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);
        });
    }
}