Allow contexts to be disposed more than once

Subsequent disposals are no-ops, rather than throwing an exception.

Author: ctrueden

src/main/java/org/scijava/Context.java

@@ -97,6 +97,12 @@ public class Context implements Disposable, AutoCloseable {
 	 */
 	private boolean strict;
 
+	/**
+	 * False if the context is currently active; true if the context
+	 * has already been disposed, or is in the process of being disposed.
+	 */
+	private boolean disposed;
+
 	/**
 	 * Creates a new SciJava application context with all available services.
 	 *
@@ -418,16 +424,8 @@ public boolean isInjectable(final Class<?> type) {
 
 	@Override
 	public void dispose() {
-		final EventService eventService = getService(EventService.class);
-		if (eventService != null) eventService.publish(new ContextDisposingEvent());
-
-		// NB: Dispose services in reverse order.
-		// This may or may not actually be necessary, but seems safer, since
-		// dependent services will be disposed *before* their dependencies.
-		final List<Service> services = serviceIndex.getAll();
-		for (int s = services.size() - 1; s >= 0; s--) {
-			services.get(s).dispose();
-		}
+		if (disposed) return;
+		doDispose(true);
 	}
 
 	// -- AutoCloseable methods --
@@ -580,6 +578,23 @@ private String createMissingServiceMessage(
 		return msg.toString();
 	}
 
+	private synchronized void doDispose(final boolean announce) {
+		if (disposed) return;
+		disposed = true;
+		if (announce) {
+			final EventService eventService = getService(EventService.class);
+			if (eventService != null) eventService.publish(new ContextDisposingEvent());
+		}
+
+		// NB: Dispose services in reverse order.
+		// This may or may not actually be necessary, but seems safer, since
+		// dependent services will be disposed *before* their dependencies.
+		final List<Service> services = serviceIndex.getAll();
+		for (int s = services.size() - 1; s >= 0; s--) {
+			services.get(s).dispose();
+		}
+	}
+
 	private static PluginIndex plugins(final boolean empty) {
 		return empty ? new PluginIndex(null) : null;
 	}

src/test/java/org/scijava/ContextDisposalTest.java

@@ -0,0 +1,51 @@
+/*
+ * #%L
+ * SciJava Common shared library for SciJava software.
+ * %%
+ * Copyright (C) 2009 - 2023 SciJava developers.
+ * %%
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ * 
+ * 1. Redistributions of source code must retain the above copyright notice,
+ *    this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ * 
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ * #L%
+ */
+
+package org.scijava;
+
+import org.junit.Test;
+
+/**
+ * Tests disposal of {@link Context}s.
+ * 
+ * @author Curtis Rueden
+ */
+public class ContextDisposalTest {
+
+	/**
+	 * Tests that a {@link Context} can be disposed more than once without
+	 * throwing an exception.
+	 */
+	@Test
+	public void testDoubleDisposal() {
+		final Context context = new Context();
+		context.dispose();
+		context.dispose();
+	}
+}