Capability to instantiate an object based on the String class name (#1074)

It is usefull when you want to set the MetricsTackerFactory from a
property.

src/main/java/com/zaxxer/hikari/util/PropertyElf.java

@@ -146,7 +146,14 @@ public final class PropertyElf
             writeMethod.invoke(target, propValue.toString());
          }
          else {
-            writeMethod.invoke(target, propValue);
+            try {
+               LOGGER.debug("Try to create a new instance of \"{}\"", propValue.toString());
+               writeMethod.invoke(target, Class.forName(propValue.toString()).newInstance());
+            }
+            catch (InstantiationException | ClassNotFoundException e) {
+               LOGGER.debug("Class \"{}\" not found or could not instantiate it (Default constructor)", propValue.toString());
+               writeMethod.invoke(target, propValue);
+            }
          }
       }
       catch (Exception e) {

src/test/java/com/zaxxer/hikari/mocks/TestObject.java

@@ -0,0 +1,27 @@
+package com.zaxxer.hikari.mocks;
+
+public class TestObject
+{
+   private TestObject testObject;
+   private String string;
+
+   public void setTestObject(TestObject testObject)
+   {
+      this.testObject = testObject;
+   }
+
+   public void setString(String string)
+   {
+      this.string = string;
+   }
+
+   public TestObject getTestObject()
+   {
+      return testObject;
+   }
+
+   public String getString()
+   {
+      return string;
+   }
+}

src/test/java/com/zaxxer/hikari/util/PropertyElfTest.java

@@ -0,0 +1,43 @@
+package com.zaxxer.hikari.util;
+
+import org.junit.Assert;
+import org.junit.Test;
+import com.zaxxer.hikari.mocks.TestObject;
+
+import java.util.Properties;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotSame;
+import static org.junit.Assert.fail;
+
+public class PropertyElfTest
+{
+   @Test
+   public void setTargetFromProperties() throws Exception
+   {
+      Properties properties = new Properties();
+      properties.setProperty("string", "aString");
+      properties.setProperty("testObject", "com.zaxxer.hikari.mocks.TestObject");
+      TestObject testObject = new TestObject();
+      PropertyElf.setTargetFromProperties(testObject, properties);
+      assertEquals("aString", testObject.getString());
+      assertEquals(com.zaxxer.hikari.mocks.TestObject.class, testObject.getTestObject().getClass());
+      assertNotSame(testObject, testObject.getTestObject());
+   }
+
+   @Test
+   public void setTargetFromPropertiesNotAClass() throws Exception
+   {
+      Properties properties = new Properties();
+      properties.setProperty("string", "aString");
+      properties.setProperty("testObject", "it is not a class");
+      TestObject testObject = new TestObject();
+      try {
+         PropertyElf.setTargetFromProperties(testObject, properties);
+         fail("Could never come here");
+      }
+      catch (RuntimeException e) {
+         assertEquals("argument type mismatch", e.getCause().getMessage());
+      }
+   }
+}