public static void main(String[] args) {
if (1 == 1) return;
// create inMemory store which does not use serialization,
// and has speed comparable to `java.util` collections
DB inMemory = new DB(new StoreHeap());
Map m = inMemory.getTreeMap("test");
Random r = new Random();
// insert random stuff, keep on mind it needs to fit into memory
for (int i = 0; i < 10000; i++) {
m.put(r.nextInt(), "dwqas" + i);
}
// now create on-disk store, it needs to be completely empty
File targetFile = UtilsTest.tempDbFile();
DB target = DBMaker.newFileDB(targetFile).make();
Pump.copy(inMemory, target);
inMemory.close();
target.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();
}