JavaPoet ======== `JavaPoet` is a Java API for generating `.java` 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. ### Example Here's a (boring) `HelloWorld` class: ```java 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: ```java 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 `MethodSpec` "main" configured with modifiers, return type, parameters and code statements. We add the main method to a `HelloWorld` class, and then add that to a `HelloWorld.java` file. In this case we write the file to `System.out`, but we could also get it as a string (`JavaFile.toString()`) or write it to the file system (`JavaFile.writeTo()`). The [Javadoc][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 (`TypeSpec`), fields (`FieldSpec`), methods & constructors (`MethodSpec`), parameters (`ParameterSpec`) and annotations (`AnnotationSpec`). 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: ```java 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: ```java 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 `addStatement()` which takes care of semicolons and newline, and `beginControlFlow()` + `endControlFlow()` which are used together for braces, newlines, and indentation: ```java 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: ```java 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, "*")`: ```java 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. ### $L for Literals The string-concatenation in calls to `beginControlFlow()` and `addStatement` is distracting. Too many operators. To address this, JavaPoet offers a syntax inspired-by but incompatible-with [`String.format()`][formatter]. It accepts **`$L`** to emit a **literal** value in the output. This works just like `Formatter`'s `%s`: ```java 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. ### $S for Strings When emitting code that includes string literals, we can use **`$S`** 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: ```java 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 $S", name) .build(); } ``` In this case, using `$S` gives us quotation marks: ```java public final class HelloWorld { String slimShady() { return "slimShady"; } String eminem() { return "eminem"; } String marshallMathers() { return "marshallMathers"; } } ``` ### $T for Types We Java programmers love our types: they make our code easier to understand. And JavaPoet is on board. It has rich built-in support for types, including automatic generation of `import` statements. Just use **`$T`** to reference **types**: ```java 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 `.java` file, complete with the necessary `import`: ```java package com.example.helloworld; import java.util.Date; public final class HelloWorld { Date today() { return new Date(); } } ``` We passed `Date.class` 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): ```java ClassName hoverboard = ClassName.get("com.mattel", "Hoverboard"); MethodSpec today = MethodSpec.methodBuilder("tomorrow") .returns(hoverboard) .addStatement("return new $T()", hoverboard) .build(); ``` And that not-yet-existent class is imported as well: ```java package com.example.helloworld; import com.mattel.Hoverboard; public final class HelloWorld { Hoverboard tomorrow() { return new Hoverboard(); } } ``` The `ClassName` type is very important, and you'll need it frequently when you're using JavaPoet. It can identify any _declared_ class. Declared types are just the beginning of Java's rich type system: we also have arrays, parameterized types, wildcard types, and type variables. JavaPoet has classes for building each of these: ```java ClassName hoverboard = ClassName.get("com.mattel", "Hoverboard"); ClassName list = ClassName.get("java.util", "List"); ClassName arrayList = ClassName.get("java.util", "ArrayList"); TypeName listOfHoverboards = ParameterizedTypeName.get(list, hoverboard); MethodSpec beyond = MethodSpec.methodBuilder("beyond") .returns(listOfHoverboards) .addStatement("$T result = new $T<>()", listOfHoverboards, arrayList) .addStatement("result.add(new $T())", hoverboard) .addStatement("result.add(new $T())", hoverboard) .addStatement("result.add(new $T())", hoverboard) .addStatement("return result") .build(); ``` JavaPoet will decompose each type and import its components where possible. ```java package com.example.helloworld; import com.mattel.Hoverboard; import java.util.ArrayList; import java.util.List; public final class HelloWorld { List beyond() { List result = new ArrayList<>(); result.add(new Hoverboard()); result.add(new Hoverboard()); result.add(new Hoverboard()); return result; } } ``` #### Import static JavaPoet supports `import static`. It does it via explicitly collecting type member names. Let's enhance the previous example with some static sugar: ```java ... ClassName namedBoards = ClassName.get("com.mattel", "Hoverboard", "Boards"); MethodSpec beyond = MethodSpec.methodBuilder("beyond") .returns(listOfHoverboards) .addStatement("$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. ```java package com.example.helloworld; import static com.mattel.Hoverboard.Boards.