Update documentation for ThreadLocalWorkSource.

Test: unit test
Change-Id: I446af267dafd069b1bf5f64a84347b69fc6fec60

core/java/android/os/ThreadLocalWorkSource.java

@@ -17,6 +17,24 @@
 package android.os;
 
 /**
+ * Tracks who triggered the work currently executed on this thread.
+ *
+ * <p>ThreadLocalWorkSource is automatically updated inside system server for incoming/outgoing
+ * binder calls and messages posted to handler threads.
+ *
+ * <p>ThreadLocalWorkSource can also be set manually if needed to refine the WorkSource.
+ *
+ * <p>Example:
+ * <ul>
+ * <li>Bluetooth process calls {@link PowerManager#isInteractive()} API on behalf of app foo.
+ * <li>ThreadLocalWorkSource will be automatically set to the UID of foo.
+ * <li>Any code on the thread handling {@link PowerManagerService#isInteractive()} can call
+ * {@link ThreadLocalWorkSource#getUid()} to blame any resource used to handle this call.
+ * <li>If a message is posted from the binder thread, the code handling the message can also call
+ * {@link ThreadLocalWorkSource#getUid()} and it will return the UID of foo since the work source is
+ * automatically propagated.
+ * </ul>
+ *
  * @hide Only for use within system server.
  */
 public final class ThreadLocalWorkSource {
@@ -24,24 +42,53 @@ public final class ThreadLocalWorkSource {
     private static final ThreadLocal<Integer> sWorkSourceUid =
             ThreadLocal.withInitial(() -> UID_NONE);
 
-    /** Returns the original work source uid. */
+    /**
+     * Returns the UID to blame for the code currently executed on this thread.
+     *
+     * <p>This UID is set automatically by common frameworks (e.g. Binder and Handler frameworks)
+     * and automatically propagated inside system server.
+     * <p>It can also be set manually using {@link #setUid(int)}.
+     */
     public static int getUid() {
         return sWorkSourceUid.get();
     }
 
-    /** Sets the original work source uid. */
+    /**
+     * Sets the UID to blame for the code currently executed on this thread.
+     *
+     * <p>Inside system server, this UID will be automatically propagated.
+     * <p>It will be used to attribute future resources used on this thread (e.g. binder
+     * transactions or processing handler messages) and on any other threads the UID is propagated
+     * to.
+     *
+     * @return a token that can be used to restore the state.
+     */
     public static long setUid(int uid) {
         final long token = getToken();
         sWorkSourceUid.set(uid);
         return token;
     }
 
-    /** Restores the state using the provided token. */
+    /**
+     * Restores the state using the provided token.
+     */
     public static void restore(long token) {
         sWorkSourceUid.set(parseUidFromToken(token));
     }
 
-    /** Clears the stored work source uid. */
+    /**
+     * Clears the stored work source uid.
+     *
+     * <p>This method should be used when we do not know who to blame. If the UID to blame is the
+     * UID of the current process, it is better to attribute the work to the current process
+     * explicitly instead of clearing the work source:
+     *
+     * <pre>
+     * ThreadLocalWorkSource.setUid(Process.myUid());
+     * </pre>
+     *
+     * @return a token that can be used to restore the state.
+     **/
     public static long clear() {
         return setUid(UID_NONE);
     }