diff --git a/src/main/java/org/codelibs/core/beans/factory/BeanDescFactory.java b/src/main/java/org/codelibs/core/beans/factory/BeanDescFactory.java
index e147cf3..e822a45 100644
--- a/src/main/java/org/codelibs/core/beans/factory/BeanDescFactory.java
+++ b/src/main/java/org/codelibs/core/beans/factory/BeanDescFactory.java
@@ -47,7 +47,7 @@ public abstract class BeanDescFactory {
     /**
      * Do not instantiate.
      */
-    private BeanDescFactory() {
+    protected BeanDescFactory() {
     }
 
     /** True if initialized */
diff --git a/src/main/java/org/codelibs/core/beans/factory/ParameterizedClassDescFactory.java b/src/main/java/org/codelibs/core/beans/factory/ParameterizedClassDescFactory.java
index 8396228..c8c4e03 100644
--- a/src/main/java/org/codelibs/core/beans/factory/ParameterizedClassDescFactory.java
+++ b/src/main/java/org/codelibs/core/beans/factory/ParameterizedClassDescFactory.java
@@ -58,7 +58,7 @@ public abstract class ParameterizedClassDescFactory {
     /**
      * Do not instantiate.
      */
-    private ParameterizedClassDescFactory() {
+    protected ParameterizedClassDescFactory() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/beans/util/BeanUtil.java b/src/main/java/org/codelibs/core/beans/util/BeanUtil.java
index 65e01ad..7839d8e 100644
--- a/src/main/java/org/codelibs/core/beans/util/BeanUtil.java
+++ b/src/main/java/org/codelibs/core/beans/util/BeanUtil.java
@@ -83,7 +83,7 @@ public abstract class BeanUtil {
     /**
      * Do not instantiate.
      */
-    private BeanUtil() {
+    protected BeanUtil() {
     }
 
     /** Default options */
diff --git a/src/main/java/org/codelibs/core/beans/util/CopyOptionsUtil.java b/src/main/java/org/codelibs/core/beans/util/CopyOptionsUtil.java
index a52241d..bce6f65 100644
--- a/src/main/java/org/codelibs/core/beans/util/CopyOptionsUtil.java
+++ b/src/main/java/org/codelibs/core/beans/util/CopyOptionsUtil.java
@@ -48,7 +48,7 @@ public abstract class CopyOptionsUtil {
     /**
      * Do not instantiate.
      */
-    private CopyOptionsUtil() {
+    protected CopyOptionsUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/collection/ArrayUtil.java b/src/main/java/org/codelibs/core/collection/ArrayUtil.java
index 5a65c60..b32028e 100644
--- a/src/main/java/org/codelibs/core/collection/ArrayUtil.java
+++ b/src/main/java/org/codelibs/core/collection/ArrayUtil.java
@@ -37,7 +37,7 @@ public abstract class ArrayUtil {
     /**
      * Do not instantiate.
      */
-    private ArrayUtil() {
+    protected ArrayUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/collection/CollectionsUtil.java b/src/main/java/org/codelibs/core/collection/CollectionsUtil.java
index f9ed720..ae8126e 100644
--- a/src/main/java/org/codelibs/core/collection/CollectionsUtil.java
+++ b/src/main/java/org/codelibs/core/collection/CollectionsUtil.java
@@ -63,7 +63,7 @@ public abstract class CollectionsUtil {
     /**
      * Do not instantiate.
      */
-    private CollectionsUtil() {
+    protected CollectionsUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/concurrent/CommonPoolUtil.java b/src/main/java/org/codelibs/core/concurrent/CommonPoolUtil.java
index d300e82..1aa704e 100644
--- a/src/main/java/org/codelibs/core/concurrent/CommonPoolUtil.java
+++ b/src/main/java/org/codelibs/core/concurrent/CommonPoolUtil.java
@@ -22,10 +22,10 @@
 /**
  * Utility for common pool operations.
  */
-public class CommonPoolUtil {
+public abstract class CommonPoolUtil {
     private static final Logger logger = Logger.getLogger(CommonPoolUtil.class);
 
-    private CommonPoolUtil() {
+    protected CommonPoolUtil() {
         // nothing
     }
 
