Feat(SystemClock): Added PreciseClock implementation to the system cl…

…ock class, for the benefit of Java 8 users.

log4j-core/src/main/java/org/apache/logging/log4j/core/util/SystemClock.java

@@ -16,10 +16,15 @@
  */
 package org.apache.logging.log4j.core.util;
 
+import java.time.Instant;
+import org.apache.logging.log4j.core.time.MutableInstant;
+import org.apache.logging.log4j.core.time.PreciseClock;
+
 /**
  * Implementation of the {@code Clock} interface that returns the system time.
+ * @since 2.25
  */
-public final class SystemClock implements Clock {
+public final class SystemClock implements Clock, PreciseClock {
 
     /**
      * Returns the system time.
@@ -29,4 +34,13 @@ public final class SystemClock implements Clock {
     public long currentTimeMillis() {
         return System.currentTimeMillis();
     }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    public void init(final MutableInstant mutableInstant) {
+        final Instant instant = java.time.Clock.systemUTC().instant();
+        mutableInstant.initFromEpochSecond(instant.getEpochSecond(), instant.getNano());
+    }
 }

log4j-core/src/main/java/org/apache/logging/log4j/core/util/package-info.java

@@ -18,7 +18,7 @@
  * Log4j 2 helper classes.
  */
 @Export
-@Version("2.24.2")
+@Version("2.25.0")
 package org.apache.logging.log4j.core.util;
 
 import org.osgi.annotation.bundle.Export;

src/changelog/.2.x.x/3217_precise_clock.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<entry xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+       xmlns="https://logging.apache.org/xml/ns"
+       xsi:schemaLocation="https://logging.apache.org/xml/ns https://logging.apache.org/xml/ns/log4j-changelog-0.xsd"
+       type="changed">
+  <issue id="3217" link="https://github.com/apache/logging-log4j2/pull/3217"/>
+  <description format="asciidoc">Currently Java 8 users do not have access to timestamps with sub-millisecond precision.
+   This change moves the SystemClock implementation from log4j-core-java9 to log4j-core, overriding the current one, so that java 8 users can benefit from it.
+  </description>
+</entry>