How to send a stacktrace to log4j?

Say you catch an exception and get the following on the standard output (like, say, the console) if you do a e.printStackTrace() :

java.io.FileNotFoundException: so.txt
        at java.io.FileInputStream.<init>(FileInputStream.java)
        at ExTest.readMyFile(ExTest.java:19)
        at ExTest.main(ExTest.java:7)

Now I want to send this instead to a logger like, say, log4j to get the following:

31947 [AWT-EventQueue-0] ERROR Java.io.FileNotFoundException: so.txt
32204 [AWT-EventQueue-0] ERROR    at java.io.FileInputStream.<init>(FileInputStream.java)
32235 [AWT-EventQueue-0] ERROR    at ExTest.readMyFile(ExTest.java:19)
32370 [AWT-EventQueue-0] ERROR    at ExTest.main(ExTest.java:7)

How can I do this?

try {
   ...
} catch (Exception e) {
    final String s;
    ...  // <-- What goes here?
    log.error( s );
}

You pass the exception directly to the logger, e.g.

try {
   ...
} catch (Exception e) {
    log.error( "failed!", e );
}

It's up to log4j to render the stack trace.

If you want to log a stacktrace without involving an exception just do this:

String message = "";

for(StackTraceElement stackTraceElement : Thread.currentThread().getStackTrace()) {                         
    message = message + System.lineSeparator() + stackTraceElement.toString();
}   
log.warn("Something weird happened. I will print the the complete stacktrace even if we have no exception just to help you find the cause" + message);

You can also get stack trace as string via ExceptionUtils.getStackTrace.

See: ExceptionUtils.java

I use it only for log.debug, to keep log.error simple.

The answer from skaffman is definitely the correct answer. All logger methods such as error(), warn(), info(), debug() take Throwable as a second parameter:

try {
...
 } catch (Exception e) {
logger.error("error: ", e);
}

However, you can extract stacktrace as a String as well. Sometimes it could be useful if you wish to take advantage of formatting feature using "{}" placeholder - see method void info(String var1, Object... var2); In this case say you have a stacktrace as String, then you can actually do something like this:

try {
...
 } catch (Exception e) {
String stacktrace = TextUtils.getStacktrace(e);
logger.error("error occurred for usename {} and group {}, details: {}",username, group, stacktrace);
}

This will print parametrized message and the stacktrace at the end the same way it does for method: logger.error("error: ", e);

I actually wrote an open source library that has a Utility for extraction of a stacktrace as a String with an option to smartly filter out some noise out of stacktrace. I.e. if you specify the package prefix that you are interested in your extracted stacktrace would be filtered out of some irrelevant parts and leave you with very consized info. Here is the link to the article that explains what utilities the library has and where to get it (both as maven artifacts and git sources) and how to use it as well. Open Source Java library with stack trace filtering, Silent String parsing Unicode converter and Version comparison See the paragraph "Stacktrace noise filter"

How to send a stacktrace to log4j?

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