diff --git a/src/main/java/org/codelibs/core/convert/BigDecimalConversionUtil.java b/src/main/java/org/codelibs/core/convert/BigDecimalConversionUtil.java
index bbab1d3..bde0620 100644
--- a/src/main/java/org/codelibs/core/convert/BigDecimalConversionUtil.java
+++ b/src/main/java/org/codelibs/core/convert/BigDecimalConversionUtil.java
@@ -30,7 +30,7 @@ public abstract class BigDecimalConversionUtil {
     /**
      * Do not instantiate.
      */
-    private BigDecimalConversionUtil() {
+    protected BigDecimalConversionUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/convert/BigIntegerConversionUtil.java b/src/main/java/org/codelibs/core/convert/BigIntegerConversionUtil.java
index d311abe..67055d4 100644
--- a/src/main/java/org/codelibs/core/convert/BigIntegerConversionUtil.java
+++ b/src/main/java/org/codelibs/core/convert/BigIntegerConversionUtil.java
@@ -27,7 +27,7 @@ public abstract class BigIntegerConversionUtil {
     /**
      * Do not instantiate.
      */
-    private BigIntegerConversionUtil() {
+    protected BigIntegerConversionUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/convert/BinaryConversionUtil.java b/src/main/java/org/codelibs/core/convert/BinaryConversionUtil.java
index 81d07bd..0d8a16e 100644
--- a/src/main/java/org/codelibs/core/convert/BinaryConversionUtil.java
+++ b/src/main/java/org/codelibs/core/convert/BinaryConversionUtil.java
@@ -27,7 +27,7 @@ public abstract class BinaryConversionUtil {
     /**
      * Do not instantiate.
      */
-    private BinaryConversionUtil() {
+    protected BinaryConversionUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/convert/BooleanConversionUtil.java b/src/main/java/org/codelibs/core/convert/BooleanConversionUtil.java
index 8065bc8..ca0d03a 100644
--- a/src/main/java/org/codelibs/core/convert/BooleanConversionUtil.java
+++ b/src/main/java/org/codelibs/core/convert/BooleanConversionUtil.java
@@ -25,7 +25,7 @@ public abstract class BooleanConversionUtil {
     /**
      * Do not instantiate.
      */
-    private BooleanConversionUtil() {
+    protected BooleanConversionUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/convert/ByteConversionUtil.java b/src/main/java/org/codelibs/core/convert/ByteConversionUtil.java
index 838992a..0459ff6 100644
--- a/src/main/java/org/codelibs/core/convert/ByteConversionUtil.java
+++ b/src/main/java/org/codelibs/core/convert/ByteConversionUtil.java
@@ -30,7 +30,7 @@ public abstract class ByteConversionUtil {
     /**
      * Do not instantiate.
      */
-    private ByteConversionUtil() {
+    protected ByteConversionUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/convert/CalendarConversionUtil.java b/src/main/java/org/codelibs/core/convert/CalendarConversionUtil.java
index e74bbeb..6f7f411 100644
--- a/src/main/java/org/codelibs/core/convert/CalendarConversionUtil.java
+++ b/src/main/java/org/codelibs/core/convert/CalendarConversionUtil.java
@@ -31,7 +31,7 @@ public abstract class CalendarConversionUtil {
     /**
      * Do not instantiate.
      */
-    private CalendarConversionUtil() {
+    protected CalendarConversionUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/convert/DateConversionUtil.java b/src/main/java/org/codelibs/core/convert/DateConversionUtil.java
index 1405757..eaf1eef 100644
--- a/src/main/java/org/codelibs/core/convert/DateConversionUtil.java
+++ b/src/main/java/org/codelibs/core/convert/DateConversionUtil.java
@@ -107,7 +107,7 @@ public abstract class DateConversionUtil {
     /**
      * Do not instantiate.
      */
-    private DateConversionUtil() {
+    protected DateConversionUtil() {
     }
 
     /** Array of styles held by {@link DateFormat}. */
diff --git a/src/main/java/org/codelibs/core/convert/DoubleConversionUtil.java b/src/main/java/org/codelibs/core/convert/DoubleConversionUtil.java
index 6ccc15d..c24aada 100644
--- a/src/main/java/org/codelibs/core/convert/DoubleConversionUtil.java
+++ b/src/main/java/org/codelibs/core/convert/DoubleConversionUtil.java
@@ -30,7 +30,7 @@ public abstract class DoubleConversionUtil {
     /**
      * Do not instantiate.
      */
-    private DoubleConversionUtil() {
+    protected DoubleConversionUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/convert/FloatConversionUtil.java b/src/main/java/org/codelibs/core/convert/FloatConversionUtil.java
index 02c246b..ec3e2f0 100644
--- a/src/main/java/org/codelibs/core/convert/FloatConversionUtil.java
+++ b/src/main/java/org/codelibs/core/convert/FloatConversionUtil.java
@@ -30,7 +30,7 @@ public abstract class FloatConversionUtil {
     /**
      * Do not instantiate.
      */
-    private FloatConversionUtil() {
+    protected FloatConversionUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/convert/IntegerConversionUtil.java b/src/main/java/org/codelibs/core/convert/IntegerConversionUtil.java
index 8c1ae63..a9c407b 100644
--- a/src/main/java/org/codelibs/core/convert/IntegerConversionUtil.java
+++ b/src/main/java/org/codelibs/core/convert/IntegerConversionUtil.java
@@ -30,7 +30,7 @@ public abstract class IntegerConversionUtil {
     /**
      * Do not instantiate.
      */
-    private IntegerConversionUtil() {
+    protected IntegerConversionUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/convert/LongConversionUtil.java b/src/main/java/org/codelibs/core/convert/LongConversionUtil.java
index 9e004f3..0d1359b 100644
--- a/src/main/java/org/codelibs/core/convert/LongConversionUtil.java
+++ b/src/main/java/org/codelibs/core/convert/LongConversionUtil.java
@@ -30,7 +30,7 @@ public abstract class LongConversionUtil {
     /**
      * Do not instantiate.
      */
-    private LongConversionUtil() {
+    protected LongConversionUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/convert/NumberConversionUtil.java b/src/main/java/org/codelibs/core/convert/NumberConversionUtil.java
index 2b3fe09..887d9d9 100644
--- a/src/main/java/org/codelibs/core/convert/NumberConversionUtil.java
+++ b/src/main/java/org/codelibs/core/convert/NumberConversionUtil.java
@@ -33,7 +33,7 @@ public abstract class NumberConversionUtil {
     /**
      * Do not instantiate.
      */
-    private NumberConversionUtil() {
+    protected NumberConversionUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/convert/ShortConversionUtil.java b/src/main/java/org/codelibs/core/convert/ShortConversionUtil.java
index 482dd3c..0e85cda 100644
--- a/src/main/java/org/codelibs/core/convert/ShortConversionUtil.java
+++ b/src/main/java/org/codelibs/core/convert/ShortConversionUtil.java
@@ -30,7 +30,7 @@ public abstract class ShortConversionUtil {
     /**
      * Do not instantiate.
      */
-    private ShortConversionUtil() {
+    protected ShortConversionUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/convert/StringConversionUtil.java b/src/main/java/org/codelibs/core/convert/StringConversionUtil.java
index 2fd46e0..e9a2ef6 100644
--- a/src/main/java/org/codelibs/core/convert/StringConversionUtil.java
+++ b/src/main/java/org/codelibs/core/convert/StringConversionUtil.java
@@ -32,7 +32,7 @@ public abstract class StringConversionUtil {
     /**
      * Do not instantiate.
      */
-    private StringConversionUtil() {
+    protected StringConversionUtil() {
     }
 
