Foreword
There have been a fair number of additions to BTrace in the last year or two which, I am afraid, might have slipped the user attention as they were not given a lot of publicity at the time they were introduced.The followup series of mini blog posts covering the potentially obscure but cool functionality available in BTrace - whimsically named 'BTrace Bits' - intends to fix it.
Ok. Let's start with something simple.
[1] Templated Variables
A template variable has a value defined at deployment time and can be referenced using ant-like format - eg. ${refresh}. The variable value will be resolved as a plain string and will be the subject of any conversions necessary for the corresponding place of use.
Example: Templated uptime probe
The following probe will run the uptime check each uptime_period_ms milliseconds.
@BTrace public class Uptime { @OnTimer(from = "${uptime_period_ms}") public static void f() { println("uptime: " + Sys.VM.vmUptime()); } }
The uptime check period value must be provided to BTrace agent - either as a javaagent argument or an argument to btrace dynamic attach launcher.
Attach on launch:
java -jar application.jar -javaagent:btrace-agent.jar=script=Uptime.class,uptime_period_ms=3000
Dynamic attach:
btrace <pid> Uptime.java uptime_period_ms=3000
Elements supporting templated variables
- @OnMethod
Templating similar to what was shown in the previous example is available for all @OnMethod attributes - clazz, method, type and location whereas again the templated variables can be used in its clazz, method and type attributes in turn. - @OnTimer
The timer period can be defined via templated variable and is to be provided in the annotation's from attribute. The usage is shown in the previous example.
