BAEL-5812 Stream Large byte[] to File with WebClient (#13119)

* first draft

* editor review 1

spring-reactive-modules/spring-5-reactive-client-2/src/main/java/com/baeldung/streamlargefile/StreamLargeFileApp.java

@@ -0,0 +1,12 @@
+package com.baeldung.streamlargefile;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+
+@SpringBootApplication
+public class StreamLargeFileApp {
+
+    public static void main(String... args) {
+        SpringApplication.run(StreamLargeFileApp.class, args);
+    }
+}

spring-reactive-modules/spring-5-reactive-client-2/src/main/java/com/baeldung/streamlargefile/client/LargeFileDownloadWebClient.java

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+package com.baeldung.streamlargefile.client;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+
+import org.springframework.core.io.buffer.DataBuffer;
+import org.springframework.core.io.buffer.DataBufferUtils;
+import org.springframework.web.reactive.function.client.WebClient;
+
+import reactor.core.publisher.Flux;
+
+public class LargeFileDownloadWebClient {
+
+    private LargeFileDownloadWebClient() {
+    }
+
+    public static long fetch(WebClient client, String destination) throws IOException {
+        Flux<DataBuffer> flux = client.get()
+            .retrieve()
+            .bodyToFlux(DataBuffer.class);
+
+        Path path = Paths.get(destination);
+
+        DataBufferUtils.write(flux, path)
+            .block();
+
+        return Files.size(path);
+    }
+
+    public static void main(String... args) throws IOException {
+        String baseUrl = args[0];
+        String destination = args[1];
+
+        WebClient client = WebClient.create(baseUrl);
+
+        long bytes = fetch(client, destination);
+        System.out.printf("downloaded %d bytes to %s", bytes, destination);
+    }
+}

spring-reactive-modules/spring-5-reactive-client-2/src/main/java/com/baeldung/streamlargefile/client/LimitedFileDownloadWebClient.java

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+package com.baeldung.streamlargefile.client;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+
+import org.springframework.web.reactive.function.client.ExchangeStrategies;
+import org.springframework.web.reactive.function.client.WebClient;
+
+import reactor.core.publisher.Mono;
+
+public class LimitedFileDownloadWebClient {
+
+    private LimitedFileDownloadWebClient() {
+    }
+
+    public static long fetch(WebClient client, String destination) throws IOException {
+        Mono<byte[]> mono = client.get()
+            .retrieve()
+            .bodyToMono(byte[].class)
+            .onErrorMap(RuntimeException::new);
+
+        byte[] bytes = mono.block();
+
+        Path path = Paths.get(destination);
+        Files.write(path, bytes);
+
+        return bytes.length;
+    }
+
+    public static void main(String... args) throws IOException {
+        String baseUrl = args[0];
+        String destination = args[1];
+
+        WebClient client = WebClient.builder()
+            .baseUrl(baseUrl)
+            .exchangeStrategies(useMaxMemory())
+            .build();
+
+        long bytes = fetch(client, destination);
+        System.out.printf("downloaded %d bytes to %s", bytes, destination);
+    }
+
+    public static ExchangeStrategies useMaxMemory() {
+        long totalMemory = Runtime.getRuntime()
+            .maxMemory();
+
+        return ExchangeStrategies.builder()
+            .codecs(configurer ->
+                configurer.defaultCodecs()
+                .maxInMemorySize((int) totalMemory))
+            .build();
+    }
+}

spring-reactive-modules/spring-5-reactive-client-2/src/main/java/com/baeldung/streamlargefile/server/LargeFileController.java

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+package com.baeldung.streamlargefile.server;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+
+import org.springframework.core.io.FileSystemResource;
+import org.springframework.core.io.Resource;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+@RestController
+@RequestMapping("/large-file")
+public class LargeFileController {
+
+    public static final Path downloadPath = Paths.get("/tmp/large.dat");
+
+    @GetMapping("size")
+    Long getSize() throws IOException {
+        return Files.size(downloadPath);
+    }
+
+    @GetMapping
+    ResponseEntity<Resource> get() {
+        return ResponseEntity.ok()
+            .body(new FileSystemResource(downloadPath));
+    }
+}

spring-reactive-modules/spring-5-reactive-client-2/src/main/resources/streamlargefile/generate-sample-files.sh

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+#!/bin/sh
+
+generate() {
+	file="$1"
+	size="$2"
+	
+	fallocate -l "$size" "$file"
+	ls -lah "$file"
+}
+
+generate /tmp/small.dat 128K
+generate /tmp/large.dat 128M

spring-reactive-modules/spring-5-reactive-client-2/src/main/resources/streamlargefile/run.sh

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+#!/bin/bash
+MYSELF="$(readlink -f "$0")"
+MYDIR="${MYSELF%/*}"
+
+client="${1:-Large}"
+url="${2:-http://localhost:8081/large-file}"
+download_destination="${3:-/tmp/download.dat}"
+xmx="${4:-32m}"
+
+module_dir="$(readlink -f "$MYDIR/../../../..")"
+
+echo "module: $module_dir"
+cd $module_dir || exit
+
+echo "packaging..."
+mvn clean package dependency:copy-dependencies
+
+echo "GET $url with $client client..."
+java -Xmx$xmx -cp target/dependency/*:target/* \
+"com.baeldung.streamlargefile.client.${client}FileDownloadWebClient" \
+"$url" "$download_destination"

spring-reactive-modules/spring-5-reactive-client-2/src/test/java/com/baeldung/streamlargefile/LargeFileControllerLiveTest.java

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+package com.baeldung.streamlargefile;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+import org.springframework.core.io.ClassPathResource;
+import org.springframework.web.reactive.function.client.WebClient;
+
+import com.baeldung.streamlargefile.client.LargeFileDownloadWebClient;
+import com.baeldung.streamlargefile.client.LimitedFileDownloadWebClient;
+import com.baeldung.streamlargefile.server.LargeFileController;
+
+class LargeFileControllerLiveTest {
+
+    private static final String BASE_URL = "http://localhost:8081/large-file";
+    private static final String DOWNLOAD_DESTINATION = LargeFileController.downloadPath.resolveSibling("download.dat")
+        .toString();
+    private static final Path downloadFile = LargeFileController.downloadPath;
+    private static final Runtime runtime = Runtime.getRuntime();
+    private static final Long xmx = runtime.maxMemory();
+
+    private WebClient client = WebClient.create(BASE_URL);
+
+    @BeforeAll
+    static void init() throws IOException {
+        if (!Files.exists(downloadFile)) {
+            ClassPathResource res = new ClassPathResource("streamlargefile/generate-sample-files.sh");
+
+            runtime.exec(res.getFile()
+                .getAbsolutePath());
+        }
+    }
+
+    @Test
+    void givenMemorySafeClient_whenFileLargerThanXmx_thenFileDownloaded() throws IOException {
+        if (xmx < Files.size(downloadFile)) {
+            long size = LargeFileDownloadWebClient.fetch(client, DOWNLOAD_DESTINATION);
+            assertTrue(size > xmx);
+        }
+    }
+
+    @Test
+    void givenLimitedClient_whenXmxLargerThanFile_thenFileDownloaded() throws IOException {
+        WebClient client = WebClient.builder()
+            .baseUrl(BASE_URL)
+            .exchangeStrategies(LimitedFileDownloadWebClient.useMaxMemory())
+            .build();
+
+        if (xmx > Files.size(downloadFile)) {
+            long size = LimitedFileDownloadWebClient.fetch(client, DOWNLOAD_DESTINATION);
+            assertTrue(size < xmx);
+        }
+    }
+}