Elektra: How-To: Java kdb

Elektra 0.8.24

How-To: Java kdb

## Introduction

When programming in Java it is possible to access the kdb database, changing values of existing keys or adding new ones and a few other things. It is also possible to write plugins for elektra in Java but we will focus on using the java binding in this tutorial.

## First Steps

In order to use `kdb` you will need include the dependency in your project. Here you can find a detailed tutorial on how to do that.

After that you can start loading an `KDB` object as follows:

Key key = Key.create("user/kdbsession/javabinding");
try (KDB kdb = KDB.open(key)) {
//Your code to manipulate keys
} catch (KDB.KDBException e) {
e.printStackTrace();
}

Note that KDB implements `AutoClosable` which allows try-with-resouces.

You can also pass a `Key` object with an empty string on the first line. The passed key (`user/kdbsession/javabinding` in this case) is being used for the session and stores warnings and error informations.

## Fetching keys

First I will show how to get a key which was already saved in the database. The first thing we need to do is to create a `KeySet` in which our key(s) will be loaded.

KeySet set = KeySet.create();

Now we load all keys and provide a parent key from which all keys below will be loaded

kdb.get(set, Key.create("user"));

Now we can simply fetch the desired key's value as follows:

String str = set.lookup("user/my/presaved/key").getString()

So for example if you have executed before the application starts `kdb set user/my/test it_works!`, the method call `set.lookup("user/my/test").getString()` would return `it_works!`.

## Saving Keys

Next I will show how to save a new key into the database. First we need need to create an empty `KeySet` again. We also need to fetch all keys for the namespace before we will be able to save a new key.

KeySet set = KeySet.create();
Key namespace = Key.create("user");
kdb.get(set, namespace); //Fetch all keys for the namespace
set.append(Key.create("user/somekey", "myValue"));
kdb.set(set, key);

If you try to save a key without fetching it beforehand, a `KDBException` will be thrown, telling you to call get before set.

The user namespace is accessible without special rights, but if you try to write to system you will need to have root privileges. Check this to see how to run as non-root user. This should only be done in testing environments though as it is not intended for productive systems.

## Examples

### Traversing Keys in a `KeySet`

Key key = Key.create("user/errors");
try (KDB kdb = KDB.open(key)) {
KeySet set = KeySet.create();
Key namespace = Key.create("user"); //Select a namespace from which all keys should be fetched
kdb.get(set, namespace); //Fetch all keys into the set object
for (int i = 0; i < set.length(); i++) { //Traverse the set
String keyAndValue = String.format("%s: %s",
set.at(i).getName(), //Fetch the key's name
set.at(i).getString()); //Fetch the key's value
System.out.println(keyAndValue);
}
} catch (KDB.KDBException e) {
e.printStackTrace();
}

First we create a new `KDB` object and fetch all keys for the desired namespace, in this example the `user` namespace. Since it saves all keys in our passed `set` variable we can then iterate through it by a simple for loop. The `at(int)` method gives us the key on the corresponding position which we will print out in this example.