Introduction to Project Lombok

1. Overview

Java is a great language, but it can sometimes get too verbose for common tasks we have to do in our code or compliance with some framework practices. This often doesn’t bring any real value to the business side of our programs, and that’s where Lombok comes in to make us more productive.

It works by plugging into our build process and auto-generating Java bytecode into our .class files according to some project annotations we introduce in our code.

We can plug the Project Lombok Java library into a build tool to automate some code, such as getter/setter methods and logging variables.

In this tutorial, we’ll discuss how to use Lombok with Maven to avail of some of these features.

2. Maven Project Setup

Including Project Lombok in our builds, in whichever system we’re using, is very straightforward. Project Lombok’s project page has detailed instructions on the specifics. We can drop the Maven Lombok dependency in the provided scope:

<dependency>
    <groupId>org.projectlombok</groupId>
    <artifactId>lombok</artifactId>
    <version>1.18.36</version>
    <scope>provided</scope>
</dependency>

Depending on Lombok won’t make users of our .jars rely on it as well, as it’s a pure build dependency, not runtime.

3. Type Inference for Final Local Variables

As of Lombok 1.16.20, Lombok supports using var to infer the type of a local variable from the initializer expression. Lombok 1.18.22 supports using val for type inference of final local variables. In other words, Lombok replaces val with final var. However, we can’t use this feature on fields.

Let’s demonstrate type inference for Lombok local variables with an example:

public String lombokTypeInferred() {

    val list = new ArrayList<String>();
    list.add("Hello, Lombok!");
    val listElem = list.get(0);
    return listElem.toLowerCase(); 
}

Automatic code generation makes list a final variable of type ArrayList<String>.

4. Getters/Setters, Constructors

Encapsulating object properties via public getter and setter methods is a common practice in Java, and many frameworks rely on this “Java Bean” pattern extensively (a class with an empty constructor and get/set methods for “properties”).

This is so common that most IDEs support auto-generating code for these patterns (and more). However, this code needs to live in our sources and be maintained when a new property is added, or a field is renamed.

Let’s consider this class we want to use as a JPA entity:

@Entity
public class User implements Serializable {

    private @Id Long id; // will be set when persisting

    private String firstName;
    private String lastName;
    private int age;

    public User() {}

    public User(String firstName, String lastName, int age) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.age = age;
    }

    // getters and setters: ~30 extra lines of code
}

This is a rather simple class, but imagine if we had added the extra code for getters and setters. We would have ended up with a definition with more boilerplate zero-value code than the relevant business information: “A User has first and last names and age.”

Let’s now Lombok-ize this class:

@Entity
@Getter @Setter @NoArgsConstructor // <--- THIS is it
public class User implements Serializable {

    private @Id Long id; // will be set when persisting

    private String firstName;
    private String lastName;
    private int age;

    public User(String firstName, String lastName, int age) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.age = age;
    }
}

By adding the @Getter and @Setter annotations, we told Lombok to generate these for all the class fields. @NoArgsConstructor will lead to an empty constructor generation.

Note that this is the whole class code; unlike the version above with the // getters and setters comment, we’re not omitting anything. For a three-relevant attributes class, this is a significant code saving!

If we add attributes (properties) to our User class, the same will happen; we apply the annotations to the type itself so they will mind all fields by default.

What if we want to refine the visibility of some properties? For example, if we want to keep our entities’ id field modifiers package or protected visible because they are expected to be read, but not explicitly set by application code, we can just use a finer-grained @Setter for this particular field:

private @Id @Setter(AccessLevel.PROTECTED) Long id;

5. Lazy Getter

Applications often need to perform expensive operations and save the results for subsequent use.

For instance, let’s say we need to read static data from a file or a database. It’s generally good practice to retrieve this data once, and then cache it to allow in-memory reads within the application. This saves the application from repeating the expensive operation.

Another common pattern is to retrieve this data only when it’s first needed. In other words, we only get the data when we call the corresponding getter the first time. We call this lazy-loading.

Let’s suppose this data is cached as a field inside a class. The class must now make sure that any access to this field returns the cached data. One possible way to implement such a class is to use the getter method to retrieve the data only if the field is null. We call this a lazy getter.

Lombok makes this possible with the lazy parameter in the @Getter annotation we saw above.

For example, consider this simple class:

public class GetterLazy {

    @Getter(lazy = true)
    private final Map<String, Long> transactions = getTransactions();

    private Map<String, Long> getTransactions() {

        final Map<String, Long> cache = new HashMap<>();
        List<String> txnRows = readTxnListFromFile();

        txnRows.forEach(s -> {
            String[] txnIdValueTuple = s.split(DELIMETER);
            cache.put(txnIdValueTuple[0], Long.parseLong(txnIdValueTuple[1]));
        });

        return cache;
    }
}

This reads some transactions from a file into a Map. Since the data in the file doesn’t change, we’ll cache it once and allow access via a getter.

