How to store a hash in extended file attributes on OS X with Java?

Question 1

I have created a JNI wrapper for accessing the extended attributes now directly over the C-API. It is a open source Java Maven project and avaiable on GitHub/xattrj

For reference, I post the interesting source pieces here. For the latest sources, please refer to the above project page.

Xattrj.java

public class Xattrj {

    /**
     * Write the extended attribute to the given file
     * @param file
     * @param attrKey
     * @param attrValue
     */
    public void writeAttribute(File file, String attrKey, String attrValue){
        writeAttribute(file.getAbsolutePath(), attrKey, attrValue);
    }

    /**
     * Read the extended attribute from the given file
     * @param file
     * @param attrKey
     * @return
     */
    public String readAttribute(File file, String attrKey){
        return readAttribute(file.getAbsolutePath(), attrKey);
    }

    /**
     * Write the extended attribute to the given file
     * @param file
     * @param attrKey
     * @param attrValue
     */
    private native void writeAttribute(String file, String attrKey, String attrValue);

    /**
     * Read the extended attribute from the given file
     * @param file
     * @param attrKey
     * @return
     */
    private native String readAttribute(String file, String attrKey);


    static {
        try {
            System.out.println("loading xattrj...");
            LibraryLoader.loadLibrary("xattrj");
            System.out.println("loaded!");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

org_securityvision_xattrj_Xattrj.cpp

#include <jni.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "org_securityvision_xattrj_Xattrj.h"
#include <sys/xattr.h>


/**
 * writeAttribute
 * writes the extended attribute
 *
 */
JNIEXPORT void JNICALL Java_org_securityvision_xattrj_Xattrj_writeAttribute
    (JNIEnv *env, jobject jobj, jstring jfilePath, jstring jattrName, jstring jattrValue){

    const char *filePath= env->GetStringUTFChars(jfilePath, 0);
    const char *attrName= env->GetStringUTFChars(jattrName, 0);
    const char *attrValue=env->GetStringUTFChars(jattrValue,0);

    int res = setxattr(filePath,
                attrName,
                (void *)attrValue,
                strlen(attrValue), 0,  0); //XATTR_NOFOLLOW != 0
    if(res){
      // an error occurred, see errno
        printf("native:writeAttribute: error on write...");
        perror("");
    }
}


/**
 * readAttribute
 * Reads the extended attribute as string
 *
 * If the attribute does not exist (or any other error occurs)
 * a null string is returned.
 *
 *
 */
JNIEXPORT jstring JNICALL Java_org_securityvision_xattrj_Xattrj_readAttribute
    (JNIEnv *env, jobject jobj, jstring jfilePath, jstring jattrName){

    jstring jvalue = NULL;

    const char *filePath= env->GetStringUTFChars(jfilePath, 0);
    const char *attrName= env->GetStringUTFChars(jattrName, 0);

    // get size of needed buffer
    int bufferLength = getxattr(filePath, attrName, NULL, 0, 0, 0);

    if(bufferLength > 0){
        // make a buffer of sufficient length
        char *buffer = (char*)malloc(bufferLength);

        // now actually get the attribute string
        int s = getxattr(filePath, attrName, buffer, bufferLength, 0, 0);

        if(s > 0){
            // convert the buffer to a null terminated string
            char *value = (char*)malloc(s+1);
            *(char*)value = 0;
            strncat(value, buffer, s);
            free(buffer);

            // convert the c-String to a java string
            jvalue = env->NewStringUTF(value);
        }
    }
    return jvalue;
}

Now the makefile which has troubled me quite a bit to get things working:

CC=gcc
LDFLAGS= -fPIC -bundle
CFLAGS= -c -shared -I/System/Library/Frameworks/JavaVM.framework/Versions/Current/Headers -m64


SOURCES_DIR=src/main/c++
OBJECTS_DIR=target/c++
EXECUTABLE=target/classes/libxattrj.dylib

SOURCES=$(shell find '$(SOURCES_DIR)' -type f -name '*.cpp')
OBJECTS=$(SOURCES:$(SOURCES_DIR)/%.cpp=$(OBJECTS_DIR)/%.o)

all: $(EXECUTABLE)

$(EXECUTABLE): $(OBJECTS)
    $(CC) $(LDFLAGS) $(OBJECTS) -o $@

$(OBJECTS): $(SOURCES)
    mkdir -p $(OBJECTS_DIR)
    $(CC) $(CFLAGS) $< -o $@



clean:
    rm -rf $(OBJECTS_DIR) $(EXECUTABLE)

Question 2

OS X's /usr/bin/xattr is probably rather slow because it's implemented as a Python script. The C API for setting extended attributes is setxattr(2). Here's an example:

if(setxattr("/path/to/file",
            attribute_name,
            (void *)attribute_data,
            attribute_size,
            0,
            XATTR_NOFOLLOW) != 0)
{
  /* an error occurred, see errno */
}

You can create a JNI wrapper to access this function from Java; you might also want getxattr(2), listxattr(2), and removexattr(2), depending on what else your app needs to do.

Question 3

I've just contributed my <sys/xattr.h> JNA wrapper to the JNA-platform project. Since it's JNA you won't need to compile any native libraries. :)

@see https://github.com/twall/jna/pull/338

Should be part of the next release JNA release.