Github url


by square

square /javapoet

A Java API for generating .java source files.

8.2K Stars 1.1K Forks Last release: Not found Apache License 2.0 834 Commits 38 Releases

Available items

No Items, yet!

The developer of this repository has not created any items for sale yet. Need a bug fixed? Help with integration? A different license? Create a request here:



is a Java API for generating


source files.

Source file generation can be useful when doing things such as annotation processing or interacting with metadata files (e.g., database schemas, protocol formats). By generating code, you eliminate the need to write boilerplate while also keeping a single source of truth for the metadata.


Here's a (boring)



package com.example.helloworld; public final class HelloWorld { public static void main(String[] args) { System.out.println("Hello, JavaPoet!"); } }

And this is the (exciting) code to generate it with JavaPoet:

MethodSpec main = MethodSpec.methodBuilder("main") .addModifiers(Modifier.PUBLIC, Modifier.STATIC) .returns(void.class) .addParameter(String[].class, "args") .addStatement("$T.out.println($S)", System.class, "Hello, JavaPoet!") .build(); TypeSpec helloWorld = TypeSpec.classBuilder("HelloWorld") .addModifiers(Modifier.PUBLIC, Modifier.FINAL) .addMethod(main) .build(); JavaFile javaFile = JavaFile.builder("com.example.helloworld", helloWorld) .build(); javaFile.writeTo(System.out);

To declare the main method, we've created a


"main" configured with modifiers, return type, parameters and code statements. We add the main method to a


class, and then add that to a


In this case we write the file to


, but we could also get it as a string (


) or write it to the file system (



The Javadoc catalogs the complete JavaPoet API, which we explore below.

Code & Control Flow

Most of JavaPoet's API uses plain old immutable Java objects. There's also builders, method chaining and varargs to make the API friendly. JavaPoet offers models for classes & interfaces (


), fields (


), methods & constructors (


), parameters (


) and annotations (



But the body of methods and constructors is not modeled. There's no expression class, no statement class or syntax tree nodes. Instead, JavaPoet uses strings for code blocks:

MethodSpec main = MethodSpec.methodBuilder("main") .addCode("" + "int total = 0;\n" + "for (int i = 0; i \< 10; i++) {\n" + " total += i;\n" + "}\n") .build();

Which generates this:

void main() { int total = 0; for (int i = 0; i \< 10; i++) { total += i; } }

The manual semicolons, line wrapping, and indentation are tedious and so JavaPoet offers APIs to make it easier. There's


which takes care of semicolons and newline, and

  • endControlFlow()
    which are used together for braces, newlines, and indentation:
    MethodSpec main = MethodSpec.methodBuilder("main") .addStatement("int total = 0") .beginControlFlow("for (int i = 0; i \< 10; i++)") .addStatement("total += i") .endControlFlow() .build();

This example is lame because the generated code is constant! Suppose instead of just adding 0 to 10, we want to make the operation and range configurable. Here's a method that generates a method:

private MethodSpec computeRange(String name, int from, int to, String op) { return MethodSpec.methodBuilder(name) .returns(int.class) .addStatement("int result = 1") .beginControlFlow("for (int i = " + from + "; i \< " + to + "; i++)") .addStatement("result = result " + op + " i") .endControlFlow() .addStatement("return result") .build(); }

And here's what we get when we call

computeRange("multiply10to20", 10, 20, "\*")


int multiply10to20() { int result = 1; for (int i = 10; i \< 20; i++) { result = result \* i; } return result; }

Methods generating methods! And since JavaPoet generates source instead of bytecode, you can read through it to make sure it's right.

