Use monotonic time for calculating the elapsed time

Resolves #110

Author: ttddyy

src/main/asciidoc/changelog-1.11.x.adoc

@@ -0,0 +1,11 @@
+[[changelog-1.11]]
+=== 1.11
+
+====  New Features
+
+====  Improvements
+
+* Use monotonic time to measure the elapsed time (https://github.com/jdbc-observations/datasource-proxy/issues/110[Issue-110]).
+
+
+====  Bug Fixes

src/main/java/net/ttddyy/dsproxy/proxy/SystemStopwatchFactory.java

@@ -1,9 +1,10 @@
 package net.ttddyy.dsproxy.proxy;
 
+import java.util.concurrent.TimeUnit;
+
 /**
- * Factory to create {@link SystemStopwatch} which uses {@code System.currentTimeMillis()}.
- *
- * The unit of time is milliseconds.
+ * Factory to create {@link SystemStopwatch} which uses monotonic time.
+ * <p> The unit of time is milliseconds.
  *
  * @author Tadaya Tsuyukubo
  * @since 1.5.1
@@ -16,17 +17,16 @@ public Stopwatch create() {
     }
 
     /**
-     * Uses {@code System.currentTimeMillis()} to calculate elapsed time.
-     *
-     * The unit of time is milliseconds
+     * Uses monotonic time to calculate elapsed time.
+     * <p>The unit of time is milliseconds.
      */
     public static class SystemStopwatch implements Stopwatch {
 
         private long startTime;
 
         @Override
         public Stopwatch start() {
-            this.startTime = System.currentTimeMillis();
+            this.startTime = System.nanoTime();
             return this;
         }
 
@@ -37,7 +37,7 @@ public Stopwatch start() {
          */
         @Override
         public long getElapsedTime() {
-            return System.currentTimeMillis() - this.startTime;
+            return TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - this.startTime);
         }
 
     }