“The Pragmatic Programmer” admonished us all to “write code that writes code”: use code generators to increase productivity and avoid duplication. The language communities have clearly caught on, as more and more frameworks generate code at compile time: Project Lombok, Google Auto, and more. This session reviews these approaches including examples of how and why we’d want to do this. We will see newest Java language tools, write our own AST tranform and look at some amazing libraries based on these techniques. Bio: Nguyen Luong is a senior java technical lead at Ekino Vietnam. He likes to research new technologies and solve security challenges.

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Writing code
that writes code
by
LUONG Hoang Nguyen
Senior Java Technical Lead
ekino.

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1. Context
2. Project Lombok
3. Alternatives
4. Conclusion
AGENDA

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1.
Context

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Developer job As programmers, we often find ourselves in a similar
position.
We need to achieve the
same functionality, but in different contexts.
We need to repeat information in different
places.
Sometimes we just need to protect ourselves from
carpal tunnel syndrome by
cutting down on repetitive typing

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Boiler Plate Code
Syntax Verbosity
Problemsof
theIndustry
Non Standardization
Not following the standard code conventions
Human Error/Dependency
Repeatable things (defining Getters, Setters
etc.)

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2.
Project Lombok

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WHAT IS IT ?

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•@Getter / @Setter
•@ToString
•@EqualsAndHashCode
•@Data
•@NoArgsConstructor
•@RequiredArgsConstructor
•@AllArgsConstructor
•@Cleanup
•@Synchronized
•@Slf4j
•@Builder
•val
Features

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@Getter / Setter
Features
• Automatic Generation of getters and setters.
• Method will be public unless you explicitly specify an AccessLevel
• @Getter and/or @Setter annotation on class
• Disable getter/setter generation for any field by using the special
AccessLevel.NONE
• @NotNull for nullity check

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import lombok.AccessLevel;
import lombok.Getter;
import lombok.Setter;
public class GetterSetterExample {
/**
* Age of the person. Water is wet.
*
* @param age New value for this person's age. Sky
is blue.
* @return The current value of this person's age.
Circles are round.
*/
@Getter @Setter private int age = 10;
/**
* Name of the person.
* -- SETTER --
* Changes the name of this person.
*
* @param name The new value.
*/
@Setter(AccessLevel.PROTECTED) private String name;
@Override public String toString() {
return String.format("%s (age: %d)", name, age);
}
}
public class GetterSetterExample {
/**
* Age of the person. Water is wet.
*
* @param age New value for this person's age. Sky is blue.
* @return The current value of this person's age. Circles are
round.
*/
private int age = 10;
/**
* Name of the person.
* -- SETTER --
* Changes the name of this person.
*
* @param name The new value.
*/
private String name;
@Override public String toString() {
return String.format("%s (age: %d)", name, age);
}
public int getAge() {
return this.age;
}
public void setAge(int age) {
this.age = age;
}
protected void setName(String name) {
this.name = name;
}
}

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@EqualsAndHash
Code
Features
• Generates hashCode and equals implementations from the fields (non-
static, non-transient)
• Parameters 

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import lombok.EqualsAndHashCode;
@EqualsAndHashCode(exclude = { "id", "shape" })
class EqualsAndHashCodeExample {
private transient int transientVar = 10;
private String name;
private double score;
private Shape shape = new Square(5, 10);
private String[] tags;
private int id;
public String getName() {
return this.name;
}
}
public class EqualsAndHashCodeExample {
private transient int transientVar = 10;
private String name;
private double score;
private Shape shape = new Square(5, 10);
private String[] tags;
private int id;
public String getName() {
return this.name;
}
public boolean equals(Object o) {
if (o == this) {
return true;
}
if (!(o instanceof EqualsAndHashCodeExample)) {
return false;
}
final EqualsAndHashCodeExample other = (EqualsAndHashCodeExample) o;
if (!other.canEqual((Object) this)) {
return false;
}
final Object this$name = this.getName();
final Object other$name = other.getName();
if (this$name == null ? other$name != null : !this$name.equals(other$name)) {
return false;
}
if (Double.compare(this.score, other.score) != 0) {
return false;
}
if (!java.util.Arrays.deepEquals(this.tags, other.tags)) {
return false;
}
return true;
}
public int hashCode() {
final int PRIME = 59;
int result = 1;
final Object $name = this.getName();
result = result * PRIME + ($name == null ? 0 : $name.hashCode());
final long $score = Double.doubleToLongBits(this.score);
result = result * PRIME + (int) ($score >>> 32 ^ $score);
result = result * PRIME + java.util.Arrays.deepHashCode(this.tags);
return result;
}
protected boolean canEqual(Object other) {
return other instanceof EqualsAndHashCodeExample;
}
}

