optimize codes

sample/src/main/java/org/xutils/sample/download/DownloadManager.java

@@ -27,7 +27,7 @@ public final class DownloadManager {
         ColumnConverterFactory.registerColumnConverter(DownloadState.class, new DownloadStateConverter());
     }
 
-    private static DownloadManager instance;
+    private static volatile DownloadManager instance;
 
     private final static int MAX_DOWNLOAD_THREAD = 2; // 有效的值范围[1, 3], 设置为3时, 可能阻塞图片加载.
 

xutils/src/main/java/org/xutils/common/task/TaskControllerImpl.java

@@ -18,7 +18,7 @@ public final class TaskControllerImpl implements TaskController {
     private TaskControllerImpl() {
     }
 
-    private static TaskController instance;
+    private static volatile TaskController instance;
 
     public static void registerInstance() {
         if (instance == null) {

xutils/src/main/java/org/xutils/http/HttpManagerImpl.java

@@ -13,7 +13,7 @@
 public final class HttpManagerImpl implements HttpManager {
 
     private static final Object lock = new Object();
-    private static HttpManagerImpl instance;
+    private static volatile HttpManagerImpl instance;
 
     private HttpManagerImpl() {
     }

xutils/src/main/java/org/xutils/image/ImageManagerImpl.java

@@ -15,7 +15,7 @@
 public final class ImageManagerImpl implements ImageManager {
 
     private static final Object lock = new Object();
-    private static ImageManagerImpl instance;
+    private static volatile ImageManagerImpl instance;
 
     private ImageManagerImpl() {
     }

xutils/src/main/java/org/xutils/view/ViewInjectorImpl.java

@@ -48,7 +48,7 @@ public final class ViewInjectorImpl implements ViewInjector {
     }
 
     private static final Object lock = new Object();
-    private static ViewInjectorImpl instance;
+    private static volatile ViewInjectorImpl instance;
 
     private ViewInjectorImpl() {
     }