Add support for dumping the heap on OpenJ9

Closes gh-26466

spring-boot-project/spring-boot-actuator-autoconfigure/src/docs/asciidoc/endpoints/heapdump.adoc

@@ -7,7 +7,10 @@ The `heapdump` endpoint provides a heap dump from the application's JVM.
 [[heapdump.retrieving]]
 == Retrieving the Heap Dump
 To retrieve the heap dump, make a `GET` request to `/actuator/heapdump`.
-The response is binary data in https://docs.oracle.com/javase/8/docs/technotes/samples/hprof.html[HPROF] format and can be large.
+The response is binary data and can be large.
+Its format depends upon the JVM on which the application is running.
+When running on a HotSpot JVM the format is https://docs.oracle.com/javase/8/docs/technotes/samples/hprof.html[HPROF]
+and on OpenJ9 it is https://www.eclipse.org/openj9/docs/dump_heapdump/#portable-heap-dump-phd-format[PHD].
 Typically, you should save the response to disk for subsequent analysis.
 When using curl, this can be achieved by using the `-O` option, as shown in the following example:
 

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/management/HeapDumpWebEndpoint.java

@@ -100,25 +100,37 @@ public class HeapDumpWebEndpoint {
 		if (this.heapDumper == null) {
 			this.heapDumper = createHeapDumper();
 		}
-		File file = createTempFile(live);
+		File file = createTempFile();
 		this.heapDumper.dumpHeap(file, live);
 		return new TemporaryFileSystemResource(file);
 	}
 
-	private File createTempFile(boolean live) throws IOException {
+	private File createTempFile() throws IOException {
 		String date = DateTimeFormatter.ofPattern("yyyy-MM-dd-HH-mm").format(LocalDateTime.now());
-		File file = File.createTempFile("heapdump" + date + (live ? "-live" : ""), ".hprof");
+		File file = File.createTempFile("heap-" + date, "." + determineDumpSuffix());
 		file.delete();
 		return file;
 	}
 
+	private String determineDumpSuffix() {
+		if (this.heapDumper instanceof OpenJ9DiagnosticsMXBeanHeapDumper) {
+			return "phd";
+		}
+		return "hprof";
+	}
+
 	/**
 	 * Factory method used to create the {@link HeapDumper}.
 	 * @return the heap dumper to use
 	 * @throws HeapDumperUnavailableException if the heap dumper cannot be created
 	 */
 	protected HeapDumper createHeapDumper() throws HeapDumperUnavailableException {
-		return new HotSpotDiagnosticMXBeanHeapDumper();
+		try {
+			return new HotSpotDiagnosticMXBeanHeapDumper();
+		}
+		catch (HeapDumperUnavailableException ex) {
+			return new OpenJ9DiagnosticsMXBeanHeapDumper();
+		}
 	}
 
 	/**
@@ -140,8 +152,8 @@ public class HeapDumpWebEndpoint {
 	}
 
 	/**
-	 * {@link HeapDumper} that uses {@code com.sun.management.HotSpotDiagnosticMXBean}
-	 * available on Oracle and OpenJDK to dump the heap to a file.
+	 * {@link HeapDumper} that uses {@code com.sun.management.HotSpotDiagnosticMXBean},
+	 * available on Oracle and OpenJDK, to dump the heap to a file.
 	 */
 	protected static class HotSpotDiagnosticMXBeanHeapDumper implements HeapDumper {
 
@@ -171,6 +183,38 @@ public class HeapDumpWebEndpoint {
 
 	}
 
+	/**
+	 * {@link HeapDumper} that uses
+	 * {@code openj9.lang.management.OpenJ9DiagnosticsMXBean}, available on OpenJ9, to
+	 * dump the heap to a file.
+	 */
+	private static final class OpenJ9DiagnosticsMXBeanHeapDumper implements HeapDumper {
+
+		private Object diagnosticMXBean;
+
+		private Method dumpHeapMethod;
+
+		@SuppressWarnings("unchecked")
+		private OpenJ9DiagnosticsMXBeanHeapDumper() {
+			try {
+				Class<?> mxBeanClass = ClassUtils.resolveClassName("openj9.lang.management.OpenJ9DiagnosticsMXBean",
+						null);
+				this.diagnosticMXBean = ManagementFactory.getPlatformMXBean((Class<PlatformManagedObject>) mxBeanClass);
+				this.dumpHeapMethod = ReflectionUtils.findMethod(mxBeanClass, "triggerDumpToFile", String.class,
+						String.class);
+			}
+			catch (Throwable ex) {
+				throw new HeapDumperUnavailableException("Unable to locate OpenJ9DiagnosticsMXBean", ex);
+			}
+		}
+
+		@Override
+		public void dumpHeap(File file, boolean live) throws IOException, InterruptedException {
+			ReflectionUtils.invokeMethod(this.dumpHeapMethod, this.diagnosticMXBean, "heap", file.getAbsolutePath());
+		}
+
+	}
+
 	/**
 	 * Exception to be thrown if the {@link HeapDumper} cannot be created.
 	 */

spring-boot-project/spring-boot-docs/src/docs/asciidoc/actuator/endpoints.adoc

@@ -97,8 +97,9 @@ If your application is a web application (Spring MVC, Spring WebFlux, or Jersey)
 | ID | Description
 
 | `heapdump`
-| Returns an `hprof` heap dump file.
-  Requires a HotSpot JVM.
+| Returns a heap dump file.
+  On a HotSpot JVM, an `HPROF`-format file is returned.
+  On an OpenJ9 JVM, a `PHD`-format file is returned.
 
 | `jolokia`
 | Exposes JMX beans over HTTP (when Jolokia is on the classpath, not available for WebFlux).