Update from Google.

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diff --git a/src/main/java/com/google/devtools/build/lib/concurrent/ThreadSafety.java b/src/main/java/com/google/devtools/build/lib/concurrent/ThreadSafety.java
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+// Copyright 2014 Google Inc. All rights reserved.
+//
+// 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.devtools.build.lib.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;
+
+/**
+ * Define some standard attributes for documenting thread safety properties.
+ *<p>
+ * The names used here are adapted from those used in Joshua Bloch's book
+ * "Effective Java", which are also described at
+ * <http://www-128.ibm.com/developerworks/java/library/j-jtp09263.html>.
+ *<p>
+ * These attributes are just documentation.  They don't have any run-time
+ * effect.  The main aim is mainly just to standardize the terminology.
+ * (However, if this catches on, I can also imagine in the future having
+ * a presubmit check that checks that all new classes have thread safety
+ * annotations :)
+ *<p>
+ * See ThreadSafetyTest for examples of how these attributes should be used.
+ */
+public class ThreadSafety {
+  /**
+   * The Immutable attribute indicates that instances of the class are
+   * immutable, or at least appear that way are far as their external API
+   * is concerned.  Immutable classes are usually also ThreadSafe,
+   * but can be ThreadHostile if they perform unsynchronized access to
+   * mutable static data.  (We deviate from Bloch's nomenclature by
+   * not assuming that Immutable implies ThreadSafe; developers should
+   * explicitly annotate classes as both Immutable and ThreadSafe when
+   * appropriate.)
+   */
+  @Documented
+  @Target(value = {ElementType.TYPE})
+  @Retention(RetentionPolicy.RUNTIME)
+  public @interface Immutable {}
+
+  /**
+   * The ThreadSafe attribute marks a class or method which can safely be used
+   * from multiple threads without any need for external synchronization.
+   *
+   * When applied to a class, this attribute indicates that instances
+   * of the class can safely be used concurrently from multiple threads
+   * without any need for external synchronization, i.e. that all non-static methods
+   * are thread-safe (except any private methods that are explicitly
+   * annotated with a different thread safety annotation).  In addition it
+   * also indicates that all non-static nested classes are thread-safe (except any private
+   * nested classes that are explicitly annotated with a different thread
+   * safety annotation). Note that no guarantees are made about static class methods or static
+   * nested classes - they should be annotated separately.
+   *
+   * When applied to a method, this attribute indicates that the
+   * method can safely be called concurrently from multiple threads.
+   * The implementation must synchronize any accesses to mutable data.
+   */
+  @Documented
+  @Target(value = {ElementType.CONSTRUCTOR, ElementType.METHOD, ElementType.TYPE})
+  @Retention(RetentionPolicy.SOURCE)
+  public @interface ThreadSafe {}
+
+  /**
+   * The ThreadCompatible attribute marks a class or method that
+   * is thread-safe provided that only one thread attempts to
+   * access each object at a time.
+   *
+   * The implementation of such a class must synchronize accesses
+   * to mutable static data, but can assume that each instance will
+   * only be accessed from one thread at a time.
+   *
+   * The client must obtain an appropriate lock before calling ThreadCompatible
+   * methods, or must otherwise ensure that only one thread calls such methods.
+   * Unless otherwise specified, an appropriate lock means synchronizing on the
+   * instance.
+   *
+   * A ThreadCompatible class may contain private methods or private nested
+   * classes that are not ThreadCompatible provided that they are explicitly
+   * annotated with a different thread safety annotation.
+   */
+  @Documented
+  @Target(value = {ElementType.METHOD, ElementType.TYPE})
+  @Retention(RetentionPolicy.SOURCE)
+  public @interface ThreadCompatible {}
+
+  /**
+   * The ThreadHostile attribute marks a class or method that
+   * can't safely be used by multiple threads, for example because
+   * it performs unsynchronized access to mutable static objects.
+   */
+  @Documented
+  @Target(value = {ElementType.CONSTRUCTOR, ElementType.METHOD, ElementType.TYPE})
+  @Retention(RetentionPolicy.SOURCE)
+  public @interface ThreadHostile {}
+
+  /**
+   * The ConditionallyThreadSafe attribute marks a class that contains
+   * some methods (or nested classes) which are ThreadSafe but others which are
+   * only ThreadCompatible or ThreadHostile.
+   *
+   * The methods (and nested classes) of a ConditionallyThreadSafe class should
+   * each have their thread safety marked.
+   */
+  @Documented
+  @Target(value = {ElementType.METHOD, ElementType.TYPE})
+  @Retention(RetentionPolicy.SOURCE)
+  public @interface ConditionallyThreadSafe {}
+
+  /**
+   * The ConditionallyThreadCompatible attribute marks a class that contains
+   * some methods (or nested classes) which are ThreadCompatible but others
+   * which are ThreadHostile.
+   *
+   * The methods (and nested classes) of a ConditionallyThreadCompatible class
+   * should each have their thread safety marked.
+   */
+  @Documented
+  @Target(value = {ElementType.METHOD, ElementType.TYPE})
+  @Retention(RetentionPolicy.SOURCE)
+  public @interface ConditionallyThreadCompatible {}
+
+}