JAVA 线程异步常见的实现方式有：
new ThreadExecutorService
当然还有其他的，比如fork-join，这些下文会有提及，下面主要针对这两种场景结合 DDTrace 和 Springboot 下进行实践。
引入 DDTrace sdk
<properties>
<java.version>1.8</java.version>
<dd.version>1.21.0</dd.version>
</properties>
<dependencies>
<dependency>
<groupId>com.datadoghq</groupId>
<artifactId>dd-trace-api</artifactId>
<version>${dd.version}</version>
</dependency>
<dependency>
<groupId>io.opentracing</groupId>
<artifactId>opentracing-api</artifactId>
<version>0.33.0</version>
</dependency>
<dependency>
<groupId>io.opentracing</groupId>
<artifactId>opentracing-mock</artifactId>
<version>0.33.0</version>
</dependency>
<dependency>
<groupId>io.opentracing</groupId>
<artifactId>opentracing-util</artifactId>
<version>0.33.0</version>
</dependency>
...
关于 DDTrace sdk 使用方式参考文档ddtrace-api使用指南
Logback 配置
配置 logback ，让其输出 traceId 和 spanId,将以下 pattern 应用到所有的 appender 中。
<property name="log.pattern" value="%d{yyyy-MM-dd HH:mm:ss.SSS} [%thread] %-5level %logger - [%method,%line] %X{dd.service} %X{dd.trace_id} %X{dd.span_id} - %msg%n" />
如果有链路信息产生，则会在日志里面输出 Trace 信息。
new Thread
实现一个简单的接口，使用 logback 输出日志信息，观察日志输出情况
@RequestMapping("/thread")
@ResponseBody
public String threadTest(){
logger.info("this func is threadTest.");
return "success";
}
请求后，日志如下
2023-10-23 11:33:09.983 [http-nio-8086-exec-1] INFO com.zy.observable.ddtrace.CalcFilter - [doFilter,28] springboot-server 7209831467195902001 958235974016818257 - START /thread
host localhost:8086
connection Keep-Alive
user-agent Apache-HttpClient/4.5.14 (Java/17.0.7)
accept-encoding br,deflate,gzip,x-gzip
2023-10-23 11:33:10.009 [http-nio-8086-exec-1] INFO com.zy.observable.ddtrace.controller.IndexController - [threadTest,277] springboot-server 7209831467195902001 2587871298938674772 - this func is threadTest.
2023-10-23 11:33:10.022 [http-nio-8086-exec-1] INFO com.zy.observable.ddtrace.CalcFilter - [doFilter,34] springboot-server 7209831467195902001 958235974016818257 - END : /thread耗时：39
日志里面有 trace 信息产生， 7209831467195902001为 traceId，2587871298938674772为 spanId。
向该接口加入 new Thread ，创建一个线程。
@RequestMapping("/thread")
@ResponseBody
public String threadTest(){
logger.info("this func is threadTest.");
new Thread(()->{
logger.info("this is new Thread.");
}).start();
return "success";
}
通过请求对应的 URL，观察日志输出情况。
2023-10-23 11:40:00.994 [http-nio-8086-exec-1] INFO com.zy.observable.ddtrace.controller.IndexController - [threadTest,277] springboot-server 319673369251953601 5380270359912403278 - this func is threadTest.
2023-10-23 11:40:00.995 [Thread-10] INFO com.zy.observable.ddtrace.controller.IndexController - [lambda$threadTest$1,279] springboot-server - this is new Thread.