Some control flow statements, such as


, can have unlimited control flow possibilities. You can handle those options using



MethodSpec main = MethodSpec.methodBuilder("main") .addStatement("long now = $T.currentTimeMillis()", System.class) .beginControlFlow("if ($T.currentTimeMillis() \< now)", System.class) .addStatement("$T.out.println($S)", System.class, "Time travelling, woo hoo!") .nextControlFlow("else if ($T.currentTimeMillis() == now)", System.class) .addStatement("$T.out.println($S)", System.class, "Time stood still!") .nextControlFlow("else") .addStatement("$T.out.println($S)", System.class, "Ok, time still moving forward") .endControlFlow() .build();

Which generates:

void main() { long now = System.currentTimeMillis(); if (System.currentTimeMillis() \< now) { System.out.println("Time travelling, woo hoo!"); } else if (System.currentTimeMillis() == now) { System.out.println("Time stood still!"); } else { System.out.println("Ok, time still moving forward"); } }

Catching exceptions using


is also a use case for



MethodSpec main = MethodSpec.methodBuilder("main") .beginControlFlow("try") .addStatement("throw new Exception($S)", "Failed") .nextControlFlow("catch ($T e)", Exception.class) .addStatement("throw new $T(e)", RuntimeException.class) .endControlFlow() .build();

Which produces:

void main() { try { throw new Exception("Failed"); } catch (Exception e) { throw new RuntimeException(e); } }

$L for Literals

The string-concatenation in calls to




is distracting. Too many operators. To address this, JavaPoet offers a syntax inspired-by but incompatible-with[


]( It accepts **``` $L





private MethodSpec computeRange(String name, int from, int to, String op) { return MethodSpec.methodBuilder(name) .returns(int.class) .addStatement("int result = 0") .beginControlFlow("for (int i = $L; i < $L; i++)", from, to) .addStatement("result = result $L i", op) .endControlFlow() .addStatement("return result") .build(); }

Literals are emitted directly to the output code with no escaping. Arguments for literals may be strings, primitives, and a few JavaPoet types described below.

### $S for Strings

When emitting code that includes string literals, we can use 
```**to emit a**string**, complete with wrapping quotation marks and escaping. Here's a program that emits 3 methods, each of which returns its own name:

public static void main(String[] args) throws Exception { TypeSpec helloWorld = TypeSpec.classBuilder("HelloWorld") .addModifiers(Modifier.PUBLIC, Modifier.FINAL) .addMethod(whatsMyName("slimShady")) .addMethod(whatsMyName("eminem")) .addMethod(whatsMyName("marshallMathers")) .build(); JavaFile javaFile = JavaFile.builder("com.example.helloworld", helloWorld) .build(); javaFile.writeTo(System.out); } private static MethodSpec whatsMyName(String name) { return MethodSpec.methodBuilder(name) .returns(String.class) .addStatement("return $S", name) .build(); }

In this case, using


 gives us quotation marks:

public final class HelloWorld { String slimShady() { return "slimShady"; } String eminem() { return "eminem"; } String marshallMathers() { return "marshallMathers"; } }

### $T for Types

We Java programmers love our types: they make our code easier to understand. And JavaPoet is on board. It has rich built-in support for types, including automatic generation of


statements. Just use 
```**to reference**types**:

MethodSpec today = MethodSpec.methodBuilder("today") .returns(Date.class) .addStatement("return new $T()", Date.class) .build(); TypeSpec helloWorld = TypeSpec.classBuilder("HelloWorld") .addModifiers(Modifier.PUBLIC, Modifier.FINAL) .addMethod(today) .build(); JavaFile javaFile = JavaFile.builder("com.example.helloworld", helloWorld) .build(); javaFile.writeTo(System.out);

That generates the following


 file, complete with the necessary 



package com.example.helloworld; import java.util.Date; public final class HelloWorld { Date today() { return new Date(); } }

We passed


 to reference a class that just-so-happens to be available when we're generating code. This doesn't need to be the case. Here's a similar example, but this one references a class that doesn't exist (yet):

ClassName hoverboard = ClassName.get("", "Hoverboard"); MethodSpec today = MethodSpec.methodBuilder("tomorrow") .returns(hoverboard) .addStatement("return new $T()", hoverboard) .build();