     /** WAVE DASH */
diff --git a/src/main/java/org/codelibs/core/convert/TimeConversionUtil.java b/src/main/java/org/codelibs/core/convert/TimeConversionUtil.java
index a2a8dd1..670fdc4 100644
--- a/src/main/java/org/codelibs/core/convert/TimeConversionUtil.java
+++ b/src/main/java/org/codelibs/core/convert/TimeConversionUtil.java
@@ -110,7 +110,7 @@ public abstract class TimeConversionUtil {
     /**
      * Do not instantiate.
      */
-    private TimeConversionUtil() {
+    protected TimeConversionUtil() {
     }
 
     /** Array of styles held by {@link DateFormat} */
diff --git a/src/main/java/org/codelibs/core/convert/TimestampConversionUtil.java b/src/main/java/org/codelibs/core/convert/TimestampConversionUtil.java
index 7e51e49..20e21f4 100644
--- a/src/main/java/org/codelibs/core/convert/TimestampConversionUtil.java
+++ b/src/main/java/org/codelibs/core/convert/TimestampConversionUtil.java
@@ -86,7 +86,7 @@ public abstract class TimestampConversionUtil {
     /**
      * Do not instantiate.
      */
-    private TimestampConversionUtil() {
+    protected TimestampConversionUtil() {
     }
 
     /** Array of styles held by {@link DateFormat} */
diff --git a/src/main/java/org/codelibs/core/io/ClassTraversalUtil.java b/src/main/java/org/codelibs/core/io/ClassTraversalUtil.java
index 621d4c9..b959ae7 100644
--- a/src/main/java/org/codelibs/core/io/ClassTraversalUtil.java
+++ b/src/main/java/org/codelibs/core/io/ClassTraversalUtil.java
@@ -39,7 +39,7 @@ public abstract class ClassTraversalUtil {
     /**
      * Do not instantiate.
      */
-    private ClassTraversalUtil() {
+    protected ClassTraversalUtil() {
     }
 
     /** The file extension for class files. */
diff --git a/src/main/java/org/codelibs/core/io/CloseableUtil.java b/src/main/java/org/codelibs/core/io/CloseableUtil.java
index e5a2c93..99bd052 100644
--- a/src/main/java/org/codelibs/core/io/CloseableUtil.java
+++ b/src/main/java/org/codelibs/core/io/CloseableUtil.java
@@ -32,7 +32,7 @@ public abstract class CloseableUtil {
     /**
      * Do not instantiate.
      */
-    private CloseableUtil() {
+    protected CloseableUtil() {
     }
 
     private static final Logger logger = Logger.getLogger(CloseableUtil.class);
diff --git a/src/main/java/org/codelibs/core/io/CopyUtil.java b/src/main/java/org/codelibs/core/io/CopyUtil.java
index 56971db..7362514 100644
--- a/src/main/java/org/codelibs/core/io/CopyUtil.java
+++ b/src/main/java/org/codelibs/core/io/CopyUtil.java
@@ -119,7 +119,7 @@ public abstract class CopyUtil {
     /**
      * Do not instantiate.
      */
-    private CopyUtil() {
+    protected CopyUtil() {
     }
 
     /** Buffer size used for copying. */
diff --git a/src/main/java/org/codelibs/core/io/FileUtil.java b/src/main/java/org/codelibs/core/io/FileUtil.java
index 32fe69a..83a3bac 100644
--- a/src/main/java/org/codelibs/core/io/FileUtil.java
+++ b/src/main/java/org/codelibs/core/io/FileUtil.java
@@ -46,7 +46,7 @@ public abstract class FileUtil {
     /**
      * Do not instantiate.
      */
-    private FileUtil() {
+    protected FileUtil() {
     }
 
     /** The encoding name for UTF-8. */
diff --git a/src/main/java/org/codelibs/core/io/InputStreamUtil.java b/src/main/java/org/codelibs/core/io/InputStreamUtil.java
index a8290a7..0a4f6ab 100644
--- a/src/main/java/org/codelibs/core/io/InputStreamUtil.java
+++ b/src/main/java/org/codelibs/core/io/InputStreamUtil.java
@@ -35,7 +35,7 @@ public abstract class InputStreamUtil {
     /**
      * Do not instantiate.
      */
-    private InputStreamUtil() {
+    protected InputStreamUtil() {
     }
 
