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BTrace is a safe, dynamic tracing tool for Java. BTrace works by dynamically (bytecode) instrumenting classes of a running Java program. BTrace inserts tracing actions into the classes of a running Java program and hotswaps the traced program classes.

BTrace is a safe, dynamic tracing tool for Java. BTrace works by dynamically (bytecode) instrumenting classes of a running Java program. BTrace inserts tracing actions into the classes of a running Java program and hotswaps the traced program classes.

BTrace Terminology

Probe Point

"location" or "event" at which a set of tracing statements are executed. Probe point is "place" or "event" of interest where we want to execute some tracing statements. Trace Actions or Actions

Trace statements that are executed whenever a probe "fires".

Action Methods

BTrace trace statements that are executed when a probe fires are defined inside a static method a class. Such methods are called "action" methods.

BTrace Program Structure

A BTrace program is a plain Java class that has one or more

public static void

methods that are annotated with BTrace annotations. The annotations are used to specify traced program "locations" (also known as "probe points"). The tracing actions are specified inside the static method bodies. These static methods are referred as "action" methods.

BTrace Restrictions

To guarantee that the tracing actions are "read-only" (i.e., the trace actions don't change the state of the program traced) and bounded (i.e., trace actions terminate in bounded time), a BTrace program is allowed to do only a restricted set of actions. In particular, a BTrace class

  • can not create new objects.
  • can not create new arrays.
  • can not throw exceptions.
  • can not catch exceptions.
  • can not make arbitrary instance or static method calls - only the public static methods of com.sun.btrace.BTraceUtils class or methods declared in the same program may be called from a BTrace program.
  • (pre 1.2) can not have instance fields and methods. Only static public void returning methods are allowed for a BTrace class. And all fields have to be static.
  • can not assign to static or instance fields of target program's classes and objects. But, BTrace class can assign to it's own static fields ("trace state" can be mutated).
  • can not have outer, inner, nested or local classes.
  • can not have synchronized blocks or synchronized methods.
  • can not have loops (for, while, do..while)
  • can not extend arbitrary class (super class has to be java.lang.Object)
  • can not implement interfaces.
  • can not contains assert statements.
  • can not use class literals.

Reference: https://github.com/btraceio/btrace/wiki