diff --git a/src/main/java/xyz/zhouxy/plusone/commons/util/RandomTools.java b/src/main/java/xyz/zhouxy/plusone/commons/util/RandomTools.java
index 69eb555..6afc578 100644
--- a/src/main/java/xyz/zhouxy/plusone/commons/util/RandomTools.java
+++ b/src/main/java/xyz/zhouxy/plusone/commons/util/RandomTools.java
@@ -20,6 +20,8 @@ import java.security.SecureRandom;
 import java.util.Random;
 import java.util.concurrent.ThreadLocalRandom;
 
+import javax.annotation.Nonnull;
+
 public final class RandomTools {
 
     public static final SecureRandom DEFAULT_SECURE_RANDOM = new SecureRandom();
@@ -30,7 +32,7 @@ public final class RandomTools {
 
     /**
      * 使用传入的随机数生成器，生成指定长度的字符串
-     * 
+     *
      * @param random           随机数生成器。根据需要可以传入
      *                         {@link java.util.concurrent.ThreadLocalRandom}、{@link java.security.SecureRandom}
      *                         等，不为空
@@ -38,7 +40,68 @@ public final class RandomTools {
      * @param length           字符串长度
      * @return 随机字符串
      */
-    public static String randomStr(Random random, char[] sourceCharacters, int length) {
+    public static String randomStr(@Nonnull Random random, @Nonnull char[] sourceCharacters, int length) {
+        AssertTools.checkArgumentNotNull(random, "Random cannot be null.");
+        AssertTools.checkArgumentNotNull(sourceCharacters, "Source characters cannot be null.");
+        AssertTools.checkArgument(length >= 0, "The length should be greater than or equal to zero.");
+        return randomStrInternal(random, sourceCharacters, length);
+    }
+
+    public static String randomStr(@Nonnull char[] sourceCharacters, int length) {
+        AssertTools.checkArgumentNotNull(sourceCharacters, "Source characters cannot be null.");
+        AssertTools.checkArgument(length >= 0, "The length should be greater than or equal to zero.");
+        return randomStrInternal(ThreadLocalRandom.current(), sourceCharacters, length);
+    }
+
+    public static String secureRandomStr(@Nonnull char[] sourceCharacters, int length) {
+        AssertTools.checkArgumentNotNull(sourceCharacters, "Source characters cannot be null.");
+        AssertTools.checkArgument(length >= 0, "The length should be greater than or equal to zero.");
+        return randomStrInternal(DEFAULT_SECURE_RANDOM, sourceCharacters, length);
+    }
+
+    /**
+     * 使用传入的随机数生成器，生成指定长度的字符串
+     *
+     * @param random           随机数生成器。根据需要可以传入
+     *                         {@link java.util.concurrent.ThreadLocalRandom}、{@link java.security.SecureRandom}
+     *                         等，不为空
+     * @param sourceCharacters 字符池。字符串的字符将在数组中选，不为空
+     * @param length           字符串长度
+     * @return 随机字符串
+     */
+    public static String randomStr(@Nonnull Random random, @Nonnull String sourceCharacters, int length) {
+        AssertTools.checkArgumentNotNull(random, "Random cannot be null.");
+        AssertTools.checkArgumentNotNull(sourceCharacters, "Source characters cannot be null.");
+        AssertTools.checkArgument(length >= 0, "The length should be greater than or equal to zero.");
+        return randomStrInternal(random, sourceCharacters, length);
+    }
+
+    public static String randomStr(@Nonnull String sourceCharacters, int length) {
+        AssertTools.checkArgumentNotNull(sourceCharacters, "Source characters cannot be null.");
+        AssertTools.checkArgument(length >= 0, "The length should be greater than or equal to zero.");
+        return randomStrInternal(ThreadLocalRandom.current(), sourceCharacters, length);
+    }
+
+    public static String secureRandomStr(@Nonnull String sourceCharacters, int length) {
+        AssertTools.checkArgumentNotNull(sourceCharacters, "Source characters cannot be null.");
+        AssertTools.checkArgument(length >= 0, "The length should be greater than or equal to zero.");
+        return randomStrInternal(DEFAULT_SECURE_RANDOM, sourceCharacters, length);
+    }
+
+    /**
+     * 使用传入的随机数生成器，生成指定长度的字符串
+     *
+     * @param random           随机数生成器。根据需要可以传入
+     *                         {@link java.util.concurrent.ThreadLocalRandom}、{@link java.security.SecureRandom}
+     *                         等，不为空
+     * @param sourceCharacters 字符池。字符串的字符将在数组中选，不为空
+     * @param length           字符串长度
+     * @return 随机字符串
+     */
+    private static String randomStrInternal(@Nonnull Random random, @Nonnull char[] sourceCharacters, int length) {
+        if (length == 0) {
+            return StringTools.EMPTY_STRING;
+        }
         final char[] result = new char[length];
         for (int i = 0; i < length; i++) {
             result[i] = sourceCharacters[random.nextInt(sourceCharacters.length)];
@@ -46,17 +109,9 @@ public final class RandomTools {
         return String.valueOf(result);
     }
 
-    public static String randomStr(char[] sourceCharacters, int length) {
-        return randomStr(ThreadLocalRandom.current(), sourceCharacters, length);
-    }
-
-    public static String secureRandomStr(char[] sourceCharacters, int length) {
-        return randomStr(DEFAULT_SECURE_RANDOM, sourceCharacters, length);
-    }
-
     /**
      * 使用传入的随机数生成器，生成指定长度的字符串
-     * 
+     *
      * @param random           随机数生成器。根据需要可以传入
      *                         {@link java.util.concurrent.ThreadLocalRandom}、{@link java.security.SecureRandom}
      *                         等，不为空
@@ -64,7 +119,10 @@ public final class RandomTools {
      * @param length           字符串长度
      * @return 随机字符串
      */
-    public static String randomStr(Random random, String sourceCharacters, int length) {
+    private static String randomStrInternal(@Nonnull Random random, @Nonnull String sourceCharacters, int length) {
+        if (length == 0) {
+            return StringTools.EMPTY_STRING;
+        }
         final char[] result = new char[length];
         for (int i = 0; i < length; i++) {
             result[i] = sourceCharacters.charAt(random.nextInt(sourceCharacters.length()));
@@ -72,14 +130,6 @@ public final class RandomTools {
         return String.valueOf(result);
     }
 
-    public static String randomStr(String sourceCharacters, int length) {
-        return randomStr(ThreadLocalRandom.current(), sourceCharacters, length);
-    }
-
-    public static String secureRandomStr(String sourceCharacters, int length) {
-        return randomStr(DEFAULT_SECURE_RANDOM, sourceCharacters, length);
-    }
-
     private RandomTools() {
         throw new IllegalStateException("Utility class");
     }
diff --git a/src/main/java/xyz/zhouxy/plusone/commons/util/StringTools.java b/src/main/java/xyz/zhouxy/plusone/commons/util/StringTools.java
index 5d788d4..951ead0 100644
--- a/src/main/java/xyz/zhouxy/plusone/commons/util/StringTools.java
+++ b/src/main/java/xyz/zhouxy/plusone/commons/util/StringTools.java
@@ -21,6 +21,8 @@ import com.google.common.annotations.Beta;
 @Beta
 public class StringTools {
 
+    public static final String EMPTY_STRING = "";
+
     public static boolean isNotBlank(final String cs) {
         if (cs == null || cs.isEmpty()) {
             return false;