`@FreeBuilder`

Automatic generation of the Builder pattern for Java 1.6+

The Builder pattern is a good choice when designing classes whose constructors or static factories would have more than a handful of parameters. — Effective Java, Second Edition, page 39

Background
How to use @FreeBuilder
Build tools and IDEs
- javac
- Maven
- Gradle
- Eclipse
- IntelliJ
Troubleshooting
Alternatives
- AutoValue vs @FreeBuilder
- Proto vs @FreeBuilder
Wait, why "free"?
License

Background

Implementing the Builder pattern in Java is tedious, error-prone and repetitive. Who hasn't seen a ten-argument constructor, thought cross thoughts about the previous maintainers of the class, then added "just one more"? Even a simple four-field class requires 39 lines of code for the most basic builder API, or 72 lines if you don't use a utility like AutoValue to generate the value boilerplate.

@FreeBuilder produces all the boilerplate for you, as well as free extras like JavaDoc, getter methods, collections support, nested builders, and partial values (used in testing), which are highly useful, but would very rarely justify their creation and maintenance burden in hand-crafted code. (We also reserve the right to add more awesome methods in future!)

[The Builder pattern] is more verbose…so should only be used if there are enough parameters, say, four or more. But keep in mind that you may want to add parameters in the future. If you start out with constructors or static factories, and add a builder when the class evolves to the point where the number of parameters starts to get out of hand, the obsolete constructors or static factories will stick out like a sore thumb. Therefore, it's often better to start with a builder in the first place. — Effective Java, Second Edition, page 39

How to use `@FreeBuilder`

Quick start

See Build tools and IDEs for how to add @FreeBuilder to your project's build and/or IDE.

Create your value type (e.g. Person) as an interface or abstract class, containing an abstract accessor method for each desired field. This accessor must be non-void, parameterless, and start with 'get' or 'is'. Add the @FreeBuilder annotation to your class, and it will automatically generate an implementing class and a package-visible builder API (Person_Builder), which you must subclass. For instance:

import org.inferred.freebuilder.FreeBuilder;

@FreeBuilder
public interface Person {
  /** Returns the person's full (English) name. */
  String getName();
  /** Returns the person's age in years, rounded down. */
  int getAge();
  /** Builder of {@link Person} instances. */
  class Builder extends Person_Builder { }
}

If you are writing an abstract class, or using Java 8, you may wish to hide the builder's constructor and manually provide instead a static builder() method on the value type (though Effective Java does not do this).

What you get

If you write the Person interface shown above, you get:

A builder class with:
- a no-args constructor
- JavaDoc
- getters (throwing IllegalStateException for unset fields)
- setters
- mergeFrom methods to copy data from existing values or builders
- a build method that verifies all fields have been set
  - see below for default values and constraint checking
An implementation of Person with:
- toString
- equals and hashCode
A partial implementation of Person for unit tests with:
- UnsupportedOperationException-throwing getters for unset fields
- toString
- equals and hashCode

Person person = new Person.Builder()
    .setName("Phil")
    .setAge(31)
    .build();
System.out.println(person);  // Person{name=Phil, age=31}

Defaults and constraints

We use method overrides to add customization like default values and constraint checks. For instance:

@FreeBuilder
public interface Person {
  /** Returns the person's full (English) name. */
  String getName();
  /** Returns the person's age in years, rounded down. */
  int getAge();
  /** Returns a human-readable description of the person. */
  String getDescription();
  /** Builder class for {@link Person}. */
  class Builder extends Person_Builder {
    public Builder() {
      // Set defaults in the builder constructor.
      setDescription("Indescribable");
    }
    @Override public Builder setAge(int age) {
      // Check single-field (argument) constraints in the setter methods.
      checkArgument(age >= 18);
      return super.setAge(age);
    }
    @Override public Person build() {
      // Check cross-field (state) constraints in the build method.
      Person person = super.build();
      checkState(!person.getDescription().contains(person.getName()));
      return person;
    }
  }
}

Optional values

If a property is optional—that is, has no reasonable default—then use the Optional type. It will default to Optional.absent(), and the Builder will gain additional convenience setter methods.

  /** Returns an optional human-readable description of the person. */
  Optional<String> getDescription();

Prefer to use explicit defaults where meaningful, as it avoids the need for edge-case code; but prefer Optional to ad-hoc 'not set' defaults, like -1 or the empty string, as it forces the user to think about those edge cases. Note that setDescription will not accept a null; instead, call clearDescription() to reset the field, or setNullableDescription(Value) if you have a potentially-null value.

Using `@Nullable`

As Java currently stands, you should strongly prefer Optional to returning nulls. Using null to represent unset properties is the classic example of Hoare's billion-dollar mistake: a silent source of errors that users won't remember to write test cases for, and which won't be spotted in code reviews. The large "air gap" between the declaration of the getter and the usage is the cause of this problem. Optional uses the compiler to force the call sites to perform explicit null handling, giving reviewers a better chance of seeing mistakes. See also Using and Avoiding Null.

