[BAEL-7607] Convert DataBuffer to Mono in Reactor

spring-reactive-modules/spring-webflux/src/main/java/com/baeldung/spring/convert/databuffertomono/DataBufferConverter.java

@@ -0,0 +1,26 @@
+package com.baeldung.spring.convert.databuffertomono;
+
+import org.springframework.core.io.buffer.DataBuffer;
+import org.springframework.core.io.buffer.DataBufferUtils;
+
+import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
+
+public class DataBufferConverter { 
+    public Mono<byte[]> toByteArray(Flux<DataBuffer> data) {
+        return DataBufferUtils
+                // Here, we'll join all DataBuffers in the Flux into a single Mono<DataBuffer>.
+                .join(data)
+                .flatMap(dataBuffer -> {
+                    try {
+                        // Next, extract the byte[] from the aggregated DataBuffer manually.
+                        byte[] bytes = new byte[dataBuffer.readableByteCount()];
+                        dataBuffer.read(bytes);
+                        return Mono.just(bytes);
+                    } finally {
+                        // Ensure the final aggregated DataBuffer is released.
+                        DataBufferUtils.release(dataBuffer);
+                    }
+                });
+    }
+}

spring-reactive-modules/spring-webflux/src/test/java/com/baeldung/spring/convert/databuffertomono/DataBufferConverterTest.java

@@ -0,0 +1,35 @@
+package com.baeldung.spring.convert.databuffertomono;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.core.io.buffer.DataBuffer;
+import org.springframework.core.io.buffer.DefaultDataBufferFactory;
+import reactor.core.publisher.Flux;
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+
+public class DataBufferConverterTest {
+
+    private final DataBufferConverter converter = new DataBufferConverter();
+    private final DefaultDataBufferFactory factory = new DefaultDataBufferFactory();
+    private final String TEST_CONTENT = "This is a long test string.";
+
+    @Test
+    void givenFluxOfDataBuffers_whenConvertedToByteArray_thenContentMatches() {
+        // Setup: First, we'll manually create two DataBuffer chunks for the input Flux
+        byte[] part1 = "This is a ".getBytes();
+        byte[] part2 = "long test string.".getBytes();
+
+        DataBuffer buffer1 = factory.allocateBuffer(part1.length);
+        buffer1.write(part1);
+
+        DataBuffer buffer2 = factory.allocateBuffer(part2.length);
+        buffer2.write(part2);
+
+        Flux<DataBuffer> sourceFlux = Flux.just(buffer1, buffer2);
+
+        // Act & Assert: Here we perform conversion and block for direct assertion
+        byte[] resultBytes = converter.toByteArray(sourceFlux).block();
+
+        byte[] expectedBytes = TEST_CONTENT.getBytes();
+        assertArrayEquals(expectedBytes, resultBytes, "The reconstructed byte array should match original");
+    }
+}