I'm newbie to logging event for java program using log4j. I would like to know what to log and what not to log inside my log file because writing to log file for everything we have done in program affects the performance. Also, sometimes we write the sensitive information to the log file, for example, database name. Thus I would like to have insight view about what we should not write and what we should write into our log file.
Avoid PCI and PII data in case your logs ever got into the wrong hands. Basically if you wouldn't want a hacker getting a piece of information, then don't log it.
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I've been able to find solutions to make the log file clear on each boot, but I don't want this to happen.
I only want to clear my log4j log file when we explicitly choose to, i.e. upon clicking a "clear log" button that calls an API to clear the log.
Is there a built-in log4j function to clear the log / how can I manually clear the log using Java code?
I use logback as well as log4j2 in my java web apps for logging. So far, I've setup log rotation (and purging) from within logback and log4j2 but now I intend to use logrotate at an infrastructure level since there are lots of services (in other languages as well) and it's relatively easier to maintain one common way of handling log files.
While doing a POC, I setup the java app to write logs to a file application.log and also setup logrotate with a size criteria (of 1 MB). As expected, when the file size reached 1 MB, the log file was rotated by way of moving it to another file named application.log.1. At this point, I expected the java app to continue writing new logs to application.log file. However, the logs kept getting written in the rotated file i.e. application.log.1.
This makes me wonder whether the component within logback/log4j2 that writes the log content in the file tracks the file by its name or something else like an inode number or a file handler. Since the original active log file was not deleted but just moved with a new name.
I'm aware of the copytruncate option in logrotate which creates a copy of the active log file and then truncates the active log file, but I don't want to use this as this can lead to loss of log events for agents running on the machines which pushes the logs to systems like Elasticsearch and CloudWatch. Since truncate can happen before the agents have processed all the log entries.
How can I get the logging component to always write logs to a file named application.log even after the original file underneath gets moved?
The logging framework opens the file for write and leaves the OutputStream open until the application ends or the file is rolled over or similar. On a Unix system you can move the file, rename it, or delete it but the application will keep writing to it. The application has no way of knowing the file was externally manipulated.
If you are using Log4j 2 you should use the RollingFileAppender and use the DeleteAction to delete old files.
I usually clear log files when I'm in developement mode and I need to have a fresh start to focus only on things I have to test.
If I clear a log file in linux (have not tested Windows), logback stops to write to that file
Maybe it's something about open handlers and file descriptors in linux.
How can I recover the situation without restarting the application?
Is it possibile to have an appender that can automatically recover this situation?
While your application is running (and Logback within your application has an open handle to the log file) ...
You won't be able to delete the file on Windows
You will be able to delete the file on Linux but as far as Logback is concerned the file still exists until Logback closes that open file handle. So, Logback won't know that the the file has been deleted but since the file has been deleted Logback cannot actually write anything to disk and this situation remains until Logback is re-initialised (and your FileAppender recreates the file). Typically, this will be done on application startup.
There's an open issue against Logback requesting a change in Logback's behaviour in this situation.
If you goal here is to have log output which focusses on recent-activity-only then you could define a rolling file appender with a minimal size and no history just to retain the (for example) last 1MB of data, this might help offer some focus on recent events only.
Alternatively, you'll have to:
Vote for this issue
Use grep/awk to find the relevant aspects of your log file (you can easily grep on timestamps) even if they are in a log file which contains the last few hours of log events
Ease the burden of restarting your application in this scenario by writing a simple script which does something like: (1) stop application; (2) delete log file; (3) start application
I am trying to find a centralized way to display errors to the user while logging them. Want to display the whole error message as it is written to the log without any customization. We are using log4j to log the errors and want to do minimal changes to the code.
Make a custom Appender as can be seen here. Add the Appender to your Logger and get notified about logging events and do with them whatever you want to.
I am trying to make an Eclipse plugin that enables a student with no knowledge of English to code. So I want to take the errors of what he coded [in his own language], translate them and put them in the Error Log for him to see and understand.
How can I write in the "Error Log"?
I'm unclear of the use case here. But based on partial understanding, here is what you can do. You can wrap each exception/error with your exception hierarchy with language/locale of your choice and then print out wrapped error/exception into it. Keep this log file in the project path, so it will be accessible from within. Parse it and display it as and when it gets updated. So instead of checking the error log panel, you can see your log file for the reference.