Write CharSequence instances to DataBuffers

Prior to this commit, one could write a `CharSequence` to an existing
`DataBuffer` instance by turning it into a byte array or `ByteBuffer`
first. This had the following disadvantages:

1. Memory allocation was not efficient (not leveraging pooled memory
when available)
2. Dealing with `CharsetEncoder` is not always easy
3. `DataBuffer` implementations, like `NettyDataBuffer` can use
optimized implementations in some cases

This commit adds a new `DataBuffer#write(CharSequence, Charset)` method
for those cases and also an `ensureCapacity` method useful for checking
that the current buffer has enough capacity to write to it..

Issue: SPR-17558

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;
@@ -150,6 +151,70 @@ public class DataBufferTests extends AbstractDataBufferAllocatingTestCase {
 		release(buffer);
 	}
 
+	@Test
+	public void writeNullString() {
+		DataBuffer buffer = createDataBuffer(1);
+		try {
+			buffer.write(null, StandardCharsets.UTF_8);
+			fail("IllegalArgumentException expected");
+		}
+		catch (IllegalArgumentException exc) {
+		}
+		finally {
+			release(buffer);
+		}
+	}
+
+	@Test
+	public void writeNullCharset() {
+		DataBuffer buffer = createDataBuffer(1);
+		try {
+			buffer.write("test", null);
+			fail("IllegalArgumentException expected");
+		}
+		catch (IllegalArgumentException exc) {
+		}
+		finally {
+			release(buffer);
+		}
+	}
+
+	@Test
+	public void writeUtf8String() {
+		DataBuffer buffer = createDataBuffer(6);
+		buffer.write("Spring", StandardCharsets.UTF_8);
+
+		byte[] result = new byte[6];
+		buffer.read(result);
+
+		assertArrayEquals("Spring".getBytes(StandardCharsets.UTF_8), result);
+		release(buffer);
+	}
+
+	@Test
+	public void writeUtf8StringOutGrowsCapacity() {
+		DataBuffer buffer = createDataBuffer(5);
+		buffer.write("Spring €", StandardCharsets.UTF_8);
+
+		byte[] result = new byte[10];
+		buffer.read(result);
+
+		assertArrayEquals("Spring €".getBytes(StandardCharsets.UTF_8), result);
+		release(buffer);
+	}
+
+	@Test
+	public void writeIsoString() {
+		DataBuffer buffer = createDataBuffer(3);
+		buffer.write("\u00A3", StandardCharsets.ISO_8859_1);
+
+		byte[] result = new byte[1];
+		buffer.read(result);
+
+		assertArrayEquals("\u00A3".getBytes(StandardCharsets.ISO_8859_1), result);
+		release(buffer);
+	}
+
 	@Test
 	public void inputStream() throws IOException {
 		DataBuffer buffer = createDataBuffer(4);

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

@@ -19,6 +19,7 @@ package org.springframework.core.io.buffer;
 import java.io.InputStream;
 import java.io.OutputStream;
 import java.nio.ByteBuffer;
+import java.nio.charset.Charset;
 import java.util.function.IntPredicate;
 
 import org.springframework.util.Assert;
@@ -139,6 +140,11 @@ class LeakAwareDataBuffer implements PooledDataBuffer {
 		return this.delegate.capacity(newCapacity);
 	}
 
+	@Override
+	public DataBuffer ensureCapacity(int capacity) {
+		return this.delegate.ensureCapacity(capacity);
+	}
+
 	@Override
 	public byte getByte(int index) {
 		return this.delegate.getByte(index);
@@ -184,6 +190,11 @@ class LeakAwareDataBuffer implements PooledDataBuffer {
 		return this.delegate.write(byteBuffers);
 	}
 
+	@Override
+	public DataBuffer write(CharSequence charSequence, Charset charset) {
+		return this.delegate.write(charSequence, charset);
+	}
+
 	@Override
 	public DataBuffer slice(int index, int length) {
 		return this.delegate.slice(index, length);