test: add CompletableFutureUseTest.kt

src/test/java/io/foldright/cffu/CompletableFutureUseTest.kt

@@ -0,0 +1,74 @@
+package io.foldright.cffu
+
+import io.kotest.core.spec.style.AnnotationSpec
+import io.kotest.matchers.shouldBe
+import io.kotest.matchers.shouldNotBe
+import io.kotest.matchers.string.shouldNotStartWith
+import io.kotest.matchers.string.shouldStartWith
+import java.util.concurrent.*
+import java.util.concurrent.atomic.AtomicLong
+import kotlin.random.Random
+import kotlin.random.nextULong
+
+class CompletableFutureUseTest : AnnotationSpec() {
+    /**
+     * - `Async` methods(e.g. `runAsync`/`thenRunAsync`, etc.) can set `executor`;
+     * if executor argument is absent use default executor of [CompletableFuture.ASYNC_POOL] (normally is a [ForkJoinPool]).
+     * - non-`Async` methods use the executor of previous [CompletableFuture].
+     */
+    @Test
+    fun executor_inheritance_behavior() {
+        val testThread = Thread.currentThread()
+        CompletableFuture
+            .runAsync {
+                Thread.currentThread() shouldNotBe testThread
+
+                Thread.currentThread().name shouldNotStartWith threadNamePrefix
+            }
+            .thenRunAsync({
+                Thread.currentThread().name shouldStartWith threadNamePrefix
+            }, executor) // !! switched executor !!
+            .thenRun {
+                // when run NOT async,
+                // executor is INHERITED after switch.
+                Thread.currentThread().name shouldStartWith threadNamePrefix
+            }
+            .thenRunAsync {
+                // when run ASYNC,
+                //
+                // - executor is NOT inherited after switch!!
+                // - use the DEFAULT EXECUTOR of CompletableFuture, if no executor specified.
+                Thread.currentThread().name shouldNotStartWith threadNamePrefix
+            }
+            .join()
+    }
+
+    ////////////////////////////////////////////////////////////////////////////////
+    // executor field
+    ////////////////////////////////////////////////////////////////////////////////
+
+    private lateinit var executor: ExecutorService
+
+    private val threadNamePrefix = "CompletableFutureUseTest_${Random.nextULong()}-"
+
+    @BeforeAll
+    fun beforeAll() {
+        val threadFactory = ThreadFactory { r ->
+            val counter = AtomicLong()
+            Thread(r).apply {
+                name = "$threadNamePrefix${counter.getAndIncrement()}"
+                isDaemon = true
+            }
+        }
+        executor = ThreadPoolExecutor(
+            4, 4, 1, TimeUnit.MINUTES,
+            ArrayBlockingQueue(10), threadFactory
+        )
+    }
+
+    @AfterAll
+    fun afterAll() {
+        executor.shutdown()
+        executor.awaitTermination(1, TimeUnit.SECONDS) shouldBe true
+    }
+}