Fredrik Fornwall's geek blog

Weekend project: Chat Pusher

2010-10-31T22:05:00.000+01:00

Needed something like this: http://chatpusher.appspot.com/.

Viewing Android Source Code in Eclipse

2010-09-26T23:50:00.002+02:00

Just wrote a python script to help build the necessary folder for viewing android source code in eclipse. Check out http://mobilepearls.com/labs/viewing-android-source-code-in-eclipse/.

Handling SSH on non-standard ports for subversion and git

2010-08-08T04:19:00.000+02:00

Working with subversion and git over SSH on a non-standard port (!=22) is troublesome as the host:port syntax does not work.

One easy way around this is to to define an entry as the following in $HOME/.ssh/config:

Host my.githost.com
Port 7822

This allows one to use the host normally through most SSH-using tools such as subversion and git!

Show&hide hidden files and folders in Mac file dialogs

2010-08-08T04:10:00.001+02:00

Keyboard shortcut of the day: Use "Cmd+Shift+." to toggle showing hidden files in Mac file dialogs.

Simple HTTP benchmarks with ab

2010-07-08T00:05:00.001+02:00

The ab tool from Apache is a simple command line program for benchmarking, included in the httpd-tools rpm package in Fedora. The below command shows how to benchmark http://example.com/ with 4 total requests using 2 concurrent connections:

ab -n 4 -c 2 http://example.com/

There are a lot of options as the -H for a custom HTTP header or the -p for POST data from a file!

Eclipse tip: Choose editor from the "Open Resource" dialog

2010-05-29T12:04:00.003+02:00

Eclipse 3.5 added the option to choose which editor to use when opening a file from the Open Resource dialog:

It's useful when a file type is associated with an external editor, but one has a need of finding it in Eclipse to work with e.g. version control.

Fixing graphical glitches with eclipse on fedora 12

2010-02-06T23:57:00.002+01:00

As noticed by https://bugzilla.redhat.com/show_bug.cgi?id=538733 and https://bugs.eclipse.org/bugs/show_bug.cgi?id=291257, eclipse has problems on fedora 12 in the form of missing icons in views. Setting GDK_NATIVE_WINDOWS as below fixes the problem:

export GDK_NATIVE_WINDOWS=1; ./eclipse

Fixing graphical glitches using Spotify&Wine in Fedora 11

2009-09-05T23:41:00.005+02:00

To reduce dual-booting to windows to a minimum, I recently setup Spotify under Wine in Fedora 11 (64-bit).
Initially, the audio went silent shortly after starting the application - this was fixed by changing the sound driver in winecfg to ALSA instead of PulseAudio.
A second problem was graphical glitches:

Googling found a nice blog post about what was missing:

yum install cups-libs.i586 fontconfig.i586 gnutls.i586 hal-libs.i586 libXcomposite.i586 libpng.i586 sane-backends-libs.i586 libXcursor.i586 libXinerama.i586 libXrandr.i586 libXrender.i586

After installing the above packages the graphical glitches disappeared and victory was declared:

Worth repeating: How to easily add certificates to your keystore in java

2009-08-05T11:08:00.004+02:00

Today I (yet again) had the below tiresome problem:

sun.security.validator.ValidatorException:
PKIX path building failed:
sun.security.provider.certpath.SunCertPathBuilderException:
unable to find valid certification path to requested target

The problem occurred when accessing WSDL files on a test server. I cursed life...

Then, a helpful post how to easily add certificates at http://blogs.sun.com/andreas/entry/no_more_unable_to_find saved the day. Yay!

Indentation problem when pasting into vim and the nopaste option

2009-05-18T15:09:00.003+02:00

When pasting content into vim, such as a code snippet, it may mess up the indentation in a nested kind of way, with indentation increasing the further down you go.

The vim paste option fixes this - just issue :set paste before pasting, and :set nopaste when done.

As noted by the vim documentation, the underlying problem is that a terminal program, in contrast to a GUI one, may not recognize the difference between pasted and typed text.

Java app to send geocaching data to your Nokia phone

2009-05-03T18:40:00.003+02:00

I have documented a small java app that's useful for me that allows me to send geocaching information from geocaching.com to Nokia phones. See http://fornwall.net/landmarksender/ for more information.

Session variables in postgresql

2009-04-29T13:45:00.004+02:00

Consider a trigger that would like to prevent certain operations when executed by a web application, while allowing the operations when performed by a nightly batch job.

