Tracing Java Applications with BTrace

BTrace is a dynamic tracing tool for Java. It allows you to insert tracing actions into the classes of a running Java program. This is especially useful, if you are using a third-party library. So, if you want to know whenever a certain method is called in your program, you can tell BTrace to print out a message whenever that method is hit. See the User Guide for more information.

Quick Start:

• Download BTrace from here
• Extract the release to a directory e.g. ~/btrace
• Write a BTrace program. There are many samples present in the samples directory. Here is one of mine: the program below prints a message whenever the target program calls the ArrayList.add() method.

import com.sun.btrace.annotations.*;
import static com.sun.btrace.BTraceUtils.*;
import java.util.*;
@BTrace
public class ArrayListTrace {

    // @OnMethod annotation tells where to probe.
    // In this example, we are interested in entry
    // into the ArrayList.add() method.
    @OnMethod(
        clazz="java.util.ArrayList",
        method="add";
    )
    // print out what was added
    public static void alert(@Self ArrayList self, Object o) {
        print("ADDED ");
        println(o);
    }
}

• Compile your script:

javac -cp "~/btrace/build/*" ArrayListTrace.java

• Start the java program you want to probe and get its pid.
• Run BTrace and watch the messages appear:

~/btrace/bin/btrace -cp ".:~/btrace/build/*" <PID> ArrayListTrace.class

Useful BTrace Programs:

Here are a few useful BTrace programs I have used in the past:

• OnThrow: Prints out the stack trace of an exception whenever a Throwable is created. This way you can capture all exceptions, even those that have been swallowed.
• LogTracer: Prints out log messages.
• AllMethods: Prints out the name of every method entered.