If we now look at the compiled code of this class, we’ll see a getter method that updates the cache if it was null and then returns the cached data:

public class GetterLazy {

    private final AtomicReference<Object> transactions = new AtomicReference();

    public GetterLazy() {
    }

    //other methods

    public Map<String, Long> getTransactions() {
        Object value = this.transactions.get();
        if (value == null) {
            synchronized(this.transactions) {
                value = this.transactions.get();
                if (value == null) {
                    Map<String, Long> actualValue = this.readTxnsFromFile();
                    value = actualValue == null ? this.transactions : actualValue;
                    this.transactions.set(value);
                }
            }
        }

        return (Map)((Map)(value == this.transactions ? null : value));
    }
}

It’s interesting to point out that Lombok wrapped the data field in an AtomicReference. This ensures atomic updates to the transactions field. The getTransactions() method also makes sure to read the file if transactions is null.

We discourage using the AtomicReference transactions field directly from within the class. We recommend using the getTransactions() method for accessing the field.

For this reason, if we use another Lombok annotation like ToString in the same class, it will use getTransactions() instead of directly accessing the field.

6. Immutable Setter With One Changed Field

Furthermore, we can use the @With annotation to automatically generate a method that can clone an object with one changed field. As an example, when we want to clone a User object with a new age value we can annotate the field with @With:

@AllArgsConstructor
public class User implements Serializable {

    private @Id Long id;  

    private final String firstName;
    private final String lastName;
    @With private final int age;

}

Subsequently, we can use the automatically generated withAge(int newAge) instance method to clone a User but with a different age:

User user = new User("John","Smith",40);
User user_updated = user.withAge(41);

7. Value Classes/DTO’s

There are many situations in which we want to define a data type with the sole purpose of representing complex “values” as “Data Transfer Objects,”. Most of the time in the form of immutable data structures we build once and never want to change.

We design a class to represent a successful login operation. We want all fields to be non-null and objects to be immutable so that we can thread-safely access their properties:

public class LoginResult {

    private final Instant loginTs;

    private final String authToken;
    private final Duration tokenValidity;
    
    private final URL tokenRefreshUrl;

    // constructor taking every field and checking nulls

    // read-only accessor, not necessarily as get*() form
}

Again, the amount of code we would have to write for the commented sections would be of a much larger volume than the information we want to encapsulate needs to be. We can use Lombok to improve this:

@RequiredArgsConstructor
@Accessors(fluent = true) @Getter
public class LoginResult {

    private final @NonNull Instant loginTs;

    private final @NonNull String authToken;
    private final @NonNull Duration tokenValidity;
    
    private final @NonNull URL tokenRefreshUrl;

}

Once we add the @RequiredArgsConstructor annotation, we’ll get a constructor for all the final fields in the class, just as we declared them. Adding @NonNull to attributes makes our constructor check for nullability and throws NullPointerExceptions accordingly. This would also happen if the fields were non-final and we added @Setter for them.

Do we want the boring old get*() form for our properties? Since we added @Accessors(fluent=true) in this example, “getters” would have the same method name as the properties; getAuthToken() simply becomes authToken().

This “fluent” form would apply to non-final fields for attribute setters, as well as allow for chained calls:

// Imagine fields were no longer final now
return new LoginResult()
  .loginTs(Instant.now())
  .authToken("asdasd")
  // and so on

8. Core Java Boilerplate

Another situation where we end up writing code we need to maintain is when generating the toString(), equals(), and hashCode() methods. IDEs try to help with templates for auto-generating these in terms of our class attributes.

We can automate this using other Lombok class-level annotations:

• @ToString: will generate a toString() method including all class attributes. No need to write one ourselves and maintain it as we enrich our data model.
• @EqualsAndHashCode: will generate both equals() and hashCode() methods by default considering all relevant fields, and according to very well thought semantics.

These generators ship very handy configuration options. For example, if our annotated classes take part of a hierarchy, we can just use the callSuper=true parameter and parent results will be considered when generating the method’s code.

8.1. A Demo

To demonstrate this, let’s say we had our User JPA entity example include a reference to events associated with this user:

@OneToMany(mappedBy = "user")
private List<UserEvent> events;

We wouldn’t want to have the whole list of events dumped whenever we call the toString() method of our User, just because we used the @ToString annotation. Instead, we can parameterize it like this, @ToString(exclude = {“events”}), and that won’t happen. This is also helpful to avoid circular references if, for example, UserEvents had a reference to a User.

