https://github.com/JetBrains/java-annotations: A set of annotations used for code inspection support and code documentation. 
/*
 * Copyright 2000-2020 JetBrains s.r.o.
 *
 * 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
 *
 * https://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.
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package org.jetbrains.annotations;

import java.lang.annotation.*;


An element annotated with Nullable claims null value is perfectly valid
to return (for methods), pass to (parameters) or hold in (local variables and fields).
Apart from documentation purposes this annotation is intended to be used by static analysis tools
to validate against probable runtime errors or element contract violations.


By convention, this annotation applied only when the value should always be checked against null because the developer could do nothing to prevent null from happening. Otherwise, too eager Nullable usage could lead to too many false positives from static analysis tools. 
 For example, Map.get(Object key) should not be annotated Nullable because someone may have put not-null value in the map by this key and is expecting to find this value there ever since. 
 On the other hand, the Reference.get() should be annotated Nullable because it returns null if object got collected which can happen at any time completely unexpectedly. 
/**
 * An element annotated with {@link Nullable} claims {@code null} value is perfectly <em>valid</em>
 * to return (for methods), pass to (parameters) or hold in (local variables and fields).
 * Apart from documentation purposes this annotation is intended to be used by static analysis tools
 * to validate against probable runtime errors or element contract violations.
 * <br>
 * By convention, this annotation applied only when the value should <em>always</em> be checked against {@code null}
 * because the developer could do nothing to prevent null from happening.
 * Otherwise, too eager {@link Nullable} usage could lead to too many false positives from static analysis tools.
 * <br>
 * For example, {@link java.util.Map#get(Object key)} should <em>not</em> be annotated {@link Nullable} because
 * someone may have put not-null value in the map by this key and is expecting to find this value there ever since.
 * <br>
 * On the other hand, the {@link java.lang.ref.Reference#get()} should be annotated {@link Nullable} because
 * it returns {@code null} if object got collected which can happen at any time completely unexpectedly.
 */
@Documented
@Retention(RetentionPolicy.CLASS)
@Target({ElementType.METHOD, ElementType.FIELD, ElementType.PARAMETER, ElementType.LOCAL_VARIABLE, ElementType.TYPE_USE})
public @interface Nullable {
  @NonNls String value() default "";
}