BAEL-3817 - Add unit tests showing the difference between the Spring ThreadPoolTaskExecutor's corePoolSize and maxPoolSize properties

spring-threads/.gitignore

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+RemoteSystemsTempFiles/
+.classpath
+.project
+.settings/
+bin/
+.metadata/
+docs/*.autosave
+.recommenders/
+build/
+.gradle/
+.DS_Store
+.idea/

spring-threads/README.md

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+## Spring Threads
+
+This module contains articles about threading using threads, specifically Spring's ThreadPoolTaskExecutor
+
+### Relevant Articles:
+- [Introduction to Thread Pools in Java](https://www.baeldung.com/thread-pool-java-and-guava)
+- [How To Do @Async in Spring](https://www.baeldung.com/spring-async)
+- [A Guide to the Spring Task Scheduler](https://www.baeldung.com/spring-task-scheduler)
+- [A Guide to the Java ExecutorService](https://www.baeldung.com/java-executor-service-tutorial)
+

spring-threads/pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://maven.apache.org/POM/4.0.0"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>spring-threads</artifactId>
+    <version>0.0.1-SNAPSHOT</version>
+    <name>spring-threads</name>
+    <packaging>jar</packaging>
+
+    <parent>
+        <groupId>com.baeldung</groupId>
+        <artifactId>parent-spring-5</artifactId>
+        <version>0.0.1-SNAPSHOT</version>
+        <relativePath>../parent-spring-5</relativePath>
+    </parent>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.springframework</groupId>
+            <artifactId>spring-context</artifactId>
+            <version>${spring.version}</version>
+        </dependency>
+    </dependencies>
+
+    <properties>
+        <assertj.version>3.12.2</assertj.version>
+    </properties>
+
+</project>

spring-threads/src/test/java/com/baeldung/threading/ThreadPoolTaskExecutorUnitTest.java

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+package com.baeldung.threading;
+
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ThreadLocalRandom;
+
+import org.junit.Assert;
+import org.junit.Test;
+import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor;
+
+public class ThreadPoolTaskExecutorUnitTest {
+
+    @Test
+    public void whenUsingDefaults_thenSingleThread() {
+        ThreadPoolTaskExecutor taskExecutor = new ThreadPoolTaskExecutor();
+        taskExecutor.afterPropertiesSet();
+
+        CountDownLatch countDownLatch = new CountDownLatch(10);
+        for (int i = 0; i < 10; i++) {
+            taskExecutor.execute(() -> {
+                try {
+                    Thread.sleep(10L * ThreadLocalRandom.current().nextLong(1, 10));
+                    countDownLatch.countDown();
+                } catch (InterruptedException e) {
+                    Thread.currentThread().interrupt();
+                }
+            });
+        }
+
+        while (countDownLatch.getCount() > 0) {
+            Assert.assertEquals(1, taskExecutor.getPoolSize());
+        }
+    }
+
+    @Test
+    public void whenCorePoolSizeFive_thenFiveThreads() {
+        ThreadPoolTaskExecutor taskExecutor = new ThreadPoolTaskExecutor();
+        taskExecutor.setCorePoolSize(5);
+        taskExecutor.afterPropertiesSet();
+
+        CountDownLatch countDownLatch = new CountDownLatch(10);
+        for (int i = 0; i < 10; i++) {
+            taskExecutor.execute(() -> {
+                try {
+                    Thread.sleep(100L * ThreadLocalRandom.current().nextLong(1, 10));
+                    countDownLatch.countDown();
+                } catch (InterruptedException e) {
+                    Thread.currentThread().interrupt();
+                }
+            });
+        }
+
+        while (countDownLatch.getCount() > 0) {
+            Assert.assertEquals(5, taskExecutor.getPoolSize());
+        }
+    }
+
+    @Test
+    public void whenCorePoolSizeFive_andMaxPoolSizeTen_thenFiveThreads() {
+        ThreadPoolTaskExecutor taskExecutor = new ThreadPoolTaskExecutor();
+        taskExecutor.setCorePoolSize(5);
+        taskExecutor.setMaxPoolSize(10);
+        taskExecutor.afterPropertiesSet();
+
+        CountDownLatch countDownLatch = new CountDownLatch(10);
+        for (int i = 0; i < 10; i++) {
+            taskExecutor.execute(() -> {
+                try {
+                    Thread.sleep(100L * ThreadLocalRandom.current().nextLong(1, 10));
+                    countDownLatch.countDown();
+                } catch (InterruptedException e) {
+                    Thread.currentThread().interrupt();
+                }
+            });
+        }
+
+        while (countDownLatch.getCount() > 0) {
+            Assert.assertEquals(5, taskExecutor.getPoolSize());
+        }
+    }
+
+    @Test
+    public void whenCorePoolSizeFive_andMaxPoolSizeTen_andQueueCapacityZero_thenTenThreads() {
+        ThreadPoolTaskExecutor taskExecutor = new ThreadPoolTaskExecutor();
+        taskExecutor.setCorePoolSize(5);
+        taskExecutor.setMaxPoolSize(10);
+        taskExecutor.setQueueCapacity(0);
+        taskExecutor.afterPropertiesSet();
+
+        CountDownLatch countDownLatch = new CountDownLatch(10);
+        for (int i = 0; i < 10; i++) {
+            taskExecutor.execute(() -> {
+                try {
+                    Thread.sleep(100L * ThreadLocalRandom.current().nextLong(1, 10));
+                    countDownLatch.countDown();
+                } catch (InterruptedException e) {
+                    Thread.currentThread().interrupt();
+                }
+            });
+        }
+
+        while (countDownLatch.getCount() > 0) {
+            Assert.assertEquals(10, taskExecutor.getPoolSize());
+        }
+    }
+
+    @Test
+    public void whenCorePoolSizeFive_andMaxPoolSizeTen_andQueueCapacityTen_thenTenThreads() {
+        ThreadPoolTaskExecutor taskExecutor = new ThreadPoolTaskExecutor();
+        taskExecutor.setCorePoolSize(5);
+        taskExecutor.setMaxPoolSize(10);
+        taskExecutor.setQueueCapacity(10);
+        taskExecutor.afterPropertiesSet();
+
+        CountDownLatch countDownLatch = new CountDownLatch(10);
+        for (int i = 0; i < 20; i++) {
+            taskExecutor.execute(() -> {
+                try {
+                    Thread.sleep(100L * ThreadLocalRandom.current().nextLong(1, 10));
+                    countDownLatch.countDown();
+                } catch (InterruptedException e) {
+                    Thread.currentThread().interrupt();
+                }
+            });
+        }
+
+        while (countDownLatch.getCount() > 0) {
+            Assert.assertEquals(10, taskExecutor.getPoolSize());
+        }
+    }
+}