Obviously, greenfield code can trivially adopt Optional, but even existing APIs can be converted to Optional via a simple refactoring sequence; see below. However, if you have compelling legacy reasons that mandate using nulls, you can disable null-checking by marking the getter method @Nullable. (Any annotation type named "Nullable" will do, but you may wish to use javax.annotation.Nullable, as used in Google Guava.)

Converting from `@Nullable`

This is the O(1), non-tedious, non–error-prone way we recomment converting @Nullable to Optional:

Load all your code in Eclipse, or another IDE with support for renaming and inlining.
[IDE REFACTOR] Rename all your @Nullable getters to getNullableX.
Add an Optional-returning getX
Implement your getNullableX methods as: return getX().orNull()
[IDE REFACTOR] Inline your getNullableX methods

At this point, you have effectively performed an automatic translation of a @Nullable method to an Optional-returning one. Of course, your code is not optimal yet (e.g. if (foo.getX().orNull() != null) instead of if (foo.getX().isPresent()) ). Search-and-replace should get most of these issues.

[IDE REFACTOR] Rename all your @Nullable setters to setNullableX.

Your API is now @FreeBuilder-compatible :)

Collections and Maps

@FreeBuilder has special support for List, Set, Multiset, Map and Multimap properties:

The Builder's setX method is removed
Mutation methods are added instead: addX (collections), putX (maps) and clearX
The Builder's getX method returns an unmodifiable view of the current values: when the Builder is changed, the view also changes
The property defaults to an empty collection
The value type returns immutable collections

Nested buildable types

@FreeBuilder has special support for buildable types like protos and other @FreeBuilder types:

The setX method gains an overload for the property type's Builder, as shorthand for setX(x.build())
The getX method is removed
A getXBuilder method is added instead, returning a mutable Builder for the property
The property inherits the defaults of its Builder type

Builder construction

Effective Java recommends passing required parameters in to the Builder constructor. While we follow most of the recommendations therein, we explicitly do not follow this one: while you gain compile-time verification that all parameters are set, you lose flexibility in client code, as well as opening yourself back up to the exact same subtle usage bugs as traditional constructors and factory methods. For the default @FreeBuilder case, where all parameters are required, this does not scale.

If you want to follow Effective Java more faithfully in your own types, however, just create the appropriate constructor in your builder subclass:

    public Builder(String name, int age) {
      // Set all initial values in the builder constructor
      setName(name);
      setAge(age);
    }

Implementation note: in javac, we spot these fields being set in the constructor, and do not check again at runtime.

Partials

A partial value is an implementation of the value type which does not conform to the type's state constraints. It may be missing required fields, or it may violate a cross-field constraint.

Person person = new Person.Builder()
    .setName("Phil")
    .buildPartial();  // build() would throw an IllegalStateException here
System.out.println(person);  // prints: partial Person{name="Phil"}
person.getAge();  // throws UnsupportedOperationException

As partials violate the (legitimate) expectations of your program, they must not be created in production code. (They may also affect the performance of your program, as the JVM cannot make as many optimizations.) However, when testing a component which does not rely on the full state restrictions of the value type, partials can reduce the fragility of your test suite, allowing you to add new required fields or other constraints to an existing value type without breaking swathes of test code.

Jackson

To create types compatible with the Jackson JSON serialization library, use the @JsonProperty annotation on your getter methods as normal, and use the builder property of @JsonDeserialize to point Jackson at your Builder class. For instance:

// This type can be freely converted to and from JSON with Jackson
@JsonDeserialize(builder = Address.Builder.class)
interface Address {
    @JsonProperty("city") String getCity();
    @JsonProperty("state") String getState();

    class Builder extends Address_Builder {}
}

@FreeBuilder will copy the @JsonProperty annotations to the relevant setter methods on the builder.

GWT

To enable GWT serialization of the generated Value subclass, just add @GwtCompatible(serializable = true) to your @FreeBuilder-annotated type, and extend/implement Serializable. This will generate a CustomFieldSerializer, and ensure all necessary types are whitelisted.

Build tools and IDEs

javac

Download freebuilder-1.2.jar and add it to the classpath (or processorpath, if you supply one) on the command line. You will also need to add guava-18.0.jar to the classpath, as it is used in the generated code.

Maven

Add the @FreeBuilder artifact as an optional dependency to your Maven POM:

<dependencies>
  <dependency>
    <groupId>org.inferred</groupId>
    <artifactId>freebuilder</artifactId>
    <version>1.2</version>
    <optional>true</optional>
  </dependency>
  <dependency>
    <groupId>com.google.guava</groupId>
    <artifactId>guava</artifactId>
    <version>18.0</version>
  </dependency>
</dependencies>

You may already have Guava, or want to use a different version; adjust accordingly.

