Revised Charset handling and common StringUtils.uriDecode delegate

Issue: SPR-14492

spring-core/src/main/java/org/springframework/util/StringUtils.java

@@ -16,6 +16,8 @@
 
 package org.springframework.util;
 
+import java.io.ByteArrayOutputStream;
+import java.nio.charset.Charset;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
@@ -690,6 +692,59 @@ public abstract class StringUtils {
 		return cleanPath(path1).equals(cleanPath(path2));
 	}
 
+	/**
+	 * Decode the given encoded URI component value. Based on the following rules:
+	 * <ul>
+	 * <li>Alphanumeric characters {@code "a"} through {@code "z"}, {@code "A"} through {@code "Z"},
+	 * and {@code "0"} through {@code "9"} stay the same.</li>
+	 * <li>Special characters {@code "-"}, {@code "_"}, {@code "."}, and {@code "*"} stay the same.</li>
+	 * <li>A sequence "{@code %<i>xy</i>}" is interpreted as a hexadecimal representation of the character.</li>
+	 * </ul>
+	 * @param source the encoded String (may be {@code null})
+	 * @param charset the character set
+	 * @return the decoded value
+	 * @throws IllegalArgumentException when the given source contains invalid encoded sequences
+	 * @since 5.0
+	 * @see java.net.URLDecoder#decode(String, String)
+	 */
+	public static String uriDecode(String source, Charset charset) {
+		if (source == null) {
+			return null;
+		}
+		int length = source.length();
+		if (length == 0) {
+			return source;
+		}
+		Assert.notNull(charset, "Charset must not be null");
+
+		ByteArrayOutputStream bos = new ByteArrayOutputStream(length);
+		boolean changed = false;
+		for (int i = 0; i < length; i++) {
+			int ch = source.charAt(i);
+			if (ch == '%') {
+				if (i + 2 < length) {
+					char hex1 = source.charAt(i + 1);
+					char hex2 = source.charAt(i + 2);
+					int u = Character.digit(hex1, 16);
+					int l = Character.digit(hex2, 16);
+					if (u == -1 || l == -1) {
+						throw new IllegalArgumentException("Invalid encoded sequence \"" + source.substring(i) + "\"");
+					}
+					bos.write((char) ((u << 4) + l));
+					i += 2;
+					changed = true;
+				}
+				else {
+					throw new IllegalArgumentException("Invalid encoded sequence \"" + source.substring(i) + "\"");
+				}
+			}
+			else {
+				bos.write(ch);
+			}
+		}
+		return (changed ? new String(bos.toByteArray(), charset) : source);
+	}
+
 	/**
 	 * Parse the given {@code localeString} value into a {@link Locale}.
 	 * <p>This is the inverse operation of {@link Locale#toString Locale's toString}.