Implement DnsCacheTtl accessors (#309)

changelog/@unreleased/pr-309.v2.yml

@@ -0,0 +1,5 @@
+type: improvement
+improvement:
+  description: Implement DnsCacheTtl accessors
+  links:
+  - https://github.com/palantir/jvm-diagnostics/pull/309

jvm-diagnostics/build.gradle

@@ -10,5 +10,8 @@ dependencies {
 }
 
 moduleJvmArgs {
-    exports 'java.management/sun.management', 'java.base/jdk.internal.platform'
+    exports 'java.management/sun.management',
+            'java.base/jdk.internal.platform',
+            // Required for access to 'sun.net.InetAddressCachePolicy' for DNS cache TTL
+            'java.base/sun.net'
 }

jvm-diagnostics/src/main/java/com/palantir/jvm/diagnostics/DnsCacheTtlAccessor.java

@@ -0,0 +1,33 @@
+/*
+ * (c) Copyright 2022 Palantir Technologies Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.palantir.jvm.diagnostics;
+
+public interface DnsCacheTtlAccessor {
+
+    /** Returns the number of seconds successful DNS results are cached before making another DNS request. */
+    int getPositiveSeconds();
+
+    /** Returns the number of seconds unsuccessful DNS results are cached before making another DNS request. */
+    int getNegativeSeconds();
+
+    /**
+     * The amount of time a previously successful DNS result will be used beyond {@link #getPositiveSeconds()} if
+     * subsequent DNS requests for the same hostname fail.
+     * Introduced in Java 21 by <a href=https://bugs.openjdk.org/browse/JDK-8306653>JDK-8306653</a>.
+     */
+    int getStaleSeconds();
+}

jvm-diagnostics/src/main/java/com/palantir/jvm/diagnostics/JvmDiagnostics.java

@@ -16,8 +16,10 @@
 
 package com.palantir.jvm.diagnostics;
 
+import java.lang.reflect.Method;
 import java.util.Optional;
 import java.util.OptionalLong;
+import java.util.function.IntSupplier;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -122,6 +124,20 @@ public static Optional<CpuSharesAccessor> cpuShares() {
         }
     }
 
+    /**
+     * Returns a {@link HotspotDnsCacheTtlAccessor}. This functionality is not supported on all java runtimes,
+     * and an {@link Optional#empty()} is returned in cases TTL information is not accessible.
+     */
+    public static Optional<DnsCacheTtlAccessor> dnsCacheTtl() {
+        try {
+            HotspotDnsCacheTtlAccessor accessor = new HotspotDnsCacheTtlAccessor();
+            return accessor.isEnabled() ? Optional.of(accessor) : Optional.empty();
+        } catch (Throwable t) {
+            log.debug("Failed to create a HotspotDnsCacheTtlAccessor", t);
+            return Optional.empty();
+        }
+    }
+
     private JvmDiagnostics() {}
 
     private static final class HotspotSafepointTimeAccessor implements SafepointTimeAccessor {
@@ -209,4 +225,54 @@ public OptionalLong getCpuShares() {
             return OptionalLong.of(result);
         }
     }
+
+    private static final class HotspotDnsCacheTtlAccessor implements DnsCacheTtlAccessor {
+
+        boolean isEnabled() {
+            try {
+                // Ensure invocations succeed. If sufficient exports aren't present, this will throw.
+                getPositiveSeconds();
+                return true;
+            } catch (Throwable t) {
+                return false;
+            }
+        }
+
+        private final IntSupplier staleAccessor = createStaleAccessor();
+
+        private static IntSupplier createStaleAccessor() {
+            if (Runtime.version().feature() >= 21) {
+                // Introduced in Java 21 by https://bugs.openjdk.org/browse/JDK-8306653
+                try {
+                    Method getStale = sun.net.InetAddressCachePolicy.class.getMethod("getStale");
+                    return () -> {
+                        try {
+                            return (Integer) getStale.invoke(null);
+                        } catch (ReflectiveOperationException roe) {
+                            log.debug("Failed to load stale InetAddressCachePolicy", roe);
+                            return 0;
+                        }
+                    };
+                } catch (ReflectiveOperationException roe) {
+                    log.debug("Failed to load stale InetAddressCachePolicy", roe);
+                }
+            }
+            return () -> 0;
+        }
+
+        @Override
+        public int getPositiveSeconds() {
+            return sun.net.InetAddressCachePolicy.get();
+        }
+
+        @Override
+        public int getNegativeSeconds() {
+            return sun.net.InetAddressCachePolicy.getNegative();
+        }
+
+        @Override
+        public int getStaleSeconds() {
+            return staleAccessor.getAsInt();
+        }
+    }
 }

jvm-diagnostics/src/test/java/com/palantir/jvm/diagnostics/JvmDiagnosticsTest.java

@@ -51,4 +51,11 @@ void testThreadUserTime() {
                         .getUserTimeNanoseconds(Thread.currentThread().getId()))
                 .isGreaterThan(0L);
     }
+
+    @Test
+    void testDnsCacheTtl() {
+        assertThat(JvmDiagnostics.dnsCacheTtl().get().getPositiveSeconds()).isEqualTo(30);
+        assertThat(JvmDiagnostics.dnsCacheTtl().get().getNegativeSeconds()).isEqualTo(10);
+        assertThat(JvmDiagnostics.dnsCacheTtl().get().getStaleSeconds()).isEqualTo(0);
+    }
 }