8360287: JFR: PlatformTracer class should be loaded lazily

Reviewed-by: mgronlun

src/jdk.jfr/share/classes/jdk/jfr/internal/JDKEvents.java

@@ -45,6 +45,7 @@ import jdk.jfr.events.InitialSecurityPropertyEvent;
 import jdk.jfr.events.MethodTimingEvent;
 import jdk.jfr.events.MethodTraceEvent;
 import jdk.jfr.internal.periodic.PeriodicEvents;
+import jdk.jfr.internal.settings.MethodSetting;
 import jdk.jfr.internal.tracing.PlatformTracer;
 import jdk.jfr.tracing.MethodTracer;
 
@@ -235,7 +236,7 @@ public final class JDKEvents {
     }
 
     private static void emitMethodTiming() {
-        if (MethodTimingEvent.enabled()) {
+        if (MethodSetting.isInitialized() && MethodTimingEvent.enabled()) {
             PlatformTracer.emitTiming();
         }
     }

src/jdk.jfr/share/classes/jdk/jfr/internal/settings/MethodSetting.java

@@ -42,6 +42,7 @@ import jdk.jfr.internal.tracing.PlatformTracer;
 @Name(Type.SETTINGS_PREFIX + "Filter")
 public final class MethodSetting extends FilterSetting {
     private final Modification modification;
+    private volatile static boolean initialized;
 
     public MethodSetting(PlatformEventType eventType, Modification modification, String defaultValue) {
         super(eventType, defaultValue);
@@ -55,6 +56,20 @@ public final class MethodSetting extends FilterSetting {
 
     @Override
     protected void apply(PlatformEventType eventType, List<String> filters) {
+        ensureInitialized();
         PlatformTracer.setFilters(modification, filters);
     }
+
+    // Expected to be called when holding external lock, so no extra
+    // synchronization is required here.
+    private static void ensureInitialized() {
+        if (!initialized) {
+            PlatformTracer.initialize();
+            initialized = true;
+        }
+    }
+
+    public static boolean isInitialized() {
+        return initialized;
+    }
 }

src/jdk.jfr/share/classes/jdk/jfr/internal/tracing/PlatformTracer.java

@@ -53,8 +53,6 @@ public final class PlatformTracer {
     private static List<Filter> timingFilters = List.of();
     private static TimedMethod OBJECT;
 
-    private static boolean initialized;
-
     public static byte[] onMethodTrace(Module module, ClassLoader classLoader, String className,
                                        byte[] oldBytecode, long[] ids, String[] names, String[] signatures,
                                        int[] modifications) {
@@ -159,7 +157,6 @@ public final class PlatformTracer {
     }
 
     public static void setFilters(Modification modification, List<String> filters) {
-        ensureInitialized();
         publishClasses(applyFilter(modification, filters));
     }
 
@@ -252,14 +249,6 @@ public final class PlatformTracer {
         timedClasses.clear();
     }
 
-    // Expected to be called when holding external lock, so no extra
-    // synchronization is required here.
-    private static void ensureInitialized() {
-        if (!initialized) {
-            initialize();
-            initialized = true;
-        }
-    }
 
     // This method has three purposes:
     //
@@ -274,7 +263,7 @@ public final class PlatformTracer {
     // This method takes 1-10 milliseconds to run and is only executed once,
     // provided a user has specified a non-empty filter for the MethodTrace or
     // MethodTiming event.
-    private static void initialize() {
+    public static void initialize() {
         try {
             Logger.log(LogTag.JFR_METHODTRACE, LogLevel.DEBUG, "Method tracer initialization started.");
             Thread current = Thread.currentThread();

test/jdk/jdk/jfr/event/tracing/TestLazyPlatformTracer.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) 2025, 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.
+ */
+package jdk.jfr.event.tracing;
+
+import jdk.internal.misc.Unsafe;
+import jdk.jfr.FlightRecorder;
+import jdk.jfr.Recording;
+/**
+* @test
+* @summary Tests that PlatformTracer is not initialized if a method filter has not been set.
+* @requires vm.flagless
+* @requires vm.hasJFR
+* @modules java.base/jdk.internal.misc jdk.jfr/jdk.jfr.internal.tracing
+* @library /test/lib
+* @run main/othervm -XX:StartFlightRecording jdk.jfr.event.tracing.TestLazyPlatformTracer
+*/
+public class TestLazyPlatformTracer {
+
+    public static void main(String... args) throws Exception {
+        // Stop recording so end chunk events are emitted
+        FlightRecorder.getFlightRecorder().getRecordings().getFirst().stop();
+        if (!Unsafe.getUnsafe().shouldBeInitialized(jdk.jfr.internal.tracing.PlatformTracer.class)) {
+            throw new AssertionError("PlatformTracer should not have been initialized");
+        }
+    }
+}