Skip to main content

Java file.canWrite always returning true

This is going to be very short. Actually does not entail a blog post. Anyways, here goes.

So, when I was trying to setup some Junit tests on an EC2 instance, I stumble into this ridiculous failure where, file.canWrite() simply returns true, no matter what. A code like below, keeps printing the "file is writable"


public class Test1 {
        public static void main(String[] args) throws Exception {
                File file = new File("hello.txt");
                if (file.canWrite()) {
                        System.out.println("File is writable!");
                } else {
                        System.out.println("File is in read only mode!");

Turns out, the culprit is that I am running as "root". Since root can do-it-all, it simply returns true even though the file permissions read

[root@myhost ~]# ls -al hello.txt
-r--r--r--. 1 root root 0 Jan 30 18:52 hello.txt

Kind of subtle, since I would expect the permissions to be the ultimate source of truth.


Popular posts from this blog

Learning Spark Streaming #1

I have been doing a lot of Spark in the past few months, and of late, have taken a keen interest in Spark Streaming. In a series of posts, I intend to cover a lot of details about Spark streaming and even other stream processing systems in general, either presenting technical arguments/critiques, with any micro benchmarks as needed.

Some high level description of Spark Streaming (as of 1.4),  most of which you can find in the programming guide.  At a high level, Spark streaming is simply a spark job run on very small increments of input data (i.e micro batch), every 't' seconds, where t can be as low as 1 second.

As with any stream processing system, there are three big aspects to the framework itself.

Ingesting the data streams : This is accomplished via DStreams, which you can think of effectively as a thin wrapper around an input source such as Kafka/HDFS which knows how to read the next N entries from the input.The receiver based approach is a little complicated IMHO, and …

Thoughts On Adding Spatial Indexing to Voldemort

This weekend, I set out to explore something that has always been a daemon running at the back of my head. What would it mean to add Spatial Indexing support to Voldemort, given that Voldemort supports a pluggable storage layer.. Would it fit well with the existing Voldemort server architecture? Or would it create a frankenstein freak show where two systems essentially exist side by side under one codebase... Let's explore..

Basic Idea The 50000 ft blueprint goes like this.

Implement a new Storage Engine on top Postgres sql (Sorry innoDB, you don't have true spatial indexes yet and Postgres is kick ass)Implement a new smart partitioning layer that maps a given geolocation to a subset of servers in the cluster (There are a few ways to do this. But this needs to be done to get an efficient solution. I don't believe in naive spraying of results to all servers)Support "geolocation" as a new standard key serializer type in Voldemort. The values will still be  opaque b…

Enabling SSL in MAMP

Open /Applications/MAMP/conf/apache/httpd.conf, Add the line LoadModule ssl_module modules/         or uncomment this out if already in the conf file
Also add lines to listen on 80, if not there alreadyListen 80ServerName localhost:80 Open /Applications/MAMP/conf/apache/ssl.conf. Remove all lines as well as . Find the line defining SSLCertificateFile and SSLCertificateKeyFile, set it to SSLCertificateFile /Applications/MAMP/conf/apache/ssl/server.crt SSLCertificateKeyFile /Applications/MAMP/conf/apache/ssl/server.keyCreate a new folder /Applications/MAMP/conf/apache/ssl. Drop into the terminal and navigate to the new foldercd /Applications/MAMP/conf/apache/sslCreate a private key, giving a password openssl genrsa -des3 -out server.key 1024Remove the password cp server.key server-pw.key openssl rsa -in server-pw.key -out server.keyCreate a certificate signing request, pressing return for default values openssl req -new -key server.key -o…