SimpleTrigger vs CronTrigger in Quartz Scheduler

SimpleTrigger
SimpleTrigger is used for scenarios in which we need to execute a job at a specific moment in time.

SimpleTrigger trigger = (SimpleTrigger) TriggerBuilder.newTrigger()
  .withIdentity("trigger1", "group1")
  .startAt(myStartTime)
  .forJob("job1", "group1")
  .build();

CronTrigger
The CronTrigger is used when we need schedules based on calendar-like statements.

CronTrigger trigger = TriggerBuilder.newTrigger()
  .withIdentity("trigger3", "group1")
  .withSchedule(CronScheduleBuilder.cronSchedule("0 0/2 8-17 * * ?"))
  .forJob("myJob", "group1")
  .build();

References
https://www.baeldung.com/quartz
https://stackoverflow.com/questions/26403391/difference-between-cron-trigger-and-simple-trigger-in-quartz-scheduler
http://www.quartz-scheduler.org/documentation/quartz-2.3.0/tutorials/crontrigger.html
https://www.freeformatter.com/cron-expression-generator-quartz.html

Volatile memory in Java

To ensure that updates to variables propagate predictably to other threads, we should apply the volatile modifier to those variables:

public class TaskRunner {

    private volatile static int number;
    private volatile static boolean ready;

    // same as before
}

This way, we communicate with runtime and processor to not reorder any instruction involving the volatile variable. Also, processors understand that they should flush any updates to these variables right away.

References
https://www.baeldung.com/java-volatile
https://www.baeldung.com/java-stack-heap

Force Spring Boot to use Gson instead of Jackson

Well, WebMvcConfigurerAdapter is deprecated. As of Spring 5.0 do this:

@SpringBootApplication(exclude = {JacksonAutoConfiguration.class})
public class Spring01Application {

    public static void main(String[] args) {
        SpringApplication.run(Spring01Application.class, args);
    }

}
@Configuration
public class WebConfig implements WebMvcConfigurer {
    @Bean
    public Gson gson() {
        GsonBuilder b = new GsonBuilder();
        b.registerTypeAdapterFactory(HibernateProxyTypeAdapter.FACTORY);
        b.registerTypeAdapterFactory(DateTypeAdapter.FACTORY);
        b.registerTypeAdapterFactory(TimestampTypeAdapter.FACTORY);
        b.registerTypeAdapterFactory(LocalDateTypeAdapter.FACTORY);
        b.registerTypeAdapterFactory(LocalDateTimeTypeAdapter.FACTORY);
        return b.create();
    }

    @Override
    public void configureMessageConverters(
        List<HttpMessageConverter<?>> converters) {
        StringHttpMessageConverter stringConverter = new StringHttpMessageConverter();
        stringConverter.setWriteAcceptCharset(false);
        stringConverter.setSupportedMediaTypes(Collections
            .singletonList(MediaType.TEXT_PLAIN));
        converters.add(stringConverter);
        converters.add(new ByteArrayHttpMessageConverter());
        converters.add(new SourceHttpMessageConverter<>());
        GsonHttpMessageConverter gsonHttpMessageConverter = new GsonHttpMessageConverter();
        gsonHttpMessageConverter.setGson(gson());
        gsonHttpMessageConverter.setSupportedMediaTypes(Arrays
            .asList(MediaType.APPLICATION_JSON));
        converters.add(gsonHttpMessageConverter);
    }
}

References
https://stackoverflow.com/questions/40786366/force-spring-boot-to-use-gson-instead-of-jackson

Parallel foreach in Java

List<Integer> listOfNumbers = Arrays.asList(1, 2, 3, 4);
listOfNumbers.parallelStream().forEach(number ->
    System.out.println(number + " " + Thread.currentThread().getName())
);

Custom Thread Pool

List<Integer> listOfNumbers = Arrays.asList(1, 2, 3, 4);
ForkJoinPool customThreadPool = new ForkJoinPool(4);
int sum = customThreadPool.submit(
    () -> listOfNumbers.parallelStream().reduce(0, Integer::sum)).get();
customThreadPool.shutdown();
assertThat(sum).isEqualTo(10);

References
https://www.baeldung.com/java-when-to-use-parallel-stream

Configuring Quartz Scheduler in Spring Boot

QuartzConfig.java

@Configuration
public class QuartzConfig {

    @Bean
    public JobDetailFactoryBean jobDetail() {
        JobDetailFactoryBean jobDetailFactory = new JobDetailFactoryBean();
        jobDetailFactory.setJobClass(GarbageCollectorJob.class);
        jobDetailFactory.setDescription("Invoke Sample Job service...");
        jobDetailFactory.setDurability(true);
        return jobDetailFactory;
    }

    @Bean
    public SimpleTriggerFactoryBean trigger(JobDetail job) {
        SimpleTriggerFactoryBean trigger = new SimpleTriggerFactoryBean();
        trigger.setJobDetail(job);
        // every one hour
        trigger.setRepeatInterval(60 * 60 * 1000);
        trigger.setRepeatCount(SimpleTrigger.REPEAT_INDEFINITELY);
        return trigger;
    }

