Polish contribution

This commit:

- fixes Checkstyle violations
- improves Javadoc
- adds missing @since tags
- renames getCurrentQueueSize() to getQueueSize()
- avoids NullPointerExceptions in getQueueSize()
- introduces tests for queue size and queue capacity

Closes gh-28583

spring-context/src/main/java/org/springframework/scheduling/concurrent/ThreadPoolTaskExecutor.java

@@ -72,6 +72,8 @@ import org.springframework.util.concurrent.ListenableFutureTask;
  * {@link org.springframework.scheduling.concurrent.ConcurrentTaskExecutor} adapter.
  *
  * @author Juergen Hoeller
+ * @author Rémy Guihard
+ * @author Sam Brannen
  * @since 2.0
  * @see org.springframework.core.task.TaskExecutor
  * @see java.util.concurrent.ThreadPoolExecutor
@@ -155,7 +157,7 @@ public class ThreadPoolTaskExecutor extends ExecutorConfigurationSupport
 
 	/**
 	 * Set the ThreadPoolExecutor's keep-alive seconds.
-	 * Default is 60.
+	 * <p>Default is 60.
 	 * <p><b>This setting can be modified at runtime, for example through JMX.</b>
 	 */
 	public void setKeepAliveSeconds(int keepAliveSeconds) {
@@ -178,7 +180,7 @@ public class ThreadPoolTaskExecutor extends ExecutorConfigurationSupport
 
 	/**
 	 * Set the capacity for the ThreadPoolExecutor's BlockingQueue.
-	 * Default is {@code Integer.MAX_VALUE}.
+	 * <p>Default is {@code Integer.MAX_VALUE}.
 	 * <p>Any positive value will lead to a LinkedBlockingQueue instance;
 	 * any other value will lead to a SynchronousQueue instance.
 	 * @see java.util.concurrent.LinkedBlockingQueue
@@ -188,6 +190,15 @@ public class ThreadPoolTaskExecutor extends ExecutorConfigurationSupport
 		this.queueCapacity = queueCapacity;
 	}
 
+	/**
+	 * Return the capacity for the ThreadPoolExecutor's BlockingQueue.
+	 * @since 5.3.21
+	 * @see #setQueueCapacity(int)
+	 */
+	public int getQueueCapacity() {
+		return this.queueCapacity;
+	}
+
 	/**
 	 * Specify whether to allow core threads to time out. This enables dynamic
 	 * growing and shrinking even in combination with a non-zero queue (since
@@ -315,19 +326,18 @@ public class ThreadPoolTaskExecutor extends ExecutorConfigurationSupport
 		}
 		return this.threadPoolExecutor.getPoolSize();
 	}
-	
-	/**
-	 * Return the current number of threads waiting in the queue
-	 */
-	public int getCurrentQueueSize() {
-		return this.getThreadPoolExecutor().getQueue().size();
-	}
 
 	/**
-	* Return the maximum capacity of the queue
-	*/
-	public int getQueueCapacity() {
-		return this.queueCapacity;
+	 * Return the current queue size.
+	 * @since 5.3.21
+	 * @see java.util.concurrent.ThreadPoolExecutor#getQueue()
+	 */
+	public int getQueueSize() {
+		if (this.threadPoolExecutor == null) {
+			// Not initialized yet: assume no queued tasks.
+			return 0;
+		}
+		return this.threadPoolExecutor.getQueue().size();
 	}
 
 	/**