TOO_MANY_CONSTRUCTORS

TOO_MANY_CONSTRUCTORS

Summary

Guice will throw a TOO_MANY_CONSTRUCTORS error when there are more than one @Inject annotated constructors in a class that uses constructor injection. When multiple constructors are annotated with @Inject, Guice does not know which one to use.

Common Causes

Copy paste error

This usually happens when a new constructor is added to the class that already has a constructor annotated with @Inject. Since @Inject by itself does not have any effect unless Guice is used to construct those objects, this mistake may only become clearer later when the class is integrated with Guice.

Example class that has too many injectable constructors:

final class Foo {
  @Inject
  Foo(Bar bar) {
    ...
  }

  @Inject
  Foo(Bar bar, Optional<Baz> baz) {
    ...
  }
}

There are two ways to fix this error:

• Simply remove @Inject from all but one constructor:

  final class Foo {
    Foo(Bar bar) { ... }
  
    @Inject // Guice will use this constructor.
    Foo(Bar bar, Optional<Baz> baz) { ... }
    }

• Remove all the @Inject annotations from those constructors and add an explicit binding for the class. The recommended way to do this is to add a @Provides method for the class:

  final class Foo {
    Foo(Bar bar) { ... }
  
    Foo(Bar bar, Optional<Baz> baz) { ... }
  }
  
  final class FooModule extends AbstractModule {
    @Provides
    Foo provideFoo(Bar bar) {
      return new Foo(bar); // Calls the one argument constructor explicitly.
    }
  }

  In rare cases that you can't use @Provides method because you need to use Guice with AOP features, you can use constructor binding to tell Guice explicitly which constructor to use.