Introduce toString(Charset) in FastByteArrayOutputStream

This commit introduces a toString() overload in
FastByteArrayOutputStream that accepts a Charset in order to mirror the
method that was introduced in ByteArrayOutputStream in JDK 10,
including a special case for when a single buffer is in use internally
to avoid the need to resize.

This commit also updates getContentAsString() in
ContentCachingRequestWrapper to use this new toString(Charset) method.

Closes gh-31737

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

@@ -19,6 +19,7 @@
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
+import java.nio.charset.Charset;
 import java.security.MessageDigest;
 import java.util.ArrayDeque;
 import java.util.Deque;
@@ -162,9 +163,26 @@ public void close() {
 	 */
 	@Override
 	public String toString() {
-		return new String(toByteArrayUnsafe());
+		return toString(Charset.defaultCharset());
 	}
 
+	/**
+	 * Converts the buffer's contents into a string by decoding the bytes using
+	 * the specified {@link java.nio.charset.Charset charset}.
+	 *
+	 * @param      charset  the {@linkplain java.nio.charset.Charset charset}
+	 *             to be used to decode the {@code bytes}
+	 * @return     a String decoded from the buffer's contents
+	 */
+	public String toString(Charset charset) {
+		if (size() == 0) {
+			return "";
+		}
+		if (buffers.size() == 1) {
+			return new String(buffers.getFirst(), 0, index, charset);
+		}
+		return new String(toByteArrayUnsafe(), charset);
+	}
 
 	// Custom methods
 

spring-core/src/test/java/org/springframework/util/FastByteArrayOutputStreamTests.java

@@ -57,6 +57,24 @@ void resize() throws Exception {
 		assertThat(this.os.size()).isEqualTo(sizeBefore);
 	}
 
+	@Test
+	void stringConversion() throws Exception {
+		this.os.write(this.helloBytes);
+		assertThat(this.os.toString()).isEqualTo("Hello World");
+		assertThat(this.os.toString(StandardCharsets.UTF_8)).isEqualTo("Hello World");
+
+		FastByteArrayOutputStream empty = new FastByteArrayOutputStream();
+		assertThat(empty.toString()).isEqualTo("");
+		assertThat(empty.toString(StandardCharsets.US_ASCII)).isEqualTo("");
+
+		FastByteArrayOutputStream outputStream = new FastByteArrayOutputStream(5);
+		// Add bytes in multiple writes to ensure we get more than one buffer internally
+		outputStream.write(this.helloBytes, 0, 5);
+		outputStream.write(this.helloBytes, 5, 6);
+		assertThat(outputStream.toString(StandardCharsets.UTF_8)).isEqualTo("Hello World");
+		assertThat(outputStream.toString()).isEqualTo("Hello World");
+	}
+
 	@Test
 	void autoGrow() throws IOException {
 		this.os.resize(1);

spring-web/src/main/java/org/springframework/web/util/ContentCachingRequestWrapper.java

@@ -205,7 +205,7 @@ public byte[] getContentAsByteArray() {
 	 * @see #getContentAsByteArray()
 	 */
 	public String getContentAsString() {
-		return new String(this.cachedContent.toByteArrayUnsafe(), Charset.forName(getCharacterEncoding()));
+		return this.cachedContent.toString(Charset.forName(getCharacterEncoding()));
 	}
 
 	/**