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@Data
Features
• All together now: A shortcut for @ToString, @EqualsAndHashCode,
@Getter on all fields, @Setter on all non-final fields, and
@RequiredArgsConstructor
• Included annotations with changed parameters can be defined along with
@Data

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import lombok.Data;
@Data
public class DataExample {
private final String name;
private double score;
}
public class DataExample {
private final String name;
private double score;
@java.beans.ConstructorProperties({ "name" })
public DataExample(String name) {
this.name = name;
}
public String getName() {
return this.name;
}
public double getScore() {
return this.score;
}
public void setScore(double score) {
this.score = score;
}
public boolean equals(Object o) {
if (o == this) {
return true;
}
if (!(o instanceof DataExample)) {
return false;
}
final DataExample other = (DataExample) o;
if (!other.canEqual((Object) this)) {
return false;
}
final Object this$name = this.name;
final Object other$name = other.name;
if (this$name == null ? other$name != null : !this$name.equals(other$name)) {
return false;
}
if (Double.compare(this.score, other.score) != 0) {
return false;
}
return true;
}
public int hashCode() {
final int PRIME = 59;
int result = 1;
final Object $name = this.name;
result = result * PRIME + ($name == null ? 0 : $name.hashCode());
final long $score = Double.doubleToLongBits(this.score);
result = result * PRIME + (int) ($score >>> 32 ^ $score);
return result;
}
protected boolean canEqual(Object other) {
return other instanceof DataExample;
}
public String toString() {
return "DataExample(name=" + this.name + ", score=" + this.score + ")";
}
}

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@NoArgsConstructor
@RequiredArgsConstructor
@AllArgsConstructor
Features
• Generates constructors that take no arguments, one argument per final /
non-null field, or one argument for every field.
• Specialized constructor can be made. Compile-time error in case of any
conflicts.

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@Cleanup
Features
• Automatic resource management: Call your close() methods safely with no
hassle.
@Cleanup
InputStream in=new FileInputStream("some/file");
• If the type of object you'd like to cleanup does not have a close() method,
but some other no-argument method like dispose() so write it as
@Cleanup("dispose")
TestClass t = new TestClass();

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@Builder
val
Features
• @Builder generates a static builder() method that returns a builder
instance. This builder instance can be used to build an object of the class
annotated with @Builder (here Person)
• Person.builder().name("Adam Savage").city("San
Francisco").job("Mythbusters").job("Unchained Reaction").build();
• val: You can use val as the type of a local variable declaration instead of
actually writing the type. This feature works on local variables and on
foreach loops only, not on fields
val example = new ArrayList<String>();
example.add("Hello, World!");

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import lombok.Builder;
@Builder
public class BuilderExample {
private String name;
private int age;
}
public class BuilderExample {
private String name;
private int age;
@java.beans.ConstructorProperties({ "name", "age" })
BuilderExample(String name, int age) {
this.name = name;
this.age = age;
}
public static BuilderExampleBuilder builder() {
return new BuilderExampleBuilder();
}
public static class BuilderExampleBuilder {
private String name;
private int age;
BuilderExampleBuilder() {
}
public BuilderExample.BuilderExampleBuilder name(String name) {
this.name = name;
return this;
}
public BuilderExample.BuilderExampleBuilder age(int age) {
this.age = age;
return this;
}
public BuilderExample build() {
return new BuilderExample(name, age);
}
public String toString() {
return "BuilderExampleBuilder(name=" + this.name + ", age=" + this.age + ")";
}
}
}

