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The Checker Framework enhances Java's type system to make it more powerful and useful. This lets software developers detect and prevent errors in their Java programs. The Checker Framework includes compiler plug-ins ("checkers") that find bugs or verify their absence. It also permits you to write your own compiler plug-ins.

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package org.checkerframework.framework.qual;

import java.lang.annotation.Annotation;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * Declares that the field may not be accessed if the receiver is of the specified qualifier type
 * (or any supertype).
 *
 * This property is verified by the checker that type-checks the {@code when} element value
 * qualifier.
 *
 * 
Example Consider a class, {@code Table}, with a locking field, {@code lock}. The lock
 * is used when a {@code Table} instance is shared across threads. When running in a local thread,
 * the {@code lock} field ought not to be used.
 *
 * 
You can declare this behavior in the following way:
 *
 * 
{@code
 * class Table {
 *   private @Unused(when=LocalToThread.class) final Lock lock;
 *   ...
 * }
 * }
 *
 * The checker for {@code @LocalToThread} would issue an error for the following code:
 *
 *   @LocalToThread Table table = ...;
 *   ... table.lock ...;
 * 
 *
 * @checker_framework.manual #unused-fields Unused fields
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.FIELD})
public @interface Unused {
  /**
   * The field that is annotated with @Unused may not be accessed via a receiver that is annotated
   * with the "when" annotation.
   */
  Class when();
}