Gradle

Add the following lines to your dependencies:

compile 'org.inferred:freebuilder:1.2'
compile 'com.google.guava:guava:18.0'

You may already have Guava, or want to use a different version; adjust accordingly.

Eclipse

Condensed from Eclipse Indigo's documentation.

Download freebuilder-1.2.jar and guava-18.0.jar and add them to your project. Select them, right-click and choose Build path > Add to Build path.

In your projects properties dialog, go to Java Compiler > Annotation Processing and ensure Enable annotation processing is checked. Next, go to Java Compiler > Annotation Processing > Factory Path, select Add JARs, and select freebuilder-1.2.jar.

IntelliJ

Condensed from the IntelliJ 14.0.3 documentation and Auto Issue #106.

Download freebuilder-1.2.jar and guava-18.0.jar, add them to your project, right-click them and select Use as Project Library.

In your Settings, go to Build, Execution, Deployment > Compiler > Annotation Processors and ensure Enable annotation processing is selected, and Store generated sources relative to is set to Module content root. (If you have specified a processor path, ensure you add the new JAR to it. Similarly, if you choose to specify processors explicitly, add org.inferred.freebuilder.processor.Processor to the list.)

Run Build > Rebuild Project, then right-click the new generated folder (this may have a different name if you have changed the Production sources directory setting) and select Mark Directory As > Generated Sources Root.

Troubleshooting

Troubleshooting javac

If you make a mistake in your code (e.g. giving your value type a private constructor), @FreeBuilder is designed to output a Builder superclass anyway, with as much of the interface intact as possible, so the only errors you see are the ones output by the annotation processor.

Unfortunately, javac has a broken design: if any annotation processor outputs any error whatsoever, all generated code is discarded, and all references to that generated code are flagged as broken references. Since your Builder API is generated, that means every usage of a @FreeBuilder builder becomes an error. This deluge of false errors swamps the actual error you need to find and fix. (See also the related issue.)

If you find yourself in this situation, search the output of javac for the string "@FreeBuilder type"; nearly all errors include this in their text.

Troubleshooting Eclipse

Eclipse manages, somehow, to be worse than javac here. It will never output annotation processor errors unless there is another error of some kind; and, even then, only after an incremental, not clean, compile. In practice, most mistakes are made while editing the @FreeBuilder type, which means the incremental compiler will flag the errors. If you find a generated superclass appears to be missing after a clean compile, however, try touching the relevant file to trigger the incremental compiler. (Or run javac.)

Online resouces

If you find yourself stuck, needing help, wondering whether a given behaviour is a feature or a bug, or just wanting to discuss @FreeBuilder, please join and/or post to the @FreeBuilder mailing list.
To see a list of open issues, or add a new one, see the @FreeBuilder issue tracker.
To submit a bug fix, or land a sweet new feature, read up on how to contribute.

Alternatives

AutoValue vs `@FreeBuilder`

Why is @FreeBuilder better than AutoValue?

It’s not! AutoValue provides an implementing class with a package-visible constructor, so you can easily implement the Factory pattern. If you’re writing an immutable type that needs a small number of values to create (Effective Java suggests at most three), and is not likely to require more in future, use the Factory pattern.

How about if you want a builder? AutoValue.Builder lets you explicitly specify a minimal Builder interface that will then be implemented by generated code, while @FreeBuilder provides a generated builder API. AutoValue.Builder is better if you must have a minimal API—for instance, in an Android project, where every method is expensive—or strongly prefer a visible-in-source API at the expense of many useful methods. Otherwise, consider using @FreeBuilder to implement the Builder pattern.

I used AutoValue, but now have more than three properties! How do I migrate to @FreeBuilder?

Ensure your getter methods start with 'get' or 'is'.
Change your annotation to @FreeBuilder.
Rewrite your factory method(s) to use the builder API.
Inline your factory method(s) with a refactoring tool (e.g. Eclipse).

You can always skip step 4 and have both factory and builder methods, if that seems cleaner!

Can I use both AutoValue and @FreeBuilder?

Not really. You can certainly use both annotations, but you will end up with two different implementing classes that never compare equal, even if they have the same values.

Proto vs `@FreeBuilder`

Protocol buffers have provided builders for ages. Why should I use @FreeBuilder?

Protocol buffers are cross-platform, backwards- and forwards-compatible, and have a very efficient wire format. Unfortunately, they do not support custom validation logic; nor can you use appropriate Java domain types, such as Instant or Range. Generally, it will be clear which one is appropriate for your use-case.

Wait, why "free"?

Free as in beer: you don't pay the cost of writing or maintaining the builder code.
Free as in flexible: you should always be able to customize the builder where the defaults don't work for you.
Free as in liberty: you can always drop @FreeBuilder and walk away with the code it generated for you.

License

Copyright 2014 Google Inc. All rights reserved.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.

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