     /** Default buffer size. */
diff --git a/src/main/java/org/codelibs/core/io/OutputStreamUtil.java b/src/main/java/org/codelibs/core/io/OutputStreamUtil.java
index 6b89e56..c6a190d 100644
--- a/src/main/java/org/codelibs/core/io/OutputStreamUtil.java
+++ b/src/main/java/org/codelibs/core/io/OutputStreamUtil.java
@@ -34,7 +34,7 @@ public abstract class OutputStreamUtil {
     /**
      * Do not instantiate.
      */
-    private OutputStreamUtil() {
+    protected OutputStreamUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/io/PropertiesUtil.java b/src/main/java/org/codelibs/core/io/PropertiesUtil.java
index 2460aca..564ab10 100644
--- a/src/main/java/org/codelibs/core/io/PropertiesUtil.java
+++ b/src/main/java/org/codelibs/core/io/PropertiesUtil.java
@@ -41,7 +41,7 @@ public abstract class PropertiesUtil {
     /**
      * Do not instantiate.
      */
-    private PropertiesUtil() {
+    protected PropertiesUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/io/ReaderUtil.java b/src/main/java/org/codelibs/core/io/ReaderUtil.java
index 7362e60..3c317bb 100644
--- a/src/main/java/org/codelibs/core/io/ReaderUtil.java
+++ b/src/main/java/org/codelibs/core/io/ReaderUtil.java
@@ -39,7 +39,7 @@ public abstract class ReaderUtil {
     /**
      * Do not instantiate.
      */
-    private ReaderUtil() {
+    protected ReaderUtil() {
     }
 
     /** Default buffer size */
diff --git a/src/main/java/org/codelibs/core/io/ResourceBundleUtil.java b/src/main/java/org/codelibs/core/io/ResourceBundleUtil.java
index 8ef5ece..7aff17f 100644
--- a/src/main/java/org/codelibs/core/io/ResourceBundleUtil.java
+++ b/src/main/java/org/codelibs/core/io/ResourceBundleUtil.java
@@ -37,7 +37,7 @@ public abstract class ResourceBundleUtil {
     /**
      * Do not instantiate.
      */
-    private ResourceBundleUtil() {
+    protected ResourceBundleUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/io/ResourceTraversalUtil.java b/src/main/java/org/codelibs/core/io/ResourceTraversalUtil.java
index 945b642..481fdf2 100644
--- a/src/main/java/org/codelibs/core/io/ResourceTraversalUtil.java
+++ b/src/main/java/org/codelibs/core/io/ResourceTraversalUtil.java
@@ -42,7 +42,7 @@ public abstract class ResourceTraversalUtil {
     /**
      * Do not instantiate.
      */
-    private ResourceTraversalUtil() {
+    protected ResourceTraversalUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/io/ResourceUtil.java b/src/main/java/org/codelibs/core/io/ResourceUtil.java
index e965a5e..daa2cd8 100644
--- a/src/main/java/org/codelibs/core/io/ResourceUtil.java
+++ b/src/main/java/org/codelibs/core/io/ResourceUtil.java
@@ -39,7 +39,7 @@ public abstract class ResourceUtil {
     /**
      * Do not instantiate.
      */
-    private ResourceUtil() {
+    protected ResourceUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/io/SerializeUtil.java b/src/main/java/org/codelibs/core/io/SerializeUtil.java
index 5be3ed4..b144813 100644
--- a/src/main/java/org/codelibs/core/io/SerializeUtil.java
+++ b/src/main/java/org/codelibs/core/io/SerializeUtil.java
@@ -37,7 +37,7 @@ public abstract class SerializeUtil {
     /**
      * Do not instantiate.
      */
-    private SerializeUtil() {
+    protected SerializeUtil() {
     }
 
     private static final int BYTE_ARRAY_SIZE = 8 * 1024;
diff --git a/src/main/java/org/codelibs/core/io/TraversalUtil.java b/src/main/java/org/codelibs/core/io/TraversalUtil.java
index 5bd5bf7..ded25fb 100644
--- a/src/main/java/org/codelibs/core/io/TraversalUtil.java
+++ b/src/main/java/org/codelibs/core/io/TraversalUtil.java
@@ -69,7 +69,7 @@ public abstract class TraversalUtil {
     /**
      * Do not instantiate.
      */
-    private TraversalUtil() {
+    protected TraversalUtil() {
     }
 
     /** An empty array of {@link Traverser}. */
diff --git a/src/main/java/org/codelibs/core/io/WriterUtil.java b/src/main/java/org/codelibs/core/io/WriterUtil.java
index 2131278..b79db30 100644
--- a/src/main/java/org/codelibs/core/io/WriterUtil.java
+++ b/src/main/java/org/codelibs/core/io/WriterUtil.java
@@ -38,7 +38,7 @@ public abstract class WriterUtil {
     /**
      * Do not instantiate.
      */
-    private WriterUtil() {
+    protected WriterUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/jar/JarFileUtil.java b/src/main/java/org/codelibs/core/jar/JarFileUtil.java
index b336d4d..7d20e5e 100644
--- a/src/main/java/org/codelibs/core/jar/JarFileUtil.java
+++ b/src/main/java/org/codelibs/core/jar/JarFileUtil.java
@@ -43,7 +43,7 @@ public abstract class JarFileUtil {
     /**
      * Do not instantiate.
      */
-    private JarFileUtil() {
+    protected JarFileUtil() {
     }
 