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• Generates Java Boilerplate
• Compile Time Only -> not affect performance for runtime
• For Eclipse and javac
• IntelliJ IDEA & NetBeans too
• Removable with delombok
• Know what is generated
Features

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How does it work? Java compilation has 3 stages:
In the Parse and Enter phase, the compiler parses source files into
an Abstract Syntax Tree (AST
In the Annotation Processing phase, custom annotation processors
are invoked
Annotation processors can do things like validate classes or
generate new resources, including source files.
In the last phase, Analyse and Generate, the compiler generates
class files (byte code) from the Abstract Syntax Trees generated in
phase 1
Project Lombok hooks itself into the compilation process as an
annotation processor. Normally, annotation processors only generate
new source files whereas Lombok modifies existing classes.

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Example • Example we create a very simple Lombok transformation that adds a
helloWorld method to any class annotated as @HelloWorld.
• The basic classes we need to write are:
- Annotation class
- Eclipse handler
- Javac handler (Intellj also supported this)
@HelloWorld
public class MyClass
{
}
public class MyClass {
public void helloWorld() {
System.out.println("Hello
World");
}
}

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Annotation
Example • At first, we just need to create an annotation called HelloWorld that can be
applied to classes.
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import
java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.SOURCE)
public @interface HelloWorld {
}

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Handler
Example • Our handler will be adding a new Method Declaration to the Type
Declaration. A Method Declaration is composed of several components.
The AST for our Method Declaration will have this :

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Handler Logic
Example • Our handle method will need to do the following:
1. Mark annotation as processed
2. Create the helloWorld method
3. Inject the helloWorld method into the AST of the annotated class

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Lombok provides our handler with the AST of the annotation node. We know
that the annotation can only be applied to a Type. To get the AST for the Type
(Class), we call annotationNode.up(). The annotation is a child of the Type
AST, so by calling up() we get the parent AST which is the Type AST we need
to modify.
Javac Handler
Example
@ProviderFor(JavacAnnotationHandler.class)
public class HandleHelloWorld implements JavacAnnotationHandler<HelloWorld>{
public boolean handle(AnnotationValues<HelloWorld> annotation, JCAnnotation ast,
JavacNode annotationNode) {
JavacHandlerUtil.markAnnotationAsProcessed(annotationNode, HelloWorld.class);
JavacNode typeNode = annotationNode.up();
if(notAClass(typeNode)) {
annotationNode.addError("@HelloWorld is only supported on a class.");
return false;
}
JCMethodDecl helloWorldMethod = createHelloWorld(typeNode);
JavacHandlerUtil.injectMethod(typeNode, helloWorldMethod);
return true;
}
…
}

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• Start with a method node.
• Add the return type, parameters, access level, throw clause, etc to the
method node.
• Create an expression statement to represent System.out.println("Hello
World")
• Add the expression to the method node.
Javac Handler
Hello world method
Example
private JCMethodDecl createHelloWorld(JavacNode type) {
TreeMaker treeMaker = type.getTreeMaker();
JCModifiers modifiers = treeMaker.Modifiers(Modifier.PUBLIC);
List<JCTypeParameter> methodGenericTypes = List.<JCTypeParameter>nil();
JCExpression methodType = treeMaker.TypeIdent(TypeTag.VOID);
Name methodName = type.toName("helloWorld");
List<JCVariableDecl> methodParameters = List.<JCVariableDecl>nil();
List<JCExpression> methodThrows = List.<JCExpression>nil();
JCExpression printlnMethod =
JavacHandlerUtil.chainDots(treeMaker, type, "System", "out", "println");
List<JCExpression> printlnArgs = List.<JCExpression>of(treeMaker.Literal("hello world"));
JCMethodInvocation printlnInvocation =
treeMaker.Apply(List.<JCExpression>nil(), printlnMethod, printlnArgs);
JCBlock methodBody =
treeMaker.Block(0, List.<JCStatement>of(treeMaker.Exec(printlnInvocation)));
JCExpression defaultValue = null;
return treeMaker.MethodDef(
modifiers,
methodName,
methodType,
methodGenericTypes,
methodParameters,
methodThrows,
methodBody,
defaultValue
);
}

