#はじめに
Beanの変換に詰め替え作業は手動で実施するとコード量は増えてしまい、メンテナンス性は良くないです。
BeanUtils.copyPropertiesのような便利メソッドがありますが、ほかのマッピングライブラリをまとめてみます。

#FasterXML ObjectMapper
https://github.com/FasterXML/jackson-databind

JSON関連する処理はこれを使うと便利です。

##使い方
###build.gradleに追加

build.gradle

// https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-databind
compile group: 'com.fasterxml.jackson.core', name: 'jackson-databind', version: '2.9.8'

###Beanクラス作成

package com.test.lombok;

import lombok.Getter;
import lombok.Setter;
import lombok.ToString;

@Getter
@Setter
@ToString
public class User {
	private String userId;
	
	private String userName;
	
	private String age;
	
	private String gender;
}

###user.jsonサンプル作成

user.json

{
"userId": "user001",
"userName": "テスト 太郎",
"age": 20,
"gender": "famale"
}

###テストコード

package com.test.lombok;

import java.io.File;
import java.util.Map;

import com.fasterxml.jackson.databind.ObjectMapper;

public class TestMain {

	public static void main(String[] args) throws Exception {
		ObjectMapper mapper = new ObjectMapper();

		// ファイルパスで指定
		User user1 = mapper.readValue(
				new File("C:\\workspace_sts\\lombok\\src\\main\\java\\com\\test\\lombok\\user.json"), User.class);
		System.out.println(user1);

		// URL形式で指定
		User user2 = mapper.readValue(TestMain.class.getResource("user.json"), User.class);
		System.out.println(user2);

		// 文字列から変換
		User user3 = mapper.readValue(
				"{\"userId\":\"user001\", \"userName\": \"テスト 太郎２\", \"age\": 22, \"gender\": \"male\"}", User.class);
		System.out.println(user3);

		// オブジェクトから文字列に変換
		String jsonString = mapper.writeValueAsString(user3);
		System.out.println(jsonString);

		// 文字列からMapに変換
		Map<String, String> userMap = mapper.readValue(jsonString, Map.class);
		System.out.println(userMap);

	}

}

結果：

User(userId=user001, userName=テスト太郎, age=20, gender=famale)
User(userId=user001, userName=テスト太郎, age=20, gender=famale)
User(userId=user001, userName=テスト太郎２, age=22, gender=male)
{"userId":"user001","userName":"テスト太郎２","age":"22","gender":"male"}
{userId=user001, userName=テスト太郎２, age=22, gender=male}

#ModelMapper
http://modelmapper.org/

##使い方
###build.gradleに追加

build.gradle

	// https://mvnrepository.com/artifact/org.modelmapper/modelmapper
	compile group: 'org.modelmapper', name: 'modelmapper', version: '2.3.5'

###元Beanクラス

package com.test.lombok;

import lombok.Data;

@Data
public class User {
	private String userId;
	
	private String userName;
	
	private String age;
	
	private String gender;
	
	private Address addr;
}

@Data
class Address {
	private String zipCode;
	private String address1;
	private String address2;
	private String address3;
}

###DTOクラス

package com.test.lombok;

import lombok.Getter;
import lombok.Setter;
import lombok.ToString;

@Getter
@Setter
@ToString
public class UserDTO {
	private String userId;

	private String userName;

	private String age;

	private String gender;
	
	private String addrZipCode;
	
	private String addrAddress1;
	
	private String addrAddress2;
	
	private String addrAddress3;
}

###テストコード

package com.test.lombok;

import org.modelmapper.ModelMapper;

public class TestMain {

	public static void main(String[] args) throws Exception {
		ModelMapper modelMapper = new ModelMapper();
		
		User user = new User();
		user.setUserId("user001");
		user.setUserName("テスト");
		user.setAge("18");
		user.setGender("male");
		Address addr = new Address();
		addr.setZipCode("984-0031");
		addr.setAddress1("○○県");
		user.setAddr(addr );
		
		UserDTO userDTO = modelMapper.map(user, UserDTO.class);
		System.out.println(userDTO);

