java实现读写json文件的示例详解

<project xmlns="http://maven.apache.org/POM/4.0.0" ...>
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.example</groupId>
  <artifactId>json-demo</artifactId>
  <version>1.0-SNAPSHOT</version>
  <dependencies>
    <!-- Jackson 核心 -->
    <dependency>
      <groupId>com.fasterxml.jackson.core</groupId>
      <artifactId>jackson-databind</artifactId>
      <version>2.15.2</version>
    </dependency>
    <!-- JUnit 5 -->
    <dependency>
      <groupId>org.junit.jupiter</groupId>
      <artifactId>junit-jupiter</artifactId>
      <version>5.9.2</version>
      <scope>test</scope>
    </dependency>
  </dependencies>
  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-surefire-plugin</artifactId>
        <version>3.0.0-M7</version>
      </plugin>
    </plugins>
  </build>
</project>

// ========== src/main/java/com/example/json/model/UserConfig.java ==========
package com.example.json.model;
 
import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.annotation.JsonProperty;
import java.util.List;
import java.util.Map;
 
/**
 * 用户配置模型
 */
@JsonInclude(JsonInclude.Include.NON_NULL)
public class UserConfig {
    @JsonProperty("user_id")
    private int id;
 
    private String name;
    private String email;
 
    private List<String> roles;
 
    private Map<String, Object> profile;
 
    // 构造方法、getter、setter 省略
}
 
// ========== src/main/java/com/example/json/util/JsonUtils.java ==========
package com.example.json.util;
 
import com.example.json.model.UserConfig;
import com.fasterxml.jackson.core.JsonFactory;
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;
 
import java.io.*;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.List;
 
/**
 * JSON 读写工具类
 */
public class JsonUtils {
 
    private static final ObjectMapper MAPPER = new ObjectMapper();
    static {
        MAPPER.configure(SerializationFeature.INDENT_OUTPUT, true);
        MAPPER.configure(JsonParser.Feature.AUTO_CLOSE_SOURCE, true);
    }
 
    /**
     * 读取 JSON 文件到指定类型对象
     */
    public static <T> T readJson(String filePath, Class<T> clazz) throws JsonException {
        try {
            return MAPPER.readValue(new File(filePath), clazz);
        } catch (IOException e) {
            throw new JsonException("读取 JSON 文件失败: " + filePath, e);
        }
    }
 
    /**
     * 写入对象为 JSON 文件
     * @param append 是否追加，true 时在文件末尾追加；false 覆盖写入
     */
    public static void writeJson(String filePath, Object obj, boolean append) throws JsonException {
        try (Writer writer = new BufferedWriter(new FileWriter(filePath, append))) {
            MAPPER.writeValue(writer, obj);
        } catch (IOException e) {
            throw new JsonException("写入 JSON 文件失败: " + filePath, e);
        }
    }
 
    /**
     * 大文件流式写入示例
     */
    public static void writeJsonStream(String filePath, List<UserConfig> list) throws JsonException {
        JsonFactory factory = new JsonFactory();
        try (JsonGenerator gen = factory.createGenerator(new File(filePath), JsonFactory.Feature.WRITE_BIGDECIMAL_AS_PLAIN)) {
            gen.writeStartArray();
            for (UserConfig cfg : list) {
                MAPPER.writeValue(gen, cfg);
            }
            gen.writeEndArray();
        } catch (IOException e) {
            throw new JsonException("流式写入 JSON 失败: " + filePath, e);
        }
    }
}
 
// ========== src/main/java/com/example/json/util/JsonException.java ==========
package com.example.json.util;
 
/**
 * JSON 操作异常
 */
public class JsonException extends Exception {
    public JsonException(String message, Throwable cause) {
        super(message, cause);
    }
}
 
// ========== src/main/java/com/example/json/App.java ==========
package com.example.json;
 
import com.example.json.model.UserConfig;
import com.example.json.util.JsonUtils;
import com.example.json.util.JsonException;
 
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
 
public class App {
    public static void main(String[] args) {
        String path = "data/user_config.json";
 
        try {
            // 1. 读取现有配置
            UserConfig cfg = JsonUtils.readJson(path, UserConfig.class);
            System.out.println("读取配置: " + cfg);
 
            // 2. 修改并写回
            cfg.setRoles(Arrays.asList("admin", "user"));
            Map<String,Object> profile = new HashMap<>();
            profile.put("age", 30);
            profile.put("address", "北京");
            cfg.setProfile(profile);
            JsonUtils.writeJson(path, cfg, false);
            System.out.println("写入配置完成");
 
        } catch (JsonException e) {
            e.printStackTrace();
        }
    }
}
 
// ========== src/test/java/com/example/json/JsonUtilsTest.java ==========
package com.example.json;
 
import com.example.json.model.UserConfig;
import com.example.json.util.JsonUtils;
import com.example.json.util.JsonException;
import org.junit.jupiter.api.*;
import java.nio.file.*;
 
@TestInstance(TestInstance.Lifecycle.PER_CLASS)
public class JsonUtilsTest {
 
    private final String testFile = "target/test_config.json";
 
    @BeforeAll
    void init() throws Exception {
        Files.createDirectories(Paths.get("target"));
    }
 
    @Test
    void testReadWrite() throws JsonException {
        UserConfig cfg = new UserConfig();
        cfg.setId(100);
        cfg.setName("测试");
        JsonUtils.writeJson(testFile, cfg, false);
 
        UserConfig read = JsonUtils.readJson(testFile, UserConfig.class);
        Assertions.assertEquals(100, read.getId());
        Assertions.assertEquals("测试", read.getName());
    }
 
    @Test
    void testReadNotFound() {
        Assertions.assertThrows(JsonException.class, () -> {
            JsonUtils.readJson("nonexistent.json", UserConfig.class);
        });
    }
}