http://findbugs.sourceforge.net/: Annotation the FindBugs tool supports 
package javax.annotation.concurrent;

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;

/*
 * Copyright (c) 2005 Brian Goetz
 * Released under the Creative Commons Attribution License
 *   (http://creativecommons.org/licenses/by/2.5)
 * Official home: http://www.jcip.net
 */


Immutable
The class to which this annotation is applied is immutable. This means that
its state cannot be seen to change by callers. Of necessity this means that
all public fields are final, and that all public final reference fields refer
to other immutable objects, and that methods do not publish references to any
internal state which is mutable by implementation even if not by design.
Immutable objects may still have internal mutable state for purposes of
performance optimization; some state variables may be lazily computed, so
long as they are computed from immutable state and that callers cannot tell
the difference.
Immutable objects are inherently thread-safe; they may be passed between
threads or published without synchronization.

/**
 * Immutable
 * 
 * The class to which this annotation is applied is immutable. This means that
 * its state cannot be seen to change by callers. Of necessity this means that
 * all public fields are final, and that all public final reference fields refer
 * to other immutable objects, and that methods do not publish references to any
 * internal state which is mutable by implementation even if not by design.
 * Immutable objects may still have internal mutable state for purposes of
 * performance optimization; some state variables may be lazily computed, so
 * long as they are computed from immutable state and that callers cannot tell
 * the difference.
 * 
 * Immutable objects are inherently thread-safe; they may be passed between
 * threads or published without synchronization.
 */
@Documented
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.CLASS)
public @interface Immutable {
}