     private static final Logger logger = Logger.getLogger(JarFileUtil.class);
diff --git a/src/main/java/org/codelibs/core/jar/JarInputStreamUtil.java b/src/main/java/org/codelibs/core/jar/JarInputStreamUtil.java
index 8c8fa42..0a2755e 100644
--- a/src/main/java/org/codelibs/core/jar/JarInputStreamUtil.java
+++ b/src/main/java/org/codelibs/core/jar/JarInputStreamUtil.java
@@ -34,7 +34,7 @@ public abstract class JarInputStreamUtil {
     /**
      * Do not instantiate.
      */
-    private JarInputStreamUtil() {
+    protected JarInputStreamUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/lang/AnnotationUtil.java b/src/main/java/org/codelibs/core/lang/AnnotationUtil.java
index 992d8e7..dec4700 100644
--- a/src/main/java/org/codelibs/core/lang/AnnotationUtil.java
+++ b/src/main/java/org/codelibs/core/lang/AnnotationUtil.java
@@ -35,7 +35,7 @@ public abstract class AnnotationUtil {
     /**
      * Do not instantiate.
      */
-    private AnnotationUtil() {
+    protected AnnotationUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/lang/ClassLoaderUtil.java b/src/main/java/org/codelibs/core/lang/ClassLoaderUtil.java
index aefd835..0072363 100644
--- a/src/main/java/org/codelibs/core/lang/ClassLoaderUtil.java
+++ b/src/main/java/org/codelibs/core/lang/ClassLoaderUtil.java
@@ -40,7 +40,7 @@ public abstract class ClassLoaderUtil {
     /**
      * Do not instantiate.
      */
-    private ClassLoaderUtil() {
+    protected ClassLoaderUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/lang/ClassUtil.java b/src/main/java/org/codelibs/core/lang/ClassUtil.java
index 07eb67c..3402515 100644
--- a/src/main/java/org/codelibs/core/lang/ClassUtil.java
+++ b/src/main/java/org/codelibs/core/lang/ClassUtil.java
@@ -42,7 +42,7 @@ public abstract class ClassUtil {
     /**
      * Do not instantiate.
      */
-    private ClassUtil() {
+    protected ClassUtil() {
     }
 
     /** Map from wrapper types to primitive types */
diff --git a/src/main/java/org/codelibs/core/lang/ConstructorUtil.java b/src/main/java/org/codelibs/core/lang/ConstructorUtil.java
index 43e99d9..3935466 100644
--- a/src/main/java/org/codelibs/core/lang/ConstructorUtil.java
+++ b/src/main/java/org/codelibs/core/lang/ConstructorUtil.java
@@ -35,7 +35,7 @@ public abstract class ConstructorUtil {
     /**
      * Do not instantiate.
      */
-    private ConstructorUtil() {
+    protected ConstructorUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/lang/FieldUtil.java b/src/main/java/org/codelibs/core/lang/FieldUtil.java
index 01ee570..c7412bc 100644
--- a/src/main/java/org/codelibs/core/lang/FieldUtil.java
+++ b/src/main/java/org/codelibs/core/lang/FieldUtil.java
@@ -35,7 +35,7 @@ public abstract class FieldUtil {
     /**
      * Do not instantiate.
      */
-    private FieldUtil() {
+    protected FieldUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/lang/GenericsUtil.java b/src/main/java/org/codelibs/core/lang/GenericsUtil.java
index 8c101ce..8da6e1e 100644
--- a/src/main/java/org/codelibs/core/lang/GenericsUtil.java
+++ b/src/main/java/org/codelibs/core/lang/GenericsUtil.java
@@ -42,7 +42,7 @@ public abstract class GenericsUtil {
     /**
      * Do not instantiate.
      */
-    private GenericsUtil() {
+    protected GenericsUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/lang/MethodUtil.java b/src/main/java/org/codelibs/core/lang/MethodUtil.java
index 028eb93..c8dfdba 100644
--- a/src/main/java/org/codelibs/core/lang/MethodUtil.java
+++ b/src/main/java/org/codelibs/core/lang/MethodUtil.java
@@ -36,7 +36,7 @@ public abstract class MethodUtil {
     /**
      * Do not instantiate.
      */
-    private MethodUtil() {
+    protected MethodUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/lang/ModifierUtil.java b/src/main/java/org/codelibs/core/lang/ModifierUtil.java
index 8f0546e..414d8dc 100644
--- a/src/main/java/org/codelibs/core/lang/ModifierUtil.java
+++ b/src/main/java/org/codelibs/core/lang/ModifierUtil.java
@@ -31,7 +31,7 @@ public abstract class ModifierUtil {
     /**
      * Do not instantiate.
      */
-    private ModifierUtil() {
+    protected ModifierUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/lang/ObjectUtil.java b/src/main/java/org/codelibs/core/lang/ObjectUtil.java
index ba36a62..4cfa714 100644
--- a/src/main/java/org/codelibs/core/lang/ObjectUtil.java
+++ b/src/main/java/org/codelibs/core/lang/ObjectUtil.java
@@ -25,7 +25,7 @@ public abstract class ObjectUtil {
     /**
      * Do not instantiate.
      */
-    private ObjectUtil() {
+    protected ObjectUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/lang/StringUtil.java b/src/main/java/org/codelibs/core/lang/StringUtil.java
index 5032c49..5017c6b 100644
--- a/src/main/java/org/codelibs/core/lang/StringUtil.java
+++ b/src/main/java/org/codelibs/core/lang/StringUtil.java
@@ -32,7 +32,7 @@ public abstract class StringUtil {
     /**
      * Do not instantiate.
      */
-    private StringUtil() {
+    protected StringUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/lang/SystemUtil.java b/src/main/java/org/codelibs/core/lang/SystemUtil.java
index 020b763..4c300e2 100644
--- a/src/main/java/org/codelibs/core/lang/SystemUtil.java
+++ b/src/main/java/org/codelibs/core/lang/SystemUtil.java
@@ -27,7 +27,7 @@ public abstract class SystemUtil {
     /**
      * Do not instantiate.
      */
-    private SystemUtil() {
+    protected SystemUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/lang/ThreadUtil.java b/src/main/java/org/codelibs/core/lang/ThreadUtil.java
index 3cce6ae..17ce6c4 100644
--- a/src/main/java/org/codelibs/core/lang/ThreadUtil.java
+++ b/src/main/java/org/codelibs/core/lang/ThreadUtil.java
@@ -31,7 +31,7 @@ public abstract class ThreadUtil {
     /**
      * Do not instantiate.
      */
-    private ThreadUtil() {
+    protected ThreadUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/message/MessageFormatter.java b/src/main/java/org/codelibs/core/message/MessageFormatter.java
index f66e912..c301406 100644
--- a/src/main/java/org/codelibs/core/message/MessageFormatter.java
+++ b/src/main/java/org/codelibs/core/message/MessageFormatter.java
@@ -34,7 +34,7 @@ public abstract class MessageFormatter {
     /**
      * Do not instantiate.
      */
-    private MessageFormatter() {
+    protected MessageFormatter() {
     }
 
