DefaultDataBuffer improvements

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

spring-core/src/main/java/org/springframework/core/io/buffer/DefaultDataBuffer.java

@@ -272,10 +272,13 @@ public class DefaultDataBuffer implements DataBuffer {
 						ByteBuffer.allocate(minCapacity));
 
 		// Explicit cast for compatibility with covariant return type on JDK 9's ByteBuffer
-		((Buffer) oldBuffer).position(this.readPosition);
+		final int remaining = readableByteCount();
+		((Buffer) oldBuffer).position(this.readPosition).limit(this.writePosition);
 		newBuffer.put(oldBuffer);
 
 		this.byteBuffer = newBuffer;
+		this.readPosition = 0;
+		this.writePosition = remaining;
 		oldBuffer.clear();
 	}
 

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);
+	}
+
+
 }