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@UUIDColumn
@JsonColumn
Custom Annotations • We can create custom annotations that helpful for our projects. It will
make standardization for our coding.
@Entity
@Data
public class TestModelEntity {
@Id
@UUIDColumn
private UUID id;
@JsonColumn
private Model model;
….
}
@Entity
public class TestModelEntity {
@Id
@Type(
type = "pg-uuid"
)
@GeneratedValue(
generator = "uuid2"
)
private UUID id;
@Type(
type = "JSONB",
parameters = { @Parameter(
name = "class",
value = "com.ekino.lombok.sample.Model"
)}
)
private Model model;
….
}

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3.
Alternatives

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Google Auto AutoFactory
JSR-330-compatible factories
AutoService
Provider-configuration files for ServiceLoader
AutoValue
Immutable value-type code generation for Java 1.6+ .
Common
Helper utilities for writing annotation processors.

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AutoValue
• You write an abstract class
• It has abstract accessors, but no fields
• Annotate it with @AutoValue
• Javac generates a concrete subclass for you
• Callers only ever see the parent type
Google Auto

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AutoValue
• What you write (plain Java).
import com.google.auto.value.AutoValue;
@AutoValue
public abstract class Foo {
public static Foo create(String text, int number) {
// defensive copies, preconditions
return new AutoValue_Foo(text, number);
}
/** Documentation here. */
public abstract String text(); // or getText(), if you like
/** Documentation here. */
public abstract int number();
}
Google Auto

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AutoValue
• What code does that generate?
final class AutoValue_Foo extends Foo { // (note: NOT public!)
private final String text;
private final int number;
AutoValue_Foo(String text, int number) {
if (text == null) {
throw new NullPointerException("text");
}
this.text = text;
this.number = number;
}
@Override public String text() {
return text;
}
@Override public int number() {
return number;
}
// and the customary equals/hashCode/toString.
}
Google Auto

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3.
Conclusion

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Advantages
• User writes only plain old Java code
• No runtime impact
• no dependency (@AutoValue has source retention)
• No magical modifying of existing classes
• Still just a single javac pass to compile!
Google Auto

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Disadvantages
• Bootstrap can be annoying—when you first type new AutoValue_Foo it may go
red in the IDE
• Some once-per-project build setup required
• Not support setter for AutoValue
• AutoValue does introduce some fragility.
• The generator has to choose the order of constructor parameters somehow,
so it uses the order in which the accessors appear in the source file.
• This means an innocent refactoring to reorder those accessors could break
your tests
Google Auto

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Advantages
• User writes only plain old Java code
• No runtime impact
• no dependency (annotations has source retention)
• Support delombok
• Maximally concise value classes.
• There is plugin support to integrate with IDE like Lombok plugin for Intellij
Project Lombok

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Disadvantages
• The compiler hacks are non-standard and fragile.
• Somebody may mention that the code is no longer really Java – the code
lose its WYSIWYG feel.
Project Lombok

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• Project Lombok https://projectlombok.org/
• Google Auto https://github.com/google/auto
•http://notatube.blogspot.com/2010/12/project-lombok-creating-custom.html
•http://www.ibm.com/developerworks/library/j-lombok/
•https://docs.google.com/presentation/d/14u_h-
lMn7f1rXE1nDiLX0azS3IkgjGl5uxp5jGJ75RE/edit#slide=id.g2a5e9c4a8_01
39
•The pragmatic programmer - book by Andy Hunt and Dave Thomas
Resources

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Thank you
• LUONG Hoang Nguyen
• Email: hoang-nguyen.luong@ekino.com
• Phone: 0914 974197