通过日志输出发现，new Thread 方式，并不能够输出 Trace 信息，也就是说 Trace 并未传递进去。
如果我们显示的把 Trace 信息传递进去是不是就可以了，说干就干。
ThreadLocal 为什么不行
ThreadLocal 本地线程变量，该变量为当前线程独有。
为了方便使用，我们创建一个工具类 ThreadLocalUtil。
public static final ThreadLocal<Span> THREAD_LOCAL = new ThreadLocal<>();
然后将当前 Span 信息存储到 ThreadLocal。
@RequestMapping("/thread")
@ResponseBody
public String threadTest(){
logger.info("this func is threadTest.");
ThreadLocalUtil.setValue(GlobalTracer.get().activeSpan());
logger.info("current traceiD:{}",GlobalTracer.get().activeSpan().context().toTraceId());
new Thread(()->{
logger.info("this is new Thread.");
logger.info("new Thread get span:{}",ThreadLocalUtil.getValue());
}).start();
return "success";
}
通过请求对应的 URL，观察日志输出情况。
2023-10-23 14:14:02.339 [http-nio-8086-exec-1] INFO com.zy.observable.ddtrace.controller.IndexController - [threadTest,278] springboot-server 4492960774800816442 4097884453719637622 - this func is threadTest.
2023-10-23 14:14:02.340 [http-nio-8086-exec-1] INFO com.zy.observable.ddtrace.controller.IndexController - [threadTest,280] springboot-server 4492960774800816442 4097884453719637622 - current traceiD:4492960774800816442
2023-10-23 14:14:02.341 [Thread-9] INFO com.zy.observable.ddtrace.controller.IndexController - [lambda$threadTest$1,283] springboot-server - this is new Thread.
2023-10-23 14:14:02.342 [Thread-9] INFO com.zy.observable.ddtrace.controller.IndexController - [lambda$threadTest$1,284] springboot-server - new Thread get span:null
在新线程内获取外部线程 ThreadLocal，获取到的值为 null。
通过分析ThreadLocal源码发现，当我们使用 ThreadLocal 的 set() 方法时，ThreadLocal 内部使用了Thread.currentThread()作为了 ThreadLocal 的数据存储的 key，也就是说当从新线程里面获取变量信息，key 发生了变化，所以取不到值。
public class ThreadLocal<T> {
...
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
map.set(this, value);
} else {
createMap(t, value);
}
}
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}
...
}
InheritableThreadLocal
InheritableThreadLocal 扩展了 ThreadLocal，以提供从父线程到子线程的值继承：当创建子线程时，子线程接收父线程具有值的所有可继承线程局部变量的初始值。
官方解释：
This class extends ThreadLocal to provide inheritance of values from parent thread to child thread: when a child thread is created, the child receives initial values for all inheritable thread-local variables for which the parent has values. Normally the child's values will be identical to the parent's; however, the child's value can be made an arbitrary function of the parent's by overriding the childValue method in this class.
Inheritable thread-local variables are used in preference to ordinary thread-local variables when the per-thread-attribute being maintained in the variable (e.g., User ID, Transaction ID) must be automatically transmitted to any child threads that are created.
Note: During the creation of a new thread, it is possible to opt out of receiving initial values for inheritable thread-local variables.
为了方便使用，我们创建一个工具类 InheritableThreadLocalUtil.java，用于存放 Span 信息
public static final InheritableThreadLocal<Span> THREAD_LOCAL = new InheritableThreadLocal<>();
将 ThreadLocalUtil 换成 InheritableThreadLocalUtil
@RequestMapping("/thread")
@ResponseBody
public String threadTest(){
logger.info("this func is threadTest.");
InheritableThreadLocalUtil.setValue(GlobalTracer.get().activeSpan());
logger.info("current traceiD:{}",GlobalTracer.get().activeSpan().context().toTraceId());
new Thread(()->{
logger.info("this is new Thread.");
logger.info("new Thread get span:{}",InheritableThreadLocalUtil.getValue());
}).start();
return "success";
}
通过请求对应的 URL，观察日志输出情况。
2023-10-23 14:37:05.415 [http-nio-8086-exec-1] INFO com.zy.observable.ddtrace.controller.IndexController - [threadTest,278] springboot-server 8754268856419787293 5276611939997441402 - this func is threadTest.
2023-10-23 14:37:05.416 [http-nio-8086-exec-1] INFO com.zy.observable.ddtrace.controller.IndexController - [threadTest,280] springboot-server 8754268856419787293 5276611939997441402 - current traceiD:8754268856419787293
2023-10-23 14:37:05.416 [Thread-9] INFO com.zy.observable.ddtrace.controller.IndexController - [lambda$threadTest$1,283] springboot-server - this is new Thread.
