DEV Community: Golov Pavel

Java and containers: best practices

Golov Pavel — Fri, 18 Dec 2020 08:54:44 +0000

Introduction

This note based on the presentation of eldermoraes.

https://www.youtube.com/watch?v=R4kxLsXkAE4.

Java and Containers

Common issues

Long build time
Huge image size
Hard maintainability
Resource allocation

Initial docker file

Let's look at the initial docker file:

FROM debian:stretch

COPY lib/* /deployment/lib/
COPY *-runner.jar /deployment/

RUN apt-get update
RUN apt-get -y install openjdk-8-jdk ssh vim

CMD ["java", "-jar", "/deployment/sample-runner.jar"]

This file has a bunch of problems, listed before. We will improve this file during the article.

How to avoid long build time

Put static layers at the top and changing layers at the bottom of the docker file

If we put changing layers at the top of the docker file and change these layers, it triggers to rebuild all the bottom layers that not been changed.

Bad practice:

COPY lib/* /deployment/lib/
COPY sample-runner.jar /deployment/

RUN apt-get update
RUN apt-get -y install openjdk-8-jdk ssh vim

Best practice:

RUN apt-get update
RUN apt-get -y install openjdk-8-jdk ssh vim

COPY lib/* /deployment/lib/
COPY sample-runner.jar /deployment/

Order of commands in the docker file is the matters!

Copy files with specific names

Try to don't use the wildcard copy command because if you create a file in the directory with the target file, the cache will be broken.

Bad practice:

COPY *-runner.jar /deployment/

Best practice:

COPY sample-runner.jar /deployment/app.jar

Group layers

Instead of having a list of commands, try to group your commands. It reduces cache size, images size and layers count.

Bad practice:

RUN apt-get update
RUN apt-get -y install openjdk-8-jdk ssh vim

Best practice:

RUN apt-get update \
 && apt-get -y install \
    openjdk-8-jdk ssh vim

How to get rid of huge image size

Use flag `--no-install-recommends` for `apt-get` command

Don't install not necessary staff into your container.

Bad practice:

RUN apt-get -y install openjdk-8-jdk

Best practice:

RUN apt-get -y install --no-install-recommends openjdk-8-jdk

Remove cache of your apt manager

For apt-get command use rm -rf /var/lib/apt/lists/* command.

Bad practice:

RUN apt-get update \
 && apt-get -y install --no-install-recommends openjdk-8-jdk

Best practice:

RUN apt-get update \
 && apt-get -y install --no-install-recommends openjdk-8-jdk
 && rm -rf /var/lib/apt/lists/*

How to stay away from hard maintainability

Use specific images

No need to use the Debian base image if you deploy your java application. Use the official language/framework image (like openjdk-alpine).

Bad practice:

FROM debian:stretch

Best practice:

FROM openjdk

Always specify base image tag

It didn't break your build and deploy in the case when the base image will be updated.

Bad practice:

FROM openjdk

Best practice:

FROM openjdk:8

Use JRE if you don't need to build your application

Bad practice:

FROM openjdk:8

Best practice:

FROM openjdk:8-jre-alpine

Java resource allocation

Before Java 8u121 Java didn't know about process cgroups, therefore Java application didn't know about resource limitations.

It's not recommended to use Java version before 8u121 in the container environment.

Result

Let's look at the result docker file.

FROM openjdk:8-jre-alpine

COPY lib/* /deployment/lib/
COPY sample-runner.jar /deployment/app.jar

CMD ["java", "-jar", "/deployment/app.jar"]

Using this file docker break the cache only if sample-runner.jar file will be changed. Moreover, it breaks only the last layer cache. Therefore, the build time was optimized.

The size of the initial docker image is 662MB and the size of the result docker image is 95.3MB. It's perfect optimization!

The result docker file looks compact and high-maintainable.

