Making a Read-Only File Writeable

[Robert Paris]

I'm sort of looking for some collaboration here. I know that java.io.File is cannot be made writeable after it's been setReadOnly(); So I made a class RWFile (Read-Write-File) that extends java.io.File and adds one method: boolean setWriteable(). I have already implemented this for windows (using JNI) but wondered if other people out there wanted to contribute code for making it work on Linux/Unix/MacOSX. I'm posting all my code below including instructions on how to compile, etc. But can someone out there on those OS's contribute that code? I'll even put it all together and maybe we could set up a sourceforge thing or something.
The cool thing is, now you can use the feature of making files read-only, because now you can set them back to writeable! So have fun with this, and anyone who solves it for the other OS's please post it here!

/*
 * File: RWFile.java
 *
 * Author: Robert Paris.
 */
import java.io.File;
import java.net.*;
public class RWFile extends File
{
/* 
 * NATIVE METHOD: ON WINDOWS THIS ACTION IS ALLOWED.
 * NO IDEA IF LINUX, ETC ALLOW IT OR NOT
 */
private native boolean setWriteableNative(String fileName);

/* 
 * LOAD THE LIBRARY WITH NATIVE CODE
 */
static 
{
System.loadLibrary("RWFile");
}

/*
 * JUST DELEGATE TO PARENT ON ALL OF THESE
 */
public RWFile(String file)
{
super( file );
}

public RWFile(File parent, String child)
{
super( parent, child );
}

public RWFile(String parent, String child)
{
super( parent, child );
}

public RWFile(URI uri)
{
super( uri );
}

public boolean setWriteable() throws SecurityException
{
/*
 * GET PATH - path IS PRIVATE IN java.io.File AND WE DON'T CACHE
 * IT BECAUSE IT CAN CHANGE AND THEN WE'D BE OUT OF SYNCH.
 * EX. renameTo() WOULD CHANGE IT.
 */
String filePath = this.getPath();

//CHECK SECURITY RIGHTS JUST LIKE JAVA EXPECTS US TO
SecurityManager security = System.getSecurityManager();
if ( security != null ) 
{
security.checkWrite( filePath );
}

return setWriteableNative( filePath );
}
//TESTING
public static void main(String[] args) throws Exception
{
RWFile fle = new RWFile( args [ 0 ] );
System.out.println( "CAN WRITE: " + fle.canWrite() );
System.out.println( "Setting read only..." );
System.out.println( fle.setReadOnly() );
System.out.println( "CAN WRITE: " + fle.canWrite() );
System.out.println( "Setting writeable..." );
System.out.println( fle.setWriteable() );
System.out.println( "CAN WRITE: " + fle.canWrite() );
}
}

/*
 * File: RWFile.c
 *
 * Author: Robert Paris
 */
#include <windows.h>
#include <jni.h>
#include "RWFile.h"
#include "excep.h"
#include <stdio.h>
JNIEXPORT jboolean JNICALL 
Java_RWFile_setWriteableNative(JNIEnv *env, jobject obj, jstring fileName) 
{
//FILE HANDLING VARIABLES
WIN32_FIND_DATA FileData; 
HANDLE hFile; 
DWORD dwAttrs;
int success = 0;
//ALLOCATE FOR OUR FILE NAME STRING CONVERSION
char buf[128];

//THE FILE NAME STRING CONVERTED
const char *cppFileName = (*env)->GetStringUTFChars(env, fileName, 0);

//START SEARCHING FOR THE FILE
hFile = FindFirstFile( cppFileName, &FileData ); 

//NO FILE FOUND - VERY UNLIKELY, IN FACT SUN'S CODE DOESN'T EVEN BOTHER CHECKING THIS
if ( hFile == INVALID_HANDLE_VALUE ) 
{  
/*
 * I INCLUDED THE CODE IN CASE ANYONE WANTS THIS, BUT I THINK THROWING THIS ERROR
 * MEANS YOU CAN'T TRUST THE API. IF YOU HAVE A FILE OBJECT, YOU SHOULD BE
 * ABLE TO EXPECT THAT WHEN YOU CALL setReadOnly() OR setWriteable() IT
 * WON'T DISSAPEAR ON YOU. THAT'S NOT AN EXCEPTION, BUT A SYSTEM FAILURE, WHICH
 * IS PROBABLY WHY SUN'S CODE DOESN'T THROW ANYTHING OR EVEN CHECK IF IT EXISTS
 */
//JNU_ThrowByName( env, "java/io/FileNotFoundException", "File was not found and thus could not be made writeable." );

