Part 1: read from String, File and InputStream

src/main/java/com/ankurm/jackson3/part1objectmapper/B02ReadJson.java

@@ -0,0 +1,49 @@
+package com.ankurm.jackson3.part1objectmapper;
+
+import tools.jackson.databind.json.JsonMapper;
+
+import java.io.ByteArrayInputStream;
+import java.io.File;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.util.List;
+
+/**
+ * Post: Jackson ObjectMapper Guide — https://ankurm.com/jackson-objectmapper-guide/
+ * Section: "Deserialising JSON to Java Objects"
+ *
+ * Reading from a String, a File and an InputStream.
+ *
+ * The post also shows readValue(new URL(...), ...). That overload is deliberately
+ * NOT reproduced here: it performs a live network call, which would make this
+ * example non-reproducible. The InputStream form below is what a real HTTP client
+ * hands you anyway.
+ */
+public class B02ReadJson {
+
+    public record Article(Long articleId, String title, List<String> tags) { }
+
+    public static void main(String[] args) throws Exception {
+        JsonMapper mapper = JsonMapper.builder().build();
+
+        String json = "{\"articleId\":1,\"title\":\"Jackson Deep Dive\",\"tags\":[\"java\",\"json\"]}";
+
+        // 1. Read from a String
+        Article fromString = mapper.readValue(json, Article.class);
+        System.out.println("from String : " + fromString.title());
+
+        // 2. Read from a File
+        File file = File.createTempFile("article", ".json");
+        Files.writeString(file.toPath(), json);
+        Article fromFile = mapper.readValue(file, Article.class);
+        System.out.println("from File   : " + fromFile.articleId());
+
+        // 3. Read from an InputStream
+        try (var in = new ByteArrayInputStream(json.getBytes(StandardCharsets.UTF_8))) {
+            Article fromStream = mapper.readValue(in, Article.class);
+            System.out.println("from Stream : " + fromStream.tags());
+        }
+
+        file.deleteOnExit();
+    }
+}