3-metaprogramming.md

Metaprogramming

Metaprogramming is the writing of computer programs that write or manipulate other programs (or themselves) as their data, or that do part of the work at compile time that would otherwise be done at run time.

• Ability to read, generate, transform or modify programs dynamically (at runtime);
• Has the objective of increasing performance, since reflection has high costs.

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JavaPoet

JavaPoet is a Java API for generating .java source files.

JavaFile

Represents a Java file containing a single top level class.

Example:

val file = JavaFile
    .builder("com.example", typeSpec)
    .build()

TypeSpec

Represents a generated class, interface, or enum declaration.

Example:

val typeSpec = TypeSpec
    .classBuilder("MyClass")
    .addField(fieldSpec)
    .addMethod(methodSpec)
    .build()

FieldSpec

Represents a generated field declaration.

Example:

val fieldSpec = FieldSpec
    .builder(String::class.java)
    .addModifiers(Modifier.PRIVATE)
    .build()

MethodSpec

Represents a generated constructor or method declaration.

Example:

val methodSpec = MethodSpec
    .methodBuilder("myMethod")
    .addModifiers(Modifier.PUBLIC)
    .addParameter(parameterSpec)
    .returns(String::class.java)
    .addStatement("return \"Hello, World!\"")
    .build()

To define a constructor, use the constructorBuilder method instead of methodBuilder.

ParameterSpec

Represents a generated parameter declaration.

It is not very used, since the parameters are defined directly in the addParameter method.