//NOT EXISTS, SO CAN'T SET WRITEABLE - AGAIN, UNLIKELY
return success;
} 

//GET THE FILE ATTRIBUTES
dwAttrs = GetFileAttributes( FileData.cFileName ); 

//IF READ ONLY CHANGE, OTHERWISE WE'LL JUST RETURN TRUE - ALREADY WRITEABLE
if ( ( dwAttrs & FILE_ATTRIBUTE_READONLY ) )
{
//NO SUCH FILE ATTRIBUTE: FILE_ATTRIBUTE_WRITEABLE - WE JUST NEGATE THE READ ONLY
if ( SetFileAttributes( FileData.cFileName, dwAttrs ^ FILE_ATTRIBUTE_READONLY ) )
success = 1; 
}

//CLOSE THE FILE HANDLE. 
FindClose(hFile);
//RELEASE STRING - DON'T WANT MEMORY LEAK
(*env)->ReleaseStringUTFChars( env, fileName, cppFileName );
//SUCCESS
return success;
}
/*
 * I'M NOT USING THIS, BUT THOUGHT IF YOU WANTED TO I'D INCLUDE IT. THIS
 * CODE IS ACTUALLY SIMILAR TO THAT IN JAVA'S JNI BOOK
 */
JNIEXPORT void JNICALL 
JNU_ThrowByName(JNIEnv *env, const char *exceptionName, const char *message)
{
jclass exceptionClass;
jthrowable previousException;

//WAS THERE ALREADY AN EXCEPTION THROWN?
if ( ( previousException = (*env)->ExceptionOccurred( env ) ) != NULL ) 
{
(*env)->DeleteLocalRef( env, previousException );
return;
}

//GET OUR EXCEPTION'S CLASS SO WE CAN "THROW" IT
exceptionClass = (*env)->FindClass(env, exceptionName);

//IF IS 0 THEN AN EXCEPTION ALREADY WAS THROWN - WHERE'S THE try-catch!! I HATE C.
if ( exceptionClass != 0 )
(*env)->ThrowNew(env, exceptionClass, message);

//CLEAN UP TIME! NO LEAKS FOR US PLEASE
(*env)->DeleteLocalRef(env, exceptionClass);
}

INSTRUCTIONS
--------------------------
1. Compile the RWFile.java with:

javac RWFile.java
2. You need to create the C/C++ Header which is basically the interface (with some precompiling stuff put in). Don't worry, just do this:
javah -classpath . -jni RWFile
(NOTE!! Even though Sun's tutorial says you can do it without the classpath, forget it, I've never gotten that to work. )
This outputs a file called "RWFile.h". Your C code needs to be able to find this at compile time.
3. Compile the C code:
cl -I<Java_SDK_Home>\include -I<Java_SDK_Home>\include\win32 -LD RWFile.c -FeRWFile.dll
So if you have your SDK at the folder C:\Java2\, then you'd have this:
cl -IC:\Java2\include -IC:\Java2\include\win32 -LD RWFile.c -FeRWFile.dll
4. Run the test program:

java RWFile "testFile.txt"
It should print out:
CAN WRITE: true
Setting read only...
true
CAN WRITE: false
Setting writeable...
true
CAN WRITE: true
That's it!
Robert
[ February 06, 2003: Message edited by: Robert Paris ]

[Robert Paris]

OK, I created a sourceforge project! Anyone who wants to contribute let me know.
http://sourceforge.net/projects/rwfile-java/
Also, I added a jar file with the source code and a test (currently only works on windows files). All you have to do is download the file, save it and double-click! I set it up to load the library from the jar file so you have only one file to download.
NOTE: Don't run it straight from a download window, it won't work since at that point it's a temp file or something. Save it and run it from the file system. java -jar RWFile.jar will also work.
The SourceForge Page