ADDITIONAL SYSTEM INFORMATION :
System: Macbook Pro 16 (2019)
OS: mac 11.5.1
JDKs:
- OpenJDK Runtime Environment AdoptOpenJDK-11.0.11+9 (build 11.0.11+9)
- OpenJDK Runtime Environment Temurin-17.0.1+12 (build 17.0.1+12)

A DESCRIPTION OF THE PROBLEM :
The ProcessBuilder makes use of a cached ThreadPoolExecutor internally to monitor the status of the forked processes. Whenever a new thread named "process reaper" is created within this pool through the defined ThreadFactory, this new thread inherits the contextClassLoader of the calling thread (default thread creation behavior).

As a result, since the "process reaper" threads can stay as long as there are processes to execute (keep alive time is set to 1 mn in java.util.concurrent.Executors.newCachedThreadPool()), such a thread retains a reference to the contextClassLoader instance of the thread which triggered the new "process reaper" thread allocation in the first place.

Such a reference to this contextClassLoader implies that the entire object graph attached to it is retained in memory and cannot be garbage-collected, even if otherwise discarded: the initial situation which triggered this investigation was indeed a classloader leak happening when a webapp making use of ProcessBuilder is restarted within a Tomcat container.

In addition to this, since these threads are shared and reused across the JVM and cannot allocate non java.lang objects, it does not look relevant to use a specific contextClassLoader for the "process reaper" threads in any case.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Build the provided test class, and run it.
This class executes a process twice, each time with a different contextClassLoader, reports the outcome and the current contextClassLoader of all the "process reaper" threads.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The contextClassLoader of idle "process reaper" threads should be the bootstrap classloader (null):

Process #32998 launched using classloader java.net.URLClassLoader@7344699f - exit code 0
Thread process reaper #22 - contextClassLoader null
Process #32999 launched using classloader java.net.URLClassLoader@2a84aee7 - exit code 0
Thread process reaper #22 - contextClassLoader null
ACTUAL -
The contextClassLoader of idle "process reaper" threads is the contextClassLoader instance used during the first run instead:

Process #32998 launched using classloader java.net.URLClassLoader@7344699f - exit code 0
Thread process reaper #22 - contextClassLoader java.net.URLClassLoader@7344699f <<<<< HERE
Process #32999 launched using classloader java.net.URLClassLoader@2a84aee7 - exit code 0
Thread process reaper #22 - contextClassLoader java.net.URLClassLoader@7344699f <<<<< HERE

---------- BEGIN SOURCE ----------
import java.io.File;
import java.io.IOException;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.concurrent.CompletableFuture;

public class ProcessBuilderClassloadingIssue {
    
    public static void main(String... args) throws Exception {
        // Run #1
        execProcessUsingClassLoaderAndReport(new ProcessLauncher(), newClassLoader());
        reportProcessReaperThreads();

        // Run #2
        execProcessUsingClassLoaderAndReport(new ProcessLauncher(), newClassLoader());
        reportProcessReaperThreads();
    }

    public static ClassLoader newClassLoader() throws Exception {
        return new URLClassLoader(new URL[]{ new File(".").toURI().toURL() });
    }
    
    public static void execProcessUsingClassLoaderAndReport(
            ProcessLauncher processLauncher,
            ClassLoader classLoader) throws Exception {
        // Switch the current thread contextClassLoader
        Thread.currentThread().setContextClassLoader(classLoader);
        
        Process process = processLauncher.exec().get();
        System.out.println("Process #" + process.pid() + " launched using classloader "
                + classLoader + " - exit code " + process.exitValue());
    }
    
    public static void reportProcessReaperThreads() {
        for (Thread thread: Thread.getAllStackTraces().keySet()) {
            if ("process reaper".equals(thread.getName())) {
                System.out.println("Thread " + thread.getName() + " #" + thread.getId()
                        + " - contextClassLoader " + thread.getContextClassLoader() );
            }
        }
    }
    
    public static class ProcessLauncher {
        public static final String COMMAND = "/bin/date";
        
        public CompletableFuture<Process> exec() throws IOException {
            return new ProcessBuilder(COMMAND).start().onExit();
        }
    }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
None found.

FREQUENCY : always