DefaultDataBuffer improvements

DefaultDataBuffer.grow(int):
- Copy only the remaining data
- Update readPosition/writePosition

spring-core/src/test/java/org/springframework/core/io/buffer/DataBufferTests.java

@@ -20,6 +20,7 @@ import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
 import java.nio.ByteBuffer;
+import java.nio.charset.StandardCharsets;
 import java.util.Arrays;
 
 import org.junit.Test;
@@ -266,4 +267,20 @@ public class DataBufferTests extends AbstractDataBufferAllocatingTestCase {
 	}
 
 
+	@Test
+	public void growDataBuffer() {
+		DataBuffer buffer = stringBuffer("Hello World!");
+
+		byte[] bytes = new byte[5];
+		buffer.read(bytes);
+		assertArrayEquals("Hello".getBytes(StandardCharsets.UTF_8), bytes);
+
+		buffer.write("!!".getBytes(StandardCharsets.UTF_8));
+
+		bytes = new byte[9];
+		buffer.read(bytes);
+		assertArrayEquals(" World!!!".getBytes(StandardCharsets.UTF_8), bytes);
+	}
+
+
 }