Added Unit Tests to increase code coverage (#2571)

apm-commons/apm-util/src/test/java/org/apache/skywalking/apm/util/StringUtilTest.java

@@ -39,5 +39,18 @@ public class StringUtilTest {
         Assert.assertNull(StringUtil.join('.'));
         Assert.assertEquals("Single part.", StringUtil.join('.', "Single part."));
         Assert.assertEquals("part1.part2.p3", StringUtil.join('.', "part1", "part2", "p3"));
+        Assert.assertEquals("E", StringUtil.join('E', new String[2]));
     }
-}
+
+    @Test
+    public void testSubstringMatchReturningTrue() {
+        StringBuffer stringBuffer = new StringBuffer("ZP~>xz1;");
+        Assert.assertTrue(StringUtil.substringMatch(stringBuffer, 0, stringBuffer));
+    }
+
+    @Test
+    public void testSubstringMatchWithPositive() {
+        Assert.assertFalse(StringUtil.substringMatch("", 4770, ""));
+    }
+
+}

oap-server/server-library/library-util/src/test/java/org/apache/skywalking/oap/server/library/util/PropertyPlaceholderHelperTest.java

@@ -80,6 +80,19 @@ public class PropertyPlaceholderHelperTest {
             yaml.load(placeholderHelper.replacePlaceholders(properties.getProperty("restPort"), properties)));
     }
 
+    @Test
+    public void testReplacePlaceholders() {
+        PropertyPlaceholderHelper propertyPlaceholderHelper = PropertyPlaceholderHelper.INSTANCE;
+        Properties properties = new Properties();
+        String resultString = propertyPlaceholderHelper.replacePlaceholders("&${[}7", properties);
+
+        Assert.assertEquals(0, properties.size());
+        Assert.assertTrue(properties.isEmpty());
+
+        Assert.assertNotNull(resultString);
+        Assert.assertEquals("&${[}7", resultString);
+    }
+
     @After
     public void afterTest() {
         //revert environment variables changes after the test for safe.