*; import static com.mattel.Hoverboard.createNimbus; import static java.util.Collections.*; import com.mattel.Hoverboard; 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; } } ``` ### $N for Names Generated code is often self-referential. Use **`$N`** to refer to another generated declaration by its name. Here's a method that calls another: ```java 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 `hexDigit()` method as an argument to the `byteToHex()` method using `$N`: ```java 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 `CodeBlock.add()`. In each example, we generate code to say "I ate 3 tacos" ```java 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. ```java CodeBlock.builder().add("I ate $2L $1L", "tacos", 3) ``` #### Named Arguments Use the syntax `$argumentName:X` where `X` is the format character and call `CodeBlock.addNamed()` with a map containing all argument keys in the format string. Argument names use characters in `a-z`, `A-Z`, `0-9`, and `_`, and must start with a lowercase character. ```java Map map = new LinkedHashMap<>(); map.put("food", "tacos"); map.put("count", 3); CodeBlock.builder().addNamed("I ate $count:L $food:L", map) ``` ### Methods All of the above methods have a code body. Use `Modifiers.ABSTRACT` to get a method without any body. This is only legal if the enclosing class is either abstract or an interface. ```java MethodSpec flux = MethodSpec.methodBuilder("flux") .addModifiers(Modifier.ABSTRACT, Modifier.PROTECTED) .build(); TypeSpec helloWorld = TypeSpec.classBuilder("HelloWorld") .addModifiers(Modifier.PUBLIC, Modifier.ABSTRACT) .addMethod(flux) .build(); ``` Which generates this: ```java public abstract class HelloWorld { protected abstract void flux(); } ``` The other modifiers work where permitted. Note that when specifying modifiers, JavaPoet uses [`javax.lang.model.element.Modifier`][modifier], a class that is not available on Android. This limitation applies to code-generating-code only; the output code runs everywhere: JVMs, Android, and GWT. Methods also have parameters, exceptions, varargs, Javadoc, annotations, type variables, and a return type. All of these are configured with `MethodSpec.Builder`. ### Constructors `MethodSpec` is a slight misnomer; it can also be used for constructors: ```java MethodSpec flux = MethodSpec.constructorBuilder() .addModifiers(Modifier.PUBLIC) .addParameter(String.class, "greeting") .addStatement("this.$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: ```java 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 `ParameterSpec.builder()` or `MethodSpec`'s convenient `addParameter()` API: ```java 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 `android` and `robot` parameters is different, the output is the same: ```java void welcomeOverlords(final String android, final String robot) { } ``` The extended `Builder` form is necessary when the parameter has annotations (such as `@Nullable`). ### Fields Like parameters, fields can be created either with builders or by using convenient helper methods: ```java 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: ```java public class HelloWorld { private final String android; private final String robot; } ``` The extended `Builder` form is necessary when a field has Javadoc, annotations, or a field initializer. Field initializers use the same [`String.format()`][formatter]-like syntax as the code blocks above: ```java FieldSpec android = FieldSpec.builder(String.class, "android") .addModifiers(Modifier.PRIVATE, Modifier.FINAL) .initializer("$S + $L", "Lollipop v.", 5.0d) .build(); ``` Which generates: ```java private final String android = "Lollipop v." + 5.0; ``` ### Interfaces JavaPoet has no trouble with interfaces. Note that interface methods must always be `PUBLIC ABSTRACT` and