And that not-yet-existent class is imported as well:

package com.example.helloworld; import; public final class HelloWorld { Hoverboard tomorrow() { return new Hoverboard(); } }



 type is very important, and you'll need it frequently when you're using JavaPoet. It can identify any _declared_ class. Declared types are just the beginning of Java's rich type system: we also have arrays, parameterized types, wildcard types, and type variables. JavaPoet has classes for building each of these:

ClassName hoverboard = ClassName.get("", "Hoverboard"); ClassName list = ClassName.get("java.util", "List"); ClassName arrayList = ClassName.get("java.util", "ArrayList"); TypeName listOfHoverboards = ParameterizedTypeName.get(list, hoverboard); MethodSpec beyond = MethodSpec.methodBuilder("beyond") .returns(listOfHoverboards) .addStatement("$T result = new $T<>()", listOfHoverboards, arrayList) .addStatement("result.add(new $T())", hoverboard) .addStatement("result.add(new $T())", hoverboard) .addStatement("result.add(new $T())", hoverboard) .addStatement("return result") .build();

JavaPoet will decompose each type and import its components where possible.

package com.example.helloworld; import; import java.util.ArrayList; import java.util.List; public final class HelloWorld { List beyond() { List result = new ArrayList<>(); result.add(new Hoverboard()); result.add(new Hoverboard()); result.add(new Hoverboard()); return result; } }

#### Import static

JavaPoet supports

import static

. It does it via explicitly collecting type member names. Let's enhance the previous example with some static sugar:

... ClassName namedBoards = ClassName.get("", "Hoverboard", "Boards"); MethodSpec beyond = MethodSpec.methodBuilder("beyond") .returns(listOfHoverboards) .addStatement("$T result = new $T<>()", listOfHoverboards, arrayList) .addStatement("result.add($T.createNimbus(2000))", hoverboard) .addStatement("result.add($T.createNimbus("2001"))", hoverboard) .addStatement("result.add($T.createNimbus($T.THUNDERBOLT))", hoverboard, namedBoards) .addStatement("$T.sort(result)", Collections.class) .addStatement("return result.isEmpty() ? $T.emptyList() : result", Collections.class) .build(); TypeSpec hello = TypeSpec.classBuilder("HelloWorld") .addMethod(beyond) .build(); JavaFile.builder("com.example.helloworld", hello) .addStaticImport(hoverboard, "createNimbus") .addStaticImport(namedBoards, "*") .addStaticImport(Collections.class, "*") .build();

JavaPoet will first add your

import static

 block to the file as configured, match and mangle all calls accordingly and also import all other types as needed.

package com.example.helloworld; import static*; import static; import static java.util.Collections.*; import; import java.util.ArrayList; import java.util.List; class HelloWorld { List beyond() { List result = new ArrayList<>(); result.add(createNimbus(2000)); result.add(createNimbus("2001")); result.add(createNimbus(THUNDERBOLT)); sort(result); return result.isEmpty() ? emptyList() : result; } }

