java runtime tracing library to replace system.out.println

Have you heard of any library which would allow me to set up tracing for specific methods at runtime?

Instead of adding (and removing) lots of System.out.println in my code (and having to re-compile and re-deploy) I would like to have a magic thing which would print out a line for each call of selected method without any change in the code. This would work without re-compiling, so some kind of JVM agent (or some non-standard JVM would be needed?). Sounds like a job for aspect programming?

A typical scenario would be to start an application, configure the traced methods dynamically (开发者_运维百科in a separate file or similar) and then everytime a selected method is called a line with its name (and arguments) is printed out to System.out (or some log file).

Naturally one could think of tens of additional features, but this basic set would be a great tool. BTW, I use Eclipse interactive debugger too, not only the System.out tracing technique, but both have some advantages and sometimes Eclipse is not enough.

Yes what you are referring to is known as Aspect oriented programming. A typical library providing this for Java is AspectJ. You define what are called pointcuts, essentially regular expressions for classes and method names, including wildcards, and the code to execute at each pointcut, known as an advice. This is useful for logging and also security checks and similar cross cutting concerns.

You can turn pointcut advices on and off through configuration. You can have an advice execute before a method call, after it returns or even after it throws an exception. Arguments are also available.

An aspectj java agent is needed for this to work.

In my experience, that kind of very detailed tracing (much more detailed than one would normally use for logging) as a debugging technique is indicative of insufficient unit testing and integration testing.

You can do this using a tool called InTrace.

NOTE: InTrace is a free and open source tool which I have written.

Log4J useful for disabling logging depending on "log-level" (DEBUG, INFO, WARN, FATAL).

You specify in configuration file what the least level you want to appear in logs, e.g., don't log anything below INFO level, and voila!

Looks like there's yet another solution - called Byteman. In their own words:

Byteman is a tool which simplifies tracing and testing of Java programs. Byteman allows you to insert extra Java code into your application, either as it is loaded during JVM startup or even after it has already started running. The injected code is allowed to access any of your data and call any application methods, including where they are private. You can inject code almost anywhere you want and there is no need to prepare the original source code in advance nor do you have to recompile, repackage or redeploy your application. In fact you can remove injected code and reinstall different code while the application continues to execute.

Jackplay is the tool you are looking for.

It allows you to enable logging on method entry and exit points without any coding or redeployment.

It also allows redefining a method body. It gives you web based UI as control panel to enable or undo tracing on your class.methods.