Close tracer on JVM shutdown

[jpkrohling]

Signed-off-by: Juraci Paixão Kröhling juraci@kroehling.de

Which problem is this PR solving?

• Resolves jaeger-client-java does not automatically flush when program exits #544

Short description of the changes

• Adds a shutdown hook to close the tracer before the JVM dies. I tested this with the same code as the reporter. When running the code without an explicit tracer.close(), I see this in the logs for the client:

12:35:51.066 [main] INFO io.jaegertracing.internal.reporters.LoggingReporter - Span reported: f9e218d52dc14279:f9e218d52dc14279:0:1 - say-hello

But nothing on the server side. Starting an all-in-one container with --log-level=debug confirms that nothing is received on that end.

With this PR applied, I do see the trace:

12:40:39.544 [main] INFO io.jaegertracing.internal.reporters.LoggingReporter - Span reported: 3ce5effda0536731:3ce5effda0536731:0:1 - say-hello

And on the server side:

{"level":"debug","ts":1536316839.6288192,"caller":"app/span_processor.go:104","msg":"Span written to the storage by the collector","trace-id":"3ce5effda0536731","span-id":"3ce5effda0536731"}

[codecov]

Codecov Report

Merging #546 into master will increase coverage by 0.12%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #546      +/-   ##
============================================
+ Coverage     88.64%   88.76%   +0.12%     
- Complexity      517      519       +2     
============================================
  Files            67       67              
  Lines          1893     1896       +3     
  Branches        243      243              
============================================
+ Hits           1678     1683       +5     
+ Misses          141      138       -3     
- Partials         74       75       +1

Impacted Files	Coverage Δ	Complexity Δ
...n/java/io/jaegertracing/internal/JaegerTracer.java	90.42% <100%> (+0.11%)	24 <0> (ø)	⬇️
...rtracing/internal/reporters/CompositeReporter.java	71.42% <0%> (ø)	6% <0%> (+1%)	⬆️
...java/io/jaegertracing/zipkin/ZipkinV2Reporter.java	100% <0%> (+28.57%)	3% <0%> (+1%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8e9492f...41a9d10. Read the comment docs.

[ackerleytng]

Thanks! Looking forward to the next release!

[yurishkuro]

what will this do to unit tests?