2023-10-23 14:37:05.417 [Thread-9] INFO com.zy.observable.ddtrace.controller.IndexController - [lambda$threadTest$1,284] springboot-server - new Thread get span:datadog.trace.instrumentation.opentracing32.OTSpan@712ad7e2
通过观测以上日志信息，线程内部已经获取到了 span 对象地址，但日志 pattern 部分并没有 Trace 信息输出，原因在于，DDTrace 对 logback 的getMDCPropertyMap()和 getMdc()方法做了插桩处理,将 Trace 信息 put 到 MDC 中。
@Advice.OnMethodExit(suppress = Throwable.class)
public static void onExit(
@Advice.This ILoggingEvent event,
@Advice.Return(typing = Assigner.Typing.DYNAMIC, readOnly = false)
Map<String, String> mdc) {
if (mdc instanceof UnionMap) {
return;
}
AgentSpan.Context context =
InstrumentationContext.get(ILoggingEvent.class, AgentSpan.Context.class).get(event);
// Nothing to add so return early
if (context == null && !AgentTracer.traceConfig().isLogsInjectionEnabled()) {
return;
}
Map<String, String> correlationValues = new HashMap<>(8);
if (context != null) {
DDTraceId traceId = context.getTraceId();
String traceIdValue =
InstrumenterConfig.get().isLogs128bTraceIdEnabled() && traceId.toHighOrderLong() != 0
? traceId.toHexString()
: traceId.toString();
correlationValues.put(CorrelationIdentifier.getTraceIdKey(), traceIdValue);
correlationValues.put(
CorrelationIdentifier.getSpanIdKey(), DDSpanId.toString(context.getSpanId()));
}else{
AgentSpan span = activeSpan();
if (span!=null){
correlationValues.put(
CorrelationIdentifier.getTraceIdKey(), span.getTraceId().toString());
correlationValues.put(
CorrelationIdentifier.getSpanIdKey(), DDSpanId.toString(span.getSpanId()));
}
}
String serviceName = Config.get().getServiceName();
if (null != serviceName && !serviceName.isEmpty()) {
correlationValues.put(Tags.DD_SERVICE, serviceName);
}
String env = Config.get().getEnv();
if (null != env && !env.isEmpty()) {
correlationValues.put(Tags.DD_ENV, env);
}
String version = Config.get().getVersion();
if (null != version && !version.isEmpty()) {
correlationValues.put(Tags.DD_VERSION, version);
}
mdc = null != mdc ? new UnionMap<>(mdc, correlationValues) : correlationValues;
}
为了让新创建的线程的日志也能够获取父线程 Trace 信息，可以通过创建 span 来实现，该 span 需要作为父线程的子 span才能完成串联。
new Thread(()->{
logger.info("this is new Thread.");
logger.info("new Thread get span:{}",InheritableThreadLocalUtil.getValue());
Span span = null;
try {
Tracer tracer = GlobalTracer.get();
span = tracer.buildSpan("thread")
.asChildOf(InheritableThreadLocalUtil.getValue())
.start();
span.setTag("threadName", Thread.currentThread().getName());
GlobalTracer.get().activateSpan(span);
logger.info("thread:{}",span.context().toTraceId());
}finally {
if (span!=null) {
span.finish();
}
}
}).start();
通过请求对应的 URL，观察日志输出情况。
2023-10-23 14:51:28.969 [http-nio-8086-exec-1] INFO com.zy.observable.ddtrace.controller.IndexController - [threadTest,278] springboot-server 2303424716416355903 7690232490489894572 - this func is threadTest.
2023-10-23 14:51:28.969 [http-nio-8086-exec-1] INFO com.zy.observable.ddtrace.controller.IndexController - [threadTest,280] springboot-server 2303424716416355903 7690232490489894572 - current traceiD:2303424716416355903
2023-10-23 14:51:28.970 [Thread-9] INFO com.zy.observable.ddtrace.controller.IndexController - [lambda$threadTest$1,283] springboot-server - this is new Thread.
