https://github.com/google/guice/guice: Guice is a lightweight dependency injection framework for Java 6 and above (Google, Inc.) 
/*
 * Copyright (C) 2006 Google 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.
 */

package com.google.inject;

import static java.lang.annotation.ElementType.CONSTRUCTOR;
import static java.lang.annotation.ElementType.FIELD;
import static java.lang.annotation.ElementType.METHOD;
import static java.lang.annotation.RetentionPolicy.RUNTIME;

import java.lang.annotation.Documented;
import java.lang.annotation.Retention;
import java.lang.annotation.Target;


Annotates members of your implementation class (constructors, methods and fields) into which the Injector should inject values. The Injector fulfills injection requests for: 
In all cases, a member can be injected regardless of its Java access specifier (private, default,
protected, public).

Author:crazybob@google.com (Bob Lee)
/**
 * Annotates members of your implementation class (constructors, methods and fields) into which the
 * {@link Injector} should inject values. The Injector fulfills injection requests for:
 *
 * <ul>
 * <li>Every instance it constructs. The class being constructed must have exactly one of its
 *     constructors marked with {@code @Inject} or must have a constructor taking no parameters. The
 *     Injector then proceeds to perform field and method injections.
 * <li>Pre-constructed instances passed to {@link Injector#injectMembers}, {@link
 *     com.google.inject.binder.LinkedBindingBuilder#toInstance(Object)} and {@link
 *     com.google.inject.binder.LinkedBindingBuilder#toProvider(javax.inject.Provider)}. In this
 *     case all constructors are, of course, ignored.
 * <li>Static fields and methods of classes which any {@link Module} has specifically requested
 *     static injection for, using {@link Binder#requestStaticInjection}.
 * </ul>
 *
 * In all cases, a member can be injected regardless of its Java access specifier (private, default,
 * protected, public).
 *
 * @author crazybob@google.com (Bob Lee)
 */
@Target({METHOD, CONSTRUCTOR, FIELD})
@Retention(RUNTIME)
@Documented
public @interface Inject {

  
If true, and the appropriate binding is not found, the Injector will skip injection of this
method or field rather than produce an error. When applied to a field, any default value
already assigned to the field will remain (guice will not actively null out the field). When
applied to a method, the method will only be invoked if bindings for all parameters are
found. When applied to a constructor, an error will result upon Injector creation.

/**
   * If true, and the appropriate binding is not found, the Injector will skip injection of this
   * method or field rather than produce an error. When applied to a field, any default value
   * already assigned to the field will remain (guice will not actively null out the field). When
   * applied to a method, the method will only be invoked if bindings for <i>all</i> parameters are
   * found. When applied to a constructor, an error will result upon Injector creation.
   */
  boolean optional() default false;
}