One way to solve this is to use a kind of session variables. The trigger lets the operation execute normally only if a session variable is set, which the batch job sets at the start of the execution. Below is an example illustrating one implementation for this in PostgreSQL.

First, we define a kind of namespace for our session variable (a customized option or variable class in postgres lingo) in postgresql.conf:

# comma-separated list of class names:
custom_variable_classes = 'myvariableclass'

Now, consider the following table:

CREATE TABLE my_table (ID INTEGER);
INSERT INTO my_table VALUES (1);

A trigger is to prevent DELETE:s from that table unless the session variable myvariableclass.idontcare is set:

CREATE OR REPLACE FUNCTION prevent_delete_unless_idontcare() RETURNS TRIGGER AS $$
DECLARE
  idontcare TEXT;
BEGIN
  SELECT INTO idontcare CURRENT_SETTING('myvariableclass.idontcare');
  IF (idontcare != 'true') THEN
    RAISE EXCEPTION 'Not allowed to delete unless myvariableclass.idontcare is true';
  END IF;
  RETURN NULL;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER prevent_delete_unless_idontcare_trigger
AFTER DELETE ON my_table
FOR EACH ROW EXECUTE PROCEDURE prevent_delete_unless_idontcare();

Now let's see it in action:

fornwall=> DELETE FROM my_table;
ERROR:  Not allowed to delete unless myvariableclass.idontcare is true
fornwall=> SELECT SET_CONFIG('myvariableclass.idontcare', 'true', FALSE);
 set_config 
------------
 true
(1 row)

fornwall=> DELETE FROM my_table;
DELETE 1

See postgres documentation for the current_setting and set_config functions.

PyS60 script to summarize top persons in SMS inbox and outbox

2009-02-23T14:57:00.003+01:00

This requires the latest version of PyS60 (1.9.x):

# -*- coding:utf-8 -*-
import appuifw, collections, e32, inbox

def summarize_box(box, box_name):
    counter = collections.defaultdict(int)
    for message_id in box.sms_messages():
            counter[box.address(message_id)] += 1

    content = "Persons in " + box_name + ":\n"
    for (name, count) in sorted(counter.iteritems(), key=lambda (x,y):(y,x), reverse=True):
        content += u"%-3d: %s\n" % (count, name)

    return content

content = summarize_box(inbox.Inbox(), 'Inbox')
content += "\n"
content += summarize_box(inbox.Inbox(inbox.ESent), 'Sent')

lock = e32.Ao_lock()
appuifw.app.body = appuifw.Text(content)
appuifw.app.exit_key_handler = lock.signal
lock.wait()

Tip: What to try when autocomplete stops working for java in eclipse

2009-02-15T02:49:00.004+01:00

Suddenly (?) the autocomplete functionality of the java editor in eclipse stopped working. Searching for "eclipse autocomplete not working" on google quickly found a cure:

Go Preferences>Java>Editor>Content Assist>Advanced
Make sure “Other Java Proposals” is ticked

Size of a postgresql database

2009-01-21T11:26:00.005+01:00

The size of a postgresql database can be obtained using the following query:

$ select pg_size_pretty(pg_database_size('database_name'));
 pg_size_pretty 
----------------
 70 GB
(1 row)

Adding a search provider for searching Gmail from the browser

2008-12-08T12:44:00.010+01:00

Adding search engines to the browser (in the top-right search field in most browsers) may be done by creating an xml file containing an Opensearch description and then adding them with a javascript method call.

The below button adds a search provider for searching gmail:

The javascript call is

window.external.AddSearchProvider('http://fornwall.net/files/blog/gmail-search-provider.xml');

where the xml file is as follows:

<OpenSearchDescription xmlns="http://a9.com/-/spec/opensearch/1.1/"
                       xmlns:moz="http://www.mozilla.org/2006/browser/search/">
  <ShortName>Gmail</ShortName>
  <Description>Search Gmail</Description>
  <InputEncoding>UTF-8</InputEncoding>
  <Image width="16" height="16">https://mail.google.com/favicon.ico</Image>
  <Url type="text/html" method="GET" template="https://mail.google.com/mail/#search/{searchTerms}"/>
  <moz:SearchForm>https://mail.google.com/mail/</moz:SearchForm>
</OpenSearchDescription>

"Universal Edit Button" for marking web pages as editable

2008-08-09T16:30:00.007+02:00

There exists an intresting suggestion for a Universal Edit Button on web pages. Basically, just as the RSS icon pops up in the browser address bar when visiting a page with a feed related to it, an edit icon shows up when visiting an editable page.

