pom.xml

pom.xml

	<parent>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>2.0.1.RELEASE</version>
	</parent>
	<dependencies>
		<dependency>
			<groupId>org.projectlombok</groupId>
			<artifactId>lombok</artifactId>
		</dependency>
	</dependencies>

ObjectWriterを使用してファイル出力

package kagamihoge.tojsonfile;

import java.nio.file.Paths;
import java.util.Arrays;
import java.util.List;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;

import com.fasterxml.jackson.core.util.DefaultPrettyPrinter;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.ObjectWriter;

import lombok.AllArgsConstructor;
import lombok.Data;

@SpringBootApplication
public class BeanToJsonFileApplication implements CommandLineRunner {
	public static void main(String[] args) throws InterruptedException {
		SpringApplication.run(BeanToJsonFileApplication.class, args).close();
	}

	@Bean
	public ObjectWriter writer() {
		ObjectMapper mapper = new ObjectMapper();
		ObjectWriter writer = mapper.writer(new DefaultPrettyPrinter());
		return writer;
	}

	@Autowired
	ObjectWriter writer;

	@Override
	public void run(String... args) throws Exception {
		List<Sub> subList = Arrays.asList(new Sub(1, true), new Sub(2, true));
		Top top = new Top("hogeId", null, subList);

		writer.writeValue(Paths.get("json.txt").toFile(), top);
	}

	@Data
	@AllArgsConstructor
	static class Top {
		private String hogeId;
		private String nullValue;
		private List<Sub> subList;
	}

	@Data
	@AllArgsConstructor
	static class Sub {
		private int subId;
		private boolean bool;
	}
}

出力例

{
  "hogeId" : "hogeId",
  "nullValue" : null,
  "subList" : [ {
    "subId" : 1,
    "bool" : true
  }, {
    "subId" : 2,
    "bool" : true
  } ]
}

他の制御項目

基本的にはjacksonなので、他にやりたいことがあれば大抵はぐぐれば出てくる。

以下はその一例。

キーをキャメルケースからスネークケースに変換

import com.fasterxml.jackson.databind.PropertyNamingStrategy;

mapper.setPropertyNamingStrategy(PropertyNamingStrategy.SNAKE_CASE);

出力例。

{
  "hoge_id" : "hogeId",
  "null_value" : null,
  "sub_list" : [ {
    "sub_id" : 1,
    "bool" : true
  }, {
    "sub_id" : 2,
    "bool" : true
  } ]
}

キーを任意の値に変更


@JsonProperty("alternateId")
private String hogeId;

Stringとして出力

System.out.println(writer.writeValueAsString(top));

JSONの整形が不要な場合

import com.fasterxml.jackson.core.util.MinimalPrettyPrinter;

ObjectWriter writer = mapper.writer(new MinimalPrettyPrinter());

出力例。

{"null_value":null,"sub_list":[{"sub_id":1,"bool":true},{"sub_id":2,"bool":true}],"alternateId":"hogeId"}