Simple java logging to file

Still have any questions about an article, leave us a comment. Signup for news, latest articles and special offers!! Additional menu. IOException ;. FileHandler ;. Logger ;. SimpleFormatter ;. XMLFormatter ;. Loggers are normally named, using a hierarchical dot-separated.

Logger names can be arbitrary strings, but they should normally be. Active Oldest Votes. PrintWriter; import java. StringWriter; import java. Date; import java. Formatter; import java. IOException; import java. FileHandler; import java. Level; import java. Improve this answer. SubOptimal SubOptimal The Java world offers a variety of choices of frameworks to help with the chore of logging. Recommendation I strongly recommend slf4j. Community Bot 1 1 1 silver badge.

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Linked Related Hot Network Questions. This time the output looks as follows:. We started with the Configuration definition. Next, we defined the appender. The Appender is responsible for delivering the LogEvent to its destination. You can have multiple of those. In our case the Appenders section contains a single appender of the Console type:. We also provided the pattern for the PatternLayout which defines the way our LogEvent will be formatted in the Console appender. The last section defines the Loggers.

We define the configuration of different loggers that we defined in our code or in the libraries that we are using. In our case this section only contains the Root logger definition:. The Root logger definition tells Log4j to use that configuration when a dedicated configuration for a logger is not found. In our Root logger definition, we say that the default log level should be set to INFO and the log events should be sent to the appender with the name Console.

If we run the example code mentioned above with our new XML configuration the output will be as follows:. If we would like to use the properties file instead of the XML one we could just create a log4j2. To achieve similar results as we got with the XML based configuration we would use the following properties:. It is less verbose and works, but is also less flexible if you want to use slightly more complicated functionalities. The Logger is the main entity that our application uses to create LogRecord, so basically to log what we want to output as the log message.

Creating the Logger is very simple. In the majority of the cases it will be a private, static object inside a class, for example:. The above part of the code uses the Log4j 2 classes to create the Logger object, but you can also base the initialization on SLF4J, so you can always change the logging framework of your choice:.

No matter which method of initialization we will chose, once we have the Logger instance we can use it to generate our log events:. A log level or log severity is a piece of information telling how important a given log event is.

It is a simple, yet very powerful way of distinguishing log events from each other. If the log levels are used properly in your application all you need is to look at the severity first.

It will tell you if you can continue sleeping during the on-call night or you need to jump out of bed right away and hit another personal best in running between your bedroom and laptop in the living room. You can think of the log levels as a way to filter the critical information about your system state and the one that is purely informative. The log levels can help in reducing the information noise and reduce alert fatigue. Log4j 2 appender is responsible for delivering LogEvents to their destination.

Some appenders only send LogEvents while others wrap other appenders to provide additional functionality. Keep in mind that this is not a full list of appenders and to see all of them look at Log4j 2 appenders documentation. You can see that I have two appenders defined — one called Console that appends the data to the System. The second one is called File. I provided a fileName configuration property to tell the appender where the data should be written and I said that I want to append the data to the end of the file.

We also have two Loggers defined. We have the root logger that appends the data to our File appender. It does that for all the log events with the INFO log level and higher. We also have a second Logger defined with the name of com. You can see that the times are exactly the same, the lines are different though just as our patterns are.

In the normal production environment, you would probably use the Rolling File appender to roll over the files daily or when they will reach a certain size. One of the useful appenders is the SyslogAppender that enables sending the log events generated by our application to a Syslog compatible destination. That can be your local Syslog server, remote Syslog server in your organization, or even one exposed as a part of a log centralization solution such as Sematext Logs.

For example, the following configuration would send out log messages to the Syslog server exposed as a part of Sematext Cloud :. You can see that we have two appenders defined. One is the already known — the Console appender writes the log events to the standard output. The second is the Syslog appender that sends data to Syslog compatible destinations available at logsene-syslog-receiver. We also included some configuration properties like port, protocol, and format. As you can see, sending log events from your Java application to Syslog is really simple.

If the appenders that come out of the box with Log4j 2 are not enough we can create our own implementations. We just need to extend one of the abstract appender classes, like AbstractAppender for example and implement the append and createAppender method. For example:.

The append methodis the one responsible for processing the LogEvent objects. The createAppender method is responsible for creating the instance of our appender. We alsoused the Plugin and PluginFactory annotations to tell Log4j how to create the appender itself. The key thing to remember is to include the packages section in the root Configuration tag, so that Log4j knows where to look for the plugin.

Layout is used by the appender to format a LogEvent into a form that is defined. By default, there are a few layouts available in Log4j 2 some of them require additional runtime dependencies :. The Log4j PatternLayout is a powerful tool. It allows us to structure our logs events without the need of writing any code — we only need to define the pattern that we will use and include that in our configuration. It is that simple.

The configuration would look as follows:. As you can see the date related to the event can be further configured using patterns. You can learn about that in the documentation related to PatternLayout.

We have used the logger name, the class, include exceptions, configure highlighting color, location of the message and many, many more. All of them are described in the documentation related to PatternLayout.

But just to give you a more sophisticated example here is another configuration:. You can see that in addition to the date and message we also have the level of the message, the class and the line where the log event was produced. We even included the process identifier.