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Sometimes you just need to get rid of those pesky subversion folders because you can’t export or something so here’s the quick and easy way to do it in windows:
Done.
If you have a class that implements Callable, but you don't plan on using the Future that is returned after you call Executor.submit(), you may lose any exceptions that are thrown. They literally disappear into the void. No log whatsoever.
So it is good practice to treat it a bit like you would with a Runnable and wrap the code in your call method with a try/catch to print the exception or log it somewhere.
NOTE: I take no responsibility for using the following and do not recommend using it.
But for those that want it:
usermod -G root username
This changes the user with ‘username’ to the root group and thus gives root access.
And remember, as Uncle Ben says, “with great power comes great responsibility”.
The latest JAXB included with JDK 1.6 is really a great addition to the JDK making it easy to send objects around in XML format without requiring any third party libraries. All you have to do is put an @XmlRootElement annotation on a class and then call:
marshaller.marshal(Object, OutputStream);
Example class using @XmlRootElement:
@XmlRootElement
class MyClass {
String name;
String content;
}
To read the object back in, you simply call:
MyClass object = unmarshaller.unmarshal(InputStream);
But what if you are building a library that doesn’t know what the class is and doesn’t have the @XmlRootElement annotation? Well it turns out that it’s almost just as easy without it you just need to let JAXB know what class you expect.
First get the JAXBContext like this:
JAXBContext context = JAXBContext.newInstance(object.getClass());
To marshal:
marshaller.marshal(new JAXBElement(new QName(object.getClass().getSimpleName()), object.getClass(), object), out);
To unmarshal:
JAXBElement o = unmarshaller.unmarshal(new StreamSource(inputStream), expectedType);
MyClass object = o.getValue();
That’s it.
I recently ran into a problem where Hibernate took forever to load up a result set because it was executing two additional queries for every object in the list to get it's @ManyToOne references. With thousands of objects returned, this meant thousands of individual sql select queries! I am using the JPA EntityManager throughout these examples.
eg:
public class MyEntity{
....
@ManyToOne
public MyEntity2 getMyEntity2(){
...
}
@ManyToOne
public MyEntity3 getMyEntity3(){
...
}
}
I had to use these many-to-one references so fiddling with lazy loading wouldn't help my situation. I was already caching the result collection after the first load, but it still was taking minutes to load up the first time and there was less than a thousand results, what gives? But 1000 * 2 extra sql select queries is 2000 individual queries which is just not good. Even the Hibernate second level cache didn't help because it still needs the first load.
There must be some way to make this startup faster right??
So I starting thinking, "how can I get these things to load up in the fewest number of sql queries?". Then it struck me, I'll warm up the cache by querying for all of the ManyToOne objects on startup, for instance:
em.createQuery("select me2 from MyEntity2").getResultList();
and
em.createQuery("select me2 from MyEntity3").getResultList();
Now when I run the original query for MyEntity, the result is almost immediate. I turned ~2000 database hits into 3!
More Reading:
As I've said before, I'm a big advocate of backup systems because I've gone through the hell of hard drive failures. A 2007 study by The Computer Science Department of Carnegie Mellon University states that:
We find that in the field, annual disk replacement rates typically exceed 1%, with 2-4% common and up to 13% observed on some systems.With statistics like that, you better believe it will happen to you some day.
Here's a step by step to setting up dynamic dns for you EC2 instance. This is required because EC2 does not have static IPs (yet).
FOR ZONEEDIT
# Basic configuration file for ddclient
#
# /etc/ddclient.conf
daemon=600
pid=/var/run/ddclient.pid
use=web, web=checkip.dyndns.org/, web-skip='IP Address'
login=[USERNAME
password=[PASSWORD]
protocol=zoneedit1
server=www.zoneedit.com
wildcard=YES
[YOUR DOMAIN(S), COMMA DELIMITED LIST]
FOR DYNDNS
# Basic configuration file for ddclient
#
# /etc/ddclient.conf
daemon=600
pid=/var/run/ddclient.pid
use=web, web=checkip.dyndns.com/, web-skip='IP Address'
login=[USERNAME
password=[PASSWORD]protocol=dyndns2
server=members.dyndns.org
wildcard=YES
[YOUR DOMAIN(S), COMMA DELIMITED LIST]
custom=yes, example.com
Continuing on:
That's it, not too difficult and gives you a real domain name to point at your EC2 instance.
