public static void main(String[] args) throws IOException {
DB db = DBMaker.newMemoryDB().make();
//
// TreeMap has build in support for lazily loaded values.
// In that case each value are not stored inside node,
// but in separate record.
//
// use DB.createTreeMap to create TreeMap with non-default parameters
Map map = db.createTreeMap("name").valuesOutsideNodesEnable().make();
map.put("key", "this string is loaded lazily with 'map.get(key)' ");
//
// Other option for lazily loaded record is to use Atomic.Var.
// In this case you have singleton record with name.
// As bonus you can update reference in thread-safe atomic manner.
//
Atomic.Var<String> record = db.createAtomicVar("lazyRecord", "aaa", db.getDefaultSerializer());
record.set("some value");
System.out.println(record.get());
// Last option is to use low level Engine storage directly.
// Each stored record gets assigned unique recid (record id),
// which is latter used to get or update record.
// Your code should store only recid as reference to object.
// All MapDB collections are written this way.
// insert new record
long recid = db.getEngine().put("something", SerializerBase.STRING_NOSIZE);
// load record
String lazyString = db.getEngine().get(recid, SerializerBase.STRING_NOSIZE);
// update record
db.getEngine().update(recid, "new value", SerializerBase.STRING_NOSIZE);
// I hope this example helped!
db.close();
}
public static void main(String[] args) throws IOException {
/** max number of elements to import */
final long max = (int) 1e6;
/** Open database in temporary directory */
File dbFile = File.createTempFile("mapdb", "temp");
DB db =
DBMaker.newFileDB(dbFile)
/** disabling Write Ahead Log makes import much faster */
.transactionDisable()
.make();
long time = System.currentTimeMillis();
/**
* Source of data which randomly generates strings. In real world this would return data from
* file.
*/
Iterator<String> source =
new Iterator<String>() {
long counter = 0;
@Override
public boolean hasNext() {
return counter < max;
}
@Override
public String next() {
counter++;
return randomString(10);
}
@Override
public void remove() {}
};
/**
* BTreeMap Data Pump requires data source to be presorted in reverse order (highest to lowest).
* There is method in Data Pump we can use to sort data. It uses temporarly files and can handle
* fairly large data sets.
*/
source =
Pump.sort(
source,
true,
100000,
Collections.reverseOrder(BTreeMap.COMPARABLE_COMPARATOR), // reverse order comparator
db.getDefaultSerializer());
/**
* Disk space used by serialized keys should be minimised. Keys are sorted, so only difference
* between consequential keys is stored. This method is called delta-packing and typically saves
* 60% of disk space.
*/
BTreeKeySerializer<String> keySerializer = BTreeKeySerializer.STRING;
/** Translates Map Key into Map Value. */
Fun.Function1<Integer, String> valueExtractor =
new Fun.Function1<Integer, String>() {
@Override
public Integer run(String s) {
return s.hashCode();
}
};
/** Create BTreeMap and fill it with data */
Map<String, Integer> map =
db.createTreeMap("map")
.pumpSource(source, valueExtractor)
// .pumpPresort(100000) // for presorting data we could also use this method
.keySerializer(keySerializer)
.make();
System.out.println(
"Finished; total time: "
+ (System.currentTimeMillis() - time) / 1000
+ "s; there are "
+ map.size()
+ " items in map");
db.close();
}