For the LoginResult example, we may want to define equality and hash code calculation just in terms of the token itself and not the other final attributes in our class. Then, we can simply write something like @EqualsAndHashCode(of = {“authToken”}).

If the features from the annotations we’ve reviewed so far are of interest, we may want to examine @Data and @Value annotations too, as they behave as if a set of them had been applied to our classes. After all, these discussed usages are very commonly put together in many cases.

8.2. (Not) Using the @EqualsAndHashCode With JPA Entities

Whether we should use the default equals() and hashCode() methods, or create custom ones for the JPA entities, is an often discussed topic among developers. There are multiple approaches we can follow, each having its pros and cons.

By default, @EqualsAndHashCode includes all non-final properties of the entity class. We can try to “fix” this by using the onlyExplicitlyIncluded attribute of the @EqualsAndHashCode to make Lombok use only the entity’s primary key. Still, the generated equals() method can cause some issues. Thorben Janssen explains this scenario in greater detail in one of his blog posts.

In general, we should avoid using Lombok to generate the equals() and hashCode() methods for our JPA entities.

9. The Builder Pattern

The following could make for a sample configuration class for a REST API client:

public class ApiClientConfiguration {

    private String host;
    private int port;
    private boolean useHttps;

    private long connectTimeout;
    private long readTimeout;

    private String username;
    private String password;

    // Whatever other options you may thing.

    // Empty constructor? All combinations?

    // getters... and setters?
}

We could take an initial approach based on using the class-default empty constructor and providing setter methods for every field; however, we ideally want configurations not to be re-set once they’ve been built (instantiated), effectively making them immutable. Therefore, we want to avoid setters, but writing such a potentially long args constructor is an anti-pattern.

Instead, we can tell the tool to generate a builder pattern, which negates us from having to write an extra Builder class and the associated fluent setter-like methods by simply adding the @Builder annotation to our ApiClientConfiguration:

@Builder
public class ApiClientConfiguration {

    // ... everything else remains the same

}

Leaving the class definition such as above (no declare constructors or setters + @Builder), we can end up using it as:

ApiClientConfiguration config = 
  ApiClientConfiguration.builder()
    .host("api.server.com")
    .port(443)
    .useHttps(true)
    .connectTimeout(15_000L)
    .readTimeout(5_000L)
    .username("myusername")
    .password("secret")
    .build();

10. Checked Exceptions Burden

Many Java APIs are designed to throw one or more of several checked exceptions; client code is forced to either catch or declare to throws. How many times have we turned these exceptions we know won’t happen into something like this:

public String resourceAsString() {
    try (InputStream is = this.getClass().getResourceAsStream("sure_in_my_jar.txt")) {
        BufferedReader br = new BufferedReader(new InputStreamReader(is, "UTF-8"));
        return br.lines().collect(Collectors.joining("\n"));
    } catch (IOException | UnsupportedCharsetException ex) {
          // If this ever happens, then its a bug.
          throw new RuntimeException(ex); <--- encapsulate into a Runtime ex.
    }
}

If we want to avoid this code pattern because the compiler won’t be happy (and we know the checked errors cannot happen), use the aptly named @SneakyThrows:

@SneakyThrows
public String resourceAsString() {
    try (InputStream is = this.getClass().getResourceAsStream("sure_in_my_jar.txt")) {
        BufferedReader br = new BufferedReader(new InputStreamReader(is, "UTF-8"));
        return br.lines().collect(Collectors.joining("\n"));
    } 
}

11. Ensure Our Resources Are Released

Java 7 introduced the try-with-resources block to ensure that resources held by instances of implementing java.lang.AutoCloseable are released when exiting.

Lombok provides an alternative and more flexible way of achieving this via @Cleanup. We can use it for any local variable whose resources we want to make sure are released. There’s no need for them to implement any particular interface, we’ll just get the close() method called:

@Cleanup InputStream is = this.getClass().getResourceAsStream("res.txt");

Our releasing method has a different name? No problem, we just customize the annotation:

@Cleanup("dispose") JFrame mainFrame = new JFrame("Main Window");

12. Annotate Our Class to Get a Logger

Many of us add logging statements to our code sparingly by creating an instance of a Logger from our framework of choice. Let’s say SLF4J:

public class ApiClientConfiguration {

    private static Logger LOG = LoggerFactory.getLogger(ApiClientConfiguration.class);

    // LOG.debug(), LOG.info(), ...

