JEP 102 Process Updates revised API draft

[Paul Sandoz]

Hi,

I am not sure everyone is aware of the sandbox repo that was setup, but it's rather handy to see the changes:

  http://hg.openjdk.java.net/jdk9/sandbox/jdk/rev/9a02f23ca186

On Feb 12, 2015, at 11:02 PM, Roger Riggs <Roger.Riggs at Oracle.com> wrote:

> Hi,
> 
> The Process and ProcessHandle API javadoc has been updated with the comments
> and suggestions including the loose coupling of the Process from the CompletableFutures
> that are linked to process termination.
> The improved implementation from Peter is incorporated and the method descriptions
> updated to reflect its behavior.
> 
> Updated javadoc:   http://cr.openjdk.java.net/~rriggs/ph-apidraft/
> 
> I ran into a issue with the generics on CompletableFuture<? extends...> completableFuture()
> in ProcessHandle and Process.
> The JDK compiles fine but the javac has a problem with the assignment in a test.
> I'll come back to it after a long weekend in a warmer climate.
> 

The use of wild cards in this context is generally discouraged because it propagates to the caller as you have found out. You cannot do what you want for the same reasons you cannot do this:

  List<? extends Number> len = Arrays.asList(1, 2, 3);

  // As this point we do not know the lower bound of elements in len 
  // Is it Integer or is it BigInteger?
  List<Number> ln = len;  // error
  ln.add(new BigInteger(...)); // Potential heap pollution

Which means you are stuck if you want to provide CF<Process> and CF<ProcessHandle> using the same overloaded method name.

You could do:

  abstract class ProcessHandle<T extends ProcessHandle<T>> {
      CompletableFuture<T> completableFuture() { ... }
  }

  class Process extends ProcessHandle<Process> {
      CompletableFuture<Process> completableFuture() { ... }
  }

But i am not sure it's worth it, especially as it exposes the sub-type of process handle.

Paul.

> Suggestions appreciated, Roger
> 
> 
> p.s.
> 
> ws/jdk9-sandbox/jdk/test/java/lang/ProcessHandle/CompletionTest.java:67: error: incompatible types: CompletableFuture<CAP#1> cannot be converted to CompletableFuture<Process>
>                CompletableFuture<Process> future = p.completableFuture();
> ^
>  where CAP#1 is a fresh type-variable:
>    CAP#1 extends ProcessHandle from capture of ? extends ProcessHandle