Improve threading by making use of green threads with Java 21

This is an experiment. The code was checked to keep blocking
of platform threads to a minimum (See [1]). This will keep
overhead to of using threads small while only I/O will
allocate more threads.

The project now uses Multi-Release Jars, so it will detect
correct version during runtime while keeping backwards
compatibility. However, your IDE might be set manually to
Java 21. A multi-project [2] architecture might be necessary,
but not tested if it really fixes it.

[1] https://openjdk.org/jeps/444
[2] https://maven.apache.org/plugins/maven-compiler-plugin/multirelease.html

core/src/main/java/com/github/games647/fastlogin/core/AsyncScheduler.java

@@ -40,20 +40,16 @@ import java.util.concurrent.atomic.AtomicInteger;
  */
 public class AsyncScheduler {
 
-    private static final int MAX_CAPACITY = 1024;
-
-    //todo: single thread for delaying and scheduling tasks
     private final Logger logger;
 
-    // 30 threads are still too many - the optimal solution is to separate into processing and blocking threads
-    // where processing threads could only be max number of cores while blocking threads could be minimized using
-    // non-blocking I/O and a single event executor
     private final Executor processingPool;
 
     private final AtomicInteger currentlyRunning = new AtomicInteger();
 
     public AsyncScheduler(Logger logger, Executor processingPool) {
         this.logger = logger;
+
+        logger.info("Using legacy scheduler");
         this.processingPool = processingPool;
     }
 

core/src/main/java21/com/github/games647/fastlogin/core/AsyncScheduler.java

@@ -0,0 +1,75 @@
+/*
+ * SPDX-License-Identifier: MIT
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2015-2023 games647 and contributors
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package com.github.games647.fastlogin.core;
+
+import org.slf4j.Logger;
+
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.Executor;
+import java.util.concurrent.Executors;
+import java.util.concurrent.atomic.AtomicInteger;
+
+/**
+ * This limits the number of threads that are used at maximum. Thread creation can be very heavy for the CPU and
+ * context switching between threads too. However, we need many threads for blocking HTTP and database calls.
+ * Nevertheless, this number can be further limited, because the number of actually working database threads
+ * is limited by the size of our database pool. The goal is to separate concerns into processing and blocking only
+ * threads.
+ */
+public class AsyncScheduler {
+
+    private final Logger logger;
+
+    private final Executor asyncPool;
+
+    private final AtomicInteger currentlyRunning = new AtomicInteger();
+
+    public AsyncScheduler(Logger logger, Executor processingPool) {
+        this.logger = logger;
+
+        logger.info("Using optimized green threads with Java 21");
+        this.asyncPool = Executors.newVirtualThreadPerTaskExecutor();
+    }
+
+    public CompletableFuture<Void> runAsync(Runnable task) {
+        return CompletableFuture
+                .runAsync(() -> process(task), asyncPool)
+                .exceptionally(error -> {
+                    logger.warn("Error occurred on thread pool", error);
+                    return null;
+                });
+    }
+
+    private void process(Runnable task) {
+        currentlyRunning.incrementAndGet();
+        try {
+            task.run();
+        } finally {
+            currentlyRunning.getAndDecrement();
+        }
+    }
+}