...w.s.m.s.DefaultHandlerExceptionResolver :
Failed to write HTTP message: org.springframework.http.converter.HttpMessageNotWritableException:
    Could not write JSON document: failed to lazily initialize a collection of role:
        com.foobar.bar.domain.Bar.manyBars, could not initialize proxy - no Session (through reference chain: java.util.ArrayList[0]-com.foobar.bar.domain.Bar["manyBars"]);
    nested exception is com.fasterxml.jackson.databind.JsonMappingException:
        failed to lazily initialize a collection of role:
        com.foobar.bar.domain.Bar.manyBars, could not initialize proxy - no Session (through reference chain: java.util.ArrayList[0]->com.foobar.bar.domain.Bar["manyBars"])

# MySQL DB - "foo"
spring.datasource.url=jdbc:mysql://localhost:3306/foo?currentSchema=public
spring.datasource.username=XXX
spring.datasource.password=XXX
spring.datasource.driver-class-name=com.mysql.jdbc.Driver
# MySQL DB - "bar"
bar.datasource.url=jdbc:mysql://localhost:3306/bar?currentSchema=public
bar.datasource.username=XXX
bar.datasource.password=XXX
bar.datasource.driver-class-name=com.mysql.jdbc.Driver
# JPA
spring.jpa.show-sql=true
spring.jpa.hibernate.ddl-auto=none
spring.jpa.properties.hibernate.dialect=org.hibernate.dialect.MySQL5Dialect

@Configuration
@EnableTransactionManagement
@EnableJpaRepositories(entityManagerFactoryRef = "entityManagerFactory",
        transactionManagerRef = "transactionManager",
        basePackages = {"com.foobar.foo.repo"})
public class FooDbConfig {

    @Primary
    @Bean(name = "dataSource")
    @ConfigurationProperties(prefix = "spring.datasource")
    public DataSource dataSource() {
        return DataSourceBuilder.create().build();
    }

    @Primary
    @Bean(name = "entityManagerFactory")
    public LocalContainerEntityManagerFactoryBean entityManagerFactory(
            EntityManagerFactoryBuilder builder, @Qualifier("dataSource") DataSource dataSource) {
        return builder
                .dataSource(dataSource)
                .packages("com.foobar.foo.domain")
                .persistenceUnit("foo")
                .build();
    }

    @Primary
    @Bean(name = "transactionManager")
    public PlatformTransactionManager transactionManager(
            @Qualifier("entityManagerFactory") EntityManagerFactory entityManagerFactory) {
        return new JpaTransactionManager(entityManagerFactory);
    }
}

@Configuration
@EnableTransactionManagement
@EnableJpaRepositories(entityManagerFactoryRef = "barEntityManagerFactory",
        transactionManagerRef = "barTransactionManager", basePackages = {"com.foobar.bar.repo"})
public class BarDbConfig {

    @Bean(name = "barDataSource")
    @ConfigurationProperties(prefix = "bar.datasource")
    public DataSource dataSource() {
        return DataSourceBuilder.create().build();
    }

    @Bean(name = "barEntityManagerFactory")
    public LocalContainerEntityManagerFactoryBean barEntityManagerFactory(
            EntityManagerFactoryBuilder builder, @Qualifier("barDataSource") DataSource dataSource) {
        return builder
                .dataSource(dataSource)
                .packages("com.foobar.bar.domain")
                .persistenceUnit("bar")
                .build();
    }

    @Bean(name = "barTransactionManager")
    public PlatformTransactionManager barTransactionManager(
            @Qualifier("barEntityManagerFactory") EntityManagerFactory barEntityManagerFactory) {
        return new JpaTransactionManager(barEntityManagerFactory);
    }
}

@RestController
public class FooBarController {

    private final FooRepository fooRepo;
    private final BarRepository barRepo;

    @Autowired
    FooBarController(FooRepository fooRepo, BarRepository barRepo) {
        this.fooRepo = fooRepo;
        this.barRepo = barRepo;
    }

    @RequestMapping("/foo")
    public List<Foo> listFoo() {
        return fooRepo.findAll();
    }

    @RequestMapping("/bar")
    public List<Bar> listBar() {
        return barRepo.findAll();
    }

    @RequestMapping("/foobar/{id}")
    public String fooBar(@PathVariable("id") Integer id) {
        Foo foo = fooRepo.findById(id);
        Bar bar = barRepo.findById(id);

        return foo.getName() + " " + bar.getName() + "!";
    }

}

@Entity
@Table(name = "foo")
public class Foo {

    @Id
    @GeneratedValue(strategy = IDENTITY)
    @Column(name = "id", unique = true, nullable = false)
    private Integer id;

    @Column(name = "name")
    private String name;

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "foo")
    @JsonIgnoreProperties({"foo"})
    private Set<ManyFoo> manyFoos = new HashSet<>(0);

    // Constructors, Getters, Setters
}

@Entity
@Table(name = "many_foo")
public class ManyFoo {

    @Id
    @GeneratedValue(strategy = IDENTITY)
    @Column(name = "id", unique = true, nullable = false)
    private Integer id;

    @Column(name = "name")
    private String name;

    @ManyToOne(fetch = FetchType.LAZY)
    @JsonIgnoreProperties({"manyFoos"})
    private Foo foo;

    // Constructors, Getters, Setters
}

@SpringBootApplication
public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}