add CowDemo

src/test/java/com/alibaba/demo/cow/CowDemo.kt

+package com.alibaba.demo.cow
+
+import com.alibaba.expandThreadPool
+import com.alibaba.ttl.TransmittableThreadLocal
+import com.alibaba.ttl.threadpool.TtlExecutors
+import java.util.concurrent.Executors
+import java.util.concurrent.TimeUnit
+
+fun main(args: Array<String>) {
+    val threadPool = Executors.newCachedThreadPool().let {
+        expandThreadPool(it)
+        TtlExecutors.getTtlExecutorService(it)
+    }!!
+
+    val traceContext = object : TransmittableThreadLocal<Trace>() {
+        override fun initialValue(): Trace = Trace("init", Span("first", 0))
+        override fun copy(parentValue: Trace): Trace = parentValue.copy() // shadow copy Trace, this is fast
+        override fun childValue(parentValue: Trace): Trace = parentValue.copy() // shadow copy Trace, this is fast
+
+        fun increaseSpan() {
+            get().run {
+                // COW the Span object in Trace
+                span = span.copy(id = "${span.id} + PONG", counter = span.counter + 1)
+            }
+        }
+
+        override fun toString(): String {
+            return "${get()}[${super.toString()}]"
+        }
+    }
+
+    fun printTtlInfo() {
+        println("${Thread.currentThread().name}: $traceContext")
+    }
+
+    printTtlInfo()
+    threadPool.execute {
+        printTtlInfo()
+        traceContext.increaseSpan()
+        printTtlInfo()
+
+        threadPool.execute {
+            printTtlInfo()
+            traceContext.increaseSpan()
+            printTtlInfo()
+        }
+    }
+
+    Thread.sleep(100)
+    threadPool.shutdown()
+    threadPool.awaitTermination(1, TimeUnit.SECONDS)
+}
+
+private data class Trace(var name: String, var span: Span)
+
+private data class Span(val id: String, val counter: Int)