Updated some comments about thread safety and fixed one easy case. Sean, please have a look.

git-svn-id: https://zxing.googlecode.com/svn/trunk@973 59b500cc-1b3d-0410-9834-0bbf25fbcc57

core/src/com/google/zxing/common/BitSource.java

@@ -20,7 +20,8 @@ package com.google.zxing.common;
  * <p>This provides an easy abstraction to read bits at a time from a sequence of bytes, where the
  * number of bits read is not often a multiple of 8.</p>
  *
- * <p>This class is not thread-safe.</p>
+ * <p>This class is thread-safe but not reentrant. Unless the caller modifies the bytes array
+ * it passed in, in which case all bets are off.</p>
  *
  * @author Sean Owen
  */

core/src/com/google/zxing/common/BlackPointEstimator.java

@@ -27,7 +27,7 @@ import com.google.zxing.ReaderException;
  * <p>For an interesting discussion of this issue, see
  * <a href="http://webdiis.unizar.es/~neira/12082/thresholding.pdf">this paper</a>.</p>
  *
- * NOTE: This class is not threadsafe.
+ * NOTE: This class is not thread-safe.
  *
  * @author Sean Owen
  * @author dswitkin@google.com (Daniel Switkin)

core/src/com/google/zxing/common/GridSampler.java

@@ -34,7 +34,7 @@ import com.google.zxing.ReaderException;
  */
 public abstract class GridSampler {
 
-  private static GridSampler gridSampler = null;
+  private static GridSampler gridSampler = new DefaultGridSampler();
 
   /**
    * Sets the implementation of {@link GridSampler} used by the library. One global
@@ -43,7 +43,7 @@ public abstract class GridSampler {
    * in the whole lifetime of the JVM. For instance, an Android activity can swap in
    * an implementation that takes advantage of native platform libraries.
    * 
-   * @param newGridSampler
+   * @param newGridSampler The platform-specific object to install.
    */
   public static void setGridSampler(GridSampler newGridSampler) {
     if (newGridSampler == null) {
@@ -56,11 +56,6 @@ public abstract class GridSampler {
    * @return the current implementation of {@link GridSampler}
    */
   public static GridSampler getInstance() {
-    // No real point in trying to make this thread-safe;
-    // doesn't matter if a second instance is created
-    if (gridSampler == null) {
-      gridSampler = new DefaultGridSampler();
-    }
     return gridSampler;
   }
 

core/src/com/google/zxing/multi/qrcode/detector/MultiFinderPatternFinder.java

@@ -35,6 +35,7 @@ import java.util.Vector;
  * markers at three corners of a QR Code.</p>
  *
  * <p>This class is not thread-safe and should not be reused.</p>
+ * TODO(dswitkin): Is this comment still true? I don't see any global variables for example.
  *
  * <p>In contrast to {@link FinderPatternFinder}, this class will return an array of all possible
  * QR code locations in the image.</p>

core/src/com/google/zxing/qrcode/detector/AlignmentPatternFinder.java

@@ -33,6 +33,7 @@ import java.util.Vector;
  * some code.</p>
  *
  * <p>This class is not thread-safe.</p>
+ * TODO(dswitkin): Is this still true?
  *
  * @author Sean Owen
  */

core/src/com/google/zxing/qrcode/detector/FinderPatternFinder.java

@@ -32,6 +32,7 @@ import java.util.Vector;
  * markers at three corners of a QR Code.</p>
  *
  * <p>This class is not thread-safe and should not be reused.</p>
+ * TODO(dswitkin): Is this still true?
  *
  * @author Sean Owen
  */