    @Bean
    public SchedulerFactoryBean scheduler(Trigger trigger, JobDetail job, JobFactory springBeanJobFactory) {
        SchedulerFactoryBean schedulerFactory = new SchedulerFactoryBean();
        schedulerFactory.setConfigLocation(new ClassPathResource("quartz.properties"));

        schedulerFactory.setJobFactory(springBeanJobFactory);
        schedulerFactory.setJobDetails(job);
        schedulerFactory.setTriggers(trigger);
        return schedulerFactory;
    }

    @Bean
    public SpringBeanJobFactory springBeanJobFactory(ApplicationContext applicationContext) {
        AutoWiringSpringBeanJobFactory jobFactory = new AutoWiringSpringBeanJobFactory();
        jobFactory.setApplicationContext(applicationContext);
        return jobFactory;
    }
}

AutoWiringSpringBeanJobFactory.java

public final class AutoWiringSpringBeanJobFactory
        extends SpringBeanJobFactory
        implements ApplicationContextAware {

    private transient AutowireCapableBeanFactory beanFactory;

    public void setApplicationContext(
            final ApplicationContext context) {
        beanFactory = context.getAutowireCapableBeanFactory();
    }

    @Override
    protected Object createJobInstance(
            final TriggerFiredBundle bundle)
            throws Exception {
        final Object job = super.createJobInstance(bundle);
        beanFactory.autowireBean(job);
        return job;
    }
}

quartz.properties

org.quartz.threadPool.class=org.quartz.simpl.SimpleThreadPool
org.quartz.threadPool.threadCount=10
org.quartz.threadPool.threadsInheritContextClassLoaderOfInitializingThread=true

org.quartz.jobStore.class=org.quartz.simpl.RAMJobStore

References
https://www.baeldung.com/spring-quartz-schedule
http://www.btmatthews.com/blog/2011/inject-application-context+dependencies-in-quartz-job-beans.html
https://www.candidjava.com/spring-boot/quartz-example/

Spring Boot – Multiple Spring Data modules found, entering strict repository configuration mode

@EnableJpaRepositories(basePackages = {"com.ecommerce.core.repository.jpa"})
@EnableElasticsearchRepositories(basePackages= {"com.ecommerce.core.repository.elastic"})
@EnableRedisRepositories(basePackages = {"org.springframework.data.redis.connection.jedis"})

Since we are explicitly enabling the repositories on specific packages we can include this in the application.properties to avoid these errors:

spring.data.redis.repositories.enabled=false

We can do the same for the other repositories as well. If you encounter similar errors:

spring.data.elasticsearch.repositories.enabled=false
spring.data.jpa.repositories.enabled=false

References
https://stackoverflow.com/questions/47002094/spring-multiple-spring-data-modules-found-entering-strict-repository-configur

Serialize and Deserialize Joda Time in Gson

register a TypeAdapter with GSON to wrap the use of a Joda preconfigured Formatters :

    public static Gson gsonDateTime() {
    Gson gson = new GsonBuilder()
            .registerTypeAdapter(DateTime.class, new JsonSerializer<DateTime>() {
                @Override
                public JsonElement serialize(DateTime json, Type typeOfSrc, JsonSerializationContext context) {
                    return new JsonPrimitive(ISODateTimeFormat.dateTime().print(json));
                }
            })
            .registerTypeAdapter(DateTime.class, new JsonDeserializer<DateTime>() {
                @Override
                public DateTime deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
                    DateTime dt = ISODateTimeFormat.dateTime().parseDateTime(json.getAsString());
                    return dt;
                }
            })
            .create();
    return gson;
}

References
https://stackoverflow.com/questions/14996663/is-there-a-standard-implementation-for-a-gson-joda-time-serialiser

Spring Data MongoDB Transactions

MongoDB Configuration

@Configuration
@EnableMongoRepositories(basePackages = "com.baeldung.repository")
public class MongoConfig extends AbstractMongoClientConfiguration{

    @Bean
    MongoTransactionManager transactionManager(MongoDatabaseFactory dbFactory) {
        return new MongoTransactionManager(dbFactory);
    }

    @Override
    protected String getDatabaseName() {
        return "test";
    }

    @Override
    public MongoClient mongoClient() {
        final ConnectionString connectionString = new ConnectionString("mongodb://localhost:27017/test");
        final MongoClientSettings mongoClientSettings = MongoClientSettings.builder()
            .applyConnectionString(connectionString)
            .build();
        return MongoClients.create(mongoClientSettings);
    }
}

Synchronous Transactions

@Test
@Transactional
public void whenPerformMongoTransaction_thenSuccess() {
    userRepository.save(new User("John", 30));
    userRepository.save(new User("Ringo", 35));
    Query query = new Query().addCriteria(Criteria.where("name").is("John"));
    List<User> users = mongoTemplate.find(query, User.class);

    assertThat(users.size(), is(1));
}

Note that we can’t use listCollections command inside a multi-document transaction – for example:

References
https://www.baeldung.com/spring-data-mongodb-transactions