Project Url: ZacSweers/inspector
Introduction: A tiny class validation library.
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Inspector is a tiny class validation tool that has a tiny, flexible, and powerful API.

If you've ever used Moshi or GSON, it should feel very familiar.

The base type is Inspector, which can contain any number of Validators or Validator.Factorys.

Like Moshi, Inspector operates on Types directly. Validation is a decentralized, pluggable system. There is a reflective class-based validator generator if you want, or you can use the inspector-compiler annotation processor to generate validator implementations for you.

Validation normally operates on method return types. Everything is assumed non-null, unless annotated with one of the number of @Nullable annotations out there. Nullability is the only validation run out of the box by the reflective adapter as well. Inspector is purposefully stupid, you know your own models!

Usage looks like this:

Inspector inspector = new Inspector.Builder()
    .add(Foo.class, new FooValidator())
    .add(...)  // Whatever you want!

boolean isValid = inspector.validator(Foo.class).isValid(myFooStance);

// Or more aggressively, but with descriptive error messages
try {
} catch (ValidationException e) {
  // Server's sending you bad stuff

Note that you have two ways of checking validation: the non-throwing isValid() method for quick boolean checks, or the throwing validate() method that throws an exception with potentially more information.


@ValidationQualifier - Like Moshi's @JsonQualifier or javax's @Qualifer. Use these on your own annotations to create custom validators with interesting implementations.

@InspectorExcluded - Use this to exclude methods from validation.

@ValidatedBy(SomeValidator.class) - Use this to designate that this should be validated by a specified validator. Note that if you use this reflectively, it must also be reflectively instantiable.



  • Annotation processor that generates validator implementations
  • Supports a service-loader-based extensions API via InspectorExtension
    • First party implementations are under the compiler-extensions directory
  • Validator implementation is generated in the same package in a Validator_<YourClass>.java

Simply add a static Validator-returning method to your model and be sure to annotate it with something to look for, such as @AutoValue (this can be done automatically via the inspector-autovalue-compiler-extension).

First party extensions:

  • inspector-android-compiler-extension: Generates validation for Android support annotations
  • inspector-autovalue-compiler-extension: Tells the inspector compiler to look for @AutoValue annotations
  • inspector-nullability-compiler-extension: Generates nullability validation based on the presence of @Nullable annotations
  • inspector-rave-compiler-extension: Generates validation for RAVE annotations
public abstract class Foo {

  public abstract String bar();

  public static Validator<Foo> validator(Inspector inspector) {
    // Generated class is in the same package, prefixed with "Validator_"
    return new Validator_Foo(insepctor);

If you have a lot of these, you may not want to manually have to hook all these up to an inspector instance. To solve this, there are two batteries-included options you can use.

The inspector-factory-compiler annotation processor can generate a factory implementation that delegates to all the types on that compilation path. Simply stub a factory class like so:

@InspectorFactory(include = AutoValue.class) 
public abstract class MyFactory implements Validator.Factory {
  public static MyFactory create() {
    return new InspectorFactory_MyFactory();
  • Your class must implement ValidatorFactory and not be final.
  • You mark which annotations you want to target your factory to, such as @AutoValue.
  • An InspectorFactory_<YourClassName> will be generated in the same package for referencing with an implementation of the create() method.

The other option is to use @GenerateValidator. Simply annotate desired types with this, and a Validator implementation will be generated (regardless of the presence of a static Validator-returning method). You can read these via optional Validator.Factory implementation in the annotation via GenerateValidator.FACTORY.

class Foo {
  public String bar() {

// Later
Inspector inspector = new Inspector.Builder()

inspector.validator(Foo.class).validate(fooInstance); // will be dynamically looked up!


There's some helpful tools available:

  • CompositeValidator for composing multiple Validators for a given type/property.
  • Types is a utility class with helpful factories for creating different Type implementations.
  • Types can implement SelfValidating to indicate that they handle their own validation, and thus Inspector will just defer to that.
  • There is an inspector-retrofit artifact with a Retrofit Converter.Factory implementation that you can drop in to your network stack.


There is a sample project under inspector-sample with nontrivial examples.

inspector-sample-android is just a for-fun proof of concept of how some non-generated support library annotation validators would look. This unfortunately is not currently possible since support annotations are not RUNTIME retained.


Your gradle file could look like this:

Maven Central

depedencies {
  // Core Inspector library
  implementation 'io.sweers.inspector:inspector:x.y.z'

  // Compiler to generate validators, in a Java/Android project
  annotationProcessor 'io.sweers.inspector:inspector-compiler:x.y.z'

  // Or Kotlin (± Android)
  kapt 'io.sweers.inspector:inspector-compiler:x.y.z'

  // Compiler annotations
  implementation 'io.sweers.inspector:inspector-compiler-annotations:x.y.z'

  // Retrofit artifact
  implementation 'io.sweers.inspector:inspector-retrofit:x.y.z'

  // Optional compiler extensions
  compileOnly 'io.sweers.inspector:inspector-android-compiler-extension:x.y.z'
  compileOnly 'io.sweers.inspector:inspector-autovalue-compiler-extension:x.y.z'
  compileOnly 'io.sweers.inspector:inspector-nullability-compiler-extension:x.y.z'
  compileOnly 'io.sweers.inspector:inspector-rave-compiler-extension:x.y.z'

Snapshots of the development version are available in Sonatype's snapshots repository.


See subprojects for more detailed READMEs about specific artifacts. Full Javadocs can be found at

This library is heavily influenced by Moshi and RAVE.


Copyright (C) 2017 Zac Sweers

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,
See the License for the specific language governing permissions and
limitations under the License.
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