Make a query with sub condition for child list via Spring Data JPA

Golov Pavel — Mon, 23 Nov 2020 10:33:43 +0000

Introduction

Recently I faced this situation when I needed to query an Entity with non-deleted children via Spring Data JPA and Hibernate.

Let's look at the domain model of my test application:

@Entity
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE)
    private Long id;

    @OneToMany(mappedBy = "user", cascade = CascadeType.ALL)
    private List<Item> items;

    ... getters, setters, equals, and hashcode.
}

@Entity
public class Item {

    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE)
    private Long id;

    @ManyToOne
    @JoinColumn(name = "user")
    private User user;

    private Boolean deleted;

    ... getters, setters, equals, and hashcode.
}

The question was: how can I query all the users with non-deleted items?

The first idea was to use JPQL and @Query annotation inside the Spring Data CrudRepository. I wrote the next query:

public interface UserRepository extends CrudRepository<User, Long> {

    @Query("from User u left join u.items i where i.deleted = false or i.deleted is null")
    List<User> findUserWithNonDeletedItems();

}

Testing

The test code is:

@SpringBootApplication
public class DemoApplication implements CommandLineRunner {

    @Autowired
    private UserService userService;

    @Autowired
    private UserRepository userRepository;

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

    @Override
    public void run(String... args) throws Exception {
        userService.createUser();
        userService.makeQuery();
    }
}

@Service
public class UserService {

    private final UserRepository userRepository;

    public UserService(UserRepository userRepository) {
        this.userRepository = userRepository;
    }

    @Transactional
    public void makeQuery() {
        var result = userRepository.findUsersWithNonDeletedItems();
        assert result.get(0).getItems().size() == 1;
    }

    @Transactional
    public void createUser() {
        User userWithItems = new User();

        var items = List.of(
                new Item(userWithItems, false),
                new Item(userWithItems, true)
        );

        userWithItems.setItems(items);
        userRepository.save(userWithItems);

        User userWithoutItems = new User();
        userRepository.save(userWithoutItems);
    }
}

When I ran this code, I received AssertionError, because findUsersWithNonDeletedItems() returned 2 items for userWithItems, including a deleted item. The reason for this appearance is the absence of the fetch keyword.

Let's explain the difference between left join and left join fetch queries.

Left join

If we make the next JPQL request:

from User u
left join u.items i
where i.deleted = false or i.deleted is null

Hibernate is going to generate the following SQL statement:

SELECT u.*
FROM user u
LEFT OUTER JOIN item i ON i.user_id = u.id
WHERE i.deleted = false OR i.deleted is null

It never requests items for every user. As result, it makes an additional query for receiving all user's items, which doesn't contain a deleted filter.

Left join fetch

If we make the next JPQL request:

from User u
left join fetch u.items i
where i.deleted = false or i.deleted is null

Hibernate is going to generate the following SQL statement:

SELECT u.*, i.*
FROM user u
LEFT OUTER JOIN item i ON i.user_id = u.id
WHERE i.deleted = false OR i.deleted is null

In this query hibernate loads users with their items and filters items by deleted column. As result, we receive users with deleted items.

Solution

Result query looks like this:

public interface UserRepository extends CrudRepository<User, Long> {

    @Query("from User u left join fetch u.items i where i.deleted = false or i.deleted is null")
    List<User> findUsersWithNonDeletedItems();

}

It requests users with only non deleted items.

DEV Community: Golov Pavel

Java and containers: best practices

Introduction

Java and Containers

Common issues

Initial docker file

How to avoid long build time

Put static layers at the top and changing layers at the bottom of the docker file

Copy files with specific names

Group layers

How to get rid of huge image size

Use flag --no-install-recommends for apt-get command

Remove cache of your apt manager

How to stay away from hard maintainability

Use specific images

Always specify base image tag

Use JRE if you don't need to build your application

Java resource allocation

Result

Make a query with sub condition for child list via Spring Data JPA

Introduction

Testing

Left join

Left join fetch

Solution

Use flag `--no-install-recommends` for `apt-get` command