showdatetime", "true");
> System.setProperty("org.apache.commons.logging.simplelog.log.org.apache.commons.httpclient", "debug");

### Log4j Examples

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.httpclient.wire.header=DEBUG
> log4j.logger.org.apache.commons.httpclient=DEBUG

Enable full wire(header and content) + 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.httpclient.wire=DEBUG
> log4j.logger.org.apache.commons.httpclient=DEBUG

Log wire to file + context logging

> log4j.rootLogger=INFO
> 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.appender.F=org.apache.log4j.FileAppender
> log4j.appender.F.File=wire.log
> log4j.appender.F.layout=org.apache.log4j.PatternLayout
> log4j.appender.F.layout.ConversionPattern =%5p [%c] %m%n
> log4j.logger.httpclient.wire=DEBUG, F
> log4j.logger.org.apache.commons.httpclient=DEBUG, stdout

Enable just 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.commons.httpclient=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

### java.util.logging Examples

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
> httpclient.wire.header.level=FINEST
> org.apache.commons.httpclient.level=FINEST

Enable full wire(header and content) + context logging

> .level=INFO
> handlers=java.util.logging.ConsoleHandler
> java.util.logging.ConsoleHandler.formatter = java.util.logging.SimpleFormatter
> httpclient.wire.level=FINEST
> org.apache.commons.httpclient.level=FINEST

Enable just context logging

> .level=INFO
> handlers=java.util.logging.ConsoleHandler
> java.util.logging.ConsoleHandler.formatter = java.util.logging.SimpleFormatter
> org.apache.commons.httpclient.level=FINEST

More detailed information is available from the Java Logging documentation.