}

This is such a common pattern that Lombok developers have simplified it for us:

@Slf4j // or: @Log @CommonsLog @Log4j @Log4j2 @XSlf4j
public class ApiClientConfiguration {

    // log.debug(), log.info(), ...

}

Many logging frameworks are supported, and of course, we can customize the instance name, topic, etc.

13. Write Thread-Safer Methods

In Java, we can use the synchronized keyword to implement critical sections; however, this is not a 100% safe approach. Other client code can eventually also synchronize on our instance, potentially leading to unexpected deadlocks.

This is where @Synchronized comes in. We can annotate our methods (both instance and static) with it, and we’ll get an auto-generated, private, unexposed field that our implementation will use for locking:

@Synchronized
public /* better than: synchronized */ void putValueInCache(String key, Object value) {
    // whatever here will be thread-safe code
}

However, when using virtual threads, introduced in Java 21, we should use the @Locked, @Locked.Read, and @Locked.Write annotations to obtain a ReentrantLock.

14. Automate Objects Composition

Java doesn’t have language-level constructs to smooth out a “favor composition inheritance” approach. Other languages have built-in concepts such as Traits or Mixins to achieve this.

Lombok’s @Delegate comes in very handy when we want to use this programming pattern. Let’s consider an example, where we:

• want Users and Customers to share some common attributes for naming and phone number
• define both an interface and an adapter class for these fields
• have our models implement the interface and @Delegate to their adapter, effectively composing them with our contact information

First, let’s define an interface:

public interface HasContactInformation {

    String getFirstName();
    void setFirstName(String firstName);

    String getFullName();

    String getLastName();
    void setLastName(String lastName);

    String getPhoneNr();
    void setPhoneNr(String phoneNr);

}

Now an adapter as a support class:

@Data
public class ContactInformationSupport implements HasContactInformation {

    private String firstName;
    private String lastName;
    private String phoneNr;

    @Override
    public String getFullName() {
        return getFirstName() + " " + getLastName();
    }
}

Now for the interesting part let’s see how easy it is to compose contact information into both model classes:

public class User implements HasContactInformation {

    // Whichever other User-specific attributes

    @Delegate(types = {HasContactInformation.class})
    private final ContactInformationSupport contactInformation =
      new ContactInformationSupport();

    // User itself will implement all contact information by delegation
    
}

The case for Customer would be so similar that we can omit the sample for brevity.

15. Constants for Field Names

When we deal with classes with many fields, especially when working with reflection, serialization frameworks, or building dynamic queries, we often create string constants representing field names. Manually managing these constants is error-prone and repetitive.

Lombok simplifies this task by providing the @FieldNameConstants annotation, which automatically generates string constants for a class’s field names.

Let’s consider the following class:

@Getter
@FieldNameConstants
public class Person {

    private final String firstName;
    private final String lastName;
    private final int age;

    public Person(String firstName, String lastName, int age) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.age = age;
    }
}

When we use the @FieldNameConstants annotation, Lombok generates a convenient nested static class containing constant field names. This allows us to safely reference fields and avoid hardcoding string literals in our code.

For instance, Lombok generates a nested class Fields with constants:

public class Person {

    public static final class Fields {
    public static final String firstName = "firstName";
    public static final String lastName = "lastName";
    public static final String age = "age";

    // Existing code

}

Now, whenever we need to refer to fields by name, we can use these constants, avoiding typos and simplifying refactoring:

CriteriaBuilder cb = entityManager.getCriteriaBuilder();
CriteriaQuery<Person> query = cb.createQuery(Person.class);
Root<Person> root = query.from(Person.class);
query.select(root).where(cb.equal(root.get(Person.Fields.lastName), "Doe"));

This feature helps us avoid string literals scattered throughout the code, making it safer, more readable, and easier to maintain.

We can also customize the name of the inner class:

@FieldNameConstants(innerTypeName = "FieldConstants")

16. Rolling Lombok Back?

Short answer: Not at all, really.

There may be a worry that if we use Lombok in one of our projects, we may later want to rollback that decision. The potential problem could be that there are a large number of classes annotated for it. In this case, we’re covered thanks to the delombok tool from the same project.

By delombok-ing our code, we get auto-generated Java source code with exactly the same features from the bytecode Lombok built. We can then simply replace our original annotated code with these new delomboked files, and no longer depend on it.

This is something we can integrate into our build.

17. Conclusion

There are still some other features we haven’t presented in this article. We can take a deeper dive into the feature overview for more details and use cases.

Furthermore, most functions we’ve shown have a number of customization options that we may find handy. The available built-in configuration system could also help us with that.

Now that we can give Lombok a chance to get into our Java development toolset, we can boost our productivity.