2023-10-23 14:51:28.971 [Thread-9] INFO com.zy.observable.ddtrace.controller.IndexController - [lambda$threadTest$1,284] springboot-server - new Thread get span:datadog.trace.instrumentation.opentracing32.OTSpan@c3a1aae
2023-10-23 14:51:28.971 [Thread-9] INFO com.zy.observable.ddtrace.controller.IndexController - [lambda$threadTest$1,292] springboot-server - thread:2303424716416355903
2023-10-23 14:51:28.971 [Thread-9] INFO com.zy.observable.ddtrace.controller.IndexController - [lambda$threadTest$1,294] springboot-server 2303424716416355903 5766505477412800739 - thread:2303424716416355903
为何线程内有两条日志的 pattern 没有输出 Trace 信息？原因在于当前线程内部的 span 是在日志输出之后创建的，只需要将日志放到 span 创建下面即可。
new Thread(()->{
Span span = null;
try {
Tracer tracer = GlobalTracer.get();
span = tracer.buildSpan("thread")
.asChildOf(InheritableThreadLocalUtil.getValue())
.start();
span.setTag("threadName", Thread.currentThread().getName());
GlobalTracer.get().activateSpan(span);
logger.info("this is new Thread.");
logger.info("new Thread get span:{}",InheritableThreadLocalUtil.getValue());
logger.info("thread:{}",span.context().toTraceId());
}finally {
if (span!=null) {
span.finish();
}
}
}).start();
通过请求对应的 URL，观察日志输出情况。
2023-10-23 15:01:00.490 [http-nio-8086-exec-1] INFO com.zy.observable.ddtrace.controller.IndexController - [threadTest,278] springboot-server 472828375731745486 6076606716618097397 - this func is threadTest.
2023-10-23 15:01:00.491 [http-nio-8086-exec-1] INFO com.zy.observable.ddtrace.controller.IndexController - [threadTest,280] springboot-server 472828375731745486 6076606716618097397 - current traceId:472828375731745486
2023-10-23 15:01:00.492 [Thread-9] INFO com.zy.observable.ddtrace.controller.IndexController - [lambda$threadTest$1,291] springboot-server 472828375731745486 9214366589561638347 - this is new Thread.
2023-10-23 15:01:00.492 [Thread-9] INFO com.zy.observable.ddtrace.controller.IndexController - [lambda$threadTest$1,292] springboot-server 472828375731745486 9214366589561638347 - new Thread get span:datadog.trace.instrumentation.opentracing32.OTSpan@12fd40f0
2023-10-23 15:01:00.493 [Thread-9] INFO com.zy.observable.ddtrace.controller.IndexController - [lambda$threadTest$1,293] springboot-server 472828375731745486 9214366589561638347 - thread:472828375731745486
ExecutorService
创建一个 API ，并通过Executors 创建 ExecutorService对象。
@RequestMapping("/execThread")
@ResponseBody
public String ExecutorServiceTest(){
ExecutorService executor = Executors.newCachedThreadPool();
logger.info("this func is ExecutorServiceTest.");
executor.submit(()->{
logger.info("this is executor Thread.");
});
return "ExecutorService";
}
通过请求对应的 URL，观察日志输出情况。
2023-10-23 15:24:41.828 [http-nio-8086-exec-1] INFO com.zy.observable.ddtrace.controller.IndexController - [ExecutorServiceTest,309] springboot-server 2170215511602500482 4370366221958823908 - this func is ExecutorServiceTest.
2023-10-23 15:24:41.832 [pool-2-thread-1] INFO com.zy.observable.ddtrace.controller.IndexController - [lambda$ExecutorServiceTest$2,311] springboot-server 2170215511602500482 4370366221958823908 - this is executor Thread.
ExecutorService 线程池方式会自动传递 Trace 信息，这种自动的能力源于 DDTrace 对相应组件埋点操作实现。
JAVA 对于很多线程组件框架都做了链路传递的支持，如：ForkJoinTask、ForkJoinPool、TimerTask、FutureTask、ThreadPoolExecutor等等。
源码
springboot-ddtrace-server