### $N for Names

Generated code is often self-referential. Use 
```** to refer to another generated declaration by its name. Here's a method that calls another:

public String byteToHex(int b) { char[] result = new char[2]; result[0] = hexDigit((b >>> 4) & 0xf); result[1] = hexDigit(b & 0xf); return new String(result); } public char hexDigit(int i) { return (char) (i < 10 ? i + '0' : i - 10 + 'a'); }

When generating the code above, we pass the


 method as an argument to the 


method using 



MethodSpec hexDigit = MethodSpec.methodBuilder("hexDigit") .addParameter(int.class, "i") .returns(char.class) .addStatement("return (char) (i < 10 ? i + '0' : i - 10 + 'a')") .build(); MethodSpec byteToHex = MethodSpec.methodBuilder("byteToHex") .addParameter(int.class, "b") .returns(String.class) .addStatement("char[] result = new char[2]") .addStatement("result[0] = $N((b >>> 4) & 0xf)", hexDigit) .addStatement("result[1] = $N(b & 0xf)", hexDigit) .addStatement("return new String(result)") .build();

### Code block format strings

Code blocks may specify the values for their placeholders in a few ways. Only one style may be used for each operation on a code block.

#### Relative Arguments

Pass an argument value for each placeholder in the format string to


. In each example, we generate code to say "I ate 3 tacos"

CodeBlock.builder().add("I ate $L $L", 3, "tacos")

#### Positional Arguments

Place an integer index (1-based) before the placeholder in the format string to specify which argument to use.

CodeBlock.builder().add("I ate $2L $1L", "tacos", 3)

#### Named Arguments

Use the syntax




 is the format character and call 


with a map containing all argument keys in the format string. Argument names use characters in






, and 


, and must start with a lowercase character.

Map map = new LinkedHashMap<>(); map.put("food", "tacos"); map.put("count", 3); CodeBlock.builder().addNamed("I ate $count:L $food:L", map)

### Methods

All of the above methods have a code body. Use


 to get a method without any body. This is only legal if the enclosing class is either abstract or an interface.

MethodSpec flux = MethodSpec.methodBuilder("flux") .addModifiers(Modifier.ABSTRACT, Modifier.PROTECTED) .build(); TypeSpec helloWorld = TypeSpec.classBuilder("HelloWorld") .addModifiers(Modifier.PUBLIC, Modifier.ABSTRACT) .addMethod(flux) .build();

Which generates this:

public abstract class HelloWorld { protected abstract void flux(); }

The other modifiers work where permitted. Note that when specifying modifiers, JavaPoet uses[


](, a class that is not available on Android. This limitation applies to code-generating-code only; the output code runs everywhere: JVMs, Android, and GWT.

Methods also have parameters, exceptions, varargs, Javadoc, annotations, type variables, and a return type. All of these are configured with


### Constructors


 is a slight misnomer; it can also be used for constructors:

MethodSpec flux = MethodSpec.constructorBuilder() .addModifiers(Modifier.PUBLIC) .addParameter(String.class, "greeting") .addStatement("this.$N = $N", "greeting", "greeting") .build(); TypeSpec helloWorld = TypeSpec.classBuilder("HelloWorld") .addModifiers(Modifier.PUBLIC) .addField(String.class, "greeting", Modifier.PRIVATE, Modifier.FINAL) .addMethod(flux) .build();

Which generates this:

public class HelloWorld { private final String greeting; public HelloWorld(String greeting) { this.greeting = greeting; } }

For the most part, constructors work just like methods. When emitting code, JavaPoet will place constructors before methods in the output file.

### Parameters

Declare parameters on methods and constructors with either




's convenient 



ParameterSpec android = ParameterSpec.builder(String.class, "android") .addModifiers(Modifier.FINAL) .build(); MethodSpec welcomeOverlords = MethodSpec.methodBuilder("welcomeOverlords") .addParameter(android) .addParameter(String.class, "robot", Modifier.FINAL) .build();

Though the code above to generate




 parameters is different, the output is the same:

void welcomeOverlords(final String android, final String robot) { }

The extended


 form is necessary when the parameter has annotations (such as 


