JDK 16 RFR of JDK-8250240: Address use of default constructors in the java.util.concurrent

[Martin Buchholz]

So these are all abstract classes where the constructor can only be
called via super() ?
In which case one would expect the constructors to be protected, not public.
But I'm probably missing some reason why "protected" would not be 100%
compatible.

Historically, we've preferred to put changes in via CVS, but in 2020,
we might prefer you make the change directly in openjdk.
Doug?

On Thu, Jul 23, 2020 at 3:04 PM Joe Darcy <joe.darcy at oracle.com> wrote:
>
> Hello,
>
> Martin, how would you prefer these changes, or equivalent ones, to get
> into the java.util.concurrent upstream sources?
>
> Several classes in java.util.concurrent rely on default constructors,
> which is not recommended.  Please review the patch below which removes
> them along with the corresponding CSR
> (https://bugs.openjdk.java.net/browse/JDK-8250241); webrev at
> http://cr.openjdk.java.net/~darcy/8250240.0/.
>
> Thanks,
>
> -Joe
>
> diff -r d62da6fc4074
> src/java.base/share/classes/java/util/concurrent/AbstractExecutorService.java
> ---
> a/src/java.base/share/classes/java/util/concurrent/AbstractExecutorService.java
> Thu Jul 23 20:25:41 2020 +0100
> +++
> b/src/java.base/share/classes/java/util/concurrent/AbstractExecutorService.java
> Thu Jul 23 15:01:27 2020 -0700
> @@ -77,6 +77,11 @@
>   public abstract class AbstractExecutorService implements ExecutorService {
>
>       /**
> +     * Constructor for subclasses to call.
> +     */
> +    public AbstractExecutorService() {}
> +
> +    /**
>        * Returns a {@code RunnableFuture} for the given runnable and default
>        * value.
>        *
> diff -r d62da6fc4074
> src/java.base/share/classes/java/util/concurrent/ForkJoinTask.java
> --- a/src/java.base/share/classes/java/util/concurrent/ForkJoinTask.java
> Thu Jul 23 20:25:41 2020 +0100
> +++ b/src/java.base/share/classes/java/util/concurrent/ForkJoinTask.java
> Thu Jul 23 15:01:27 2020 -0700
> @@ -242,6 +242,11 @@
>       private static final int SIGNAL   = 1 << 16; // true if joiner waiting
>       private static final int SMASK    = 0xffff;  // short bits for tags
>
> +    /**
> +     * Constructor for subclasses to call.
> +     */
> +    public ForkJoinTask() {}
> +
>       static boolean isExceptionalStatus(int s) {  // needed by subclasses
>           return (s & THROWN) != 0;
>       }
> diff -r d62da6fc4074
> src/java.base/share/classes/java/util/concurrent/RecursiveAction.java
> ---
> a/src/java.base/share/classes/java/util/concurrent/RecursiveAction.java
> Thu Jul 23 20:25:41 2020 +0100
> +++
> b/src/java.base/share/classes/java/util/concurrent/RecursiveAction.java
> Thu Jul 23 15:01:27 2020 -0700
> @@ -166,6 +166,11 @@
>       private static final long serialVersionUID = 5232453952276485070L;
>
>       /**
> +     * Constructor for subclasses to call.
> +     */
> +    public RecursiveAction() {}
> +
> +    /**
>        * The main computation performed by this task.
>        */
>       protected abstract void compute();
> diff -r d62da6fc4074
> src/java.base/share/classes/java/util/concurrent/RecursiveTask.java
> ---
> a/src/java.base/share/classes/java/util/concurrent/RecursiveTask.java
> Thu Jul 23 20:25:41 2020 +0100
> +++
> b/src/java.base/share/classes/java/util/concurrent/RecursiveTask.java
> Thu Jul 23 15:01:27 2020 -0700
> @@ -69,6 +69,11 @@
>       private static final long serialVersionUID = 5232453952276485270L;
>
>       /**
> +     * Constructor for subclasses to call.
> +     */
> +    public RecursiveTask() {}
> +
> +    /**
>        * The result of the computation.
>        */
>       @SuppressWarnings("serial") // Conditionally serializable
> diff -r d62da6fc4074
> src/java.base/share/classes/java/util/concurrent/locks/AbstractQueuedLongSynchronizer.java
> ---
> a/src/java.base/share/classes/java/util/concurrent/locks/AbstractQueuedLongSynchronizer.java
> Thu Jul 23 20:25:41 2020 +0100
> +++
> b/src/java.base/share/classes/java/util/concurrent/locks/AbstractQueuedLongSynchronizer.java
> Thu Jul 23 15:01:27 2020 -0700
> @@ -65,6 +65,11 @@
>
>       private static final long serialVersionUID = 7373984972572414692L;
>
> +    /**
> +     * Constructor for subclasses to call.
> +     */
> +    public AbstractQueuedLongSynchronizer() {}
> +
>       /*
>        * To keep sources in sync, the remainder of this source file is
>        * exactly cloned from AbstractQueuedSynchronizer, replacing class
>