Marking the page as editable is done by adding a <link/> element with content as follows:

<link rel="alternate" type="application/wiki" title="Edit this page!" href="wiki?edit=WelcomeVisitors"/>

The values of the rel and type attributes are currently a bit strange and are being discussed.

Apparently, Wikipedia has already adopted it and there is a Firefox extension available to watch it live.

Servlet to create java heap dumps

2008-08-04T23:18:00.006+02:00

To create a heap dump for analyzing memory usage one needs to use non-portable API:s. This is example code using suns HotSpotDiagnosticMXBean, but using reflection to avoid compile-time dependency on the sun class library:

public class HeapDumpServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        File tmpFile = File.createTempFile("heapdump", ".hprof");
        try {
            MBeanServer server = ManagementFactory.getPlatformMBeanServer();
            // use reflection to avoid compile time dependency on sun classes
            String className = "com.sun.management.HotSpotDiagnosticMXBean";
            Class diagClass = Class.forName(className);
            Object diagBean = ManagementFactory.newPlatformMXBeanProxy(server,
                "com.sun.management:type=HotSpotDiagnostic", diagClass);
            Method dumpHeapMethod = diagClass.getMethod("dumpHeap", String.class, boolean.class);
            dumpHeapMethod.invoke(diagBean, tmpFile.getAbsolutePath(), true);
            response.setContentType("application/octet-stream");
            response.addHeader("Content-Disposition", "attachment; filename=heapdump.hprof");
            IOUtils.copy(new FileInputStream(tmpFile), response.getOutputStream());
        } catch (Exception e) {
            response.setContentType("text/plain");
            response.getWriter().println("ERROR: " + e.getMessage() + "\n" + ExceptionUtils.getFullStackTrace(e));
        } finally {
            tmpFile.delete();
        }
    }

}

Monitoring java threads programmatically

2008-08-04T22:20:00.004+02:00

Using the java.lang.management package introduced in java 1.5 one may monitor some aspects of the threads of a java process:

Find all active threads and get their stack traces
Find deadlocked threads
Find out what lock a thread is waiting for (and which thread owns it)
(Optional, depending on JVM): Monitor thread contention - how many times and for how long have each thread waited or blocked
(Optional, depending on JVM): Measure thread CPU time (and how much of that time was spent in user mode)

The code to do this looks as follows:

ThreadMXBean threadBean = ManagementFactory.getThreadMXBean();
if (threadBean.isThreadCpuTimeSupported())
    threadBean.setThreadCpuTimeSupported(true);
if (threadBean.isThreadContentionMonitoringSupported())
    threadBean.setThreadContentionMonitoringEnabled(true);

out.println("There are " + threadBean.getThreadCount() +
    " threads of which " + threadBean.getDaemonThreadCount() +
    " are daemons.");

ThreadInfo[] infos
    = threadBean.getThreadInfo(threadBean.getAllThreadIds(), Integer.MAX_VALUE);

for (ThreadInfo info : infos) {
    out.println("Thread(id=" + info.getThreadId() + "): " +
        info.getThreadName() + ":");

    out.println("\tState: " + info.getThreadState());
    if (info.getLockOwnerId() != null)
        out.println("\tWaiting on lock " + info.getLockName() + " owned by " + info.getLockOwnerId());

    if (info.getBlockedTime() != -1) {
        out.println("\tBlocked time: " + (info.getBlockedTime() / 1000.) + " seconds during " +
            info.getBlockedCount() + " blocks");
        out.println("\tWaited time: " + (info.getWaitedTime() / 1000.) + " seconds during " +
            info.getWaitedCount() + " blocks");
    }

    out.println("\tStack trace:");
    for (int j = 0; j < threadInfo.getStackTrace().length; j++) {
        StackTraceElement e = threadInfo.getStackTrace()[j];
        out.println(e.getClassName() + "#" + e.getMethodName() + ":" + e.getLineNumber());
    }
}

Better shell scripts: set -u

2008-08-04T01:27:00.004+02:00

Beginning shell scripts with "set -u" could save you serious time. From bash(1):

Treat unset variables as an error when performing parameter expansion. If expansion is attempted on an unset variable, the shell prints an error message, and, if not interactive, exits with a non-zero status.