### Fields

Like parameters, fields can be created either with builders or by using convenient helper methods:

FieldSpec android = FieldSpec.builder(String.class, "android") .addModifiers(Modifier.PRIVATE, Modifier.FINAL) .build(); TypeSpec helloWorld = TypeSpec.classBuilder("HelloWorld") .addModifiers(Modifier.PUBLIC) .addField(android) .addField(String.class, "robot", Modifier.PRIVATE, Modifier.FINAL) .build();

Which generates:

public class HelloWorld { private final String android; private final String robot; }

The extended


 form is necessary when a field has Javadoc, annotations, or a field initializer. Field initializers use the same [


]( syntax as the code blocks above:

FieldSpec android = FieldSpec.builder(String.class, "android") .addModifiers(Modifier.PRIVATE, Modifier.FINAL) .initializer("$S + $L", "Lollipop v.", 5.0d) .build();

Which generates:

private final String android = "Lollipop v." + 5.0;

### Interfaces

JavaPoet has no trouble with interfaces. Note that interface methods must always be


 and interface fields must always be 


. These modifiers are necessary when defining the interface:

TypeSpec helloWorld = TypeSpec.interfaceBuilder("HelloWorld") .addModifiers(Modifier.PUBLIC) .addField(FieldSpec.builder(String.class, "ONLY_THING_THAT_IS_CONSTANT") .addModifiers(Modifier.PUBLIC, Modifier.STATIC, Modifier.FINAL) .initializer("$S", "change") .build()) .addMethod(MethodSpec.methodBuilder("beep") .addModifiers(Modifier.PUBLIC, Modifier.ABSTRACT) .build()) .build();

But these modifiers are omitted when the code is generated. These are the defaults so we don't need to include them for


's benefit!

public interface HelloWorld { String ONLY_THING_THAT_IS_CONSTANT = "change"; void beep(); }

### Enums



 to create the enum type, and 


 for each value:

TypeSpec helloWorld = TypeSpec.enumBuilder("Roshambo") .addModifiers(Modifier.PUBLIC) .addEnumConstant("ROCK") .addEnumConstant("SCISSORS") .addEnumConstant("PAPER") .build();

To generate this:

public enum Roshambo { ROCK, SCISSORS, PAPER }

Fancy enums are supported, where the enum values override methods or call a superclass constructor. Here's a comprehensive example:

TypeSpec helloWorld = TypeSpec.enumBuilder("Roshambo") .addModifiers(Modifier.PUBLIC) .addEnumConstant("ROCK", TypeSpec.anonymousClassBuilder("$S", "fist") .addMethod(MethodSpec.methodBuilder("toString") .addAnnotation(Override.class) .addModifiers(Modifier.PUBLIC) .addStatement("return $S", "avalanche!") .returns(String.class) .build()) .build()) .addEnumConstant("SCISSORS", TypeSpec.anonymousClassBuilder("$S", "peace") .build()) .addEnumConstant("PAPER", TypeSpec.anonymousClassBuilder("$S", "flat") .build()) .addField(String.class, "handsign", Modifier.PRIVATE, Modifier.FINAL) .addMethod(MethodSpec.constructorBuilder() .addParameter(String.class, "handsign") .addStatement("this.$N = $N", "handsign", "handsign") .build()) .build();

Which generates this:

public enum Roshambo { ROCK("fist") { @Override public String toString() { return "avalanche!"; } }, SCISSORS("peace"), PAPER("flat"); private final String handsign; Roshambo(String handsign) { this.handsign = handsign; } }

### Anonymous Inner Classes

In the enum code, we used


. Anonymous inner classes can also be used in code blocks. They are values that can be referenced with 



TypeSpec comparator = TypeSpec.anonymousClassBuilder("") .addSuperinterface(ParameterizedTypeName.get(Comparator.class, String.class)) .addMethod(MethodSpec.methodBuilder("compare") .addAnnotation(Override.class) .addModifiers(Modifier.PUBLIC) .addParameter(String.class, "a") .addParameter(String.class, "b") .returns(int.class) .addStatement("return $N.length() - $N.length()", "a", "b") .build()) .build(); TypeSpec helloWorld = TypeSpec.classBuilder("HelloWorld") .addMethod(MethodSpec.methodBuilder("sortByLength") .addParameter(ParameterizedTypeName.get(List.class, String.class), "strings") .addStatement("$T.sort($N, $L)", Collections.class, "strings", comparator) .build()) .build();

This generates a method that contains a class that contains a method:

void sortByLength(List strings) { Collections.sort(strings, new Comparator() { @Override public int compare(String a, String b) { return a.length() - b.length(); } }); }

