Add some task test

HMCLCore/src/main/java/org/jackhuang/hmcl/task/Task.java

@@ -76,6 +76,11 @@ public abstract class Task<T> {
     // last exception
     private Exception exception;
 
+    /**
+     * When task has been cancelled, task.exception will be null.
+     *
+     * @return the exception thrown during execution, possibly from dependents or dependencies.
+     */
     public final Exception getException() {
         return exception;
     }

HMCLCore/src/main/java/org/jackhuang/hmcl/task/TaskExecutor.java

@@ -80,8 +80,10 @@ public final class TaskExecutor {
             return future.get();
         } catch (InterruptedException e) {
             Thread.currentThread().interrupt();
-        } catch (ExecutionException | CancellationException e) {
+        } catch (ExecutionException ignore) {
-            e.printStackTrace();
+            // We have dealt with ExecutionException in exception handling and uncaught exception handler.
+        } catch (CancellationException e) {
+            Logging.LOG.log(Level.INFO, "Task " + firstTask + " has been cancelled.", e);
         }
         return false;
     }
@@ -202,8 +204,6 @@ public final class TaskExecutor {
                             }
                             task.onDone().fireEvent(new TaskEvent(this, task, true));
                             taskListeners.forEach(it -> it.onFailed(task, e));
-                        } else if (e instanceof CancellationException || e instanceof RejectedExecutionException) {
-                            task.setException(e);
                         } else {
                             task.setException(e);
                             exception = e;

HMCLCore/src/test/java/org/jackhuang/hmcl/util/TaskTest.java

@@ -3,10 +3,14 @@ package org.jackhuang.hmcl.util;
 import javafx.embed.swing.JFXPanel;
 import org.jackhuang.hmcl.task.Schedulers;
 import org.jackhuang.hmcl.task.Task;
+import org.jackhuang.hmcl.task.TaskExecutor;
 import org.jackhuang.hmcl.util.platform.JavaVersion;
 import org.junit.Assert;
+import org.junit.Assume;
 import org.junit.Test;
 
+import java.util.concurrent.CancellationException;
+import java.util.concurrent.RejectedExecutionException;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicReference;
 
@@ -68,4 +72,36 @@ public class TaskTest {
         Assert.assertTrue("Task does not succeed", result);
         Assert.assertTrue("ThenAccept has not been executed", flag.get());
     }
+
+    @Test
+    public void testCancellation() {
+        Task<?> task = Task.runAsync(() -> Thread.sleep(200));
+        TaskExecutor executor = task.executor();
+        Lang.thread(() -> {
+            try {
+                Thread.sleep(100);
+                executor.cancel();
+            } catch (InterruptedException e) {
+                Assume.assumeNoException(e);
+            }
+        });
+        Assert.assertFalse("Task should fail because we have cancelled it", executor.test());
+        Assert.assertNull("CancellationException should not be recorded.", executor.getException());
+        Assert.assertNull("CancellationException should not be recorded.", task.getException());
+    }
+
+    @Test
+    public void testRejectedExecutionException() {
+        Schedulers.defaultScheduler();
+        Schedulers.shutdown();
+
+        Task<?> task = Task.runAsync(() -> {
+            Thread.sleep(1000);
+        });
+
+        boolean result = task.test();
+
+        Assert.assertFalse("Task should fail since ExecutorService is shut down and RejectedExecutionException should be thrown", result);
+        Assert.assertTrue("RejectedExecutionException should be recorded", task.getException() instanceof RejectedExecutionException);
+    }
 }