--- a/src/java.base/share/classes/java/lang/Process.java	Sat Mar 16 13:44:30 2019 -0700
+++ b/src/java.base/share/classes/java/lang/Process.java	Sat Mar 16 15:05:21 2019 -0700
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1995, 2017, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1995, 2019, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -206,20 +206,20 @@
     public boolean waitFor(long timeout, TimeUnit unit)
         throws InterruptedException
     {
-        long startTime = System.nanoTime();
-        long rem = unit.toNanos(timeout);
+        long remainingNanos = unit.toNanos(timeout); // throw NPE before other conditions
+        if (hasExited())
+            return true;
+        if (timeout <= 0)
+            return false;
 
+        long deadline = System.nanoTime() + remainingNanos;
         do {
-            try {
-                exitValue();
+            Thread.sleep(Math.min(TimeUnit.NANOSECONDS.toMillis(remainingNanos) + 1, 100));
+            if (hasExited())
                 return true;
-            } catch(IllegalThreadStateException ex) {
-                if (rem > 0)
-                    Thread.sleep(
-                        Math.min(TimeUnit.NANOSECONDS.toMillis(rem) + 1, 100));
-            }
-            rem = unit.toNanos(timeout) - (System.nanoTime() - startTime);
-        } while (rem > 0);
+            remainingNanos = deadline - System.nanoTime();
+        } while (remainingNanos > 0);
+
         return false;
     }
 
@@ -320,11 +320,20 @@
      * @since 1.8
      */
     public boolean isAlive() {
+        return !hasExited();
+    }
+
+    /**
+     * This is called from the default implementation of
+     * {@code waitFor(long, TimeUnit)}, which is specified to poll
+     * {@code exitValue()}.
+     */
+    private boolean hasExited() {
         try {
             exitValue();
+            return true;
+        } catch (IllegalThreadStateException e) {
             return false;
-        } catch(IllegalThreadStateException e) {
-            return true;
         }
     }
 

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/test/jdk/java/lang/Process/WaitFor.java	Sat Mar 16 15:05:21 2019 -0700
@@ -0,0 +1,66 @@
+/*
+ * Copyright (c) 2019, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug 8220684
+ * @summary Process.waitFor(long, TimeUnit) can return false for a process
+ *          that exited within the timeout
+ * @run main/othervm WaitFor
+ */
+
+import java.io.InputStream;
+import java.io.OutputStream;
+import java.util.concurrent.TimeUnit;
+
+public class WaitFor {
+    public static void main(String[] args) throws Throwable {
+        int failCnt = 0;
+        for (int i = 0; i < 30; ++i) {
+            Process proc = new MyProcess(new ProcessBuilder("true").start());
+            boolean exited = proc.waitFor(100, TimeUnit.MILLISECONDS);
+            if (!exited && !proc.isAlive()) failCnt++;
+        }
+        if (failCnt > 10) {
+            throw new RuntimeException(failCnt + " processes were still alive"
+                + " after timeout");
+        }
+    }
+}
+
+/**
+ * This class uses the default implementation of java.lang.Process#waitFor(long,
+ * TimeUnit), and delegates all other calls to the actual implementation of
+ * Process.
+ */
+class MyProcess extends Process {
+    Process impl;
+    public MyProcess(Process impl) { this.impl = impl; }
+    public OutputStream getOutputStream() { return impl.getOutputStream(); }
+    public InputStream getInputStream() { return impl.getInputStream(); }
+    public InputStream getErrorStream() { return impl.getErrorStream(); }
+    public int waitFor() throws InterruptedException { return impl.waitFor(); }
+    public int exitValue() { return impl.exitValue(); }
+    public void destroy() { impl.destroy(); }
+    public ProcessHandle toHandle() { return impl.toHandle(); }
+}