MapStruct requires Impl class

Question

I have next classes:

Mapper

public interface DeviceTokensMapper {

    DeviceTokensMapper INSTANCE = Mappers.getMapper(DeviceTokensMapper.class);

    @Mappings({
            @Mapping(source = "tokenName", target = "tokenName"),
            @Mapping(source = "userOsType", target = "osType"),
    })

    DeviceTokensDTO toDeviceTokensDTO(DeviceTokens deviceTokens);
}

Entity:

@Entity
public class DeviceTokens {

    @Id
    @Column(name="token_name", nullable = false)
    private String tokenName;

    @Column(name = "os", nullable = false)
    @Enumerated
    private UserOSType userOsType;

    public DeviceTokens() {}

    public DeviceTokens(String tokenName, UserOSType userOSType) {
        this.tokenName = tokenName;
        this.userOsType = userOSType;
    }

    public String getTokenName() {
        return tokenName;
    }

    public void setTokenName(String tokenName) {
        this.tokenName = tokenName;
    }

    public UserOSType getUserOsType() {
        return userOsType;
    }

    public void setUserOsType(UserOSType userOsType) {
        this.userOsType = userOsType;
    }
}

DTO:

public class DeviceTokensDTO {

    private String tokenName;

    private UserOSType osType;

    public DeviceTokensDTO() {}

    public DeviceTokensDTO(String tokenName, UserOSType osType) {
        this.tokenName = tokenName;
        this.osType = osType;
    }

    public UserOSType getOsType() {
        return osType;
    }

    public void setOsType(UserOSType osType) {
        this.osType = osType;
    }

    public String getTokenName() {
        return tokenName;
    }

    public void setTokenName(String tokenName) {
        this.tokenName = tokenName;
    }
}

And method from spring service class where I use this mapping:

@Transactional
public DeviceTokensDTO storeToken(String tokenId, UserOSType userOsType) {
    DeviceTokens deviceTokens = new DeviceTokens(tokenId, userOsType);
    DeviceTokens newDevice = deviceTokensRepository.save(deviceTokens);

    return DeviceTokensMapper.INSTANCE.toDeviceTokensDTO(newDevice);
}

When I run above method I see next exception:

ERROR [dispatcherServlet]:? - Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Handler processing failed; nested exception is java.lang.ExceptionInInitializerError] with root cause java.lang.ClassNotFoundException: dto.DeviceTokensMapperImpl

So why mapper require implementation class? Could please someone advise? Thanks.

Answer 1

if you use maven, you need to add mapstruct-processor dependency as follows:

<dependency>
    <groupId>org.mapstruct</groupId>
    <artifactId>mapstruct-jdk8</artifactId>
    <version>1.2.0.Final</version>
</dependency>
<dependency>
    <groupId>org.mapstruct</groupId>
    <artifactId>mapstruct-processor</artifactId>
    <version>1.2.0.Final</version>
</dependency>

Answer 2

MapStruct generates code at compile time, and the call to Mappers.getMapper(DeviceTokensMapper.class); will look for the generated implementation of the mapper interface. For some reason it seems to be missing in your deployment unit (WAR etc.).

Btw. when working with Spring as your DI container, you can use @Mapper(componentModel="spring") and you will be able to obtain mapper instances via dependency injection instead of using the Mappers factory.

Answer 3

Do you have both mapstruct-processor-xx and mapstruct-xx libraries included in your project?

I had the same problem and I realized that I forgot to include mapstruct-processor-xx.

Answer 4

Are you using Maven? If yes, then most probably you have missed the mapstruct-processor configuration under the maven compiler plugin.

The proper configuration is as follows:

<dependencies>
    <dependency>
        <groupId>org.mapstruct</groupId>
        <artifactId>mapstruct</artifactId> <!-- use mapstruct-jdk8 for Java 8 or higher -->
        <version>${org.mapstruct.version}</version>
    </dependency>
</dependencies>

<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.5.1</version>
            <configuration>
                <source>1.6</source> <!-- or higher, depending on your project -->
                <target>1.6</target> <!-- or higher, depending on your project -->
                <annotationProcessorPaths>
                    <path>
                        <groupId>org.mapstruct</groupId>
                        <artifactId>mapstruct-processor</artifactId>
                        <version>${org.mapstruct.version}</version>
                    </path>
                </annotationProcessorPaths>
            </configuration>
        </plugin>
    </plugins>
</build>