[LOGGING-135] Use double-checked locking idiom to improve thread-safety of Log4JLogger and LogKitLogger.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/logging/trunk@1448119 13f79535-47bb-0310-9956-ffa450edef68

src/main/java/org/apache/commons/logging/impl/Log4JLogger.java

@@ -53,7 +53,7 @@ public class Log4JLogger implements Log, Serializable {
     private static final String FQCN = Log4JLogger.class.getName();
 
     /** Log to this logger */
-    private transient Logger logger = null;
+    private transient volatile Logger logger = null;
 
     /** Logger name */
     private final String name;
@@ -254,10 +254,16 @@ public class Log4JLogger implements Log, Serializable {
      * Return the native Logger instance we are using.
      */
     public Logger getLogger() {
-        if (logger == null) {
-            logger = Logger.getLogger(name);
+        Logger result = logger;
+        if (result == null) {
+            synchronized(this) {
+                result = logger;
+                if (result == null) {
+                    logger = result = Logger.getLogger(name);
                 }
-        return this.logger;
+            }
+        }
+        return result;
     }
 
     /**

src/main/java/org/apache/commons/logging/impl/LogKitLogger.java

@@ -41,7 +41,7 @@ public class LogKitLogger implements Log, Serializable {
     // ------------------------------------------------------------- Attributes
 
     /** Logging goes to this <code>LogKit</code> logger */
-    protected transient Logger logger = null;
+    protected transient volatile Logger logger = null;
 
     /** Name of this logger */
     protected String name = null;
@@ -65,12 +65,16 @@ public class LogKitLogger implements Log, Serializable {
      * Return the underlying Logger we are using.
      */
     public Logger getLogger() {
-
-        if (logger == null) {
-            logger = Hierarchy.getDefaultHierarchy().getLoggerFor(name);
+        Logger result = logger;
+        if (result == null) {
+            synchronized(this) {
+                result = logger;
+                if (result == null) {
+                    logger = result = Hierarchy.getDefaultHierarchy().getLoggerFor(name);
                 }
-        return logger;
-
+            }
+        }
+        return result;
     }
 
     // ----------------------------------------------------- Log Implementation