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20130708 (Monday, 08 July 2013)
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No launcher defined for extension 'odt'
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:ref:`davlink` doesn't work (out of the box) on an Ubuntu client.
It says "No launcher defined for extension 'odt'" even though 
libreoffice is installed.
Re-learning to develop and maintain a Java applet.

How to have DavLink re-scan your system for launchers::

  $ rm /etc/.java/.systemPrefs/lino/davlink/prefs.xml

(This will later cause a warning "Prefs file removed in background 
/etc/.java/.systemPrefs/lino/davlink/prefs.xml", but I didn't 
observe other problems than this)

Added a main method to DavLink applet so I can run it from a 
command line like this::

  $ cd ~/hgwork/lino/java
  $ java davlink.DavLink file:///path/to/document.odt


Enhanced :srcref:`java/Makefile` so that it reads my keystore password 
from a file :file:`~/.keystore_password` to avoid typing it again and 
again when working on the DavLink applet.


One question was: how to make error messages more evident?
I tried displaying them in a `javax.swing.JOptionPane` but didn't 
like the result. Want to get a standard Javascript alert message.

Solution: The applet method `DavLink.open` now returns the error 
message as a string. A return value f `null` means that there was no 
error. And `Lino.davlink_open` then calls alert on this.
Learned that I cannot return the plain Exception object,
it must be some simple data type like String.

The actual problem is still not solved: how can I configure my 
machine so that DavLink has permission to do what I want it to do?
When the applet's init method calls 
`setSecurityManager
<http://docs.oracle.com/javase/7/docs/api/java/lang/System.html#setSecurityManager(java.lang.SecurityManager)>`__
to install my own security manager, then I get (client using IcedTea)::

    java.lang.SecurityException: Changing the SecurityManager is not allowed.
        at net.sourceforge.jnlp.runtime.JNLPSecurityManager.checkPermission(JNLPSecurityManager.java:270)
        at java.lang.System.setSecurityManager0(System.java:295)
        at java.lang.System.setSecurityManager(System.java:286)
        at davlink.DavLink.init(DavLink.java:229)
        at sun.applet.AppletPanel.run(AppletPanel.java:436)
        at java.lang.Thread.run(Thread.java:722)
        
A customer (client using Oracle JRE) reports a similar message::
        
    java.lang.SecurityException: JVM Shared, not allowed to set security manager
        at sun.plugin2.applet.SecurityManagerHelper.checkPermissionHelper(Unknown Source)
        at sun.plugin2.applet.AWTAppletSecurityManager.checkPermission(Unknown Source)
        at java.lang.System.setSecurityManager0(Unknown Source)
        at java.lang.System.setSecurityManager(Unknown Source)
        at davlink.DavLink.init(DavLink.java:215)
        at com.sun.deploy.uitoolkit.impl.awt.AWTAppletAdapter.init(Unknown Source)
        at sun.plugin2.applet.Plugin2Manager$AppletExecutionRunnable.run(Unknown Source)
        at java.lang.Thread.run(Unknown Source)
        
This problem has appeared after upgrading from Java 6 to 7.

Note that the visible result varies heavily depending 
on the circumstances 
because exceptions during the init method are silently 
ignored (don't ask me why).

Note: `Setting the Security Level of the Java Client
<http://docs.oracle.com/javase/7/docs/technotes/guides/jweb/client-security.html>`_


Some docs
(e.g. `JOGL not launching on Ubuntu 12.04 LTS
<http://forum.jogamp.org/JOGL-not-launching-on-Ubuntu-12-04-LTS-td4005531.html>`__)
indicate that the answer must be to add something like the following to your 
:file:`java.policy` file::

    grant codeBase "http://127.0.0.1:8000/-" {
            permission java.security.AllPermission;
    };

One difficulty is to find the location of your :file:`java.policy`
file. A `ps aux | grep icedtea` says::

  luc      12676  4.5  0.7 1382728 60392 ?       Sl   19:11   0:02 /usr/lib/jvm/java-7-openjdk-i386/bin/java -Xbootclasspath/a:/usr/share/icedtea-web/netx.jar:/usr/share/icedtea-web/plugin.jar -classpath /usr/lib/jvm/java-7-openjdk-i386/lib/rt.jar sun.applet.PluginMain /tmp/icedteaplugin-luc/12666-icedteanp-plugin-to-appletviewer /tmp/icedteaplugin-luc/12666-icedteanp-appletviewer-to-plugin

So I guessed that my file is 
:file:`/usr/lib/jvm/java-7-openjdk-i386/jre/lib/security/java.policy`
or its link
:file:`/etc/java-7-openjdk/security/java.policy`.

Yes, this is a solution, the DavLink applet now works 
from an Ubuntu 12.04 LTS using IcedTea and Java7.

Note that the "Changing the SecurityManager is not allowed." 
exception still comes. But the whole init method has become 
useless. 

Explanation:
It was a security bug in Java6 to not raise this exception...
Users *must* edit their :file:`java.policy` 
file in order to allow DavLink to work.

Downgrading Java 7 to 6
-----------------------

A last proof of the above explanation would be to switch back to Java 6::

  $ sudo aptitude remove openjdk-7-jre
  
Now Chromium said 
"Java(TM) is required to display some elements on this page",
and an "Install plugins" button leads me to 
http://java.com/en/download/index.jsp
which suggests "Version 7 Update 25".
Not exactly what I want.

Ah, this one is better::

  $ sudo aptitude remove openjdk-7-jre icedtea-7-jre-jamvm icedtea-7-plugin

But there are still two automatic packages remaining::

    i A openjdk-7-jre-headless                                              - OpenJDK Java runtime, using Hotspot JIT (headless)                           
    i A openjdk-7-jre-lib                                                   - OpenJDK Java runtime (architecture independent libraries)      

How to find out which package(s) caused a given 
package to install automatically?