listFiles() returns null when it shouldn't. It used to work properly until recently and hasn't been modified

Question

``` public static void main(String[] args) throws InterruptedException { // TODO Auto-generated method stub while(true) { ArrayList wallpapers = new ArrayList(); File dir = new File("C:/Windows/System32/oobe/info/backgrounds/"); if(dir.listFiles() == null) System.out.println("Empty"); for(File img : dir.listFiles()) { if(img.getName().endsWith(".jpg") && img.getName() != "backgroundDefault.jpg") wallpapers.add(img); } File current = new File("C:/Windows/System32/oobe/info/backgrounds/backgroundDefault.jpg"); int i = 1; for(File img : wallpapers) { File f = new File("C:/Windows/System32/oobe/info/backgrounds/"+ i++ +".jpg"); current.renameTo(f); File file = new File("C:/Windows/System32/oobe/info/backgrounds/backgroundDefault.jpg"); img.renameTo(file); Thread.sleep(60000); } } } } } ``` This code changes the background image of the Windows Log In screen every minute. listFiles() returns null for dir and I get a NullPointerException on for(File img : dir.listFiles()). I thou…

Accepted Answer

On Windows 7 (and later) the process will have to run with elevated privileges to write to `C:/Windows` and similar directories. But if that was the problem it would result in a different error message.

What I suspect: When running a 32-bit JVM under 64-bit Windows `new File("C:/Windows/System32")` will point to `C:\Windows\SysWOW64` and there is no `info`-Folder under `C:\Windows\SysWOW64\oobe`

As a test:

```
public static void main(String[] args) {
    File sysdir = new File("C:/Windows/System32/oobe/info");
    for(File file:sysdir.listFiles()) {
        System.out.println(file.getName());
    }
}
```

runs fine with 64-bit-JRE and throws `NullPointerException` under 32-bit-JRE on Windows 7 64-bit.

So perhaps you or another application recently installed a 32-bit-jre or changed your path to point to a 32-bit-jre and thus broke your application.

listFiles() returns null when it shouldn't. It used to work properly until recently and hasn't been modified

Solution

Problem