	}

}

結果：

UserDTO(userId=user001, userName=テスト, age=18, gender=male, addrZipCode=984-0031, addrAddress1=○○県, addrAddress2=null, addrAddress3=null)

###明示的にマッピング（Explicit Mapping）

package com.test.lombok;

import org.modelmapper.ModelMapper;
import org.modelmapper.TypeMap;

public class TestMain {

	public static void main(String[] args) throws Exception {
		ModelMapper modelMapper = new ModelMapper();

		User user = new User();
		user.setUserId("user001");
		user.setUserName("テスト");
		user.setAge("18");
		user.setGender("male");
		Address addr = new Address();
		addr.setZipCode("984-0031");
		addr.setAddress1("○○県");
		user.setAddr(addr);

		TypeMap<User, UserDTO> typeMap = modelMapper.typeMap(User.class, UserDTO.class).addMappings(mapper -> {
			mapper.map(src -> src.getAddr().getZipCode(), UserDTO::setAddrZipCode);
			mapper.map(src -> src.getAddr().getAddress1(), UserDTO::setAddrAddress1);
		});

		System.out.println(typeMap.map(user));

	}

}

結果：

UserDTO(userId=user001, userName=テスト, age=18, gender=male, addrZipCode=984-0031, addrAddress1=○○県, addrAddress2=null, addrAddress3=null)

SpringBootのControllerにてFormのBeanから自分のDTOに変換するに使うと便利です。

#MapStruct
https://mapstruct.org/

##使い方
###build.gradleに追加

build.gradle

plugins {
    id 'java'
    id 'net.ltgt.apt' version '0.20'
}

repositories {
    jcenter()
}

dependencies {
    // lombok
    compileOnly "org.projectlombok:lombok:1.18.12"
    annotationProcessor "org.projectlombok:lombok:1.18.12"
    
	// https://mvnrepository.com/artifact/org.mapstruct/mapstruct
	implementation  group: 'org.mapstruct', name: 'mapstruct', version: '1.3.1.Final'
	
	// https://mvnrepository.com/artifact/org.mapstruct/mapstruct-processor
	annotationProcessor group: 'org.mapstruct', name: 'mapstruct-processor', version: '1.3.1.Final'
    
}

[compileJava, compileTestJava]*.options*.encoding= "UTF-8"

sourceSets {
    main.java.srcDirs += "build/generated/sources/annotationProcessor/java/main"
}

###Mapperインタフェース

package com.test.lombok;

import org.mapstruct.Mapper;
import org.mapstruct.Mapping;
import org.mapstruct.factory.Mappers;

@Mapper
public interface UserMapper {
	UserMapper INSTANCE = Mappers.getMapper(UserMapper.class);

	@Mapping(source = "addr.zipCode", target = "addrZipCode")
	@Mapping(source = "addr.address1", target = "addrAddress1")
	@Mapping(source = "addr.address2", target = "addrAddress2")
	UserDTO userToUserDTO(User user);
}

上記定義し、ビルドするとbuild\generated\sources\annotationProcessor\java\main\com\test\lombokにUserMapperImpl.javaクラスが生成されます。

UserMapperImpl.java

package com.test.lombok;

import javax.annotation.Generated;

@Generated(
    value = "org.mapstruct.ap.MappingProcessor",
    date = "2020-06-28T21:47:47+0900",
    comments = "version: 1.3.1.Final, compiler: javac, environment: Java 1.8.0_201 (Oracle Corporation)"
)
public class UserMapperImpl implements UserMapper {

    @Override
    public UserDTO userToUserDTO(User user) {
        if ( user == null ) {
            return null;
        }

