[#3652] Improve performance of StringUtil.simpleClassName()

Motivation:

static Package getPackage(Class<?> c) uses synchronized block internally.
Thanks to @jingene for the hint and initial report of the issue.

Modifications:

-Use simple lastIndexOf(...) and substring for a faster implementation

Result:

No more lock condition.

common/src/main/java/io/netty/util/internal/StringUtil.java

@@ -42,6 +42,7 @@ public final class StringUtil {
      * 5 - Extra allowance for anticipated escape characters that may be added.
      */
     private static final int CSV_NUMBER_ESCAPE_CHARACTERS = 2 + 5;
+    private static final char PACKAGE_SEPARATOR_CHAR = '.';
 
     static {
         // Determine the newline character of the current platform.
@@ -314,16 +315,12 @@ public final class StringUtil {
      * with anonymous classes.
      */
     public static String simpleClassName(Class<?> clazz) {
-        if (clazz == null) {
-            return "null_class";
-        }
-
-        Package pkg = clazz.getPackage();
-        if (pkg != null) {
-            return clazz.getName().substring(pkg.getName().length() + 1);
-        } else {
-            return clazz.getName();
+        String className = ObjectUtil.checkNotNull(clazz, "clazz").getName();
+        final int lastDotIdx = className.lastIndexOf(PACKAGE_SEPARATOR_CHAR);
+        if (lastDotIdx > -1) {
+            return className.substring(lastDotIdx + 1);
         }
+        return className;
     }
 
     /**

common/src/test/java/io/netty/util/internal/StringUtilTest.java

@@ -277,4 +277,26 @@ public class StringUtilTest {
         }
     }
 
+    @Test
+    public void testSimpleClassName() throws Exception {
+        testSimpleClassName(String.class);
+    }
+
+    @Test
+    public void testSimpleInnerClassName() throws Exception {
+        testSimpleClassName(TestClass.class);
+    }
+
+    private static void testSimpleClassName(Class<?> clazz) throws Exception {
+        Package pkg = clazz.getPackage();
+        String name;
+        if (pkg != null) {
+            name = clazz.getName().substring(pkg.getName().length() + 1);
+        } else {
+            name = clazz.getName();
+        }
+        assertEquals(name, simpleClassName(clazz));
+    }
+
+    private static final class TestClass { }
 }