Improve usage of ByteArrayOutputStream/ByteArrayInputStream

Closes gh-24805

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

@@ -25,6 +25,7 @@ import java.io.InputStream;
 import java.io.InputStreamReader;
 import java.io.OutputStream;
 import java.io.OutputStreamWriter;
+import java.io.UnsupportedEncodingException;
 import java.io.Writer;
 import java.nio.charset.Charset;
 
@@ -238,6 +239,24 @@ public abstract class StreamUtils {
 		return new NonClosingOutputStream(out);
 	}
 
+	/**
+	 * More effective equivalent of {@code new String(baos.toByteArray(), charset)}
+	 * As far as at invocation point {@code charset} is already available,
+	 * no exception is expected to be thrown.
+	 *
+	 * @param baos    {@link ByteArrayOutputStream} to be flushed into String
+	 * @param charset applicable {@link Charset}
+	 * @return String represenation of bytes stored in {@code baos}
+	 */
+	public static String baosToString(ByteArrayOutputStream baos, Charset charset) {
+		Assert.notNull(baos, "No ByteArrayOutputStream specified");
+		Assert.notNull(charset, "No Charset specified");
+		try {
+			return baos.toString(charset.name());
+		} catch (UnsupportedEncodingException e) {
+			throw new RuntimeException(e);
+		}
+	}
 
 	private static class NonClosingInputStream extends FilterInputStream {
 

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

@@ -774,7 +774,7 @@ public abstract class StringUtils {
 				bos.write(ch);
 			}
 		}
-		return (changed ? new String(bos.toByteArray(), charset) : source);
+		return changed ? StreamUtils.baosToString(bos, charset) : source;
 	}
 
 	/**

spring-core/src/testFixtures/java/org/springframework/core/testfixture/io/SerializationTestUtils.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.
@@ -55,7 +55,6 @@ public class SerializationTestUtils {
 			oos.writeObject(o);
 			oos.flush();
 		}
-		baos.flush();
 		byte[] bytes = baos.toByteArray();
 
 		ByteArrayInputStream is = new ByteArrayInputStream(bytes);