This library is a Java platform API for OpenTracing.

In order to understand the Java platform API, one must first be familiar with the OpenTracing project and terminology more specifically.

Initialization is OpenTracing-implementation-specific. Generally speaking, the pattern is to initialize a Tracer once for the entire process and to use that Tracer for the remainder of the process lifetime. It is a best practice to set the GlobalTracer, even if also making use of cleaner, more modern dependency injection. (See the next section below for rationale)

Where possible, use some form of dependency injection (of which there are many) to access the Tracer instance. For vanilla application code, this is often reasonable and cleaner for all of the usual DI reasons.

That said, instrumentation for packages that are themselves statically configured (e.g., JDBC drivers) may be unable to make use of said DI mechanisms for Tracer access, and as such they should fall back on GlobalTracer. By and large, OpenTracing instrumentation should always allow the programmer to specify a Tracer instance to use for instrumentation, though the GlobalTracer is a reasonable fallback or default value.

For any thread, at most one Span may be "active". Of course there may be many other Spans involved with the thread which are (a) started, (b) not finished, and yet (c) not "active": perhaps they are waiting for I/O, blocked on a child Span, or otherwise off of the critical path.

It's inconvenient to pass an active Span from function to function manually, so OpenTracing requires that every Tracer contains a ScopeManager that grants access to the active Span along with a Scope to signal deactivation. Any Span may be transferred to another callback or thread, but not Scope; more on this below.

Access to the active span is straightforward:

io.opentracing.Tracer tracer = ...;
...
Span span = tracer.scopeManager().activeSpan();
if (span != null) {
    span.log("...");
}

The common case starts a Span and then sets it as the active instance via ScopeManager:

io.opentracing.Tracer tracer = ...;
...
Span span = tracer.buildSpan("someWork").start();
try (Scope scope = tracer.scopeManager().activate(span)) {
    // Do things.
} catch(Exception ex) {
    Tags.ERROR.set(span, true);
    span.log(Map.of(Fields.EVENT, "error", Fields.ERROR_OBJECT, ex, Fields.MESSAGE, ex.getMessage()));
} finally {
    span.finish();
}

If there is already an active Span, it will act as the parent to any newly started Span unless the programmer invokes ignoreActiveSpan() at buildSpan() time or specified parent context explicitly:

io.opentracing.Tracer tracer = ...;
...
Span span = tracer.buildSpan("someWork").ignoreActiveSpan().start();

Consider the case where a Span's lifetime logically starts in one thread and ends in another. For instance, the Span's own internal timing breakdown might look like this:

 [ ServiceHandlerSpan                                 ]
 |·FunctionA·|·····waiting on an RPC······|·FunctionB·|
            
---------------------------------------------------------> time

The "ServiceHandlerSpan" is active while it's running FunctionA and FunctionB, and inactive while it's waiting on an RPC (presumably modelled as its own Span, though that's not the concern here).

The ScopeManager API makes it possible to fetch the span in FunctionA and re-activate it in FunctionB. Note that every Tracer contains a ScopeManager. These are the steps:

1. Start a Span via start.
2. At the beginning of the closure/Runnable/Future/etc itself, invoke tracer.scopeManager().activate(span) to re-activate the Span and get a new Scope, then close() it when the Span is no longer active (or use try-with-resources for less typing).
3. Invoke span.finish() when the work is done.

Here is an example using CompletableFuture:

io.opentracing.Tracer tracer = ...;
...
// STEP 1 ABOVE: start the Span.
final Span span = tracer.buildSpan("ServiceHandlerSpan").start();
try (Scope scope = tracer.scopeManager().activate(span)) {
    // Do work.
    ...

    future = CompletableFuture.supplyAsync(() -> {

        // STEP 2 ABOVE: reactivate the Span in the callback.
        try (Scope scope = tracer.scopeManager().activate(span)) {
            ...
        }
    }).thenRun(() -> {
        // STEP 3 ABOVE: finish the Span when the work is done.
        span.finish();
    });
}

Observe that passing Scope to another thread or callback is not supported. Only Span can be used under this scenario.

In practice, all of this is most fluently accomplished through the use of an OpenTracing-aware ExecutorService and/or Runnable/Callable adapter; they factor out most of the typing.

ScopeManager.active(Span, boolean) and SpanBuilder.startActive() have been deprecated as part of removing automatic Span finish upon Scope close, as doing it through try-with statements would make it hard to properly handle errors (Span objects would get finished before a catch block would be reached). This improves API safety, and makes it more difficult to do the wrong thing and end up with unexpected errors.

Scope.span() and ScopeManager.scope() have been deprecated in order to prevent the anti-pattern of passing Scope objects between threads (Scope objects are not guaranteed to be thread-safe). Now Scope will be responsible for Span deactivation only, instead of being a Span container.

This project has a working design of interfaces for the OpenTracing API. There is a MockTracer to facilitate unit-testing of OpenTracing Java instrumentation.

Packages are deployed to Maven Central under the io.opentracing group.

This is a maven project, and provides a wrapper, ./mvnw to pin a consistent version. Run ./mvnw clean install to build, run tests, and create jars.

This wrapper was generated by mvn -N io.takari:maven:wrapper -Dmaven=3.5.0

Apache 2.0 License.

See Contributing for matters such as license headers.