     /** Length of the numeric part of the message code */
diff --git a/src/main/java/org/codelibs/core/misc/AssertionUtil.java b/src/main/java/org/codelibs/core/misc/AssertionUtil.java
index 1719a8b..ee56ab3 100644
--- a/src/main/java/org/codelibs/core/misc/AssertionUtil.java
+++ b/src/main/java/org/codelibs/core/misc/AssertionUtil.java
@@ -38,7 +38,7 @@ public abstract class AssertionUtil {
     /**
      * Do not instantiate.
      */
-    private AssertionUtil() {
+    protected AssertionUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/misc/Base64Util.java b/src/main/java/org/codelibs/core/misc/Base64Util.java
index 0d1e2f2..dcf6805 100644
--- a/src/main/java/org/codelibs/core/misc/Base64Util.java
+++ b/src/main/java/org/codelibs/core/misc/Base64Util.java
@@ -28,7 +28,7 @@ public abstract class Base64Util {
     /**
      * Do not instantiate.
      */
-    private Base64Util() {
+    protected Base64Util() {
     }
 
     private static final char[] ENCODE_TABLE = { 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R',
diff --git a/src/main/java/org/codelibs/core/misc/DisposableUtil.java b/src/main/java/org/codelibs/core/misc/DisposableUtil.java
index 0089a57..2481221 100644
--- a/src/main/java/org/codelibs/core/misc/DisposableUtil.java
+++ b/src/main/java/org/codelibs/core/misc/DisposableUtil.java
@@ -35,7 +35,7 @@ public abstract class DisposableUtil {
     /**
      * Do not instantiate.
      */
-    private DisposableUtil() {
+    protected DisposableUtil() {
     }
 
     /** Registered {@link Disposable} */
diff --git a/src/main/java/org/codelibs/core/misc/LocaleUtil.java b/src/main/java/org/codelibs/core/misc/LocaleUtil.java
index aac59ff..5883cab 100644
--- a/src/main/java/org/codelibs/core/misc/LocaleUtil.java
+++ b/src/main/java/org/codelibs/core/misc/LocaleUtil.java
@@ -28,7 +28,7 @@ public abstract class LocaleUtil {
     /**
      * Do not instantiate.
      */
-    private LocaleUtil() {
+    protected LocaleUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/naming/InitialContextUtil.java b/src/main/java/org/codelibs/core/naming/InitialContextUtil.java
index d4aac9c..d4eb40c 100644
--- a/src/main/java/org/codelibs/core/naming/InitialContextUtil.java
+++ b/src/main/java/org/codelibs/core/naming/InitialContextUtil.java
@@ -35,7 +35,7 @@ public abstract class InitialContextUtil {
     /**
      * Do not instantiate.
      */
-    private InitialContextUtil() {
+    protected InitialContextUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/net/JarURLConnectionUtil.java b/src/main/java/org/codelibs/core/net/JarURLConnectionUtil.java
index 39d2ed5..ca9b948 100644
--- a/src/main/java/org/codelibs/core/net/JarURLConnectionUtil.java
+++ b/src/main/java/org/codelibs/core/net/JarURLConnectionUtil.java
@@ -33,7 +33,7 @@ public abstract class JarURLConnectionUtil {
     /**
      * Do not instantiate.
      */
-    private JarURLConnectionUtil() {
+    protected JarURLConnectionUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/net/MimeTypeUtil.java b/src/main/java/org/codelibs/core/net/MimeTypeUtil.java
index b977588..500d926 100644
--- a/src/main/java/org/codelibs/core/net/MimeTypeUtil.java
+++ b/src/main/java/org/codelibs/core/net/MimeTypeUtil.java
@@ -35,7 +35,7 @@ public abstract class MimeTypeUtil {
     /**
      * Do not instantiate.
      */
-    private MimeTypeUtil() {
+    protected MimeTypeUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/net/URLUtil.java b/src/main/java/org/codelibs/core/net/URLUtil.java
index 01c4c35..80d4524 100644
--- a/src/main/java/org/codelibs/core/net/URLUtil.java
+++ b/src/main/java/org/codelibs/core/net/URLUtil.java
@@ -43,7 +43,7 @@ public abstract class URLUtil {
     /**
      * Do not instantiate.
      */
-    private URLUtil() {
+    protected URLUtil() {
     }
 
