diff --git a/org.eclipse.jgit.junit/src/org/eclipse/jgit/junit/MockSystemReader.java b/org.eclipse.jgit.junit/src/org/eclipse/jgit/junit/MockSystemReader.java
index 65551d657..d24dd44ff 100644
--- a/org.eclipse.jgit.junit/src/org/eclipse/jgit/junit/MockSystemReader.java
+++ b/org.eclipse.jgit.junit/src/org/eclipse/jgit/junit/MockSystemReader.java
@@ -47,6 +47,7 @@
 
 import java.io.File;
 import java.io.IOException;
+import java.lang.reflect.Field;
 import java.text.DateFormat;
 import java.text.SimpleDateFormat;
 import java.util.HashMap;
@@ -170,6 +171,7 @@ public DateFormat getDateTimeInstance(int dateStyle, int timeStyle) {
 	 * Assign some properties for the currently executing platform
 	 */
 	public void setCurrentPlatform() {
+		resetOsNames();
 		setProperty("os.name", System.getProperty("os.name"));
 		setProperty("file.separator", System.getProperty("file.separator"));
 		setProperty("path.separator", System.getProperty("path.separator"));
@@ -180,6 +182,7 @@ public void setCurrentPlatform() {
 	 * Emulate Windows
 	 */
 	public void setWindows() {
+		resetOsNames();
 		setProperty("os.name", "Windows");
 		setProperty("file.separator", "\\");
 		setProperty("path.separator", ";");
@@ -191,10 +194,25 @@ public void setWindows() {
 	 * Emulate Unix
 	 */
 	public void setUnix() {
+		resetOsNames();
 		setProperty("os.name", "*nix"); // Essentially anything but Windows
 		setProperty("file.separator", "/");
 		setProperty("path.separator", ":");
 		setProperty("line.separator", "\n");
 		setPlatformChecker();
 	}
+
+	private void resetOsNames() {
+		Field field;
+		try {
+			field = SystemReader.class.getDeclaredField("isWindows");
+			field.setAccessible(true);
+			field.set(null, null);
+			field = SystemReader.class.getDeclaredField("isMacOS");
+			field.setAccessible(true);
+			field.set(null, null);
+		} catch (Exception e) {
+			e.printStackTrace();
+		}
+	}
 }
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/util/SystemReader.java b/org.eclipse.jgit/src/org/eclipse/jgit/util/SystemReader.java
index b4233b6cc..4795c89e7 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/util/SystemReader.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/util/SystemReader.java
@@ -71,6 +71,11 @@
  */
 public abstract class SystemReader {
 	private static final SystemReader DEFAULT;
+
+	private static Boolean isMacOS;
+
+	private static Boolean isWindows;
+
 	static {
 		SystemReader r = new Default();
 		r.init();
@@ -148,6 +153,8 @@ public static SystemReader getInstance() {
 	 *            the default instance.
 	 */
 	public static void setInstance(SystemReader newReader) {
+		isMacOS = null;
+		isWindows = null;
 		if (newReader == null)
 			INSTANCE = DEFAULT;
 		else {
@@ -293,26 +300,31 @@ public DateFormat getDateTimeInstance(int dateStyle, int timeStyle) {
 	 * @return true if we are running on a Windows.
 	 */
 	public boolean isWindows() {
-		String osDotName = AccessController
-				.doPrivileged(new PrivilegedAction<String>() {
-					public String run() {
-						return getProperty("os.name"); //$NON-NLS-1$
-					}
-				});
-		return osDotName.startsWith("Windows"); //$NON-NLS-1$
+		if (isWindows == null) {
+			String osDotName = getOsName();
+			isWindows = Boolean.valueOf(osDotName.startsWith("Windows")); //$NON-NLS-1$
+		}
+		return isWindows.booleanValue();
 	}
 
 	/**
 	 * @return true if we are running on Mac OS X
 	 */
 	public boolean isMacOS() {
-		String osDotName = AccessController
-				.doPrivileged(new PrivilegedAction<String>() {
-					public String run() {
-						return getProperty("os.name"); //$NON-NLS-1$
-					}
-				});
-		return "Mac OS X".equals(osDotName) || "Darwin".equals(osDotName); //$NON-NLS-1$ //$NON-NLS-2$
+		if (isMacOS == null) {
+			String osDotName = getOsName();
+			isMacOS = Boolean.valueOf(
+					"Mac OS X".equals(osDotName) || "Darwin".equals(osDotName)); //$NON-NLS-1$ //$NON-NLS-2$
+		}
+		return isMacOS.booleanValue();
+	}
+
+	private String getOsName() {
+		return AccessController.doPrivileged(new PrivilegedAction<String>() {
+			public String run() {
+				return getProperty("os.name"); //$NON-NLS-1$
+			}
+		});
 	}
 
 	/**