Example:

$ cat hello.sh
#!/bin/sh

NAME=$1
echo Hello $NAME
$ sh hello.sh
Hello

$ cat hellou.sh
#!/bin/sh

set -u

NAME=$1
echo Hello $NAME
$ sh hellou.sh
hellou.sh: line 5: $1: unbound variable

Java thread safety: Memory Consistency Properties

2008-08-02T14:33:00.004+02:00

From the words-to-live-by department (or, the java.util.concurrent javadoc):

Chapter 17 of the Java Language Specification defines the happens-before relation on memory operations such as reads and writes of shared variables. The results of a write by one thread are guaranteed to be visible to a read by another thread only if the write operation happens-before the read operation. The synchronized and volatile constructs, as well as the Thread.start() and Thread.join() methods, can form happens-before relationships. In particular:

Each action in a thread happens-before every action in that thread that comes later in the program's order.
An unlock (synchronized block or method exit) of a monitor happens-before every subsequent lock (synchronized block or method entry) of that same monitor. And because the happens-before relation is transitive, all actions of a thread prior to unlocking happen-before all actions subsequent to any thread locking that monitor.
A write to a volatile field happens-before every subsequent read of that same field. Writes and reads of volatile fields have similar memory consistency effects as entering and exiting monitors, but do not entail mutual exclusion locking.
A call to start on a thread happens-before any action in the started thread.
All actions in a thread happen-before any other thread successfully returns from a join on that thread.

The methods of all classes in java.util.concurrent and its subpackages extend these guarantees to higher-level synchronization. In particular:

Actions in a thread prior to placing an object into any concurrent collection happen-before actions subsequent to the access or removal of that element from the collection in another thread.
Actions in a thread prior to the submission of a Runnable to an Executor happen-before its execution begins. Similarly for Callables submitted to an ExecutorService.
Actions taken by the asynchronous computation represented by a Future happen-before actions subsequent to the retrieval of the result via Future.get() in another thread.
Actions prior to "releasing" synchronizer methods such as Lock.unlock, Semaphore.release, and CountDownLatch.countDown happen-before actions subsequent to a successful "acquiring" method such as Lock.lock, Semaphore.acquire, Condition.await, and CountDownLatch.await on the same synchronizer object in another thread.
For each pair of threads that successfully exchange objects via an Exchanger, actions prior to the exchange() in each thread happen-before those subsequent to the corresponding exchange() in another thread.
Actions prior to calling CyclicBarrier.await happen-before actions performed by the barrier action, and actions performed by the barrier action happen-before actions subsequent to a successful return from the corresponding await in other threads.

Send IM messages from the command line with iChat, Adium or Pidgin

2008-07-23T03:44:00.021+02:00

In my office there was a tradition of using the zwrite command line tool for writing IM to eachother. This is useful to send snippets such as

grep FATAL logfile | zwrite coworker

It's quick, avoids tedious copy and paste and allows sending messages from batch run scripts. However, zwrite started to show its age long ago, being difficult to setup on various systems and having poor desktop integration and power.

Using XMPP (mostly over the Google Talk servers) works nicely, having good native clients, a web client (gmail) and a great java library for programmatic access. Missing was a zwrite-like command line way of sending snippets. I wrote a small shell script working with a running instance of either Pidgin under *nix (using D-Bus python bindings) and iChat or Adium (using applescript) on OS X:

#!/bin/sh

test $# -eq 0 && echo "Usage: echo hi | $0 user [otherusers] " && exit 1

export MESSAGE="`cat`"

while test "$1"; do

BUDDY=$1

# aliases
test $BUDDY = friend1 && BUDDY=friend1.account
test $BUDDY = friend2 && BUDDY=friend2.account
# ..

# default to @gmail.com
test `echo $BUDDY | grep @` || BUDDY=$BUDDY@gmail.com

export BUDDY

OS=`uname`
if [ "$OS" = "Darwin" ]; then
 # use applescript to call either ichat or adium
/usr/bin/osascript <<EOL
set stdinText to do shell script "echo \"\$MESSAGE\"" without altering line endings