interface fields must always be `PUBLIC STATIC FINAL`. These modifiers are necessary when defining the interface: ```java TypeSpec helloWorld = TypeSpec.interfaceBuilder("HelloWorld") .addModifiers(Modifier.PUBLIC) .addField(FieldSpec.builder(String.class, "ONLY_THING_THAT_IS_CONSTANT") .addModifiers(Modifier.PUBLIC, Modifier.STATIC, Modifier.FINAL) .initializer("$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 `javac`'s benefit! ```java public interface HelloWorld { String ONLY_THING_THAT_IS_CONSTANT = "change"; void beep(); } ``` ### Enums Use `enumBuilder` to create the enum type, and `addEnumConstant()` for each value: ```java TypeSpec helloWorld = TypeSpec.enumBuilder("Roshambo") .addModifiers(Modifier.PUBLIC) .addEnumConstant("ROCK") .addEnumConstant("SCISSORS") .addEnumConstant("PAPER") .build(); ``` To generate this: ```java 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: ```java 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 = $N", "handsign", "handsign") .build()) .build(); ``` Which generates this: ```java public enum Roshambo { ROCK("fist") { @Override public String toString() { return "avalanche!"; } }, SCISSORS("peace"), PAPER("flat"); private final String handsign; Roshambo(String handsign) { this.handsign = handsign; } } ``` ### Anonymous Inner Classes In the enum code, we used `Types.anonymousInnerClass()`. Anonymous inner classes can also be used in code blocks. They are values that can be referenced with `$L`: ```java 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: ```java 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: `TypeSpec.anonymousClassBuilder("")`. To pass different parameters use JavaPoet's code block syntax with commas to separate arguments. ### Annotations Simple annotations are easy: ```java MethodSpec toString = MethodSpec.methodBuilder("toString") .addAnnotation(Override.class) .returns(String.class) .addModifiers(Modifier.PUBLIC) .addStatement("return $S", "Hoverboard") .build(); ``` Which generates this method with an `@Override` annotation: ```java @Override public String toString() { return "Hoverboard"; } ``` Use `AnnotationSpec.builder()` to set properties on annotations: ```java MethodSpec logRecord = MethodSpec.methodBuilder("recordEvent") .addModifiers(Modifier.PUBLIC, Modifier.ABSTRACT) .addAnnotation(AnnotationSpec.builder(Headers.class) .addMember("accept", "$S", "application/json; charset=utf-8") .addMember("userAgent", "$S", "Square Cash") .build()) .addParameter(LogRecord.class, "logRecord") .returns(LogReceipt.class) .build(); ``` Which generates this annotation with `accept` and `userAgent` properties: ```java @Headers( accept = "application/json; charset=utf-8", userAgent = "Square Cash" ) LogReceipt recordEvent(LogRecord logRecord); ``` When you get fancy, annotation values can be annotations themselves. Use `$L` for embedded annotations: ```java 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: ```java @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 `addMember()` 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: ```java 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: ```java /** * 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); ``` Use `$T` when referencing types in Javadoc to get automatic imports. Download -------- Download [the latest .jar][dl] or depend via Maven: ```xml com.squareup javapoet 1.11.1 ``` or Gradle: ```groovy compile 'com.squareup:javapoet:1.11.1' ``` Snapshots of the development version are available in [Sonatype's `snapshots` repository][snap]. 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 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. JavaWriter ========== JavaPoet is the successor to [JavaWriter][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 `.java` 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][javawriter] and [Maven Central][javawriter_maven]. [dl]: https://search.maven.org/remote_content?g=com.squareup&a=javapoet&v=LATEST [snap]: https://oss.sonatype.org/content/repositories/snapshots/com/squareup/javapoet/ [javadoc]: https://square.github.io/javapoet/1.x/javapoet/ [javawriter]: https://github.com/square/javapoet/tree/javawriter_2 [javawriter_maven]: https://search.maven.org/#artifactdetails%7Ccom.squareup%7Cjavawriter%7C2.5.1%7Cjar [formatter]: https://developer.android.com/reference/java/util/Formatter.html [modifier]: https://docs.oracle.com/javase/8/docs/api/javax/lang/model/element/Modifier.html