Avoid repeated Charset resolution in MimeType

Closes gh-25808

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -103,6 +103,9 @@ public class MimeType implements Comparable<MimeType>, Serializable {
 
 	private final Map<String, String> parameters;
 
+	@Nullable
+	private Charset resolvedCharset;
+
 	@Nullable
 	private volatile String toStringValue;
 
@@ -138,6 +141,7 @@ public class MimeType implements Comparable<MimeType>, Serializable {
 	 */
 	public MimeType(String type, String subtype, Charset charset) {
 		this(type, subtype, Collections.singletonMap(PARAM_CHARSET, charset.name()));
+		this.resolvedCharset = charset;
 	}
 
 	/**
@@ -150,6 +154,7 @@ public class MimeType implements Comparable<MimeType>, Serializable {
 	 */
 	public MimeType(MimeType other, Charset charset) {
 		this(other.getType(), other.getSubtype(), addCharsetParameter(charset, other.getParameters()));
+		this.resolvedCharset = charset;
 	}
 
 	/**
@@ -197,7 +202,7 @@ public class MimeType implements Comparable<MimeType>, Serializable {
 	 * @see <a href="https://tools.ietf.org/html/rfc2616#section-2.2">HTTP 1.1, section 2.2</a>
 	 */
 	private void checkToken(String token) {
-		for (int i = 0; i < token.length(); i++ ) {
+		for (int i = 0; i < token.length(); i++) {
 			char ch = token.charAt(i);
 			if (!TOKEN.get(ch)) {
 				throw new IllegalArgumentException("Invalid token character '" + ch + "' in token \"" + token + "\"");
@@ -210,8 +215,9 @@ public class MimeType implements Comparable<MimeType>, Serializable {
 		Assert.hasLength(value, "'value' must not be empty");
 		checkToken(attribute);
 		if (PARAM_CHARSET.equals(attribute)) {
-			value = unquote(value);
-			Charset.forName(value);
+			if (this.resolvedCharset == null) {
+				this.resolvedCharset = Charset.forName(unquote(value));
+			}
 		}
 		else if (!isQuotedString(value)) {
 			checkToken(value);
@@ -279,8 +285,7 @@ public class MimeType implements Comparable<MimeType>, Serializable {
 	 */
 	@Nullable
 	public Charset getCharset() {
-		String charset = getParameter(PARAM_CHARSET);
-		return (charset != null ? Charset.forName(unquote(charset)) : null);
+		return this.resolvedCharset;
 	}
 
 	/**