     /** Map for normalizing protocols */
diff --git a/src/main/java/org/codelibs/core/net/UuidUtil.java b/src/main/java/org/codelibs/core/net/UuidUtil.java
index cccb6de..291f0ff 100644
--- a/src/main/java/org/codelibs/core/net/UuidUtil.java
+++ b/src/main/java/org/codelibs/core/net/UuidUtil.java
@@ -31,7 +31,7 @@ public abstract class UuidUtil {
     /**
      * Do not instantiate.
      */
-    private UuidUtil() {
+    protected UuidUtil() {
     }
 
     private static final byte[] DEFAULT_ADDRESS = new byte[] { (byte) 127, (byte) 0, (byte) 0, (byte) 1 };
diff --git a/src/main/java/org/codelibs/core/nio/ChannelUtil.java b/src/main/java/org/codelibs/core/nio/ChannelUtil.java
index da89eb8..ca5f0b6 100644
--- a/src/main/java/org/codelibs/core/nio/ChannelUtil.java
+++ b/src/main/java/org/codelibs/core/nio/ChannelUtil.java
@@ -35,7 +35,7 @@ public abstract class ChannelUtil {
     /**
      * Do not instantiate.
      */
-    private ChannelUtil() {
+    protected ChannelUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/security/MessageDigestUtil.java b/src/main/java/org/codelibs/core/security/MessageDigestUtil.java
index 65ffe73..a3aee79 100644
--- a/src/main/java/org/codelibs/core/security/MessageDigestUtil.java
+++ b/src/main/java/org/codelibs/core/security/MessageDigestUtil.java
@@ -35,7 +35,7 @@ public abstract class MessageDigestUtil {
     /**
      * Do not instantiate.
      */
-    private MessageDigestUtil() {
+    protected MessageDigestUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/sql/DriverManagerUtil.java b/src/main/java/org/codelibs/core/sql/DriverManagerUtil.java
index adfdcda..68453ca 100644
--- a/src/main/java/org/codelibs/core/sql/DriverManagerUtil.java
+++ b/src/main/java/org/codelibs/core/sql/DriverManagerUtil.java
@@ -36,7 +36,7 @@ public abstract class DriverManagerUtil {
     /**
      * Do not instantiate.
      */
-    private DriverManagerUtil() {
+    protected DriverManagerUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/sql/PreparedStatementUtil.java b/src/main/java/org/codelibs/core/sql/PreparedStatementUtil.java
index b98ee6f..b1a2038 100644
--- a/src/main/java/org/codelibs/core/sql/PreparedStatementUtil.java
+++ b/src/main/java/org/codelibs/core/sql/PreparedStatementUtil.java
@@ -33,7 +33,7 @@ public abstract class PreparedStatementUtil {
     /**
      * Do not instantiate.
      */
-    private PreparedStatementUtil() {
+    protected PreparedStatementUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/sql/ResultSetUtil.java b/src/main/java/org/codelibs/core/sql/ResultSetUtil.java
index 1b5d510..8eb2a69 100644
--- a/src/main/java/org/codelibs/core/sql/ResultSetUtil.java
+++ b/src/main/java/org/codelibs/core/sql/ResultSetUtil.java
@@ -36,7 +36,7 @@ public abstract class ResultSetUtil {
     /**
      * Do not instantiate.
      */
-    private ResultSetUtil() {
+    protected ResultSetUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/sql/StatementUtil.java b/src/main/java/org/codelibs/core/sql/StatementUtil.java
index efeda95..d5d1279 100644
--- a/src/main/java/org/codelibs/core/sql/StatementUtil.java
+++ b/src/main/java/org/codelibs/core/sql/StatementUtil.java
@@ -38,7 +38,7 @@ public abstract class StatementUtil {
     /**
      * Do not instantiate.
      */
-    private StatementUtil() {
+    protected StatementUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/stream/StreamUtil.java b/src/main/java/org/codelibs/core/stream/StreamUtil.java
index c58022c..0cbc5a8 100644
--- a/src/main/java/org/codelibs/core/stream/StreamUtil.java
+++ b/src/main/java/org/codelibs/core/stream/StreamUtil.java
@@ -26,8 +26,8 @@
 /**
  * Utility class for {@link Stream}.
  */
-public class StreamUtil {
-    private StreamUtil() {
+public abstract class StreamUtil {
+    protected StreamUtil() {
         // nothing
     }
 
