Create Span

Use Tracer.createEntrySpan() API to create entry span, and then use SpanRef to contain the reference of created span in agent kernel. The first parameter is operation name of span and the second parameter is the ContextCarrierRef instance which is the reference of contextcarrier in agent kernel. If the second parameter is not null, the process of creating entry span will do the extract operation which will be introduced in inject/extract scenario.
```
import org.apache.skywalking.apm.toolkit.trace.Tracer;
...

SpanRef spanRef = Tracer.createEnteySpan("${operationName}", null);
```

Use Tracer.createLocalSpan() API to create local span, the only parameter is the operation name of span.

import org.apache.skywalking.apm.toolkit.trace.Tracer;
...

SpanRef spanRef = Tracer.createLocalSpan("${operationName}");

Use Tracer.createExitSpan() API to create exit span
- two parameters case: the first parameter is the operation name of span, the second parameter is the remote peer which means the peer address of exit operation.
```
import org.apache.skywalking.apm.toolkit.trace.Tracer;
...

SpanRef spanRef = Tracer.createExitSpan("${operationName}", "${remotePeer}");
```
- three parameters case: the first parameter is the operation name of span, the second parameter is the ContextCarrierRef instance and the third parameter is the remote peer. This case will be introduced in inject/extract scenario.

Use Tracer.stopSpan() API to stop current span

import org.apache.skywalking.apm.toolkit.trace.Tracer;
...

Tracer.stopSpan();

Inject/Extract Context Carrier

The Inject/extract is to pass context information between different process. The ContextCarrierRef contains the reference of ContextCarrier and the CarrierItemRef contains the reference of CarrierItem. The CarrierItem instances compose a linked list.

Use Tracer.inject() to inject information of current context into carrier
Use Tracer.extract() to extract info from contextCarrier.
Use items() of ContextCarrierRef instance to get head CarrierItemRef instance.
Use hasNext() of CarrierItemRef instance to judge if the CarrierItemRef has next item.
Use next() of CarrierItemRef instance to get next item
Use getHeadKey of CarrierItemRef instance to get key of current item
Use getHeadValue of CarrierItemRef instance to get value of current item
Use setHeadValue of CarrierItemRef instance to set value of current item

/* 
	You can consider map as the message's header/metadata, such as Http, MQ and RPC. 
	Do the inject operation in one process and then pass the map in header/metadata.
*/
ContextCarrierRef contextCarrierRef = new ContextCarrierRef();
Tracer.inject(contextCarrierRef);
Map<String, String> map = new HashMap<>();
CarrierItemRef next = contextCarrierRef.items();
while (next.hasNext()) {
    next = next.next();
    map.put(next.getHeadKey(), next.getHeadValue());
}
...

// Receive the map representing a header/metadata and do the extract operation in another process. 
...

ContextCarrierRef contextCarrierRef = new ContextCarrierRef();
CarrierItemRef next = contextCarrierRef.items();
for (Map.Entry<String, String> entry : map.entrySet()) {
    if (next.hasNext()) {
        next = next.next();
        if (entry.getKey().equals(next.getHeadKey()))
            next.setHeadValue(entry.getValue());
    }
}
Tracer.extract(contextCarrierRef);

Also, you can do the inject/extract operation when creating exit/entry span.

ContextCarrierRef contextCarrierRef = new ContextCarrierRef();
SpanRef spanRef = Tracer.createExitSpan("operationName", contextCarrierRef, "remotePeer");
Map<String, String> map = new HashMap<>();
CarrierItemRef next = contextCarrierRef.items();
while (next.hasNext()) {
    next = next.next();
    map.put(next.getHeadKey(), next.getHeadValue());
}
...

...

ContextCarrierRef contextCarrierRef = new ContextCarrierRef();
CarrierItemRef next = contextCarrierRef.items();
for (Map.Entry<String, String> entry : map.entrySet()) {
    if (next.hasNext()) {
        next = next.next();
        if (entry.getKey().equals(next.getHeadKey()))
            next.setHeadValue(entry.getValue());
    }
}
SpanRef spanRef = Tracer.createEntrySpan("${operationName}", contextCarrierRef);

Capture/Continue Context Snapshot

Use Tracer.capture() to capture the segment info and store it in ContextSnapshotRef, and then use Tracer.continued() to load the snapshot as the ref segment info. The capture/continue is used for tracing context in the x-thread tracing.

import org.apache.skywalking.apm.toolkit.trace.Tracer;
...

ContextSnapshotRef contextSnapshotRef = Tracer.capture();
Thread thread = new Thread(() -> {
    SpanRef spanRef = Tracer.createLocalSpan("${operationName}");
    Tracer.continued(contextSnapshotRef);
      ...

});
thread.start();
thread.join();

Add Span’s Tag and Log

Use log of SpanRef instance to record log in span

import org.apache.skywalking.apm.toolkit.trace.SpanRef;
...

SpanRef spanRef = Tracer.createLocalSpan("${operationName}");

// Throwable parameter
spanRef.log(new RuntimeException("${exception_message}"));  

// Map parameter
Map<String, String> logMap = new HashMap<>();
logMap.put("event", "${event_type}");
logMap.put("message", "${message_value}");
spanRef.log(logMap);

Use tag of SpanRef instance to add tag to span, the parameters of tag are two String which are key and value respectively.

import org.apache.skywalking.apm.toolkit.trace.SpanRef;
...

SpanRef spanRef = Tracer.createLocalSpan(operationName);
spanRef.tag("${key}", "${value}");

Async Prepare/Finish

Use prepareForAsync of SpanRef instance to make the span still alive until asyncFinish called, and then in specific time use asyncFinish of this SpanRef instance to notify this span that it could be finished.

import org.apache.skywalking.apm.toolkit.trace.SpanRef;
...

SpanRef spanRef = Tracer.createLocalSpan("${operationName}");
spanRef.prepareForAsync();
// the span does not finish because of the prepareForAsync() operation
Tracer.stopSpan();
Thread thread = new Thread(() -> {
    ...

    spanRef.asyncFinish();
});
thread.start();
thread.join();

ActiveSpan

You can use the ActiveSpan to get the current span and do some operations.

Add custom tag in the context of traced method, ActiveSpan.tag("key", "val").
ActiveSpan.error() Mark the current span as error status.
ActiveSpan.error(String errorMsg) Mark the current span as error status with a message.
ActiveSpan.error(Throwable throwable) Mark the current span as error status with a Throwable.
ActiveSpan.debug(String debugMsg) Add a debug level log message in the current span.
ActiveSpan.info(String infoMsg) Add an info level log message in the current span.
ActiveSpan.setOperationName(String operationName) Customize an operation name.

ActiveSpan.tag("my_tag", "my_value");
ActiveSpan.error();
ActiveSpan.error("Test-Error-Reason");

ActiveSpan.error(new RuntimeException("Test-Error-Throwable"));
ActiveSpan.info("Test-Info-Msg");
ActiveSpan.debug("Test-debug-Msg");

ActiveSpan.setOperationName("${opetationName}");

Sample codes only