        UserDTO userDTO = new UserDTO();

        userDTO.setAddrAddress2( userAddrAddress2( user ) );
        userDTO.setAddrAddress1( userAddrAddress1( user ) );
        userDTO.setAddrZipCode( userAddrZipCode( user ) );
        userDTO.setUserId( user.getUserId() );
        userDTO.setUserName( user.getUserName() );
        userDTO.setAge( user.getAge() );
        userDTO.setGender( user.getGender() );

        return userDTO;
    }

    private String userAddrAddress2(User user) {
        if ( user == null ) {
            return null;
        }
        Address addr = user.getAddr();
        if ( addr == null ) {
            return null;
        }
        String address2 = addr.getAddress2();
        if ( address2 == null ) {
            return null;
        }
        return address2;
    }

    private String userAddrAddress1(User user) {
        if ( user == null ) {
            return null;
        }
        Address addr = user.getAddr();
        if ( addr == null ) {
            return null;
        }
        String address1 = addr.getAddress1();
        if ( address1 == null ) {
            return null;
        }
        return address1;
    }

    private String userAddrZipCode(User user) {
        if ( user == null ) {
            return null;
        }
        Address addr = user.getAddr();
        if ( addr == null ) {
            return null;
        }
        String zipCode = addr.getZipCode();
        if ( zipCode == null ) {
            return null;
        }
        return zipCode;
    }
}

###テストコード

package com.test.lombok;

public class TestMain {

	public static void main(String[] args) throws Exception {

		User user = new User();
		user.setUserId("user001");
		user.setUserName("テスト");
		user.setAge("18");
		user.setGender("male");
		Address addr = new Address();
		addr.setZipCode("984-0031");
		addr.setAddress1("○○県");
		user.setAddr(addr);


		UserDTO userDTO = UserMapper.INSTANCE.userToUserDTO(user);

		System.out.println(userDTO);

	}
}

結果：

UserDTO(userId=user001, userName=テスト, age=18, gender=male, addrZipCode=984-0031, addrAddress1=○○県, addrAddress2=null, addrAddress3=null)

###ClassNotFoundエラー

Exception in thread "main" java.lang.ExceptionInInitializerError
	at com.test.lombok.TestMain.main(TestMain.java:18)
Caused by: java.lang.RuntimeException: java.lang.ClassNotFoundException: Cannot find implementation for com.test.lombok.UserMapper
	at org.mapstruct.factory.Mappers.getMapper(Mappers.java:61)
	at com.test.lombok.UserMapper.<clinit>(UserMapper.java:9)
	... 1 more
Caused by: java.lang.ClassNotFoundException: Cannot find implementation for com.test.lombok.UserMapper
	at org.mapstruct.factory.Mappers.getMapper(Mappers.java:75)
	at org.mapstruct.factory.Mappers.getMapper(Mappers.java:58)

build.gradleにsourceSetsを追加

build.gradle

sourceSets {
    main.java.srcDirs += "build/generated/sources/annotationProcessor/java/main"
}

詳細Doc：https://mapstruct.org/documentation/stable/reference/html/

#Dozer
https://github.com/DozerMapper/dozer

##使い方
###build.gradleに追加

build.gradle

// https://mvnrepository.com/artifact/com.github.dozermapper/dozer-core
compile group: 'com.github.dozermapper', name: 'dozer-core', version: '6.5.0'

###テストコード

package com.test.lombok;

import org.modelmapper.ModelMapper;
import org.modelmapper.TypeMap;

import com.github.dozermapper.core.DozerBeanMapperBuilder;
import com.github.dozermapper.core.Mapper;

public class TestMain {

	public static void main(String[] args) throws Exception {

		User user = new User();
		user.setUserId("user001");
		user.setUserName("テスト");
		user.setAge("18");
		user.setGender("male");
		Address addr = new Address();
		addr.setZipCode("984-0031");
		addr.setAddress1("○○県");
		user.setAddr(addr);

		Mapper mapper = DozerBeanMapperBuilder.buildDefault();
		UserDTO userDTO = mapper.map(user, UserDTO.class);

		System.out.println(userDTO);

	}
}

結果：

UserDTO(userId=user001, userName=テスト, age=18, gender=male, addrZipCode=null, addrAddress1=null, addrAddress2=null, addrAddress3=null)

同じ項目名はコピーされましたが、それ以外nullでした。

###違う項目の場合はXML、API、アノテーションで定義して対応可能
違う項目名の場合は @Mapping("binaryData")のように設定で対応可能ですが、複雑のタイプの場合はXMLで設定する必要があります。

好きなライブラリを選択すればと思います。

以上