Log4j where is the output

It then defines a static logger variable with the name MyApp which happens to be the fully qualified name of the class. The invocation of the BasicConfigurator. This method is hardwired to add to the root logger a ConsoleAppender.

The figure below depicts the object diagram of MyApp after just having called the BasicConfigurator. As a side note, let me mention that in log4j child loggers link only to their existing ancestors. In particular, the logger named com. Bar is linked directly to the root logger, thereby circumventing the unused com or com. This significantly increases performance and reduces log4j's memory footprint.

The MyApp class configures log4j by invoking BasicConfigurator. Other classes only need to import the org. Logger class, retrieve the loggers they wish to use, and log away. The previous example always outputs the same log information. Fortunately, it is easy to modify MyApp so that the log output can be controlled at run-time.

Here is a slightly modified version. This version of MyApp instructs PropertyConfigurator to parse a configuration file and set up logging accordingly. Here is a sample configuration file that results in identical output as the previous BasicConfigurator based example. Suppose we are no longer interested in seeing the output of any component belonging to the com. The following configuration file shows one possible way of achieving this.

As the logger com. Bar does not have an assigned level, it inherits its level from com. The log statement from the Bar. Consequently, doIt method's log request is suppressed. Calling the enhanced MyApp with the this configuration file will output the following on the console. In addition, as the root logger has been allocated a second appender, output will also be directed to the example. This file will be rolled over when it reaches KB. When roll-over occurs, the old version of example. Note that to obtain these different logging behaviors we did not need to recompile code.

The log4j library does not make any assumptions about its environment. In particular, there are no default log4j appenders. Under certain well-defined circumstances however, the static inializer of the Logger class will attempt to automatically configure log4j. The Java language guarantees that the static initializer of a class is called once and only once during the loading of a class into memory. It is important to remember that different classloaders may load distinct copies of the same class.

These copies of the same class are considered as totally unrelated by the JVM. The default initialization is very useful in environments where the exact entry point to the application depends on the runtime environment. For example, the same application can be used as a stand-alone application, as an applet, or as a servlet under the control of a web-server.

See Loader. String for the list of searched locations. You can optionaly specify a custom configurator. The value of the log4j. The custom configurator you specify must implement the Configurator interface. The default log4j initialization is particularly useful in web-server environments.

Under Tomcat 3. Log4j will find the properties file and initialize itself. This is easy to do and it works. You can also choose to set the system property log4j. For Tomcat 3. For Tomcat 4. Different web-applications will load the log4j classes through their respective classloaderss. Thus, each image of the log4j environment will act independetly and without any mutual synchronization.

For example, FileAppenders defined exactly the same way in multiple web-application configurations will all attempt to write the same file. The results are likely to be less than satisfactory. You must make sure that log4j configurations of different web-applications do not use the same underlying system resource. Writing an initialization servlet is the most flexible way for initializing log4j. There are no constraints on the code you can place in the init method of the servlet.

Most real-world systems have to deal with multiple clients simultaneously. In a typical multithreaded implementation of such a system, different threads will handle different clients. Logging is especially well suited to trace and debug complex distributed applications. A common approach to differentiate the logging output of one client from another is to instantiate a new separate logger for each client.

This promotes the proliferation of loggers and increases the management overhead of logging. A lighter technique is to uniquely stamp each log request initiated from the same client interaction. Martin, D. Riehle, and F. Buschmann Addison-Wesley, To uniquely stamp each request, the user pushes contextual information into the NDC, the abbreviation of Nested Diagnostic Context.

The NDC class is shown below. The NDC is managed per thread as a stack of contextual information. Note that all methods of the org. NDC class are static. Assuming that NDC printing is turned on, every time a log request is made, the appropriate log4j component will include the entire NDC stack for the current thread in the log output. This is done without the intervention of the user, who is responsible only for placing the correct information in the NDC by using the push and pop methods at a few well-defined points in the code.

In contrast, the per-client logger approach commands extensive changes in the code. To illustrate this point, let us take the example of a servlet delivering content to numerous clients. The servlet can build the NDC at the very beginning of the request before executing other code.

The contextual information can be the client's host name and other information inherent to the request, typically information contained in cookies. Hence, even if the servlet is serving multiple clients simultaneously, the logs initiated by the same code, i. Contrast this with the complexity of passing a freshly instantiated logger to all code exercised during the client's request.

Nevertheless, some sophisticated applications, such as virtual hosting web servers, must log differently depending on the virtual host context and also depending on the software component issuing the request. Recent log4j releases support multiple hierarchy trees. This enhancement allows each virtual host to possess its own copy of the logger hierarchy. Connect and share knowledge within a single location that is structured and easy to search. No Web servers running. Only the main of the class is run.

In the main I have this statement,. Nothing is shown in the Eclipse Console when the error is encountered. The error log is written, but somewhere else. Where is it written to? It depends on your logging configuration, check your log4j. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow. You can try log4j - Sample Program with the above configuration. You may want to write your log messages into multiple files for certain reasons, for example, if the file size reached to a certain threshold.

To write your logging information into multiple files, you would have to use org. RollingFileAppender class which extends the FileAppender class and inherits all its properties.

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 RollingFileAppender. This example configuration demonstrates that the maximum permissible size of each log file is 5 MB.

Upon exceeding the maximum size, a new log file will be created. You can use SMTPAppender to notify the exceptions in the logs to the required stakeholders through emails. But you do not need to send every exception thrown by the application through emails as it may stall the smooth operation of your application as the system may get busy in sending emails. There could be scenarios when you will see emails bursting for lots of recurring exceptions which may result into more errors at the application level which may impact its performance very badly.

This will not only provide the meaningful information but also there will be no impact on our application performance. You can use the error tracking product in order to send alerts about exceptions thrown by your application by using NTEventLogAppender.

Such a product can also dedupe the errors so you can figure out when an error is truly new, track its history, and track error rates. You can capture application logs and write them to a file on a disk which could be compressed and archived periodically. But when you want to search through the logs across multiple servers and applications then you need to send all of our logs to a central repository.

There are a lot of log management solutions that can help you with this, or you can even setup your own elastic search cluster for it. My suggestion is to use Splunk which provides fast, Google search engine like searching functionalities across bulky terabytes of logs. You can easily filter the logs by log level or date which makes it easy to correlate into transactions of multiple related log events, etc.

You can use filters which can be configured to suppress specific log messages. The following is the configuration details for log4j. If you want to do something that the standard Appenders do not support, you can either search online or write your own customized Appender.

For example, you can make your own custom log4j appender by extending the AppenderSkeleton class.