One particularly tricky part of defining anonymous inner classes is the arguments to the superclass constructor. In the above code we're passing the empty string for no arguments:


. To pass different parameters use JavaPoet's code block syntax with commas to separate arguments.
### Annotations

Simple annotations are easy:

MethodSpec toString = MethodSpec.methodBuilder("toString") .addAnnotation(Override.class) .returns(String.class) .addModifiers(Modifier.PUBLIC) .addStatement("return $S", "Hoverboard") .build();

Which generates this method with an



@Override public String toString() { return "Hoverboard"; }



 to set properties on annotations:

MethodSpec logRecord = MethodSpec.methodBuilder("recordEvent") .addModifiers(Modifier.PUBLIC, Modifier.ABSTRACT) .addAnnotation(AnnotationSpec.builder(Headers.class) .addMember("accept", "$S", "application/json; charset=utf-8") .addMember("userAgent", "$S", "Square Cash") .build()) .addParameter(LogRecord.class, "logRecord") .returns(LogReceipt.class) .build();

Which generates this annotation with





@Headers( accept = "application/json; charset=utf-8", userAgent = "Square Cash" ) LogReceipt recordEvent(LogRecord logRecord);

When you get fancy, annotation values can be annotations themselves. Use


 for embedded annotations:

MethodSpec logRecord = MethodSpec.methodBuilder("recordEvent") .addModifiers(Modifier.PUBLIC, Modifier.ABSTRACT) .addAnnotation(AnnotationSpec.builder(HeaderList.class) .addMember("value", "$L", AnnotationSpec.builder(Header.class) .addMember("name", "$S", "Accept") .addMember("value", "$S", "application/json; charset=utf-8") .build()) .addMember("value", "$L", AnnotationSpec.builder(Header.class) .addMember("name", "$S", "User-Agent") .addMember("value", "$S", "Square Cash") .build()) .build()) .addParameter(LogRecord.class, "logRecord") .returns(LogReceipt.class) .build();

Which generates this:

@HeaderList({ @Header(name = "Accept", value = "application/json; charset=utf-8"), @Header(name = "User-Agent", value = "Square Cash") }) LogReceipt recordEvent(LogRecord logRecord);

Note that you can call


 multiple times with the same property name to populate a list of values for that property.
### Javadoc

Fields, methods and types can be documented with Javadoc:

MethodSpec dismiss = MethodSpec.methodBuilder("dismiss") .addJavadoc("Hides {@code message} from the caller's history. Other\n" + "participants in the conversation will continue to see the\n" + "message in their own history unless they also delete it.\n") .addJavadoc("\n") .addJavadoc("

Use {@link #delete($T)} to delete the entire\n" + "conversation for all participants.\n", Conversation.class) .addModifiers(Modifier.PUBLIC, Modifier.ABSTRACT) .addParameter(Message.class, "message") .build();

Which generates this:

/** * Hides {@code message} from the caller's history. Other * participants in the conversation will continue to see the * message in their own history unless they also delete it. * *

Use {@link #delete(Conversation)} to delete the entire * conversation for all participants. */ void dismiss(Message message);



 when referencing types in Javadoc to get automatic imports.
## Download

Download [the latest .jar]( or depend via Maven:

xml com.squareup javapoet 1.13.0

or Gradle:

groovy compile 'com.squareup:javapoet:1.13.0'

Snapshots of the development version are available in [Sonatype's 



## License

Copyright 2015 Square, Inc. 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 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.

# JavaWriter

JavaPoet is the successor to [JavaWriter]( New projects should prefer JavaPoet because it has a stronger code model: it understands types and can manage imports automatically. JavaPoet is also better suited to composition: rather than streaming the contents of a


``` file top-to-bottom in a single pass, a file can be assembled as a tree of declarations.

JavaWriter continues to be available in GitHub and Maven Central.

We use cookies. If you continue to browse the site, you agree to the use of cookies. For more information on our use of cookies please see our Privacy Policy.