tell application "System Events" to set iChatRunning to (name of processes) contains "iChat"
if iChatRunning then
tell application "iChat"
 send stdinText to buddy "$BUDDY"
 activate
end tell
else
tell application "System Events" to set adiumRunning to (name of processes) contains "Adium"
if adiumRunning then
 tell application "Adium"
  tell the first account of the service "GTalk" to ¬¨
   set newChat to ¬¨
    make new chat with contacts {contact "$BUDDY"} ¬¨
    with new chat window
  send newChat message stdinText
  activate
 end tell
else
 display dialog "ERROR: You must run either iChat or Adium" buttons "OK" default button "OK"
end if
end if
EOL
else
 # calling pidgin over dbus
 python -t - <<EOL
import os, sys
try:
import dbus
except ImportError: sys.exit('ERROR: You need dbus with python bindings installed')

try:
bus = dbus.SessionBus()
service = bus.get_object("im.pidgin.purple.PurpleService", "/im/pidgin/purple/PurpleObject")
purple = dbus.Interface(service, "im.pidgin.purple.PurpleInterface")
except: sys.exit('ERROR: Could not connect - is pidgin up and running?')

xmppAccount = None
for account in purple.PurpleAccountsGetAllActive():
      if 'XMPP' == purple.PurpleAccountGetProtocolName(account): xmppAccount = account
if xmppAccount == None: sys.exit('ERROR: Cannot find any xmpp account')

recipient = os.environ['BUDDY']
matches = purple.PurpleFindBuddies(xmppAccount, recipient)
if len(matches) == 0: sys.exit('ERROR: Buddy "%s" not found!' % recipient)

conversationType = 1 # http://developer.pidgin.im/doxygen/dev/html/conversation_8h-source.html#l00051
conversation = purple.PurpleConversationNew(conversationType, xmppAccount, recipient)
im = purple.PurpleConvIm(conversation)
message = os.environ['MESSAGE']
purple.PurpleConvImSend(im, message)
EOL
 test $? -ne 0 && exit 1
fi

shift
done

OS X command line tools: open

2008-07-23T03:27:00.005+02:00

The /usr/bin/open command is a command line utility deriving from NextStep, and opens a file (or a directory or URL), just as if you had double-clicked the file's icon. It is the equivalent of the freedesktop xdg-open command but has a few options making it a bit more useful:

-a specifies the application to use, so "open -a firefox http://osnews.com/" opens the site in firefox.
-e Causes the file to be opened with /Applications/TextEdit
-t Causes the file to be opened with the default text editor, as determined via LaunchServices
-f Reads input from standard input and opens the results in the default text editor. End input by sending EOF character (type Control-D). Also useful for piping output to open and having it open in the default text editor.
-n Open a new instance of the application(s) even if one is already running.
-W Causes open to wait until the applications it opens (or that were already open) have exited. Use with the -n flag to allow open to function as an appropriate app for the $EDITOR environment variable.

Note especially that you can launch applications bundles, so

open $HOME/Downloads/GLTron.app

allows you to try out http://www.gltron.org/ directly after unpacking.

Promising "Cola: Real-Time Shared Editing"

2008-07-17T00:25:00.002+02:00

With eclipse 3.4 comes ECF (eclipse communications framework) version 2, which allows this:

The only "collaborative real-time editor" I've seen before is SubEthaEdit which is mac-only (and does not have all those IDE features one wants for coding).

Eclipse change sets

2008-07-16T23:59:00.001+02:00

The release of subversion 1.5 brought basic support for "changelists" in the command-line client. It's a quick and easy way to organize work instead of at commit time having to go through the diffs and trying to remember what belonged to the issue at hand.

Working in eclipse, I have the need for something similar, and was surprised that up until now I've never noticed the existing support for change sets in eclipse. It feels great to "discover" a small but useful feature, even if it is a couple of years old :).

dyndns.com update script

2006-09-29T23:29:00.000+02:00

For using a dynamic ip address, dyndns.com provides a great free service. Below is a script which takes care of updating the account host ip periodically, taking care only to update when the local address and host ip differs. It uses whatismyip.org for looking up the local ip address - useful if you are sitting behind a router.

#!/bin/sh
USER=xxx
PASS=xxx
HOST=xxx
HOSTIP=`host $HOST | cut -d ” ” -f4`
MYIP=`wget -q -O - whatismyip.org`
if [ “$HOSTIP” != “$MYIP” ];
then wget –http-user=$USER –http-password=$PASS -O /dev/null \
 http://members.dyndns.org:8245/nic/update?system=dyndns&hostname=${HOST}&myip=${MYIP}&wildcard=ON”
fi