[#391] Cope with Thread::stop being unavailable in JDK 20+ (#393)

* [#391] Cope with Thread::stop being unavailable in JDK 20+

In JDK 20+, the long deprecated Thread.stop() (since JDK 1.2) has been
removed and will throw an UnsupportedOperationException. This will be
handled gracefully when using option 'stopUnresponsiveDaemonThreads',
yielding a log warning "Thread.stop() is unavailable in this JRE
version, cannot force-stop any threads" once and not trying to stop any
further threads during the same execution.

Tests and documentation have been adjusted accordingly.

Closes #391.

* [#391] Add JDK 21 to CI build

Because #391 introduces special handling of the fact that in JDK 20+
there is no more Thread::stop, we should also run the tests on JDK 21.

.github/workflows/maven.yml

@@ -24,5 +24,6 @@ jobs:
     name: Verify
     uses: apache/maven-gh-actions-shared/.github/workflows/maven-verify.yml@v3
     with:
+      jdk-matrix: '[ "8", "17", "21" ]'
       ff-maven: "3.9.5"                              # Maven version for fail-fast-build
       maven-matrix: '[ "3.6.3", "3.8.8", "3.9.5" ]'  # Maven versions matrix for verify builds

src/main/java/org/codehaus/mojo/exec/ExecJavaMojo.java

@@ -50,6 +50,10 @@
 public class ExecJavaMojo
     extends AbstractExecMojo
 {
+    // Implementation note: Constants can be included in javadocs by {@value #MY_CONST}
+    private static final String THREAD_STOP_UNAVAILABLE =
+        "Thread.stop() is unavailable in this JRE version, cannot force-stop any threads";
+
     @Component
     private RepositorySystem repositorySystem;
 
@@ -171,6 +175,10 @@ public class ExecJavaMojo
      * this to <code>true</code> if you are invoking problematic code that you can't fix. An example is
      * {@link java.util.Timer} which doesn't respond to interruption. To have <code>Timer</code> fixed, vote for
      * <a href="http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6336543">this bug</a>.
+     * <p>
+     * <b>Note:</b> In JDK 20+, the long deprecated {@link Thread#stop()} (since JDK 1.2) has been removed and will
+     * throw an {@link UnsupportedOperationException}. This will be handled gracefully, yielding a log warning
+     * {@value #THREAD_STOP_UNAVAILABLE} once and not trying to stop any further threads during the same execution.
      * 
      * @since 1.1-beta-1
      */
@@ -522,6 +530,7 @@ private void terminateThreads( ThreadGroup threadGroup )
                 thread.interrupt();
             }
             // Now join with a timeout and call stop() (assuming flags are set right)
+            boolean threadStopIsAvailable = true;
             for ( Thread thread : threads )
             {
                 if ( !thread.isAlive() )
@@ -543,10 +552,18 @@ private void terminateThreads( ThreadGroup threadGroup )
                     continue;
                 }
                 uncooperativeThreads.add( thread ); // ensure we don't process again
-                if ( stopUnresponsiveDaemonThreads )
+                if ( stopUnresponsiveDaemonThreads && threadStopIsAvailable )
                 {
                     getLog().warn( "thread " + thread + " will be Thread.stop()'ed" );
-                    thread.stop();
+                    try
+                    {
+                        thread.stop();
+                    }
+                    catch ( UnsupportedOperationException unsupportedOperationException )
+                    {
+                        threadStopIsAvailable = false;
+                        getLog().warn( THREAD_STOP_UNAVAILABLE );
+                    }
                 }
                 else
                 {

src/test/java/org/codehaus/mojo/exec/ExecJavaMojoTest.java

@@ -51,6 +51,9 @@ public class ExecJavaMojoTest
 
     private static final File LOCAL_REPO = new File( "src/test/repository" );
 
+    private static final int JAVA_VERSION_MAJOR =
+        Integer.parseInt( System.getProperty( "java.version" ).replaceFirst( "[.].*", "" ) ); 
+
     /*
      * This one won't work yet public void xxtestSimpleRunPropertiesAndArguments() throws MojoExecutionException,
      * Exception { File pom = new File( getBasedir(), "src/test/projects/project2/pom.xml" ); String output = execute(
@@ -198,19 +201,36 @@ public void testWaitNonInterruptibleDaemonThreads()
     }
 
     /**
-     * See <a href="http://jira.codehaus.org/browse/MEXEC-15">MEXEC-15</a>. FIXME: this sometimes fail with
-     * unit.framework.ComparisonFailure: expected:&lt;...&gt; but was:&lt;...3(f)&gt;
+     * See <a href="http://jira.codehaus.org/browse/MEXEC-15">MEXEC-15</a>,
+     * <a href="https://github.com/mojohaus/exec-maven-plugin/issues/391">GitHub-391</a>.
+     * <p>
+     * FIXME: This sometimes fails with {@code unit.framework.ComparisonFailure: expected:<...>; but was:<...3(f)>}.
      * 
      * @throws Exception if any exception occurs
      */
     public void testUncooperativeThread()
         throws Exception
     {
+        // FIXME:
+        //   This will fail the test, because Assume is a JUnit 4 thing, but we are running in JUnit 3 mode, because
+        //   AbstractMojoTestCase extends PlexusTestCase extends TestCase. The latter is a JUnit 3 compatibility class.
+        //   If we would simply use JUnit 4 annotations, conditional ignores via Assume would just work correctly in
+        //   Surefire and IDEs. We could than have two dedicated test cases, one for each JDK 20+ and one for older
+        //   versions. In JUnit 5, we could even conveniently use @EnabledOnJre.
+        // Assume.assumeTrue( JAVA_VERSION_MAJOR < 20 );
+
         File pom = new File( getBasedir(), "src/test/projects/project10/pom.xml" );
         String output = execute( pom, "java" );
         // note: execute() will wait a little bit before returning the output,
         // thereby allowing the stop()'ed thread to output the final "(f)".
-        assertEquals( MainUncooperative.SUCCESS, output.trim() );
+        if ( JAVA_VERSION_MAJOR < 20 )
+        {
+            assertEquals( MainUncooperative.SUCCESS, output.trim() );
+        }
+        else
+        {
+            assertEquals( MainUncooperative.INTERRUPTED_BUT_NOT_STOPPED, output.trim() );
+        }
     }
 
     /**

src/test/java/org/codehaus/mojo/exec/MainUncooperative.java

@@ -9,6 +9,9 @@ public class MainUncooperative
 {
     public static final String SUCCESS = "1(interrupted)(f)2(f)";
 
+    // In JDK 20+, Thread::stop has been removed and just throws an UnsupportedOperationException
+    public static final String INTERRUPTED_BUT_NOT_STOPPED = "1(interrupted)(f)2";
+
     public static void main( String... args )
         throws InterruptedException
     {