diff --git a/src/main/java/org/codelibs/core/text/DecimalFormatSymbolsUtil.java b/src/main/java/org/codelibs/core/text/DecimalFormatSymbolsUtil.java
index 3d07c08..28ae695 100644
--- a/src/main/java/org/codelibs/core/text/DecimalFormatSymbolsUtil.java
+++ b/src/main/java/org/codelibs/core/text/DecimalFormatSymbolsUtil.java
@@ -36,7 +36,7 @@ public abstract class DecimalFormatSymbolsUtil {
     /**
      * Do not instantiate.
      */
-    private DecimalFormatSymbolsUtil() {
+    protected DecimalFormatSymbolsUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/text/DecimalFormatUtil.java b/src/main/java/org/codelibs/core/text/DecimalFormatUtil.java
index 01de267..cdf10f3 100644
--- a/src/main/java/org/codelibs/core/text/DecimalFormatUtil.java
+++ b/src/main/java/org/codelibs/core/text/DecimalFormatUtil.java
@@ -33,7 +33,7 @@ public abstract class DecimalFormatUtil {
     /**
      * Do not instantiate.
      */
-    private DecimalFormatUtil() {
+    protected DecimalFormatUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/xml/DocumentBuilderFactoryUtil.java b/src/main/java/org/codelibs/core/xml/DocumentBuilderFactoryUtil.java
index 85658e7..b89c640 100644
--- a/src/main/java/org/codelibs/core/xml/DocumentBuilderFactoryUtil.java
+++ b/src/main/java/org/codelibs/core/xml/DocumentBuilderFactoryUtil.java
@@ -35,7 +35,7 @@ public abstract class DocumentBuilderFactoryUtil {
     /**
      * Do not instantiate.
      */
-    private DocumentBuilderFactoryUtil() {
+    protected DocumentBuilderFactoryUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/xml/DocumentBuilderUtil.java b/src/main/java/org/codelibs/core/xml/DocumentBuilderUtil.java
index eb4a6f7..1bc9292 100644
--- a/src/main/java/org/codelibs/core/xml/DocumentBuilderUtil.java
+++ b/src/main/java/org/codelibs/core/xml/DocumentBuilderUtil.java
@@ -37,7 +37,7 @@ public abstract class DocumentBuilderUtil {
     /**
      * Do not instantiate.
      */
-    private DocumentBuilderUtil() {
+    protected DocumentBuilderUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/xml/DomUtil.java b/src/main/java/org/codelibs/core/xml/DomUtil.java
index e8a5f80..92a8cde 100644
--- a/src/main/java/org/codelibs/core/xml/DomUtil.java
+++ b/src/main/java/org/codelibs/core/xml/DomUtil.java
@@ -42,7 +42,7 @@ public abstract class DomUtil {
     /**
      * Do not instantiate.
      */
-    private DomUtil() {
+    protected DomUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/xml/SAXParserFactoryUtil.java b/src/main/java/org/codelibs/core/xml/SAXParserFactoryUtil.java
index 117de4e..0e5a198 100644
--- a/src/main/java/org/codelibs/core/xml/SAXParserFactoryUtil.java
+++ b/src/main/java/org/codelibs/core/xml/SAXParserFactoryUtil.java
@@ -38,7 +38,7 @@ public abstract class SAXParserFactoryUtil {
     /**
      * Do not instantiate.
      */
-    private SAXParserFactoryUtil() {
+    protected SAXParserFactoryUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/xml/SAXParserUtil.java b/src/main/java/org/codelibs/core/xml/SAXParserUtil.java
index cb97c6b..3bf312f 100644
--- a/src/main/java/org/codelibs/core/xml/SAXParserUtil.java
+++ b/src/main/java/org/codelibs/core/xml/SAXParserUtil.java
@@ -39,7 +39,7 @@ public abstract class SAXParserUtil {
     /**
      * Do not instantiate.
      */
-    private SAXParserUtil() {
+    protected SAXParserUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/xml/SchemaFactoryUtil.java b/src/main/java/org/codelibs/core/xml/SchemaFactoryUtil.java
index 2d0c86f..8134639 100644
--- a/src/main/java/org/codelibs/core/xml/SchemaFactoryUtil.java
+++ b/src/main/java/org/codelibs/core/xml/SchemaFactoryUtil.java
@@ -32,7 +32,7 @@ public abstract class SchemaFactoryUtil {
     /**
      * Do not instantiate.
      */
-    private SchemaFactoryUtil() {
+    protected SchemaFactoryUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/xml/SchemaUtil.java b/src/main/java/org/codelibs/core/xml/SchemaUtil.java
index c657a51..a34fc04 100644
--- a/src/main/java/org/codelibs/core/xml/SchemaUtil.java
+++ b/src/main/java/org/codelibs/core/xml/SchemaUtil.java
@@ -37,7 +37,7 @@ public abstract class SchemaUtil {
     /**
      * Do not instantiate.
      */
-    private SchemaUtil() {
+    protected SchemaUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/zip/ZipFileUtil.java b/src/main/java/org/codelibs/core/zip/ZipFileUtil.java
index 94f55eb..81592d6 100644
--- a/src/main/java/org/codelibs/core/zip/ZipFileUtil.java
+++ b/src/main/java/org/codelibs/core/zip/ZipFileUtil.java
@@ -43,7 +43,7 @@ public abstract class ZipFileUtil {
     /**
      * Do not instantiate.
      */
-    private ZipFileUtil() {
+    protected ZipFileUtil() {
     }
 
     /**
diff --git a/src/main/java/org/codelibs/core/zip/ZipInputStreamUtil.java b/src/main/java/org/codelibs/core/zip/ZipInputStreamUtil.java
index 32775d8..67c23db 100644
--- a/src/main/java/org/codelibs/core/zip/ZipInputStreamUtil.java
+++ b/src/main/java/org/codelibs/core/zip/ZipInputStreamUtil.java
@@ -37,7 +37,7 @@ public abstract class ZipInputStreamUtil {
     /**
      * Do not instantiate.
      */
-    private ZipInputStreamUtil() {
+    protected ZipInputStreamUtil() {
     }
 
     /**