How does log4j work

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Javatpoint Services JavaTpoint offers too many high quality services. Property Description layout. Appender uses the Layout objects and the conversion pattern associated with them to format the logging information. Appender can contain a threshold level associated with it independent of the logger level. All users are advised to migrate to Log4j 2. In this blog post, we will help you understand your current Log4j setup — specifically, the log4j 2. Make a plan to transition to Log4j 2.

Your application will thank you for that. You will get the security fixes, performance improvements, and far more features after migration. Just use the Log4j 2. If you need help with that, check out this Java logging tutorial where I explain everything you need.

There is no magic behind logging in Java. It all comes down to using an appropriate Java class and its methods to generate log events. As we discussed in the Java logging guide there are multiple ways you can start. Of course, the most naive one and not the best path to follow is just using the System.

Yes, you can do that and your messages will just go to the standard output and standard error. An example of such code could look like this:. The output of the above code execution would be as follows:.

There are other things that are missing that are not really connected to debugging. Think about the execution environment, multiple applications or microservices, and the need to unify logging to simplify the log centralization pipeline configuration.

Using the System. Of course, logging to System. Remember about that! Because of all the mentioned reasons and many more that we do not even think of, you should look into one of the possible logging libraries, like Log4 j , Log4j 2 , Logback , or even the java.

For this blog post, we will use Log4j. The topic of choosing the right logging solution for your Java application is something that we already discussed in our tutorial about logging in Java. We highly recommend reading at least the mentioned section. The Simple Logging Facade provides bindings for common logging frameworks like Log4j , Logback , and java. Imagine the process of writing a log message in the following, simplified way:.

You may ask why use an abstraction layer at all? Well, the answer is quite simple — eventually, you may want to change the logging framework, upgrade it, unify it with the rest of your technology stack. When using an abstraction layer such operation is fairly simple — you just exchange the logging framework dependencies and provide a new package.

If we were not to use an abstraction layer — we would have to change the code, potentially lots of code. Each class that logs something. Not a very nice development experience. The code of your Java application will be interacting with a standard set of key elements that allow log event creation and manipulation. The Logger is the main entity that an application uses to make logging calls — create log events.

The Logger object is usually used for a single class or a single component to provide context-bound to a specific use case. It provides methods to create log events with an appropriate log level and pass it on for further processing.

You will usually create a static object that you will interact with, for example like this:. The simplest way to start with Log4j is to include the library in the classpath of your Java application.

To do that we include the newest available log4j library, which means version 1. We use Gradle and in our simple application and the dependencies section for Gradle build file looks as follows:. We can start developing the code and include logging using Log4j :. As you can see in the above code we initialized the Logger object by using the static getLogger method and we provided the name of the class. After doing that we can easily access the static Logger object and use it to produce log events.

We can do that in the main method. One side note — the getLogger method can be also called with a String as an argument, for example:.

It would mean that we want to create a logger and associate the name of com. If we will use the same name anywhere else in the code Log4j will return the same Logger instance. That is useful if we wish to combine logging from multiple different classes in a single place. Since maxBackupIndex is defined as 2, once the second log file reaches the maximum size, the first log file will be erased and thereafter, all the logging information will be rolled back to the first log file.

There may be a requirement to generate your log files on a daily basis to keep a clean record of your logging information. To write your logging information into files on a daily basis, you would have to use org.

There is only one important configurable parameter in addition to the ones mentioned above for FileAppender:. Following is a sample configuration file log4j. If you wish to have an XML configuration file, you can generate the same as mentioned in the initial section and add only additional parameters related to DailyRollingFileAppender.

The log4j API provides the org. JDBCAppender object, which can put logging information in a specified database. Before you start using JDBC based logging, you should create a table to maintain all the log information. JDBCAppender does not need a layout to be defined explicitly. The following Java class is a very simple example that initializes and then uses the Log4J logging library for Java applications. Follow the given steps:. Note: Here x is used to output the Nested diagnostic Context NDC associated with the thread that generated the logging event.

We use NDC to distinguish clients in server-side components handling multiple clients. Check Log4J Manual for more information on this. Previous Page. Next Page. Previous Page Print Page. Save Close. Dashboard Logout. Appender uses the Layout objects and the conversion pattern associated with them to format the logging information. Appender can have a threshold level associated with it independent of the logger level.

The Appender ignores any logging messages that have a level lower than the threshold level. The Filter objects can analyze logging information beyond level matching and decide whether logging requests should be handled by a particular Appender or ignored. Designates informational messages that highlight the progress of the application at coarse-grained level. This flag is by default set to true, which means the output stream to the file being flushed with each append operation. It is possible to use any character-encoding.

By default, it is the platform-specific encoding scheme. This is by default set to true, which means the logging information being appended to the end of the same file.

This is the critical size of the file above which the file will be rolled. Default value is 10 MB. This indicates when to roll over the file and the naming convention to be followed. By default, roll over is performed at midnight each day. Sets the driver class to the specified string. If no driver class is specified, it defaults to sun. A custom log4j2 configuration can be created either programmatically or through a configuration file.

First, you can override the default configuration by simply creating a log4j2. In the log4j2 architecture, an appender is basically responsible for sending log messages to a certain output destination. Logging everything into a single file is, of course, not ideal. The triggering policy determines when the log file is rolled, meaning a new file is created, while the rollover strategy determines how the file is rolled.

You can see just how flexible this style of configuration is, and how you can tune the exact semantics of your logging strategy — down to the last detail. Along with the ConnectionSource , you can define the table and the columns to be used for storing the log data.

For example, you can configure a primary JDBCAppender , with a secondary the RollingFile and Console appenders in case a database connection cannot be established:. In a production environment, having a failover strategy for your logging mechanism is always a good idea. While the appenders are responsible for sending log messages to a destination, the layouts are used by appenders to define how a log message will be formatted.

This is quite a flexible solution that gives you a lot of control over the final output of the log message. The mechanism is primarily driven by a conversion pattern which contains conversion specifiers. You can read more about the full set of options for defining patterns in the log4j2 documentation on PatternLayout.

Logging data using the JSON format has some significant advantages, such as making the logs easier to be analyzed and processed by logging tools down the line. To be able to produce JSON, you also need to add the jackson-databind library to the classpath:.

Filters in log4j2 are used to determine if a log message should be processed or skipped.